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H / â m s iN S T IT U U T v o o r d e z e e I
IC li S Journal u l ' M a r in e S
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A v a ila b le o n l in e a t w w w .s c ie n c e d ire c t.c o m
IN S T IT U T E
.. B e ( g j u m
S C I E N C E
D I R E C T '
Partial fishing m ortality per fishing trip: a useful in d ic a to r
o f eifective fishing effort in mixed dem ersal fisheries
A driuan D. R ijnsdorp, N iels D aan, and W illem D ekker
R ijn s d o rp , A . D ., D a a n , N „ and D e k k e r, W . 2 0 0 ft. Pa n ia I fishing m o rta lity per fish in g trip:
a u se fu l in d ic a to r o r e f l e c ti v e fish in g effort in m ix e d d e m e rsa l fisheries. - R T .S Jo u rn a l o f
M a rin e S c ie n c e , 6 3 : 5 5 6 —sem.
ITTorl m a n a g e m e n t Inis b e e n p ro p o se d as a n a lte rn a tiv e fo r quota inam igem cnl in m ix ed d e ­
m e rs a l fish e rie s. It re q u ire s a m e tric to e s tim a te th e fishing m o rta lity im posed by a g iv en
q u a n tity o f n o m in a l fish in g effort. H ere, w e e s tim a te the partial fishing m o rta lity rate im ­
p o se d by o n e u n it o f fish in g effort (/''|„„-) d u rin g in d iv id u a l fishing trip s an d e x p lo re the u s e ­
fu ln e s s o f th is in d ic a to r fo r m a n a g in g N o rth S eu beam tra w le rs -.JOI) lip targ etin g so le
(S o le n s o h u i) a n d p la ic e ( F le u r o n c c ic s p la te s s a ). /■'|im. is p o sitiv e ly related lo vessel en g in e
p o w e r , a n d in c r e a s e d a n n u a lly by 2.K'!ii (s o le ) an d I .()"« (p la ic e ). The p o sitiv e trend w a s d u e
lo a n in c re a se in s k ip p e r sk ills a n d in v e stm e n t in a u x ilia ry e q u ip m en t, th e re p la c e m e n t o f
o ld v e s se ls b y n e w o n e s a n d . lo a le sse r e x te n t, lo u p g ra d e en g in es, T h e av erag e
im ­
p o se d p e r d a y at s e a by a 2 0 0 0 h p b e a m tra w le r w a s e s tim a te d lo be 1.0 • IO ' (so le ) an d
0 .6 X- [() ’’ ( p la ic e ), am i it sh o w e d su b sta n tia l se a so n a l a m i sp a tia l v ariatio n s, T h e
of
so le and p la ic e w e re n e g a tiv e ly re la te d in s u m m e r and sh o w e d n o re latio n sh ip in w inter.
The e x is te n c e o f p re d ic tiv e se a so n a l and sp a tia l p a tte rn s in
o p en s up the p o ss ib ility
o f fin e -tu n in g m a n a g e m e n t by d ire c te d effort re s tric tio n s a n d u n c o u p lin g m an ag em en t o f
p in ico and so le ,
«> 2.0(15 I n t e r n a t i o n a l C o u n c i l f o r th e l i x p l o i a t i o n o f t h e S e a . P u b l i s h e d b y f b e n e i l . u l A l l r i g h t s r en ei v e i l .
K e y w o rd s: b e a m tra w l, c a lc h a h ility , effort m a n a g e m e n t, fishing effort, fishing m o rta lity ,
m ix e d fish ery , p la ic e , so le , te c h n o lo g y c reep .
R e c e iv e d 12 M ay 2 0 0 5 ; a c c e p te d 2 2 O c to b e r 20 0 5 .
.■¡. /). R ijn sd o rp . N , ¡ h u n t, m iii II'. I h i k v r : R i l ’! h N e th e rla n d s In stiló te fo r F ish erie s R e ­
s e a r c h . A n im a l S c i a n i' (¡ro u p , Il'i/gf/i/ngcM -HA'. F O R o \ rt.V, /V ?// A li Ijinnideii. The N eihe r h m d s . C o r r e s p o n d e n c e I n .-I. I). R ijn s d o r p lei: I 31 255 .fiWA'/rt.- /<n. t M 255 57i<M-/-/,
i '-m a il: m Iria a n . ri ju s t lo r/ Kirr i c nr. n I.
