- *-**■ (V2W)
VUAMS I N S T I T U U T i / n n D
- u s i e n d e - Belgium
Potential pressure indicators for fishing, and
their data requirements
G. J. Piet, F. J. Q uirijns, L. R o b in so n , a n d S. P. R. G r e e n s t r e e t
Piet, G. J., Quirijns, F. J., R obinson, L., a n d G reen street, S. P. R. 2007. P otential p ressure in d icato rs f o r fishing, a n d th e ir d a ta req u irem en ts. ICES Jo u rn al o f M arine Science, 6 4 :1 1 0 - 1 2 1 .
In d ic a to rs o f fish in g p re ss u re a r e n e c e s sa ry t o s u p p o r t a n e c o s y ste m a p p r o a c h t o fish eries m a n a g e m e n t (EAFM ). W e p r e s e n t a fram ew o rk t h a t d istin g u is h e s f o u r levels o f p r e s s u re in d ic a to r s t h a t m o v e fro m b e in g a sim p le d e s c r ip tio n o f a n th r o p o g e n ic a c tiv ity to
m o r e p recisely d e s c rib in g t h e a c tu a l p re s s u re o n t h e e c o s y s te m a n d its c o m p o n e n ts , b u t w h ic h re q u ire in creasin g ly m o r e in fo rm a tio n
t o b e q u a n tifie d . W e u se t h e e x a m p le o f t h e D u tc h b e a m traw l fle e t in th e N o r th S ea t o c o m p a r e th e s e p re s s u re in d ic a to rs, a s th e
level o f in fo rm a tio n u s e d is in c re a se d . T h e first level is t h a t o f fle e t c a p a c ity (e.g. n u m b e r o f vessels), t h e s e c o n d is fish in g effort,
u su ally e x p re ss e d a s th e n u m b e r o f h o u r s fish in g o r d ays a t sea, th e t h ir d in c o r p o ra te s fish in g p a r a m e te r s s u c h as th e p r o p o r tio n o f
tim e a c tu a lly s p e n t fis h in g fish in g s p e e d , o r g e a r c h a ra c te ris tic s , e.g. t h e size o f t h e b e a m tr a w l in o r d e r t o d e te r m in e t h e fre q u e n c y
w ith w h ic h a n a r e a is fish ed , a n d a t t h e f o u r th level, t h e m o s t in fo rm a tiv e m e a s u re o f f is h in g p re ssu re , a n n u a l fish in g m o rta lity , is
a v a ila b le fo r a few c o m m e rc ia l s p e c ie s fro m s to c k a s se s sm e n ts . F or o t h e r sp ecies, it c a n b e c a lc u la te d fro m t h e lo w e r level p re ssu re
in d ic a to r s th r o u g h t h e in c o r p o r a tio n o f t h e c h a n c e o f in d iv id u a ls o f a sp e c ie s c o m in g in to c o n t a c t w ith t h e fishing g e a r a n d th e
e n c o u n t e r m o rta lity , w h ic h is t h e p o r tio n o f m o r ta lity c a u s e d by t h e p a s sin g o f t h e g ear. C o m p a r is o n o f t r e n d s a n d a b s o lu te v alu es
s h o w s t h a t t h e p re ss u re in d ic a to r s a t d if fe r e n t levels d iffer c o n s id e ra b ly in t h e i r d e s c rip tio n o f b o th p r e s e n t a n d h isto ric a l fishing
im p a c t in t h e N o rth Sea. T h e re fo re , fo r a n EAFM, w e ad v ise u sin g th e h ig h e s t level p re s s u re in d ic a to r t h a t c a n b e o b ta in e d w ith th e
d a t a av ailab le.
K eyw ords: ecosystem a p p ro a c h to fisheries m a n a g e m e n t, fishing im p act, fishing m o rtality , m a rin e ecosystem , m étier, p ressure in d icato r.
Received 22 February 2006; a c c e p te d 28 A ugust 2006; ad v an ce access publication 25 O c to b e r 2006.
G. J. Piet a n d F. J. Quirijns: W ageningen IM ARES, PO Box 68, 1970 AB Ijmuiden, The N etherlands. L Robinson a n d S. P. R. Greenstreet: Fisheries
Research Seri/ices, M arine Laboratory, PO Box 701, Victoria Road, A berdeen, A B U 9DB, UK. C orrespondence to G. J. Piet: tel: + 3 7 255 564699;
fa x: + 3 7 255 564644; e-mail: gerjan.piet@ w ur.nl.
Introduction
b e tw e e n “c a u s e ” a n d “e ffe c t” b e w ell e n o u g h u n d e r s to o d to
W h e n im p le m e n tin g a n e c o s y ste m a p p r o a c h to fish eries m a n a g e ­
p r o v id e th e a d v ic e r e q u ire d ( D a a n , 2005).
m e n t (E A F M ), in d ic a to r s a re r e q u ire d to d e s c rib e th e p re ss u re s
S everal
fra m e w o rk s
have
been
p ro p o sed
fo r
classifying
a ffe c tin g th e eco sy stem , th e s ta te o f th e ec o sy ste m , a n d th e
e n v ir o n m e n ta l m a n a g e m e n t in d ic a to r s o n th is b a sis, e.g. th e
re s p o n s e o f m a n a g e rs (Je n n in g s , 2 0 0 5 ). S u c h in d ic a to r s c a n be
p re s s u re s ta te r e s p o n s e (P S R ) sy stem (G a rc ia a n d S tap les, 200 0 ).
u s e d to s u p p o r t m a n a g e m e n t d e c is io n -m a k in g , to tra c k p ro g re s s
T h is f ra m e w o rk u se s p re s s u re in d ic a to r s (P ) to m e a s u re th e
to w a rd s m e e tin g m a n a g e m e n t o b je c tiv e s, a n d to a id c o m m u n i ­
p re s s u re im p a c tin g a n ec o sy ste m c o m p o n e n t, sta te in d ic a to r s (S)
c a tio n w ith n o n -sp e c ia lis t a u d ie n c e s (G a rc ia e t a l , 2000; R ice,
to m e a s u re th e s ta te o f th e ec o sy ste m c o m p o n e n t, a n d re sp o n se
20 0 0 ; R o c h e t a n d T ren k el, 2 0 0 3 ). M a n y in d ic a to r s h av e b e e n p r o ­
in d ic a to r s (R ) t o m e a s u re th e r e s p o n s e o f m a n a g e rs to t h e ch an g e
p o s e d (e.g. R ice, 2000; L in k , 2002; L in k e t a l , 2002; R o c h e t a n d
in sta te . T h is is in lin e w ith t r a d itio n a l fish e rie s m a n a g e m e n t, in
T ren k el, 2 0 0 3 ), b u t few ( i f a n y ) o f th o s e t h a t tr a c k c h a n g e s in th e
w h ic h a r e p o r t is m a d e fo r ea c h sto c k o n w h a t is c o n s id e re d th e
“ s ta te ” o f th e m a r in e e n v ir o n m e n t o r o f d iffe re n t e c o s y ste m c o m ­
b e st s ta te in d ic a to r (SSB, s p a w n in g -s to c k b io m a s s ) a n d best
p o n e n ts (e.g. fish, b e n th o s , h a b ita t) c a n s u p p o r t m a n a g e m e n t
p re s s u re in d ic a to r (F, fish in g m o r ta lity ) , w h e re a s th e re sp o n se
d ire c tly (R ice, 2 0 0 0 ). T h is is larg ely b e c a u s e t h e p re c ise cau ses o f
in d ic a to r is u su a lly th e to ta l allo w ab le c a tc h set b y m a n a g e rs. As
a n y c h a n g e s in “s ta te ” m a y b e p o o r ly u n d e r s to o d , m a k in g it d iffi­
p o lic y c o m m itm e n ts a n d a s so c ia te d o b jectiv es g e n e ra lly re la te to
c u lt to sp e cify a p p r o p ria te m a n a g e m e n t a c tio n . T o im p le m e n t a n
“s ta te ”, fo r e x a m p le , to c o n s e rv e b io d iv e rsity , re fe re n c e p o in ts ,
E A FM su ccessfully, th e re fo re , it is n o t o n ly n e c e ssa ry to h av e a
ta rg e ts , o r i n d ic a to r tre n d s n e e d e d to m e a s u re p ro g re s s to w a rd s
s u ite o f in d ic a to r s th a t a c c u ra te ly a n d c o m p re h e n s iv e ly p o r tra y
m e e tin g m a n a g e m e n t objectiv es te n d m o s t f re q u e n tly to b e se t for
th e “ s ta te ” o f v a rio u s e c o s y ste m c o m p o n e n ts , b u t it is also c ritic a l
“ s ta te ” in d ic a to rs . H o w ev er, th e “ s ta te ” o f d iffe re n t c o m p o n e n ts
to h av e in d ic a to r s th a t d e s c rib e c h a n g e s in th e level o f d iffe re n t
o f m a r in e e c o sy ste m s can rare ly , if ever, b e m a n a g e d d irectly . All
m a n a g e a b le a n th r o p o g e n ic a c tiv itie s, a n d t h a t in d ic a te th e im p a c t
m a n a g e rs c a n realistically h o p e
o f each a c tiv ity o n th e v a r io u s e c o s y ste m c o m p o n e n ts . O n ly b y
“p r e s s u re ” s u c h t h a t “sta te ” in d ic a to r s are k e p t w ith in , o r m o v e d
a d e q u a te ly
to w a rd s , a c c e p ta b le lim its. “ P re s s u re ” a n d “ r e s p o n s e ” in d ic a to rs
c o v e rin g b o t h
a sp ects w ill th e
m e c h a n is tic lin k s
to
a c h iev e is to
£ ’2 0 0 6 I n te r n a tio n a l C o u n c il fo r t h e E x p lo r a tio n o f th e S ea. P u b lis h e d b y O x fo rd J o u rn a ls . A ll rig h ts reserv ed .
F o r P e rm is sio n s , p lease em a il: jo u r n a ls .p e r m is s io n s @ o x fo r d jo u r n a ls .o r g
m a n ip u la te
109
O p tim iz in g a stra tifie d s a m p lin g design w h e n fa c e d w ith m u ltip le objectives
V in th er, M ., a n d L arsen, F. 2004. U p d a te d estim ates o f h a r b o u r p o r ­
po ise (Phocoena phocoena). Jo u rn a l o f C etacean R esearch an d
M a n ag em en t, 6: 1 9 -2 4 .
W o rm , B., Lotze, H . K., a n d M yers, R. A. 2003. P re d a to r diversity h o t­
sp o ts in th e b lu e ocean. P roceedings o f th e N a tio n a l A cadem y o f
Sciences o f th e USA, 100: 9 8 8 4 -9 8 8 8 .
ob jectiv e fu n c tio n [E q u a tio n ( 7 ) ] c a n be w r itte n as
_
©; = £
p -e ;
Yates, F. 1981. S am p ling M ethods fo r C ensuses a n d Surveys, 4 th edn.
M acM illan, N ew York. 458 pp.
S=]
S
P
S=1
p = 1
A
wpAps
S=1 p = 1 W p j s
S=1
S=1
A
S=1
w e h ave th e sa m e f o r m a t a s E q u a tio n (2 ), a n d th e o p tim a l
Appendix 1
sa m p lin g fra c tio n s [ E q u a tio n ( 8 )] follow .
Derivation o f optim al observer coverage rates
T h e o p tim a l s a m p lin g fra c tio n s c a n b e f o u n d u s in g th e C a u c h y S c h w a rtz I n e q u a lity o r L ag ran g e m u ltip lie rs . T h e s e c o n d c o m ­
p o n e n t o f E q u a tio n (2) a n d th e first c o m p o n e n t o f E q u a tio n (3)
a r e n o t a f u n c tio n o f fi, so th e m i n i m u m o f E q u a tio n (2 ) w ith
Appendix 2
Derivation o f th e unbiasedness of
T h e e s tim a to r S©>v h a s a r a n d o m c o m p o n e n t:
re sp e c t to f i w ill n o t be affe cted b y th o s e c o m p o n e n ts . B y th e
C a u c h y - S c h w a r tz In e q u a lity ( C o c h r a n , 1977, p p . 9 6 - 9 8 ; C asella
£(ê,-êv)2=è®?-n„ ê ; = ^ £ ê ï
a n d B erg er, 2 0 0 2 , p . 187),
f=i
Uv
t= i
nv
,
('T - T o
- rHv£í= £i «=i® Ä -
=h £ w ) ( £ y
V ar
t=i
nv
l#w
T h e e x p e c ta tio n s w e r e q u ire a r e
>
J 2 V A 'T M
£ ( £ ® î ) = 4 £ [ ® ? + V ar(Ô ,)
G iv e n A s a n d N s fo r all size classes a n d t h e c o s t p e r tr ip (T „),
e q u a lity is a c h ie v e d w h e n
^ £ [® ;+ V a r(® ,
A
1/2
and
I£ \
(H )
T n fsN sJ
I
fo r e a c h s are e q u a l (i.e. x$ — A)- L e ttin g fifi d e n o te th e s a m p lin g
f ra c tio n fo r th e s th size class t h a t m in im iz e s th e v a ria n c e , w e can
nv
nv
£ £ © ,©
f= l
\\
,
U— 1
\
(
=
t= I
ƒ
/
re w rite E q u a tio n (1 1 ) as
nv
nv
£ £ © .© .
b
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\
n v( n v -
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t^ u
( 12 )
s j A sT nN s = x T n N s f ; .
1)
w h ic h rely o n th e facts t h a t V a r(x ) = E { x 2) - E ( x ) 2, a n d th e
S u m m in g E q u a tio n (12) o v e r all S a n d so lv in g f o r x> w e can
e s tim a to rs 0 , a re i n d e p e n d e n t a c ro ss e le m e n ts in th e s tra tu m . W e
s u b s titu te f o r x in E q u a tio n ( 11) to o b ta in
also rely o n th e first a n d s e c o n d - o r d e r in c lu s io n p ro b a b ilitie s in
SRS, n v/ N v a n d ^ ( « v — l ) / N v (N v — 1), resp ec tiv ely , w h ic h are
p r o v e n elsew h ere (S ä rn d a l e t a l , 1992, p p . 3 1 - 3 2 ) . N o w ,
T -T
ƒ/ =
q
(AS/ N Q 1/2
r* Ell (U)172
< > = ¿ r r £
£ ( ê '-
R e co g n izin g
©
Nv
N,
wy £f=1 ®U ñtatttI)) £t=1 £U=1 010“+ ñ £f=1 Var(ê'
M
t* u
T - T 0 = Tn ¿
fisN s = T„ ¿
ns = T„n
= s L + ^ - £ Var(®>)y ie ld s th e re s u lt i n E q u a tio n (4 ).
w h ic h gives u n b ia s e d n e s s o f A v (i.e. £ (A V) = N vE(S@tV) = Av ).
T h e sa m e a p p r o a c h is u se d t o o b ta in th e o p tim a l s a m p lin g
fra c tio n s w ith m u ltip le e s tim a tio n o b jectiv es. N o tin g t h a t th e
d o i:1 0 .1 0 9 3 / icesjm s / fslO 13
P o te n tia l p ressu re indicators fo r fish in g , a n d th eir d a ta re q u irem en ts
a r e clearly e sse n tia l in th is p ro c e ss. S u c h in d ic a to r s also o fte n h av e
th e
d e s ira b le p r o p e rtie s o f ease o f m e a s u re m e n t a n d
111
s o u rc e o f m o rta lity . C learly, to b e able t o p r e d ic t re a listic a lly th e
ra p id
re sp o n s e o f eco sy stem c o m p o n e n t s to fish eries im p a c t, o n e m u s t
re s p o n s e tim e s. C o n s e q u e n tly , g u id a n c e fo r y e a r - o n -y e a r m a n a g e ­
firs t e s ta b lis h th e level o f m o r t a l i t y e x p e rie n c e d b y th e s e c o m ­
m e n t d e c is io n -m a k in g is o f te n b e tte r b a s e d o n p r e s s u re a n d
p o n e n ts b e fo re in p u ttin g th is t o a n o v erall m o d e l o f t h e fa c to rs
t h a t s tr u c tu r e th e m .
r e s p o n s e in d ic a to r s , w ith c h a n g e s in s ta te assessed less f re q u e n tly
to c o n f ir m t h a t p re s s u re a n d re s p o n s e are affe ctin g s ta te as p r e ­
d ic te d (N ic h o ls o n a n d Je n n in g s, 2004).
I n te r m s o f in d ic a to r s r e q u i r e d to im p le m e n t a n E A F M ,
p re s s u re in d ic a to rs s h o u ld id e a lly a c c o u n t f o r m o r ta lity t o a n e c o ­
T h e d e v e lo p m e n t o f p re ss u re in d ic a to r s fo r fish eries h a s in th e
s y ste m c o m p o n e n t t h a t r e s u lts d ir e c tly f ro m fish in g , s ta te in d i ­
p a s t t e n d e d to b e h a m p e r e d b y c o n f u s io n o v e r th e d iffe re n c e
c a to rs fo r th e o v erall s ta te o f t h a t c o m p o n e n t, a n d re s p o n s e
b e tw e e n t h e a c tu a l e c o lo g ic a l im p a c t o f fish in g ( m o rta lity a n d
in d ic a to r s fo r th e r e s p o n s e o f m a n a g e r s re q u ire d to a lte r th e level
h a b ita t c h a n g e ) a n d th e c o m m u n ity level c h a n g e s t h a t are la te r
o f p re s s u re w h e re th e r e h av e b e e n u n a c c e p ta b le c h a n g e s i n state.
s e e n as a c o n s e q u e n c e o f th is im p a c t (e.g. a c h a n g e in th e size
I n case m a n a g e m e n t o b jectiv es a r e se t fo r p o p u la tio n s o f eco sy s­
s tr u c tu r e o f th e c o m m u n ity ). T h e ec o sy ste m c o m p o n e n ts for
te m c o m p o n e n ts o th e r th a n t h e ta r g e t fish sp e cies (e.g. th r e a te n e d
w h ic h m o s t in f o r m a tio n is av ailab le o n th e d ir e c t effects o f fish in g
a n d d e c lin in g species, se n sitiv e b e n t h i c sp e cies) o r a t th e c o m m u ­
a r e fish a n d b e n th ic in v e rte b ra te s (D a y to n e t a l , 1995; Je n n in g s
n ity level (e.g. m e a n w e ig h t o f t h e fish c o m m u n ity ), th e k n o w n
a n d K aiser, 1998; H a ii, 1999; C ollie e t a l , 2000; K a ise r a n d de
m o r ta lity e s tim a te s o f a few t a r g e t fish sp e cies th a t c o m e fro m
G r o o t, 2 0 0 0 ). M a n y in d ic a to r s h av e b e e n p r o p o s e d th a t d e sc rib e
s to c k a s se s sm e n ts m a y n o t b e a d e q u a t e to lin k p r e s s u re w ith s u c h
t h e s ta te o f th e s e c o m p o n e n ts a t d iffe re n t h ie ra rc h ic a l levels (e.g.
s ta te in d ic a to rs . F o r th o s e c a se s, th e m o r ta lity o f all ec o sy ste m
p o p u la tio n a n d c o m m u n ity levels, see F rid et a l , 2005; P ie t a n d
c o m p o n e n ts can b e c a lc u la te d fo llo w in g t h e sw e p t-a re a m e th o d
J e n n in g s , 2 0 0 5 ). H o w e v e r, th e s ta te o f th e in d iv id u a l c o m p o n e n ts
in tr o d u c e d sim u lta n e o u s ly b y P o p e e t a l (2 0 0 0 ) fo r fish a n d P ie t
e ta l. (2 0 0 0 ) fo r b e n th o s .
is n o t o n ly d e te r m in e d b y th e effects o f a p a r tic u la r a n th r o p o g e n ic
a c tiv ity s u c h as fish in g , b u t b y a c o m b in a tio n o f all in tr in s ic a n d
S ta r tin g f ro m th e p re m is e t h a t fish in g m o r ta lity is th e m o s t
e x trin s ic f a c to rs th a t c o m b in e to s tr u c tu r e p o p u la tio n s a n d c o m ­
a c c u ra te m e a s u re to d e sc rib e f is h in g im p a c t b u t a c k n o w le d g in g
m u n itie s . T h e s e in c lu d e a c o m b in a tio n o f b io tic (e.g. c o m p e titio n ,
t h a t d a ta lim ita tio n s o fte n fo rc e u s to u se le s s -in fo rm a tiv e p ro x ie s,
p r e d a tio n , a n d la rv a l d isp e rs a l) a n d a b io tic fa c to rs (e.g. c lim a ti­
w e in tr o d u c e a f ra m e w o rk (F ig u r e 1) t h a t e n c o m p a s se s th e m o s t
c a lly d r iv e n c h a n g e s in te m p e r a tu re a n d p ro d u c tiv ity ; M u ra w sk i,
c o m m o n p re ss u re in d ic a to r s . T h e m a in o b je c tiv e o f th is p a p e r is
1993; C la rk a n d F rid , 2001; K rö n c k e , 2 0 0 1 ). I n th e o re tic a l eco lo g y
t o d e s c rib e th is f ra m e w o rk a n d h o w th e d iffe re n t e x is tin g p re s s u re
te r m s , im p a c t o r d is tu r b a n c e is th e m o r ta lity c a u s e d b y p e r t u r ­
in d ic a to r s c o m p a re w ith o n e a n o th e r , b y in c re m e n ta lly a d d in g
b a tio n s to th e e c o s y ste m , a n d fish eries im p a c t is a n a n th r o p o g e n ic
in f o r m a tio n
to th e m o s t b a s ic p r e s s u re in d ic a to r , u ltim a te ly
le a d in g to in d ic a to r s t h a t d e s c rib e th e a c tu a l ec o lo g ic a l im p a c t o f
fish in g , i.e. th e level o f m o r ta lity in f lic te d o n a p a r tic u la r eco sy s­
te m c o m p o n e n t. T h is a p p r o a c h m a k e s e x p lic it th e a s s u m p tio n s
C a p a c ity
(e .g. n u m b e r o f v e s s e ls )
t h a t a re m a d e a t lo w e r levels o f in f o r m a tio n c o n t e n t a n d h o w
th e s e in flu e n c e th e p re ss u re in d ic a to r .
W e c h o se th e D u tc h b e a m tra w l fish e ry in th e s o u t h e r n N o r th
A c tiv ity
S ea as a case stu d y . B eam tra w lin g a c c o u n ts fo r a h ig h p r o p o r t i o n
o f a ll fish in g activity, p a r tic u la r ly in t h e s o u th e r n N o r t h Sea
F is h in g e ffo rt
(e.g. d a y s a t s e a )
(J e n n in g s e t a l , 1999), a n d th e D u t c h b e a m tra w l fle e t is r e s p o n ­
sib le fo r > 7 0 %
o f to ta l b e a m tra w l e ffo rt in th e N o r t h Sea.