Introduction
advice, Effort m a n a g e m e n t h a s be en p r o p o s e d as a possible
m e a n s o f re solv ing th e s e p ro b le m s a n d i m p ro v i n g the effec­
In E u r o p e a n U n io n w aters, fish stock s arc m a n a g e d by se t­
tiveness o f m a n a g e m e n t (D an n , DA)7; U lrich c t u l ., 2002;
ting a n n u a l c a tc h lim its (total a llo w a b le ca tc h , TAC,’), ac ­
S hepherd, 200.1).
A p rerequisite for effort m a n a g e m e n t is that th e relation
com panied
by
technical
m easures
such
as
gear
restriction s, m e s h si/.e regu latio ns, a n d c lo s e d a re a s and
betw een fishing effort a n d fishing m o r ta l it y o f a species is
se a s o n s ( H o l d e n , DAM). M a n a g e m e n t has b een u n su cce s s­
ful for m i x e d fisheries ( I lolden, DAM; E u ro p e a n C o m m i s ­
know n. A sse ss in g this relation sh ip is n o t w ith o u t problems
sion, 2 0 0 1 ), an d s e v e r a l d e m e r s a l stock s h a v e d ec lin ed to
h is to ric ally lo w levels while ex p lo ita tio n levels rem ain
vidual vessels o w in g lo d ifferences in the skill o f the fishers
loo high (I C E S , 2 00 4).
T h e m a i n p r o b l e m in m a n a g i n g m ix e d fisheries b y sin-
(Marteli a n d W alters, 2 0 0 2 ). E ffic ie n c y varies a m o n g indi­
and vessel c h a ra c te ristic s ( l l i l b o r n , DM5; S quires and
Kirkley, DAW), an d it m a y in c re a se o v e r t i m e th rou gh con­
tinuous d e v e lo p m e n ts
in the
fis h in g
in d u stry
(Marchai
g le - sp e e ie s T A C ,s is that the lutte r d o not restrict the catch
el u l., 2001, 2 0 0 2 a ; E asco e c t u l., 2 0 0 1 ; U lrich cl ul.,
but o nly th e (official) land in gs, the o v e r - q u o ta catc h o f a
2002; O 'N e ill c l ul., 2 0 0 3 ). H o w e v e r , the elficicn ey may
sp e c ie s p e r h a p s b e in g d is card ed o r landed illegally {I lolden,
be affected also by m a n a g e m e n t r e g u l a ti o n s , fo r instance
DAM; D aan , 1007). T h is o c c u rs b e c a u se th e T A C for differ­
by the in tro d u c tio n o f a c lo se d
en t targe t s p e c i e s m a y not be d e p le te d in sy nch ron y, so fish­
2002a, h), c lo s e d s e a s o n s, c h a n g e s m m e s h si/.e. or cuts
ing c o n ti n u e s w h e n the T A C for o n e sp e c ie s h a s been
d e p le te d . T h i s h a s tw o m ain effects; (i) fishing m o rtality
ellieiency o f vessels in c o r p o ra le s t h r e e in terre lated aspects:
is n o t c o n s t r a in e d b y the T A C ; (ii) d e te r io r a tin g ca tc h sta­
one technical, o n e b io lo g ica l, and o n e e c o n o m i c , (ionr effi­
tistics lead to in a c c u ra te a ss e s s m e n ts a n d u n c e rta in ly in the
cien c y m a y he d efined a s the fra c tio n o f the fish present in
III5-Ï-.II m . o u i o
a r e a ( M a r c h a i et a i,
in quota (E asco e cl u l., 2 0 0 1 ; Eous c t u i , 2001 ). T herefore,
V e t t l i i l i n e i l u i t i i i i K i l ( ( m u n i I.II i tic Í
u l ' tiw S e a
s u b ito e.V . lic e n se d c u s to m e r co p y su p p lie d a n d p rin ted fo r F la n d e rs M arine In stitu te t.ib ia ry (S L I05X 00225E )
1’u b l u . h c i l b y î - . l i c v i c i l . u l
A l l n u b e , u m t v c i I.