M o re o v e r, th e fish e ry a n d fleet h a v e b e e n in te n siv e ly s tu d ie d in
F is h in g p a ra m e te rs
G e a r c h a ra c te ris tic s
re c e n t y ears, r e s u ltin g in a h ig h lev el o f k n o w le d g e r e g a r d in g its
p re c ise o p e r a tio n ( R ijn s d o rp et a l , 1998, 2 000a, 2 0 0 0 b ; P ie t e t a l ,
F re q u e n c y tra w le d
2 0 0 0 ). M o r ta lity is o n ly e s tim a te d f o r th e tw o ta rg e t sp e c ie s o f th e
b e a m tra w l fishery, p la ic e (P leuronectes pla tessa ) a n d so le (Solea
vu lg a ris), d e s p ite th e fish e ry ’s p o te n tia l to cau se c o lla te ra l d a m a g e
to o t h e r c o m p o n e n ts o f th e m a r in e ec o sy ste m . F o r e s tim a te s o f
C h a n c e o f e n c o u n te r a nd
E n c o u n te r m o rta lity
fis h in g p re s s u re o n th e s e o th e r c o m p o n e n ts , o n ly o n e o f t h e least
in f o rm a tiv e m e a s u re s o f e ffo rt (i.e. d a y s a t sea) a t a sp a tia l sc ale o f
A n n u a l fis h in g m o rta lity
(% y 1)
IC E S re c ta n g le s ( ~ 3 0 x 30 n a u tic a l m ile s) is available. I n o r d e r to
e x e m p lify h o w th is c a n b e im p ro v e d , w e u se d th e sw e p t-a re a
m e t h o d to e s tim a te th e m o r ta lity o f a “v ir tu a l p o p u l a t i o n ”, w ith
Figure 1. S c h e m a tic re p re se n ta tio n o f fishing p ressu re in d icato rs a t
d ifferen t levels o f in fo rm atio n c o n te n t. A ctivity in d icates th e
n u m b e r o f fishing h o u rs o r days a t sea p e r vessel. T h e fishing
p a ra m e te rs a n d g ear ch aracteristics d e te rm in e h o w m u c h area is
c o v e re d by a u n it o f fishing effort, w hich tra n sla te s in to th e
fre q u e n c y w ith w h ich a specific are a is traw led. T h e c h a n c e of
e n c o u n te r d e te rm in e d by th e spatial d istrib u tio n o f t h e p o p u latio n
relative t o t h a t o f th e fishery g ear a n d th e e n c o u n te r m ortality,
ex p ressed as th e p ro p o rtio n a l m o rta lity o f individuals in th e p a th o f
th e gear, d e te rm in e th e e x te n t to w hich a p o p u la tio n is actually
affe cted by th e fishery (i.e. th e a n n u a l fishing m o rta lity rate).
sp e cific c h a ra c te ristic s t h a t d e te r m in e th e im p a c t th e f is h e ry h a s
o n it.
T h e c o n s e q u e n c e s o f th e s e fin d in g s f o r th e c o lle c tio n o f d a ta
n e e d e d to s u p p o r t a n E A FM are d isc u sse d .
Material and m ethods
T h e D u tc h b e a m tra w l fleet o p e ra te s m a in ly in t h e s o u th e a s te r n
N o r th Sea a n d ta rg e ts th e flatfish sp e c ie s p la ic e a n d so le . T w o
p r in c ip a l fish in g m é tie rs are u su ally d is tin g u is h e d : “la rg e v essels”
w ith a n e n g in e p o w e r o f 221 kW o r m o r e , a n d “e u r o c u tte r s ”, w ith
112
G. J. P ie t e t al.
a n e n g in e p o w e r < 2 2 1 kW . T h e tw o m é tie rs d iffe r m a r k e d ly in
A t Level 3, p re ss u re is d e s c r ib e d b y t h e tra w lin g fre q u e n c y a n d
fish in g p ra c tic e a n d g ear c h a ra c te ristic s. T ypically, th e la rg e vessels
in c lu d e s in f o rm a tio n o n f is h in g p ra c tic e a n d g e a r c h a ra c te ristic s ,
d e p lo y 2 x 1 2 -m b e a m traw ls a n d a re n o t allo w ed to fish in s id e
a llo w in g , fo r e x a m p le , t h e to t a l are a o f se a b e d s w e p t b y t h e gear,
t h e 12 m ile c o a s ta l z o n e o r th e “ plaice b o x ”, w h e re a s t h e e u r o c u t-
o r th e v o lu m e o f w a te r f ilte r e d , in a g iv e n p e r io d o f tim e to b e cal­
te rs d e p lo y 2 x 4 - m b e a m traw ls a n d a re a llo w e d to fish in sid e
c u la te d . A t th is level, it b e c o m e s r e le v a n t i f in f o r m a tio n o n th e
th o s e areas.
B e cau se b e a m tra w lin g h a s s u c h a h ig h p o te n tia l to c a u s e c o l­
s p a tia l d is tr ib u tio n o f e f f o r t exists a n d w h e n th is in f o r m a tio n is
la te ra l d a m a g e t o o t h e r c o m p o n e n ts o f m a r in e e co sy stem s, in c lu d ­
tin g u is h in g b etw een : (i) n o s p a tia l in f o r m a tio n av ailab le; (ii) lo w
in g fish a n d b e n t h i c in v e rte b ra te c o m m u n itie s a n d se a b e d h a b ita t,
sp a tia l re s o lu tio n ; o r (iii) h i g h s p a tia l re s o lu tio n . F inally , a t Level
it h a s lo n g b e e n t h e fo c u s o f c o n sid e ra b le sc ie n tific a tte n tio n . T h is
4, w e h av e th e u ltim a te m e a s u r e o f fis h in g im p a c t: a n n u a l fish in g
h a s led to t h e c o lle c tio n o f a w id e ra n g e o f d a ta , in c lu d in g in f o r ­
m o r ta lity . F o r th e ta rg e t s p e c ie s o f th e b e a m tra w l fish ery , p laice
m a tio n o n th e ca p a c ity , s p a tia l co verage, a n d b e h a v io u r o f d iffe r­
a n d sole, th is is av ailab le f r o m s to c k a s se s sm e n ts . A lte rn a tiv e ly , it
e n t ty p e s o f vessels w ith in th e fleet, a n d te m p o r a l a n d s p a tia l
c a n b e c a lc u la te d fo llo w in g th e sw e p t a re a m e th o d (P ie t e t a l ,
av ailab le, a t w h a t sp a tia l re s o lu tio n . W e e v a lu a te d th is b y d is­
v a r ia tio n in th e s e a t d iffe re n t scales ( R ijn s d o rp e t a l , 1998; P ie t
2000; P o p e e t a l , 20 0 0 ) f o r all e c o s y ste m c o m p o n e n ts , f r o m th e
e t a l , 2 0 0 0 ). T w o d a ta b a s e s th a t d iffe r in th e ir s p a tia l re s o lu tio n
lo w e r level p re ss u re in d i c a t o r s th r o u g h th e in c o r p o r a tio n o f in f o r ­
w e re a n a ly s e d i n th is stu d y .
m a tio n o n th e s p e c ie s-sp e c ific m o r ta lity ra te p e r in d iv id u a l
c o n ta c t w ith th e gear, c o m b in e d w ith in f o r m a tio n o n t h e o v e rla p
(1 ) L o w sp a tia l resolution. T h e V IR IS d a ta b a s e c o n ta in s in f o r ­
m a t i o n o n fish in g activ ities o f th e e n tire D u tc h fle e t a t a
sp a tia l r e s o lu tio n o f IC E S re ctan g les s to re d in in d iv id u a l
fish e rs’ E C lo g b o o k s. D a ta w ere e x tra c te d f o r th e y e a rs 1 9 9 4 2 0 0 4 . T h e d a ta b a s e d istin g u is h e s d iffe re n t se g m e n ts o f th e
in s p a tia l d is tr ib u tio n o f p o p u la tio n s o f d iffe re n t o r g a n is m s o f
c o n c e r n w ith fish in g a c tiv ity .
Level 1: Fleet cap acity
T h e n u m b e r o f re g iste re d D u tc h b e a m tra w l vessels b e lo n g in g to
fle e t b a s e d o n th e ir e n g in e p o w er, a n d c o n ta in s in f o r m a tio n
ea c h o f th e tw o m é tie rs b a s e d o n e n g in e - p o w e r w as d e te r m in e d
o n th e tim e a n d d a te o f th e s ta rt a n d e n d o f th e fish in g tr ip ,
f o r ea c h y e a r f ro m th e V IR IS d a ta b a se .
th e g e a r u s e d , t h e IC E S rectan g les fish e d , a n d th e la n d in g s b y
fish sp e cies. I t is d e s ig n e d fo r q u o ta m a n a g e m e n t, b u t is av ail­
Level 2: Fishing effort
ab le f o r re se a rc h p u rp o s e s , a n d s im ila r d a ta b a s e s a re av ailab le
T o ta l a n n u a l fish in g e f fo r t, i n te r m s o f th e n u m b e r o f d a y s a t sea,
fo r o t h e r E C c o u n trie s .
w as d e te r m in e d fo r e a c h m é ti e r w ith in th e D u t c h b e a m tra w l fleet
(2) H ig h sp a tia l resolution. T h e A P R /V M S d a ta b a se c o n sists o f
a u to m a te d p o s itio n re g istra tio n (A P R ) a n d vessel m o n ito r in g
th ro u g h
sa tellite
(V M S )
d a ta
at
a
r e s o lu tio n
of
1 m in
la titu d e x 2 m in lo n g itu d e ( ~ l x 1 n a u tic a l m ile) s p a tia l u n its,
w h ic h a re re fe rre d to su b se q u e n tly i n te x t a s sq u a res. A P R d a ta
b a s e d o n th e V IR IS d a ta b a s e .
Level 3: Frequency tr a w led
T h e to ta l are a o f se a b e d s w e p t b y th e D u tc h b e a m tra w l fleet in
a n y g iv en y ear (sw e p t a r e a , SA , in m 2 y ~ 1) w as e s tim a te d fro m
are b a s e d o n a sa m p le o f a b o u t 10% o f th e D u tc h b e a m traw l
fleet t h a t w as e q u ip p e d w ith A P R e q u ip m e n t fo r th e p e rio d
SA = £ * H F * S * 1 8 5 2 * 2 W
(l)
1 9 9 3 - 2 0 0 0 , d u r in g w h ic h th e p o s itio n o f th e vessels w as
r e c o rd e d ev ery 6 m in (R ijn sd o rp e t a l , 1998). T h e V M S d a ta
b e c a m e av ailab le f ro m 2000, w h e n p o s itio n s o f all E U vessels
>24 m
w ere
re c o rd e d
fo r
e n fo rc e m e n t
p u rp o s e s .
F ro m
S e p te m b e r 2 0 0 3 , th is w as ex te n d e d to vessels > 18 m , a n d s u b ­
s e q u e n tly fro m th e 1 J a n u a ry 2005 to vessels > 15 m . P o sitio n s
are re c o r d e d a p p ro x im a te ly every 2 h . A lth o u g h th e d a ta are
c o lle c te d b y all E C c o u n trie s fo r e n fo rc e m e n t p u rp o se s , n o t all
c o u n tr ie s h a v e access to V M S d a ta fo r rese a rc h p u rp o s e s . F o r
th e D u tc h b e a m tra w l fleet, V M S d a ta f ro m o n ly a s u b s e t o f
~ 4 0 % o f t h e vessels th a t allow ed th e u s e o f th e ir V M S reg is­
tr a tio n s a re available fo r resea rch p u rp o s e s . I n a d d itio n to
d e ta ile d d a ta o n tra c k p o sitio n s , so m e vessels p ro v id e d d a ta o n
a h a u l- b y -h a u l (H B H ) basis o f th e c a tc h o f th e ta rg e t species,
traw lin g sp e e d , a n d tim e s o f sh o o tin g a n d h a u lin g o f th e gear.