P a rtia l fis h in g m o rta lity a s in d ic a to r o f e ffective fis h in g effort
lile p a th o f a traw l thai is reta in ed by th e g ear. It m a y vary
557
with phy sical co n d itio n s, the skill o f the skip pe r, a n d fish
da tab a se (N R V I) c o m p risin g sh ip iden tity co de, engine
pow er, vin tag e o f hull, v in ta g e o f en g in e, a n d ownership.
behaviour, but it is largely in d e p e n d e n t o f th e a m o u n t o f
For the analysis, individual v e sse ls w e r e u n iq u ely coded
lish p re s e n t (alth o u g h it m a y he influenced b y c a tc h rates).
for ea c h period that no c han ge o c c u rr e d in the hull, engine,
o r ow nership. T h e tim e that s u c h a u n it has b e e n in opera­
Catchability o f a spe cies refe rs to the c h a n c e that a n indi­
vidual lish in a p o p u la tio n is c a u g h t by a g ear, and th erefore
depends on both g e a r efficiency and th e d istribution o f lish
in relatio n to the d is trib u tio n o f the fleet. A s such it is an
aggregated m e a s u re that is alTectcd by the sk ills o f skip pe rs
io lo ca te the greatest den sitie s, f i n a l l y , e c o n o m ic efficiency
combines g e a r efficiency a n d ca tc h ability, b u t it c o n strain s
tion w as calculated relative to the d a te o f entry o f the unit
a n d expressed in decim al y ear. For vessels that entered the
fleet befo re the study p eriod , 1 J a n u a r y 1990 w a s taken as
d a te o f entry.
R e ctan g les w e r e grou ped into fishing a re a s that reflect
the spatial distribution o f th e tw o flatfish sp e c ie s in relation
individual v essels to fish on ly u n d e r profitable c o n d ition s
by ta k in g into account c osts an d returns.
to m a n a g e m e n t (Figure 1), H e n c e , a re a s w e r e distinguished
T ra d itio n a l sto c k a s s e s s m e n t p ro v id e s e stim a te s o f the
b o x (6, 7, 8), th e offshore fish ing g ro u n d s in the southern
(3 ), s o u th w e s te rn (4), and s o u th ea stern N o rth Sea (5, 10),
the D o g g e r B ank (13, 14), a n d the r e m a in i n g central North
catchability (</) b y age g ro u p in tu n in g series, b ased on
ihc a s s u m e d lin e a r re latio n sh ip b e tw e e n fishing m ortality
(/■') a n d effort (F ):
th a t co m p r ise d the 12-nautical m ile z o n e (1, 2), the plaice
S ea (16). In this classification, w e took a c c o u n t o f m anage­
m en t re g u la tio n s regarding m e s h size (8 0 m m south o f
5 5 ° N ; 100 m m north o f 5 5 ° N until 2 0 00 , w h en the line
w as sh ifte d to 5 6 °N fo r w a te rs ea st o f 5 ° E ) . T h e seasonal
(( X
U s in g the ratio o f the c a tc h o f fleets o r individual vessels
relative lo the internation al catc h, the ann ual fishing m o rtal­
and spatial distributions o f th e fish in g trips analysed are
given in T ab le 1.
ity m a y b e d e c o m p o s e d into the partial /•' im p osed b y these
lleels o r v essels ( D everlon anil llo lt, 1957) d u ri n g a specific
D utch b e a m tra w l fleet
period, e x p re s s e d as the fishing m o rtality in d u ced p e r unit
A m o n g the international fleets, the D u tc h fleet is responsi­
iil'oflbrl (partial
I) and integ rates all aspects
ble for > 8 0 % o f the F o f sole, w h ile for p laice a peak o f
about 6 0 % w as reached in the 1980s, fo llo w e d by a decline
of g e a r efficiency, crew skill, an d the p ro p o rtio n o f a species
to ab o u t 5 0 % in the 1990s. T h e D u tch b o tto m trawl fleet is
availab le o n the fishing g rou n d , T h e re fo re , the partial /*' a p ­
proach sh o u ld a llo w e v a lu a tio n o f the so u r c e s o f variability
d o m in a te d by large be a m traw lers ( > 3 0 0 lip), w hich con­
tribute 5 1 % o f the total n u m b e r o f fish in g day s, and 89%
in th e relatio n sh ip b e tw e e n lishing m ortality an d effort.