w h e re E is th e m e a s u re o f e ffo rt, i.e. th e n u m b e r o f d ay s re c o rd e d
a t sea b y th e e n tire fleet ( d y —x), H F th e m e a n n u m b e r o f h o u r s
fish e d in a d ay (h d ~ ') , S th e m e a n tra w lin g s p e e d ( k n o ts , c o n ­
v e r te d to m h - 1 b y m u ltip ly in g b y 1852), a n d W is t h e w id th o f
e a c h b e a m (m ) w ith tw o b e a m tra w ls to w e d b y ea c h v essel. T h ese
p a r a m e te r v alu es, d e te r m in e d u s in g in f o r m a tio n h e ld in b o th
d a ta b a s e s, v aries b e tw e e n t h e d iffe re n t m é tie rs in th e fle e t, so SA
n e e d s to b e c a lc u la te d f o r ea c h m é tie r in d e p e n d e n tly . S u m m in g
t h e e s tim a te s o f SA fo r e a c h m é tie r p r o d u c e d th e e s tim a te o f SA
fo r t h e e n tire D u tc h b e a m tra w l fleet. I n a d d itio n to th e EC
lo g b o o k d a ta , tra w lin g s p e e d w as r e c o r d e d b y a s a m p le o f fish in g
vessels in th e H B H d a ta , a llo w in g th e d e te r m in a tio n o f t h e ran g e
o f p o ss ib le fish in g sp e e d s. V essel s p e e d c o u ld also b e c a lc u la te d
f r o m th e A P R d a ta b y c a lc u la tin g th e d ista n c e b e tw e e n su b s e q u e n t
p o s itio n s a n d d iv id in g b y 0.1 h (i.e. 6 - m in tim e in te rv a ls ). T h e
T h e e c o lo g ic a l im p a c t o f fish in g b y th e D u tc h b e a m tra w l fleet c a n
V M S d a ta b a s e p r o v id e d t h e m e a s u re d sp e e d a t ea c h p o s itio n fo r
b e d e s c rib e d b y p re s s u re in d ic a to rs a t f o u r levels o f in c re a sin g
m o s t re c o rd s. T h e n u m b e r o f h o u r s s p e n t fish in g i n e a c h d ay
in f o r m a tio n c o n t e n t (F ig u re 1). L evel 1 q u a n tif ie s fleet ca p a c ity ,
(H F ) r e q u ire d fish in g a c tiv ity t o b e d is tin g u is h e d f r o m o th e r
i.e. th e n u m b e r o f vessels in a fish ery , w h e re d if fe r e n t fish in g
vessel activ ity , i.e. s te a m in g , s h o o tin g , o r h a u lin g th e g ear, o r d r if t­
m é tie rs (d e fin e d b y ta rg e t species, fish in g g e a r u s e d , a n d are a
in g . T h is d is tin c tio n w as m a d e o n th e b asis o f vessel s p e e d w h ere,
v isite d ; L a u re e e t a l , 1991) m a y b e d e fin e d as n ecessa ry . L evel 2 is
f o r ea c h m é tie r in th e fle e t, th e m e a n p r o p o r t io n o f re c o r d s p e r
th e m e a s u re m o s t c o m m o n ly re fe rre d t o as f is h in g e ffo rt, c a lc u ­
2 4 h p e r io d th a t lay w ith in th e fish in g s p e e d ra n g e w as in te r p re te d
la te d as fle e t c a p a c ity (u su a lly in n u m b e r o f vessels, b u t it m a y
as th e p r o p o r tio n o f t h e tim e s p e n t fishing. T h e m e a n tim e p e r
also ta k e a c c o u n t o f vessel to n n a g e o r e n g in e p o w e r) m u ltip lie d
d a y s p e n t fish in g w as c a lc u la te d fro m th e H B H d a ta t h a t c o n ­
b y th e ir a c tiv ity (e.g. n u m b e r o f h o u r s fish in g o r d a y s a t se a).
ta i n e d th e tim e s o f s h o o tin g a n d h a u lin g o f th e gear.
P o te n tia l p ressu re in dicators fo r fish in g , a n d their d a ta req u irem en ts
113
T h e A P R /V M S d a ta b a s e h o ld s re g istra tio n d a ta r e c o rd e d at
F o r all n o n - ta r g e t sp ecies f o r w h ic h m o r ta lity e s tim a te s c a n n o t
d iffe re n t tim e in te rv a ls. I n o r d e r to c o m b in e th e s e d a ta sets, th e
b e o b ta in e d fro m th e s to c k a s s e s s m e n ts , w e u se th e sw e p t-a re a
to ta l n u m b e r o f h o u r s fish in g p e r y e a r w as c a lc u la te d fo r e a c h set
m e th o d (P ie t e t a l , 2000; P o p e e t a l , 2000) a n d c a lc u la te d A F M
(% ) s u m m e d o v e r all s p a tia l u n i t s as
as follow s:
FR *TI
E * HF =
_ i o p * E f = i N <U - (1 - E M )TF'}
A FM =
(2)
PD
( 5)
w h e re F R is th e n u m b e r o f fish in g re g istra tio n s, tim e in te rv a l (T I)
is e q u a l to 0.1 h (6 m in ) f o r A P R a n d ~ 2 h fo r V M S , a n d P D is
th e p r o p o r t i o n o f th e fleet in th e A P R o r V M S s a m p le (i.e. fo r
w h ic h d a ta a re r e c o r d e d in th e d a ta b a s e ). N o te t h a t th e rig h t sid e
o f th is e q u a tio n c a n b e s u b s titu te d d ire c tly in to E q u a tio n (1 ). T h e
m e a n tra w lin g fre q u e n c y (T F ) w ith in th e are a o f D u tc h b e a m
w h e re N ¡ is th e p o p u la tio n a b u n d a n c e a n d T F , th e tra w lin g fre ­
q u e n c y in s p a tia l u n i t i, a n d E M is th e e n c o u n te r m o rta lity , w h ic h
d iffers fro m g e a r efficiency b e c a u s e i t e n cap su lates all fo rm s o f m o r ­
ta lity c a u s e d b y th e fishing gear, in c lu d in g th o se th a t d o n o t re su lt
in r e te n tio n o f th e a n im a l ( B e r g m a n a n d v a n S a n tb rin k , 200 0 ).
E n c o u n te r m o rta lity d e p e n d s o n c h aracteristics o f th e sp ecies (e.g.
tra w lin g o p e r a tio n s w as c a lc u la te d as
size, p o s itio n in th e w a te r c o lu m n o r se d im e n t, fragility, a n d s w im ­
TF =
m in g sp e e d ) in re la tio n to g e a r c h aracteristics (e.g. ty p e o f gear,
E ÏÏ tS A y /A ,)
(3)
SU
Level 3, th e re so lu tio n a t w in c h s p a tia l in fo rm a tio n is av ailab le is
w h e re SA,- is t h e are a o f se a b e d sw e p t b y th e D u tc h b e a m traw l
fle e t in sp a tia l u n i t i w ith to ta l a re a A,-. T h e are a A , w as c a lc u la te d
u s in g G IS ( p r o je c tio n U T M - 1 9 8 4 , z o n e 31 N ). W h e th e r o r n o t
s p a tia l in f o r m a tio n o n fish in g a c tiv itie s w as av ailab le, a n d i f so at
w h a t r e s o lu tio n , d e te r m in e d th e n u m b e r o f s p a tia l u n its (S U ). I f
b a s e d o n th e V IR IS d a ta b a s e o r A P R /V M S d a ta b a s e , S U e q u alled ,
resp ec tiv ely ,
th e
num ber
of
IC E S
m e s h size; W ard le, 1988; Casey, 1996; C ollie e t a l , 2 0 0 0 ). S im ila r to
sta tis tic a l
re ctan g les
or
1 x 2 m in s q u a re s in w h ic h D u tc h b e a m traw l a c tiv ity h a d b e e n
re c o r d e d fo r th a t y ear. T h e N o r t h Sea w as d e fin e d a s IC E S a re a IV,
m in u s th e a re a d e e p e r t h a n 2 0 0 m , a n d if n o s p a tia l in f o rm a tio n
w as av ailab le, S U = 1 a n d A x e q u a lle d th e are a o f th e N o r th Sea as
relev an t, b u t a t Level 4 th is a p p lie s b o th to th e fish ery a n d th e
species c o n c e rn e d . M o reo v er, t h e re s o lu tio n o f th e fish ery in fo r­
m a tio n d e te rm in e s th e r e q u ire d re s o lu tio n o f th e species in f o r ­
m a tio n fo r o p tim a l e s tim a tio n o f th e a n n u a l fish in g m o rta lity .
T h e re fo re , th e re is n o p o in t in a c q u ir in g sp a tia l in f o rm a tio n o f th e
species at a r e s o lu tio n b e y o n d t h a t av ailab le f o r th e fishery.
I n o r d e r to illu stra te h o w e n c o u n te r m o r ta lity a n d o v e rla p in
d is tr ib u tio n o f th e sp ecies w ith t h e fish e ry d e te r m in e th e v a lu e o f
th e in d ic a to r , w e d is tin g u is h v ir tu a l p o p u la tio n s th a t d iffe r in
th e s e tw o c h a ra c te ristic s . P u b lis h e d g ear efficien cy ra te s o f b e n th ic
in v e rte b ra te s lie w ith in th e r a n g e 0 - 9 0 %
(B e rg m a n a n d v a n
S a n tb r in k , 2000; C o llie e t a l , 2 0 0 0 ). I n o u r ex a m p le , th e re fo re , w e
d e fin e d .
u s e d e n c o u n te r m o r ta lity rates o f 2 0 a n d 8 0% . F o r th e o v e rla p in
d is tr ib u tio n , w e d is tin g u is h e d a sp e c ie s w ith “h ig h o v e rla p ” ( H O ),
Level 4: A n n u al fish in g m o rta lity (%)
w h ic h is a g g re g a te d in a reas w h e re fish in g a c tiv ity is h ig h e s t (i.e.
Id eally , th e effect o f a fish e ry o n a n y e c o sy ste m c o m p o n e n t s h o u ld
b e ex p re sse d a s a n a n n u a l fish in g m o r ta lity (i.e. th e p e rc e n ta g e
f is h in g -in d u c e d n u m b e r o f d e a th s p e r y e a r in re la tio n to p o p u ­
la tio n a b u n d a n c e ). I n t r a d itio n a l fish eries sc ien ce, th e i n s ta n ­
ta n e o u s fish in g m o r ta lity r a te F is u s u a lly c a lc u la te d as p a r t o f th e
s to c k a s se s sm e n t p ro c e ss , a n d i t c a n b e c o n v e r te d easily to th e
m o r e re a d ily u n d e r s to o d c o n c e p t o f a n n u a l fish in g m o r ta lity
th r o u g h
in s p a tia l u n its w h e re T F ; is g r e a te r th a n th e m e a n T F ac ro ss all
sp a tia l u n its ) , a n d a sp e cies w ith “ lo w o v e rla p ” (L O ), w h ic h is
a g g re g a te d in a reas w h e re fish in g a c tiv ity is le a s t (TF,- < m e a n
T F ). I n all cases, th e p o p u la tio n w ith a fixed n u m b e r o f o rg a n is m s
w a s u n if o r m ly d is tr ib u te d o v e r th e s p a tia l u n its t h a t m a tc h e d th e
c rite ria . M o re sp a tia l u n its re s u lt in less o rg a n is m s p e r u n it.
W e e x p lo re d th e s a m e s c e n a rio s as fo r th e Level 3 in d ic a to r: n o
sp a tia l in f o r m a tio n , sp a tia l in f o r m a tio n a t lo w r e s o lu tio n (IC E S
A F M = 100 * (1 - e x p “
(4)
re c ta n g le s ), a n d sp a tia l in f o r m a tio n a t h ig h r e s o lu tio n ( 1 x 2 m in
s q u a re s ). F o r c o m p a r is o n o f in d ic a to r s acro ss levels, w e fitte d
lin e a r re g re ssio n s to th e c e n tr e d tim e -s e rie s (c e n tre d a t y e a r 2 0 0 0 ),
F c a n b e e x p re ss e d as c a tc h a b ility x effo rt, w h e re c a tc h a b ility
refe rs to th e c h a n c e th a t a n in d iv id u a l in th e p o p u la tio n is killed
b y t h e g e a r (B e v e rto n a n d H o lt, 1957). T h is d e p e n d s o n (i) th e
a n d u s e d th e slo p e s a n d in te rc e p ts t o c a lcu late th e e s tim a te d v a lu e
in y e a r 200 0 ( in te rc e p t) , a n d th e re lativ e c h a n g e (% ) o v e r tim e
(s lo p e X 1 0 0 /in te rc e p t).
c h a n c e s o f in d iv id u a ls o f a sp e cies c o m in g in to c o n ta c t w ith th e
fish in g g ear, w h ic h is d e te r m in e d b y th e d is tr ib u tio n o f th e sp ecies
in r e la tio n t o th e d is tr ib u tio n o f th e fleet, a n d (ii) g ear efficiency,
Results
w h ic h is t h e p r o p o r t i o n o f t h e p o p u l a t io n i n th e p a th o f th e g e a r
Level 1: F leet ca p acity
t h a t is r e ta in e d b y th e g e a r (R ick er, 1975). C a tc h a b ility th e re fo re
A c c o rd in g to th e V IR IS d a ta b a se , th e to ta l n u m b e r o f re g iste re d
in te g ra te s all a s p e c ts o f th e d i s t r ib u t io n o f th e p o p u la tio n in
beam
r e la tio n to t h a t o f th e fish in g fleet, c re w skills, vessel c h a ra c te r­
(F ig u re 2). T h e r e d u c tio n in th e n u m b e r o f larg e vessels w as m u c h
istics, a n d g e a r effic ie n c y (R ijn s d o r p e t a l , 2 0 0 6 ).
s m a lle r in re lativ e te r m s th a n th e r e d u c tio n in th e n u m b e r o f
F ish in g m o r ta lity e s tim a te s o f p la ic e a n d so le w ere av ailab le
f r o m th e IC E S s to c k a sse s sm e n ts . F o r p laice, F e s tim a te s are b a se d
tra w le rs d e c lin e d
fro m
378
in
1995 to
224
in
2004
e u r o c u tte rs , so th e p r o p o r tio n o f la rg e vessels w ith in th e fleet
in c re a se d f r o m 5 5 % i n 1995 to 63% in 200 4 (F ig u re 2).
o n b o t h la n d in g s a n d d isc a rd s , fo r so le o n la n d in g s only. A s th e
s to c k a s se s sm e n ts fo r th e s e sp e c ie s a re k n o w n to h av e u n d e r e s ti­
Level 2: Fishing effort
m a te d m o r ta lity in th e p a s t fe w y e a rs, t h e v alu es fo r th e fin al year
O n th e b a sis o f th e V IR IS d atab ase, th e a c tiv ity p e r vessel (d ay s a t
w e re n o t p re s e n te d .
sea p e r y e a r) v a rie d c o n s id e ra b ly w ith in a n d b e tw e e n m é tie rs
G. J. P ie t e t al.
114
220
- O
200
0
w
c
o
ro
L arg e 1
■Q- Euro 1
180
■
(/)
0
' E u ro 2
O)
CD
1 160
'S
0
-Q
E
140
3C
0>
O
VMS
-X— V IR IS
■
APR
40
c
o
tt
o
Q.