Here, we ex p lo re the v a rio u s c o m p o n e n t s o f the process
and 9 2 % o f the landings o f sole a n d plaic e, respectively.
T h is partial
directly a s c a tc h a b ility (P.
c a n be interpreted
(seasonal dffiurcnees, spatial differences, effects o f engine
T h e fleet ope rate s in the offshore w a te rs o f th e N orth Sea
b eyond the 12-m ile coastal zone a n d o u ts id e the plaice
p o w er, v intag e o f hull an d e n g in e , tim e that a vessel has
box (Pastoo rs e t a l, , 2000). The n u m b e r o f large beam
heen in o p e ra t io n ) that lead s from the o pe ratio n o f a single
traw lers has d e c lin e d pro g ressiv ely b u t step w ise since
vessel ou a lishing g ro u n d lo the a n n u a l total fishing m o r ­
1990 (F ig u re 2a), intersp aced w ith p e r i o d s o f relative sta­
bility ( 199 3— 1995, 1998—2000), p a rtly in resp on se to the
tality im p o s e d o n a sp e c ie s by a m ix ed fishery, based on
ava ilab le data for the D u tc h b o tto m trawl Heels targeting
North S ea sole (S o lea s o le a ) and p laice (P le u r o n e c te s p la ­
tessa). H a v in g quantified th ese c o m p o n e n ts , we c o m m e n t
on th e u se fu ln e ss o f partial l-]wc as an in d ic a to r o f effective
57 _
lishing effort in an elfort m a n a g e m e n t context.
56 -
Material and m ethods
D ata
D ata on la n d in g s and elfort b y trip are a v a ila b le for the
1990, T h is databa se
53 -
( V IR IS ) is h eld by the natio nal fisheries insp ecto ra te and
c o m p r is e s d a ily records o f s h ip id entity c o d e , lan din gs by
D u tch b o tto m trawl fisheries from
52 -
sp ecies, a r e a fished (K T IS recta n g le), g ear, and h o u rs out
o f p o rt. A s the daily rec o rd s are not a lw a y s accurate,
c a tc h e s w e r e s u m m e d by trip an d ass ig n e d to the rectangle
h a v i n g the larg est sh a re o f th e trip ca tc h . T e c h n ic a l ch ara c­
te ristics o f th e vessels are reg istered in a national vessel
•2
-I
0
I
2
3
4
5
6
7
8
9
L o n g itu d e (d e g re e s )
F ig u re 1, M ap o f th e fish in g a re a s d is tin g u is h e d in th is study.
s u b ito e V. lic e n se d c u s to m e r c o p y s u p p lie d a n d p rin ted fo r F la n d e rs M arine In stitu te l ib ra ry (SfclftSXe e g g f f i f
A. D. R ijn sd o rp ct al.
558
T a b ic 1. N u m b e r o f f i s h m g tr ip s w ith a trip d u ra tio n b etw e e n 2 an d
but g ra d u a l in c r e a s e in m e a n e n g in e p o w e r , fro m 1996hp
< 5 d ay s o u t o f p o rt b y a re a a n d q u a rte r d u r in g the s tu d y p erio d
in 1990 to 2 2 7 7 hp in 2 0 0 3 . T h e ag eing o f the b e a m trawl
1 9 9 0 - 2 0 0 3 . A re a c o d e s are s h o w n in F ig u re 1.
fleet is reflected in t h e in crea se in the p ro p o r tio n o f vessels
w ith a hull (o r e n g i n e ) < 1 0 y e ars old fro m (>2% (79% ) jn
Q u a rte r
A rea c o d e
2
1
01
215
02
2 168
1990 to 28%. ( 5 3 % ) in 2 0 0 0 . The m e a n v in t a g e o f the (leei
4
3
243
2998
S um
175
169
802
I 45 I
1 344
7 961
03
7 336
5435
4 157
04
67(1
443
436
4 905
407
21 83.3
05
6 134
4 486
5 5 18
5710
21 848
06
1 071
1 138
I 14 4
1071
4424
in crea sed b etw e en 1 9 9 0 a n d 2003 at an a n n u a l rate o f 0.64
(hull) a n d 0 .76 (e n g i n e ) . T h e n u m b e r o f y e a r s th a t a modal
vessel h a s b e e n in o p e r a t io n w a s e s tim a te d at 21 years
(hull) a n d 16 y e a rs ( e n g i n e ) u sin g the “ p ro e I i Iciest" of
S AS ( S À S , 1999; F i g u r e 3).
1956
Partial lish in g m o r ta lity
07
479
14 5 3
I 499
831
4 262
T otal I ' b y q u a rt e r f o r b o th sp e c ie s was ta k e n fro m multi­
08
27
336
184
430
977
sp ec ies virtual p o p u la t io n a n a ly s is (M S V P A ) o f quarterly
10
3 497
3416
849
ca lc h -a t- a g e d a ta f r o m the total in tern ation al licet (K.TS,
730
4 486
668
13 742
11
2 343
731
2 978
2002), u p d a te d to 2 0 0 3 . T h e a p p ro a c h is s i m i la r (o quarterly
12
39
12-1
lift
397
s in g le -s p e c ie s V P A , b e c a u s e in th is m o d e l, p laic e and sole
13
14
966
974
118
I 601
a re not p re y e d up o n . B e c a u s e o f the h i g h e r tem p oral réso­
8II
471
1 287
319
4 828
230
16
86
240
81)
54
460
23 6 4 8
2 1 755
2 i 988
20 908
88 299
Sum
1 831
lution, the q u arterly m o d e l w a s p referred to the a n n u a l VPA
ro utin ely p e rf o rm e d f o r m a n a g e m e n t a d v ice . Q u arte rly /■',
a v e ra g e d o v e r ag e c la s s e s 2
8, w a s d e c o m p o s e d into par­
tial i ' a c c o r d i n g lo p ro p o r tio n a l catch, first b y country,
th en by fleet, in d iv id u a l v e ss el, an d finally b y w e e k ( - trip).
fleet-red u ctio n p o lic y o f th e lu iro p e a n U nion . T h e decline
T h e re fo r e , the p artial /•’ im p o s e d b y an in d iv id u a l vessel /'
h a s g e n e ra lly b e e n th r o u g h th e e x it o f o l d e r v essels with
p e r day in w eek j {/>«.„) w a s
rela tiv e ly low e n g i n e p o w e r s that h av e not b een c om p ensated
f o r by r e p l a c e m e n t s . N e w v e s s e ls m a in ly entered the fleet
b e tw e e n 1990 a n d 1995. H o w e v e r , m a n y m ore have re­
<*„
/*'
C‘
iii,'
n e w e d th e ir e n g i n e ( F i g u r e 2b). T h i s resulted in a small
w here c„ are th e la n d in g s o f vessel / in w e e k /, ( ' the total
(a)
q u arterly land in gs, /-'the q u a rte rly / ' ' o f t h e total international
V in ta g e o f h u ll
S3 21)0.1
I')')(!
H 2002
O lO'M
.. 21)0(1
■ |')i)i)
■
u m
p p m .)
|g|
E ] IVHO-K'
H
i w
I'J H itW
7 ')
■ I'l‘)S
■ IO')I
I'JUV
licet, a n d d,¡ is the n u m b e r o f d a y s fished by vessel i in
IV'JO
w e e k /.
■ •-(w
is d e t e r m i n e d b y the g ear efficiency o f the in­
d iv id u al vessel an d the n u m b e r o f lish a v a ilab le in the arca,
25(1
a n d is a m e a s u r e o f the local c a tc h a b ility o f a particular
vessel.
5 200
T his a p p r o a c h is a r a t h e r c ru d e a p p ro x im a tio n o f the ac­
« ■ « M
111
tual co n trib u tio n o f e a c h trip to total fishing mortality,
b eca u se w e had to a s s u m e sim ila rity in e x p lo ita tio n patterns
a m o n g v essels a n d a c r o s s w eeks.
illii
V in ta g e o f e n g in e
(1.75 -
ÿ 20(1 -
llü iü s i
(U K )
—
•—■
■■
—
—
«i
«i
o
5
It)
F ig u re 2. C h a n g e s in th e c o m p o s itio n o f th e a c tiv e beam traw l fleet
in re sp e c t o f (a) h u ll v in ta g e , a n d ( b ) e n g in e v in ta g e , b e f o re 1990,
v in ta g e is g ro u p e d b y 5 - y e a r p e r io d s .