O
Q_
120
0
- 1 100
2002
2000
1998
1996
20
... o,
1
2
3
4
5
6
7
8
9 10 11 12
2004
S p e e d ( k n o ts )
Figure 2. T im e-series o f t h e pressure in d icato r a t Level 1 (fleet
capacity, o p e n sym bols) a n d Level 2 (fishing effort, sh a d e d sym bols)
for tw o m é tie rs w ithin th e D u tch b eam traw l fleet. (Large 1 a n d
Large 2 are, respectively, th e Levels 1 a n d 2 in d icato rs for "Large
vessels"; Euro 1 a n d Euro 2 are, respectively, th e Levels 1 a n d 2
in d icato rs for "E u rocutters").
O - VMS
VIR S
(F ig u re 3). O v e ra ll, 8 7 % o f th e la rg e vessels s p e n t 1 5 0 - 2 5 0 d at
sea p e r y ear, w ith a n av erag e o f 170 d a t sea p e r y ear. F o r e u r o c u t­
te rs, m e a n a c tiv ity w as m u c h less, w ith a n average o f j u s t 6 7 d a t
sea p e r y ear, b u t th e d is tr ib u tio n w as sk e w ed b e c a u se 2 5 % o f e u r o ­
c u tte r s re g iste re d few er t h a n 10 d a t sea p e r year, m a n y re g iste rin g
o n ly 1 d a t se a p e r y ear. F o r b o th m é tie rs, a c tiv ity p e r vessel
d e c re a se d b y a b o u t 1.5 d a t sea p e r y ear. T o ta l D u tc h b e a m traw l
e ffo rt d e c re a s e d f ro m 49 765 d a t sea in 1995 to 2 6 034 d a t sea in
2 004. O v e r t h e s a m e p e r io d , th e p r o p o r ti o n o f to ta l D u tc h b e a m
traw l fish in g e ffo rt u n d e r ta k e n b y larg e vessels in c re a se d fro m 76
to 8 2 % (F ig u re 2 ).
Level 3: F requen cy tra w led
F o r each o f t h e tw o fish in g m e tie rs, fre q u e n c y d is tr ib u tio n s o f th e
e s tim a te d vessel s p e e d fo r all A P R a n d V M S re g istra tio n s w ere
c o m p a re d w ith fre q u e n c y d is tr ib u tio n s o f r e c o rd e d traw l sp e e d s in
th e H B H d a ta (F ig u re 4 ). F ish in g sp e ed s in th e H B H d a ta ra n g e d
fro m 3 to 6 k n o ts fo r e u r o c u tte rs , a n d f ro m 5 t o 8 k n o ts fo r large
vessels. A P R a n d V M S r e c o rd s g iving sp e e d e stim ates w ith in th e s e
ra n g e s w ere c o n s id e re d to b e “ fish in g ” re c o rd s, a n d th e p r o p o r tio n
2
3
4
5
6
7
8
9
10 11
12
S p e e d (knots)
Figure 4. E stim ates o f traw lin g sp e ed d erived fro m each o f th e
th re e d a ta b a se sources fo r "e u ro c u tte r" (u p p e r p a n e l) an d "Large
vessel" (low er p an el) m é tie rs w ithin th e D u tch b eam traw l fleet.
o f re c o rd s falling in to th is categ o ry p ro v id e d a n e stim a te o f th e
p r o p o r tio n o f tim e s p e n t fish in g , a llo w in g th e n u m b e r o f h o u rs
s p e n t fish in g b y each m é tie r in a n average 2 4 h p e r io d to b e c a lc u ­
la te d (T ab le 1). T h e re w a s little d ifferen ce b e tw e e n th e e s tim a te d
fish in g sp e e d s o r th e p r o p o r ti o n o f tim e s p e n t fish in g d e riv e d fro m
e ith e r th e A P R o r th e V M S d atab ases (T ab le 1), so t h e v alues
d e riv e d f ro m th e V M S d a ta b a s e w ere u se d in E q u a tio n ( 1 ) to esti­
m a te th e a re a o f se ab ed sw e p t p e r d a y b y a fish in g vessel b e lo n g in g
t o each m é tie r. E u ro c u tte rs sw ep t a n are a o f 1.2 k m 2 o n average
25
ea c h d ay , a n d la rg e vessels sw ep t a n area o f 5.3 k m 2.
20
th e IC E S re c ta n g le s in w h ic h D u tc h b e a m tra w le rs w e re re c o rd e d
T h e V IR IS d a ta b a s e p r o v id e d in f o rm a tio n th a t id e n tifie d all
fish in g i n e a c h y ear. L ikew ise, th e A P R /V M S d a ta b a s e id e n tifie d
E u ro c u tte rs
th e fish e d re ctan g les. K n o w in g th e a re a o f e a c h re c ta n g le o r
L a rg e v e s s e ls
sq u a re , a n d s u m m in g o v e r all re ctan g les o r sq u a re s in w h ic h
fish in g o c c u r r e d , allo w ed th e p r o p o r tio n o f th e to ta l a re a fish ed
a n n u a lly b y D u tc h b e a m tra w lin g o p e r a tio n s to b e e s tim a te d ,
d e p e n d in g o n th e s p a tia l r e s o lu tio n o f th e d a ta (F ig u re 5 a). T h e
lo w -re s o lu tio n d a ta in d ic a te d t h a t ju s t o v e r 100 IC E S re ctan g les,
a m o u n tin g to a b o u t 5 8 % o f th e N o r th Sea, w as fish e d a n n u a lly at
th e s ta r t o f t h e tim e -s e rie s , d e c lin in g t o a b o u t 5 0% a n n u a lly a t th e
0
50
100
150
200
250
A c tiv ity (d y _1 )
Figure 3. Vessel activity freq u e n cy d istrib u tio n s d e te rm in e d over
th e p erio d 1 9 9 5 - 2 0 0 4 for ea c h m é tie r o f fishing vessel w ith in th e
D u tch b eam traw l fleet.
e n d . I n c o n tr a s t, th e h ig h re s o lu tio n d a ta in d ic a te d th a t c o n s id e r ­
ab ly less o f t h e N o r th Sea w as fish e d a n n u a lly , re sp e c tiv e ly 2 0 %
(a b o u t 26 000 re ctan g les) a n d 14% . F ig u re 5 b sh o w s th e tim e se ries o f th e Level 3 in d ic a to r, th e fre q u e n c y fish ed (T F ). T h e
d iffe re n c e in fre q u e n c y d is tr ib u tio n b e tw e e n th e V IR IS a n d A P R /
V M S d a ta se ts is re fle c te d in F ig u re 6, w h ic h sh o w s th e o c c u rre n c e
P o te n tia l pressure in dicators fo r fish in g , a n d th e ir d a ta req u irem en ts
115
T ab le 1. Fishing p a ra m e te rs o f tw o m étiers o f th e D u tch b eam traw l fleet in th e N o rth Sea based on d iffe re n t d a ta sources.
M étier
E u r o c u tte r s
L a rg e v essels
Data source
Speed (knots)
Hours fishing
per 2 4 h
Proportion o f the
day sp en t fishing (%)
Area (km 2)
sw ep t per day
HBH
4.7
19.4
8 0 .9
1.3
A PR
4.4
17.1
71.1
1.1
VMS
4.2
19.3
8 0 .4
1.2
HBH
6.1
20.4
85.1
5.5
A PR
6.1
16.3
6 7 .8
4 .4
VMS
6.7
17.7
7 3 .9
5.3
o f sp a tia l u n its w ith sp ecific tra w lin g fre q u e n c ie s ( p e r y ear).
in c lu d e d in th e sa m p le . F ro m 200 0 o n , th is in c re a se d t o 6 6 - 1 4 3
F re q u e n c ie s ab o v e 20 p e r y e a r w ere o n ly o b s e rv e d fo r th e h ig h -
larg e vessels a n d 1 7 - 3 7 e u r o c u tte r s p e r y ear.
re s o lu tio n A P R /V M S d a ta . T h e tw o m é tie rs a re s tr o n g ly se g re­
g a te d sp a tia lly , b e c a u s e th e e u r o c u tte rs , w h ic h a re a llo w e d to fish
Level 4: A n n u al fish in g m ortality
a n y w h e re , a p p e a r to a v o id th e a reas w h e re t h e larg e vessels fish
T h e fish in g m o r ta lity ( F ) o f p la ic e a n d so le is e s tim a te d as p a r t o f
a n d c o n c e n tra te in th e 1 2 -m ile z o n e a n d th e p la ic e b o x , w h e re th e
th e s to c k a s se s sm e n t p ro c e s s (IC E S , 2005) a n d tra n s la te d in to th e
la rg e vessels are n o t p e r m itte d (F ig u re 7).
a n n u a l fish in g m o r ta lity u sin g E q u a tio n (4) (F ig u re 9 ). T h e tim e -
T h e d eg re e t o w h ic h th e s u b s e t o f t h e D u t c h b o tt o m tra w lin g
se ries o f th is in d ic a to r reveals th a t c o n tr a ry to w h a t w as o b se rv e d
fleet fo r w h ic h A P R /V M S d a ta w e re av ailab le is re p re se n ta tiv e o f
fo r th e lo w e r level in d ic a to r s , th e re w as n o d ecrease in a n n u a l
th e e n tire fleet d iffers c o n s id e ra b ly b e tw e e n th e p e r io d w h e n A P R
fish in g m o r ta lity o v e r th e 10 y 1 9 9 5 - 2 0 0 4 , b u t r a th e r a slig h t
d a ta w e re u se d a n d
th a t w h e n V M S d a ta b e c a m e available
( F ig u re 8). I n t h e first p e r io d (b e fo re 2 0 0 0 ), m a in ly la rg e b e a m
tra w le rs ( 1 5 - 2 4 ) a n d a few e u r o c u tte r s ( 1 - 6 ) p e r y e a r w ere
( b u t n o t s ig n ific a n t) in c re a se o f 0 .0 4 % f o r so le a n d 0 .5 5 % fo r
p la ic e (T a b le 2 ).
F o r n o n - ta r g e t sp e cies, th e r e a re n o s to c k a sse s sm e n ts , so
a n n u a l f is h in g m o r ta litie s w ere e s tim a te d fo llo w in g th e sw e p t-a re a
m e th o d . T h is re s u lte d in tim e -s e rie s o f a n n u a l fish in g m o r ta lity
(a) 60
fo r d iffe re n t v ir tu a l p o p u la tio n s , w ith v a rio u s sc e n a rio s d e p e n d in g
T5
r e s o lu tio n o f th e v ir tu a l p o p u la tio n b e in g im p a c te d re la te d to th a t
o n th e s p a tia l r e s o lu tio n o f th e fish in g d a ta a n d h o w t h e s p a tia l
o f th e fish e ry (F ig u re 10). T h e tim e -s e rie s o f a n n u a l fish in g m o r ­
ta lity d iffe re d c o n s id e ra b ly b etw e e n sc e n a rio s , b o t h in te r m s o f
a b s o lu te v a lu e a n d th e re la tiv e ch a n g e (F ig u re 10, T ab le 2 ).