.to
>1(1
A g e (y e a rs)
F ig u re 3. S u rv iv o rs h ip c u rv e ( m e a n a n d 9 5 % c o n fid e n c e lim its) of
a hull und e n g in e o p e ra tin g in th e D u tc h beam traw l (leei.
s u b ito e.V . lic e n se d c u s to m e r co p y s u p p lie d a n d p rin ted for F la n d e rs M arine In stitu te Library (S L I0 5 X 0 0 2 2 5 E )
P a rtia l fis h in g m o rta lity a s in d ic a to r o f effective fis h in g effort
S tatistics
Results
G e n e ra liz e d linear m odels were used to analyse the contri­
b u t i o n o f tem po ral and spatial covariables as w ell as ship
c h a rac teristics to the variability in F pue (SAS, 1999 P r o c genm od). As the F estim ates from V P A have not
y e t converged in the m ost recent years, th ey w ere dow nw e ig h te d b y applying an arbitrary w eig h t vector
1 - exp - 0 . 6 8 X ( 2 0 0 4 - y e a r ) that increased the w eight
f r o m 50% in the final year (2003) to > 9 3 % in y ears up
P artial fishing m ortality
t o 2000.
T w o typ es o f m odels were fitted, both using a log link
f u n c tio n a n d a Poisson error. A lthough residuals were
s lig h tly correlated to m odel predictions, w e preferred to re­
t a i n the Po isso n distribution rather th an a lognorm al or
G a m m a distribution, because o f the com plications in h a n ­
d l i n g zero observations in the latter. O ur focus is on the d e ­
v e lo p m e n t o f n ew m ethodology, rather than on data-fitting
p ro c e d u r e s .
T he first, m o re parsim onious, o n e m o delled the seasonal
p a t t e r n s in F pue for each fishing area by a periodic regres­
s i o n model, allow ing for a linear time trend (model 1):
iw
= exp ln ( P ) + area x V ' ( s i n " cp + eos" cp) + 1 .
( 1)
w h e r e P is the engine pow er o f the vessel, area the code for
t h e fishing area, cp the 2tc x w eek num ber/52, representing
a periodic function w ith a period o f 1 year, and t is the time
i n d ecim al year. H igher-order terms w e re included to fit
s e a s o n a l p e a k s and lows o f different levels a t different tim es
d u r i n g the year.
Second, to check the fit o f the periodic model, a m o re
c o m p l e x m o d e l was applied including each w e e k x year
a n d area x w e e k com bination separately, ac counting for
d e t a i l e d tem p oral variation (m odel 2):
— ex p [ln (P ) + ye ar x w e e k + area x week].
(2 )
In the th ird step, the contribution to the time trend in F pue
o f the recruitm ent o f a n ew vessel (vintage o f hull, vh), the
u p g r a d e o f th e engine (vintage o f engine, ve), and the au­
t o n o m o u s tr e n d (time that a vessel h as b een in operation,
559
E n gin e p ow er, seasonal p a tte m , and fishing area all ex­
p lain ed a significant part o f the v arian ce in F puc (Table 2).
F pue increased with engine pow er. T h e slo p e o f the log—log
regression (sole, ß = 0.809, s.e. = 0.006; plaice, ß = 0.516,
s.e. = 0.006) was < 1 , indicating th a t an increase in engine
p o w e r did n o t enhance the F paa to th e s a m e extent. Also, the
slopes o f the regressions differed b etw e en species. Hence,
the increase in m ean e n g in e po w er in the fleet observed be­
tw e e n 1990 and 2 00 3 , fro m 1996 to 2277 hp (+ 1 4 % ), re­
sulted in an increase in F puti o f 11% in sole and 7% in
plaice.
T h e periodic regression m odel (m od el 1) included the
statistically significant first, secon d, and third order terms.