X ■■ Low
• O - H igh
°
O
O verall, a n n u a l fish in g m o rta lity in c re a se d w ith in c re a sin g
o v e rla p o f th e p o p u la tio n w ith th e fishery, e n c o u n te r m o rta lity , a n d
Q
sp a tia l re s o lu tio n , a n d in th e ex am p le v aries b e tw e e n 0.7 a n d 80.1% .
T h e relative c h a n g e in a n n u a l fishing m o rta lity o v e r tim e also v a rie d
b etw e e n sc en ario s, f ro m a 1.8% decrease t o a 1.8% increase.
1994
1996
1998
2000
2002
2004
( b ) 2 .5
? 20
O ....Q
S
a'
°
o
1.0
i
$
0 .5
H ig h
"o...
< /
! 15
I
¡ = ]
O.
.O-..
" • O - ...
•o
N one
— X— Low
H igh
-X — X -
X
X
-X -
0.0
1994
1996
1998
2000
2002
2004
Figure 5. T im e-series o f th e p ressure in d ic a to r a t Level 3. The
figures sh o w (a) th e v ariatio n in th e p ro p o rtio n o f th e to ta l area of
D u tc h b eam traw l o p e ra tio n s fished in ea c h year a n d (b) th e m ean
fre q u e n c y th a t th e "fishable" are a w as traw led each year, d e p e n d in g
o n w h e th e r o r n o t in fo rm atio n o n th e spatial d istrib u tio n is
available a n d a t w h a t reso lu tio n : high (ICES statistical rectan g les) o r
low ( 1 x 2 m in sq u ares). If n o spatial in fo rm a tio n w as available, th e
p ro p o rtio n o f th e are a fished is by d efin itio n 100%.
c\j in
x
cm
x
m
F re q u e n c y c l a s s (y~^)
Figure 6. T h e d istrib u tio n o f fishing frequencies o f th e individual
fished spatial u n its across th e w hole are a o f D u tch b e a m traw l
o p e ra tio n s a t tw o spatial resolutions: low (ICES rectangles b ased on
VIRIS) a n d high ( 1 x 2 m in squares based o n APR/V M S).
118
G. J. P ie t e t al.
T able 2. S u m m ary o f a b so lu te values a n d relative tre n d s o f th e pressure in d icato rs a t differen t levels o f in fo rm atio n c o n te n t, a n d for
p o p u la tio n s t h a t differ in vulnerability t o t h a t fishery.
Value in 2000
Relative change (%)
1
276
- 6 .5
2
34 829
- 6 .8
Level
Encounter m ortality (%)
Distribution population
NS
0.3
Low
0.4
-4 .1
High
1.8
- 1 .6
Plaice
50.9
0.5
Sole
43.9
0.0
0.7
- 0 .2
Low
5.1
- 0 .2
High
14.2
0.5
2.8
- 0 .6
Low
15.4
- 0 .4
High
47.2
1.8
0.7
- 0 .2
Low
5.5
- 0 .4
High
26.1
0.2
NS
2.8
- 0 .6
Low
18.4
- 1 .1
High
64.2
1.4
0.7
- 0 .2
Low
5.9
- 0 .5
High
37.2
- 0 .8
2.8
- 0 .6
Low
20.7
- 1 .8
High
80.1
0.1
3
4
Low overlap
20
NS
80
Even distribution
NS
NS
20
80
High overlap
Spatial resolution
20
NS
80
NS
- 5 .7
Level 1 is t h e fle e t capacity. Level 2 t h e e ffo rt in day s a t sea, Level 3 th e fre q u e n c y tra w le d ( p e r y ear), a n d Level 4 th e a n n u a l fish in g m o r ta lity (% ). For th e
sp atial d is trib u tio n o f th e fle et, t h e d is tin c tio n is b a s e d o n th e in fo rm a tio n c o n t e n t o f t h e in p u t, e.g. n o in fo rm a tio n , o r sp atial in fo rm a tio n a t h ig h o r low
re so lu tio n . For t h e sp atial d is trib u tio n o f th e p o p u la tio n , th e d is tin c tio n is b a se d o n th e d is tr ib u tio n in re la tio n to t h a t o f t h e fle et.
c a lc u la te d s p e e d a c c o r d in g to th e A P R d a ta is u n d e r e s tim a te d
C o m p a r is o n o f th e fre q u e n c y d is tr ib u tio n s o f th ese d a ta sets
b e c a u se (i) th e vessel d o e s n o t fo llo w a s tr a ig h t lin e b e tw e e n tw o
sh o w s th a t t h e r e s o lu tio n u se d m a k e s a b ig d ifferen ce. A c c o rd in g
s u b s e q u e n t r e g istra tio n s, a n d
t o lo w r e s o lu tio n d a ta , > 5 0 % o f th e N o r t h S ea is fish ed , o f w h ic h
b e tw e e n
tw o
r e g is tra tio n s ,
(ii) if t h e vessel sta rts h a u lin g
th e
c a lc u la te d
m ean
speed
w ill
o n e - th ir d o f th e IC E S re c ta n g le s are fish e d m o r e th a n o n c e a y ear
d ecrease. T h e se p o in ts w ill b e c o m e in c re a sin g ly im p o r ta n t a n d
a n d m a x im u m fre q u e n c y is a b o u t five tim e s p e r y ear. A c c o rd in g
re su lt in a n u n d e r e s tim a tio n o f th e s p e e d as th e tim e in te rv a l
t o t h e h ig h re s o lu tio n d a ta , o n ly s o m e 2 0 % o f th e N o r th S ea is
b e tw e e n re g is tra tio n s is in c re a s e d (as in t h e V M S d a ta ) . T h e re fo re ,
f ish e d , o f w h ic h tw o - th ir d s are fish e d m o r e t h a n o n ce a y e a r a n d
in o r d e r to u s e sp e e d t o d is tin g u is h fish in g re g istra tio n s fro m
t h e m a x im u m fre q u e n c y is u p to 100 tim e s p e r y e a r. I n th is
o th e r a c tiv itie s t h a t d o n o t im p a c t th e e c o s y ste m , it is n e c e ssa ry to
e x a m p le , th e p r o p o r t io n o f th e a re a fish e d a c c o rd in g to t h e h ig h
o b ta in in f o r m a tio n o n th e a c tiv ity o f a vessel (e.g. H B H ), a n d to
r e s o lu tio n d a ta m a y b e slig h tly u n d e r e s tim a te d , b e c a u s e n o t all
c o m b in e th is w ith re a l s p e e d m e a s u re m e n ts (i.e. n o t c a lc u la te d
b e a m tra w le rs a re p a r t o f th e A P R /V M S d a ta b a s e a v a ila b le fo r
fro m in te rv a ls b e tw e e n r e g is tr a tio n s ) . I f th is n e e d s to b e b a s e d o n
sc ie n tific p u rp o s e s . T h e c a lc u la te d fre q u e n c y w as n o t a ffe c te d b y
c a lc u la te d s p e e d v a lu e s, t h e in te rv a l n e e d s to b e se t as s h o r t as
th is , th o u g h , b e c a u s e i t w as w e ig h te d b y a fa c to r th a t r a is e d th e
p o ssib le. T h is a p p r o a c h to u se s p e e d to d is tin g u is h fish in g f ro m
fre q u e n c y o f th e s a m p le to th a t o f th e e n tir e fleet. T h e i m p o r t a n t
o th e r a c tiv itie s w ill n e v e r b e a b s o lu te ly p e rfe c t, b e c a u s e vessels
ta k e - h o m e m essag e h e re is th a t as th e sp a tia l re s o lu tio n o f fish in g
m a y en g ag e in activ ities o th e r th a n f is h in g a t sp e e d s w ith in th e
a c tiv ity d a ta in crease s, th e p r o p o r ti o n o f th e a re a fish ed d eclin es,
fish in g s p e e d ra n g e , r e s u ltin g in s p u r io u s fish in g p o s itio n reg is­
w h e re a s th e tra w lin g fre q u e n c y w ith in th e fish e d a re a in c re a se s
tr a tio n s a n d a n o v e r e s tim a tio n o f im p a c t.
o n average.
I f av ailab le, in f o r m a tio n o n th e sp a tia l d is tr ib u tio n o f th e
fish ery a t th e h ig h e s t p o s s ib le r e s o lu tio n n e e d s to be in c o r p o ra te d
Level 4: A n n u al fish in g m ortality
w h e n assessin g th e p r e s s u re o n th e eco sy ste m . W e u s e d tw o
T h e a n n u a l fish in g m o r ta lity differs f r o m th e lo w er lev el in d i ­
so u rc e s o f d a ta t h a t d iffe re d i n th e ir sp a tia l r e s o lu tio n : V IR IS d a ta
c a to r s , b e c a u s e it e x p lic itly in c o r p o ra te s b o t h t h e c h a n c e o f in d i ­
a re a t a re la tiv e ly lo w r e s o lu tio n o f IC E S re c ta n g le ( ~ 3 0 x 30 n a u ­
v id u a ls o f a sp e cies c o m in g in to c o n ta c t w ith th e fish in g g e a r a n d
tic a l m ile s), w h e re a s th e A P R /V M S d a ta w e re a g g re g a te d a t a re la ­
t h e e n c o u n te r m o r ta lity , a n d also b e c a u s e it is th e o n ly in d ic a to r
tiv ely h ig h r e s o lu tio n o f 1 x 2 m in s q u a re s ( ~ 1 x 1 n a u tic a l m ile ).
t h a t d e s c rib e s th e p re s s u re relativ e to th e a b u n d a n c e o f th e
P o te n tia l pressu re ind icators fo r fish in g , a n d th eir d a ta req u irem en ts
119
20
80
70
.O
60 -
>> 15 -
9 -0 -
50 -
,- Q
,.o
40 P
10 -
Low
20
-
10
-
m
1995 1997 1999 2001
2003
1995 1997 1999 2001
o.
30 -
p - o -.o
. .CL
/ °-0
o -o.^o-o-o-ö'
ro
E 20
— ■ —
O)
60 i
N one
o-. o . o-o-0"0"00'
40
TO 10
20
1995 1997 1999 2001
2003
HI—
N one
HK—
Low S R
O —
H ig h S R
Even
-
2003
1995 1997 1999 2001 2003
100
80 -
O)
<
30 -
20
-
40 -
10
-
20
1995 1997 1999 2001
2003
High
-
1995 1997 1999 2001 2003
Figure 10 . T im e-series o f th e pressure in d ic a to r a t Level 4 expressed as an n u a l fishing m o rta lity (%) d e p e n d in g on th e e n c o u n te r m o rtality ,
i.e. 20 (left) vs. 80% (right), a n d th e d istrib u tio n o f t h e p o p u la tio n in relation to th e fishery, i.e. low overlap (to p ), even d istrib u tio n (m iddle),
a n d high o v erlap (b o tto m ). In each plot, th e tim e-series w ere calcu lated w ith o u t spatial in fo rm atio n o n th e fishery (N one), o r a spatial
reso lu tio n o f ICES sq u a res (Low SR), o r a spatial re so lu tio n o f 1 x 2 m in sq u a res (H igh SR).
sp e cies. A s su c h , it s h o u ld fo rm a lly b e c o n s id e re d a n im p a c t in d i­
m o r e lik ely w h e n c o n s id e rin g th e sy s te m a tic u n d e r e s tim a tio n o f
c a to r , b u t th e d is tin c tio n b e tw e e n p r e s s u re a n d im p a c t in d ic a to rs
m o r ta lity in th e p a s t few y ears b y th e s to c k a s s e s s m e n t p ro cess
is n o t m a d e in th e P S R fra m e w o rk .
(P ie t a n d R ice, 2 0 0 4 ). T h e p a ra m e te rs t h a t d e s c rib e th e a n n u a l
T h e tim e -s e rie s o f a n n u a l fish in g m o r ta lity b a s e d o n s to c k
p o p u la tio n m o r ta litie s o f p laice a n d so le b a s e d o n th e s to c k assess­
a s se s sm e n ts o f p la ic e a n d so le sh o w t h a t p r e s s u re o n th e s e sto c k s
m e n ts fall w ith in th e ra n g e o b se rv e d fo r th e d iffe re n t p o p u la tio n s
in t h e 10 y 1 9 9 5 - 2 0 0 4 p r o b a b ly in c re a se d . T h is b e c o m e s even
a t h ig h
sp a tia l
r e s o lu tio n
b a se d
on
th e
m o d e lle d
p re ss u re
120
G. J. P iet e t al.
in d ic a to r s , su g g e stin g th a t a h ig h e r s p a tia l r e s o lu tio n p ro v id e s
(i.e. a n n u a l fish in g m o r ta lit y ) w ill d iffe r b e tw e e n sp ecies, b o th in
m o r e a c c u ra te estim ates.
te r m s o f its a b s o lu te v a l u e a n d its t r e n d o v e r tim e , a n d th a t th is in
A lth o u g h th e in crease in p o p u la tio n m o r ta lity o f p la ic e a n d
t u r n m a y r e s p o n d d if f e r e n tly to m a n a g e m e n t m e a s u re s t h a n th e
so le is sy stem atically u n d e r e s tim a te d in t h e ir s to c k a ssessm en ts,
lo w e r
th e in c re a se in p o p u la tio n m o r ta lity b a s e d o n th e s w e p t-a re a
b e tw e e n sp ecies, th e p r e s s u r e e x p re ss e d a s a n n u a l fish in g m o r ta lity
level
p re s s u re
in d ic a to r s .
C o n s id e r in g
th is
d ifferen ce
m e th o d is p ro b a b ly also u n d e r e s tim a te d , b e c a u s e w e a s s u m e d
o f o n e o r tw o c o m m e r c ia l sp e cies w ill n o t b e re p re se n ta tiv e fo r th e
e n c o u n te r m o r ta lity to b e sta b le o v e r tim e , fa ilin g to ta k e a c c o u n t
w h o le c o m m u n ity . T h e r e f o r e , if m a n a g e m e n t o b jectiv es a re se t for
o f te c h n ic a l c re e p a n d in crease s in s k ip p e r skills w h ic h , a c c o rd in g
c o m m u n ity level in d ic a t o r s s u c h as m e a n w e ig h t, m e a n m a x im u m
to R ijn s d o rp e t al. (2 0 0 6 ), in c re a se th e p a r tia l fish in g m o r ta lity
le n g th , o r b io d iv e rs ity ( P i e t a n d J e n n in g s, 2 0 0 5 ), th e c o m m u n ity
ra te in th e b e a m tra w l fish ery ( > 2 2 1 k W ) b y 2 .8 % p e r y e a r fo r
m o r ta lity w ill n e e d to b e d e te r m in e d as a n in te g ra l o f all th e p o p u ­
so le a n d 1.6% fo r p laice. To w h a t e x te n t th e s e v a lu e s a p p ly to n o n ­
la tio n m o r ta litie s t h a t m a k e u p th e c o m m u n ity .
H e re w e h a v e illu s tr a te d th e p ro c e s s f o r a n u m b e r o f “v irtu a l
ta r g e t sp ecies, h o w e v e r, r e m a in s to b e assessed.
'W h en u sin g th e sw e p t-a re a m e th o d t o c a lcu late th e a n n u a l
p o p u la tio n s ” o f m a r in e s p e c ie s, o n e t h a t su ffers 2 0 % m o r ta lity in
fish in g m o rta lity , it m a y th e o re tic a lly v a ry b e tw e e n 0 a n d 100%
a b e a m traw l g e a r a n d t h e o th e r 8 0 % . U ltim a te ly , it s h o u ld b e
d e p e n d in g o n e n c o u n te r m o rta lity o f th a t p a r tic u la r sp e c ie s-
p o ssib le to c a lc u la te L e v e l 4 in d ic a to r s f o r real p o p u la tio n s a n d
m é tie r c o m b in a tio n , th e sp a tia l d is tr ib u tio n o f th e p o p u la tio n rela­
c o m m u n itie s u s in g d i s t r ib u ti o n m a p s o f b e n th o s a n d d e m e rsa l
tive to th a t o f t h e fleet, a n d th e r e s o lu tio n o f th e s p a tia l d is tr ib u tio n
fish
d a ta . I n th is stu d y , th e a n n u a l fish in g m o r ta lity in c re a se s w ith
c h a ra c te ristic sp e cies o r ta x o n - le v e l in f o r m a tio n o n th e e n c o u n te r
in c re a sin g e n c o u n te r m o rta lity (i.e. 20 —» 8 0 % ), in c re a sin g o v erlap
m o r ta lity o f th e s p e c ie s m a k in g u p th e c o m m u n ity in a g iv en
(i.e. lo w —» ev en —» h ig h ), a n d in c re a sin g sp a tia l re s o lu tio n (i.e.
fish ed are a . T h is is lik e ly to be m o r e a c h ie v a b le in th e n e a r fu tu re
n s —> lo w —» h ig h ). T h e in crease o f m o r ta lity w ith s p a tia l re so l­
f o r b e n th ic in v e r te b ra te s , b e c a u s e d is tr ib u tio n m a p s o fte n exist
u tio n is b ec a u se , in th e calcu latio n s, th e p o p u la tio n is d is trib u te d
f o r b o th fish a n d b e n t h o s , b u t e n c o u n te r m o r ta lity h a s o n ly b een
relativ e to th e m e a n traw lin g fre q u e n c y (T F ) w h ic h , as F igure 5
d e te r m in e d fo r s o m e o f th e c o m m o n b e n th ic sp e cies ( fo r review
sh o w s, d e p e n d s o n th e sp a tia l re s o lu tio n . T h e c h o ic e o f a fixed TF,
a n d m e ta -a n a ly sis , see C o llie , 20 0 0 , a n d K aiser e t a l , 2 0 0 5 ), u sin g
in d e p e n d e n t o f th e sp a tia l re s o lu tio n , w o u ld p r o b a b ly re su lt in an
s a m p lin g te c h n iq u e s t h a t a llo w th e d e te r m in a tio n o f a b s o lu te p re -
in v erse re la tio n s h ip w ith sp a tia l re s o lu tio n , b e c a u s e a t a h ig h e r res­
a n d p o s t- h a u l a b u n d a n c e s (see e x a m p le fo r a lim ite d n u m b e r o f
o lu tio n , less o f th e are a is fished, a lth o u g h w ith a h ig h e r frequency.
b e n th ic sp e cies in P ie t e t a l , 2 0 0 0 ). F o r fish , th e r e a re as yet n o
I n g en eral, a t lo w e r sp a tia l re s o lu tio n , effo rt is ev en ly d is trib u te d
e s tim a te s o f th e e n c o u n te r m o r ta lity c a u s e d b y th e p a s s in g o f th e
o v er a larg e sp a tia l u n it. W ith in th a t u n it, th e re w ill b e h ig h reso l­
g ear d e s p ite th e fact t h a t in fish e ry sc ien ce, c a tc h a b ility a n d g ear
u tio n s p a tia l u n its w ith m u c h h ig h e r fre q u e n c ie s, w h e re a s o th e r
efficiency a re k n o w n c o n c e p ts (e.g. D ic k s o n , 1993). T o f u rth e r
h ig h re s o lu tio n sp a tia l u n its h av e lo w e r fre q u e n c ie s o r are n o t
c o m p lic a te t h e m a tte r , e n c o u n te r m o r ta lity c o n s ists n o t o n ly o f
fish ed a t all. B ecause o f th is a n d th e fact th a t ev ery h a u l in a heavily
m o r ta lity o f a n im a ls c a u g h t in t h e n e t (i.e. c a tc h a b ility ), b u t also
fish ed are a cau ses less m o r ta lity th a n a s im ila r h a u l in a n are a th a t
m o r ta lity th r o u g h
w as h a rd ly fish e d o r p ristin e , th e n e t effect o f in c re a sin g s p a tia l res­
t h r o u g h th e n e t) . T h is is m o r e re le v a n t fo r b e n th o s t h a n it is fo r
o lu tio n s h o u ld b e a d ecrease in p o p u la tio n m o rta lity .
fish (B e rg m a n a n d v a n S a n tb r in k , 2 0 0 0 ).
( fo r e x a m p le , se e
w w w .m a fc o n s .o rg )
c o n t a c t w ith
th e
and
in c o r p o ra tin g
g e a r (e.g. a f te r p assage
T h e slo p e o f th e tr e n d o v e r tim e in p o p u l a ti o n m o r ta lity
d ec re a se s w ith in c re a sin g o v erlap : f o r a n e n c o u n te r m o r ta lity o f
8 0 % a n d lo w s p a tia l re s o lu tio n , th is is - 0 . 4 %
a t lo w o v erlap
General discussion
to - 1 . 8 % at h ig h o v erlap ; a t h ig h sp a tia l r e s o lu tio n it is + 1 .8 % at
T h e u se fu ln e ss in a n E A F M o f th is fra m e w o rk o f p re s s u re in d i­
lo w o v e rla p to + 0 .1 % at h ig h o v e rla p (T a b le 2 ). T h e s e larg e d iffe r­
c a to rs b e c o m e s a p p a r e n t w h e n t h e m in im u m level o f in fo rm a tio n
e n ces sh o w t h a t w ith in a n E A F M , ta r g e t levels fo r m o r ta lity can
c o n te n t re q u ire d to e v a lu a te a p a r tic u la r ty p e o f m a n a g e m e n t
o n ly b e se t re a listic a lly a t th e h ig h e s t level o f in f o r m a tio n c o n te n t,
m e a s u re s c a n b e id e n tifie d . E ffo rt c o n tro l t h r o u g h d e c o m m is s io n ­
a n d th a t w h e n m o r ta lity n e e d s to b e c a lc u la te d (e.g. fo r n o n - ta r g e t
in g is a lre a d y re fle c te d a t t h e lo w e st level, w h ile it w ill s h o w a t th e
sp e c ie s), th e s p a tia l r e s o lu tio n is im p o r ta n t. I t is th e r e fo r e p r o ­
s e c o n d level if it is im p le m e n te d th r o u g h a r e d u c tio n in days at
p o s e d t h a t a s ta n d a r d s p a tia l re s o lu tio n b e u s e d a n d , as lo n g as
sea. T e c h n ic a l m e a s u re s m a y s h o w u p a t th e t h i r d level if it
th is is n o t a c h ie v e d , th e s p a tia l r e s o lu tio n s h o u l d a t least b e
in v o lv es c h a n g e s in th e g e a r c h a ra c te ristic s th a t d e te r m in e th e area
r e p o rte d w h e n e v e r fre q u e n c ie s (L evel 3 ) o r c a lc u la te d m o r ta lity
fish ed in re la tio n to e f fo r t (e.g. if th e w id th o f a b e a m traw l is
v alu es (L ev el 4 ) a re u sed .
r e d u c e d ) , b u t u su a lly o n ly affect th e e n c o u n te r m o rta lity , a n d w ill
D e te r m in in g th e e n c o u n te r m o r ta lity o f a sp e c ie s in th e g ear
u se d b y a sp e cific m é tie r m a y b e d iffic u lt, b e c a u s e a lm o s t ev ery
th e re fo re o n ly s h o w u p a t Level 4. S p a tia l m e a s u re s s u c h a s m a rin e
p r o te c te d a re a s c a n o n ly b e e v a lu a te d a t Level 4.
vessel w ill fish w ith g ear w ith slig h tly d iffe re n t c h a ra c te ris tic s a n d
T h is case s tu d y d e m o n s tr a te s th a t it is p o ss ib le to d ev elo p
r ig g in g (e.g. fo r a b e a m traw l, th e m e s h size, t h e n u m b e r o f tic k le r
p re ss u re in d ic a to r s th a t d e s c rib e th e im p a c t in d u c e d b y fish in g
c h a in s , t h e u se o f a c h a in m a t o r f lip - u p r o p e ), w h ic h c o u ld affect
a c tiv itie s o n a p a r tic u la r ec o sy ste m c o m p o n e n t o r sy ste m (e.g. th e
e n c o u n te r m o rta lity . I t m ig h t se e m a p p r o p r ia te th e r e fo r e to d is ­
d e m e rsa l sy ste m h e re ), a n d w h ic h are a p p r o p r ia te fo r u se as p a r t
tin g u is h ea c h o f th e s e g e a r ty p es, b u t th e a m o u n t o f w o r k re q u ire d
o f a n E A FM . H o w e v e r, th e b e st p re ss u re in d ic a to r s also c o m e w ith
to e s tim a te e n c o u n te r m o r ta lity w o u ld th e n b e p r o h ib itiv e . W e
ex ten siv e d a ta r e q u ire m e n ts th a t a t p re s e n t a re o n ly m a rg in a lly
su g g e st th a t a lim ite d n u m b e r o f m é tie r s b e d is tin g u is h e d w ith
a v ailab le even in o n e o f th e m o s t d a ta - ric h m a r in e e n v iro n m e n ts
s ta n d a r d g ear ty p e s a s s u m e d fo r w h ic h e n c o u n te r m o r ta lity can
i n th e w o rld . A n E A FM c a n o n ly b e su ccessfu lly im p le m e n te d a n d
b e d e te r m in e d .
m o n ito r e d if th e f is h in g p re s s u re can b e d e s c rib e d a t a level o f
T h e c o n s e q u e n c e s o f th e
a b o v e f o r th e
u s e o f p re ss u re
in d ic a to r s a s p a r t o f a n E A FM is t h a t th e b e s t p r e s s u re in d ic a to r
in f o rm a tio n
c o n te n t t h a t is a d e q u a te to
d e c is io n -m a k in g .
g u id e m a n a g e m e n t
P o te n tia l pressu re indicators fo r fish in g , a n d their d a ta req u irem en ts
A cknow ledgem ents
T h e w o r k w as f u n d e d b y th e E u ro p e a n C o m m is s io n : “ E u ro p e a n
F ish e rie s E c o sy ste m P la n ” (EFEP, E C p r o je c t Q 5 R S - 2 0 0 1 - 0 1 6 8 5 )
a n d “M a n a g in g F ish eries to C o n s e rv e G ro u n d fis h a n d B e n th ic
I n v e rte b r a te S p ecies D iv e rsity (M A F C O N S , E C p r o je c t Q 5 R S 2 0 0 2 - 0 0 8 5 6 ) . W e th a n k A d ria a n R ijn s d o rp , N ie ls D a a n , a n d
W ille m D e k k e r f o r h e lp fu l c o m m e n ts o n a n e a rlie r d r a ft o f th e
p ap er.
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122
The physiological status and mortality associated with
otter-trawl capture, transport, and captivity o f an
exploited elasmobranch, Squalus acanthias
J o h n W. M a n d e l m a n a n d M a r i a n n e A. F a rr in g to n
M a n d elm an , J. M., a n d Farrington, M. A. 2007. T he physiological s ta tu s a n d m o rta lity asso ciated w ith o tte r-tra w l c a p tu re , tra n sp o rt, a n d
cap tiv ity o f a n ex p lo ited elasm obranch, Squalus acanthias. ICES Journal o f M arin e Science, 64: 1 2 2 - 1 3 0 .
T o assess t h e p h y sio lo g ical re sp o n s e s a n d a s s o c ia te d m o r ta lity in sp in y d o g fish (S q u a lu s a c a n th ia s ) fo llo w in g c a p t u r e in a n o t t e r traw l
a n d e x p o s u re t o a d d itio n a l c o n d itio n s , b lo o d s a m p le s w e re o b ta in e d s u b s e q u e n t t o th r e e s a m p lin g in terv als: c a p tu r e (T1), t r a n s p o r t
(T2), a n d c a p tiv ity (T3). T h e re su lts in d ic a te t h a t m a rk e d d iffe re n c e s e x iste d in b lo o d c h e m is tr y a t e a c h s a m p lin g in terv al. A c i d - b a s e
p a r a m e te r s (v a sc u la r pH , p 0 2, p C 0 2), s e ru m C a 2+ a n d C F , a n d h a e m a to c r it w e re m a x im a lly d is r u p te d a t T l , b u t p ro g ressiv ely
reso lv ed t o p r e s u m e d b asal v alu es by T3. C o n c e n tr a tio n s o f w h o le - b lo o d la c ta te , p la s m a t o t a l p ro te in , a d d itio n a l se ra e le c tro ly te s
(N a + , K+ , M g 2 ' ), a n d BUN (u re a ) w e re m a x im a lly c o m p r o m is e d a t T2, b u t also re c o v e re d b y T 3. In c o n tr a s t, s e ru m g lu c o se levels
w e re sim ila r a t T l a n d T2 b u t ro se t o p e a k levels by T3. A lth o u g h b lo o d p a r a m e te r s w e re s u b s ta n tia lly a lte re d , d o g fish m o r ta lity w as
lo w (2 o u t o f 34; 5.9% ), su g g e stin g a s tr o n g d e g re e o f re silie n c e t o c o m p o u n d e d stre s so rs a s s o c ia te d w ith c a p tu r e , tr a n s p o r t, a n d
c a p tiv ity .
Keywords: b lo o d chem istry, captivity, m ortality, spiny dogfish, stress, tra n sp o rt, trawl.
Received 21 M arch 2006; acce p te d 2 A ugust 2006; advance access publication 24 O c to b e r 2006.
J. W. M a n d e lm a n a n d M. A. Farrington: Research D epartm ent, N ew England A quarium , Central W harf, Boston, M A 02110, USA. Correspondence
to j. W. M a n d elm a n: tel: + 1 617 2833177; fa x: + 1 617 7236207; e-mail: jm andelm an@ neaq.org
Introduction
stre s so rs e ith e r in d iv id u a lly o r co llectiv ely in te le o s ts (e.g. B a rto n
T h e sp in y d o g fis h (S q u a lu s acanthias) is a c o a sta l s q u a lo id w ith a
e t a l , 2003; S u lik o w sk i a n d H o w ell, 2 0 0 3 ), few er h av e d o n e so in
ra n g e e x te n d in g fro m L a b ra d o r to F lo rid a in th e w e s te rn A tla n tic
e la s m o b ra n c h s (C liff a n d T h u r m a n , 1984; T o rre s et a l , 1986;
(S o seb ee,
d is tr ib u tio n
S m ith , 1992). M o re o v e r, n o in v e s tig a tio n to d a te h a s a d d re ss e d
( N a m m a c k e t a l , 1985). L ike m o s t e la s m o b ra n c h s , d o g fish e x h ib it
th e p h y sio lo g ic a l th r e s h o ld o f d o g fish re la te d t o th e r ig o u r s o f
K -se le c te d life h is to r y c h a ra c te ristic s , w h ic h in c lu d e slo w g ro w th ,
c a tc h a n d release, a n d to o u r k n o w le d g e , n o s tu d y h a s d o c u m e n ­
la te m a tu r ity , a n d lo w fe c u n d ity (N a m m a c k e t al., 1985; A S M F C ,
te d th e p o s t- c a p tu r e p h y s io lo g ic a l im p lic a tio n s o f m o b ile -fish in g
2 0 0 2 ). F e m a le d o g fish also d isp lay a p r o lo n g e d ( 1 8 - 2 2 m o n th )
c a p tu r e in a n e la s m o b ra n c h . In o r d e r to g a in g re a te r u n d e r s t a n d ­
p e r io d o f g e s ta tio n a n d w h e n a d ire c te d fish e ry ex ists f o r th e
in g o f p h y sio lo g ic a l stre s s re s p o n s e s a n d th e re silie n c e o f d o g fish ,
sp ecies, a re se le c te d o v e r m a le s as a f u n c tio n o f th e ir la rg e r
a sa m p le o f tr a w l-c a p tu r e d d o g fish w as tr a n s p o r te d , h e ld c a p tiv e
m a x im u m
fo r 30 d , a n d assessed f o r p h y sio lo g ic a l s ta tu s a n d m o r ta lity
2000)
and
a
c irc u m b o re a l
g lo b a l
b o d y sizes a t m a tu r ity (S o seb ee, 2 0 0 0 ). P rim a r ily
b e c a u s e o f th e s e fa c to rs, h e ig h te n e d fish in g p re s s u re in
th e
fo llo w in g th e c o m p le tio n o f ea c h s tu d y p h a se .
w e s te rn N o r t h A tla n tic d u r in g re c e n t d e c a d e s led to a r e p o r te d
7 5 % d e c lin e in m a tu r e fem ale sto c k s b e tw e e n 1998 a n d 2 0 0 3 , a n d
a c o n c o m ita n t sc a rc ity in r e c r u itm e n t (A S M F C , 2002; N E F S C ,
Material and m eth od s
2 0 0 3 ). I n c id e n ta l c a p tu r e o f d o g fish is a lso ex ten siv e. L o w tr ip
A n im al co lle c tio n , tra n sp o r t, a n d h o ld in g in tan ks
lim its a n d lim ite d c o m m e rc ia l v a lu e h av e le d to th e d is c a rd in g o f
D o g fish w e re c a u g h t in s ix , 45 m in , m o d e r a te ly p a c k e d ( ~ 2 7 0 -
c o n s is te n tly la rg e q u a n titie s o f d o g fish in w e s te rn N o r t h A tla n tic
300 kg) o tte r tra w ls d u r i n g
c o m m e r c ia l
and
S e p te m b e r 20 0 4 ) s o u th e a s t o f C h a th a m I n le t (4 1 ° 3 8 'N 6 9 ° 4 8 'W )
d isc a rd su rv iv a b ility o f d o g fish h o ld m a jo r im p lic a tio n s r e g a r d in g
a b o a r d th e c o m m e rc ia l fis h in g vessel “J o a n n e A I I I ” ( C h a th a m ,
s to c k h e a lth a n d a s so c ia te d m a n a g e m e n t. A u g m e n tin g th e assess­
M A , U S A ). A 350 h p , s e m i- h ig h - ris e D a n is h o tt e r traw l c o n t a i n ­
fish eries.
T h e re fo re , p o s t- c a p tu r e
c o n d itio n
tw o
c o n s e c u tiv e days (2 a n d
3
m e n t o f c a p tu r e stress w ith a n in v e s tig a tio n o f t r a n s p o r t a n d c a p ­
in g 302 m e sh e s in th e fis h in g circle a n d a 15.2 c m m e sh w a s u sed .
tiv ity c a n p r o v id e in sig h t r e g a rd in g a sp e cies’ c a p a b ility t o re co v er
T h e n e t also p o sse sse d 1 5 .0 f a th o m to p a n d b o t to m legs a n d 20.0
fo llo w in g a p a r tic u la r f o rm o f c a p tu r e a n d a v a rie ty o f a d d itio n a l
f a th o m s o f g r o u n d cable. T h e tra w l d o o r s w e ig h e d 454 kg. A h a rd -
stre s so rs, a n i m p o r t a n t fa c to r w h e n assessin g h o w re s ilie n t p o p u ­
b o t to m
la tio n s a re w h e n c a p tu r e d a n d d is c a rd e d as b y c a tc h .
b o u ld e rs . T h e d e p th o f tr a w lin g ra n g e d b e tw e e n 50 a n d 65 m o n a
A lth o u g h m a n y s tu d ie s have in v e s tig a te d th e p h y sio lo g ic a l
re sp o n s e s
to
c a p tu r e ,
h a n d lin g ,
tra n s p o rt
and
c o n f in e m e n t
sw eep o n
th e
b o so m
se c tio n w as u tiliz e d to
av o id
c o b b le a n d s a n d se ab ed , w ith 1 3 .0 - 1 4 .0 ° C b o tto m - w a te r te m p ­
e ra tu re . T h e to ta l le n g th (T L ) o f d o g fish u tiliz e d in th e s tu d y
T 2006 I n te r n a tio n a l C o u n c il fo r th e E x p lo ra tio n o f th e Sea. P u b lis h e d b y O x fo rd J o u rn a ls . All r ig h ts reserv ed .
F o r P e rm is s io n s , p le a se em ail: jo u r n a ls .p e r m is s io n s @ o x fo r d jo u r n a ls .o r g