T h is m odel explained 3 9 % (sole) and 4 3 % (plaice) of the
v ariance in Apue, but p erfo rm ed p o o re r th a n model 2, in­
c luding the w eek x area a n d y e a r x w e e k interaction terms
(5 7 % and 56% , respectively; T a b le 2); the change in AIC
am o un ted to 53 176 fo r sole an d 24 8 74 fo r plaice, Obviously,
m o d e l 2 is p referred in term s o f v arian ce explained.
F ig ure 4 sh ow s a rath er close c orrespo nd en ce between
the fitted seasonal patterns o f th e tw o m o d e ls for plaice.
In sole, ho w ever, the perio dic m odel sh o w e d discrepancies
d u rin g som e periods. F o r instance, in 1996 and autumn
1998, the w eek ly estim ate s w ere consistently higher than
the fitted valu es o f th e p erio d ic m o del, w h ere as the weekly
estim ates in w in ter 1995 an d 2001 w e re consistently lower,
In s o m e years, the autu m n p e a k in sole also occurred earlier
(1 999) or later (1995, 2 0 02 ) th an p re d ic te d by the periodic
Table 2. Percentage of the deviance in F puc explained by the
covariables; ln-transformed engine power (ln(F), lip), fishing area
(area; cf. Figure 1), season and time trend (/: model 1 only). The
seasonal pattern was included as a periodic function (model 1:
£ "(sin " cp + eos" cp)) or as class variables week and year (model
2). All explanatory variables explained a significant part of the var­
iance (p < 0.001).
Sole
Covariable
% Explained
Plaice
d.f.
% Explained
d.f.
t u ) w ere estim ated b y replacing t in Equation (1) with
v h + ve + tu (model 3):
Fvuc =
Model 1
LnCP)
Area x period
Time trend
exp l n ( P ) + area x 5 3 ( s i n " V + cos,i <p)
Total
+ v h + v e + tu
( 3)
T h e statistical fit o f the m od els w as com p ared by the p e r ­
c e n t a g e e xp lain ed deviance, as w ell as A k a ik e ’s in fo rm a­
Model 2
ln (P)
Area X week
Year x week
Total
20.4
14.8
3,8
1
53
I
6.0
35.8
1.0
1
77
1
39.0
55
42.8
79
20.4
17.2
19.5
1
468
676
6.0
38.2
11.6
1
670
676
57.1
1 145
55.8
1347
t i o n criterion (AIC) (Akaike, 1973).
s u b ito e .V . lic e n se d c u s to m e r c o p y s u p p lie d a n d p rin te d for F la n d e rs M arin e in stitu te Library (SLIÖ 5X 0Ö 225E)
A . D. R ijn sd o rp et al.
560
(a)
Sole
2 5 -■
20
-
X
(b )
P laice
20
15
IO
5
0
1990
1991
F ig u re 4. S e a s o n a l p attern in p re d ic te d
1992
1993
1994
1995
1999
1997
1998
1999
2000
2001
2002
2003
200-1
lo r (a) s o le and (b ) p la ic e lor a 2000 h p b eam tr a w le r lis h in g in area 3 (S o u th e rn H ig h i) from
(p erio d ic) m o d e l I (lin e s) a n d m o d el 2 (d iam o n d s).
model. D e s p it e these d ifferen ces, the perio d ic model per­
w in te r (b lu e ) a h ig h Z',)UC o n p la i c e m a y c o in c id e w ith either
form ed sufficiently well t o be u se d a s th e basis for the fur­
a hig h or a low F pw o n sole.
ther analysis. T h e results also revealed a significant positive
trend in th e
o v e r tim e .
T h e s e a s o n a l pattern estim a te d by th e p erio d ic m odel dif­
fers a m o n g fishing areas. Sole F puc p e a k s in a u tu m n and
T i m e t r e n d in c a t c h a b i l i t y
A c c o rd in g to m o d e l 1, F pyic in creased an n u a lly b y 2 .8 %
bo ttom s o u t in su m m e r i n all areas, e xcep t in the eastern
central N o rth Sea (F ig u re 5a). T h e re , F pxi0 p e a k s in late a u ­
(s.e. ~ 0,04) in so le an d 1.6% (s.c. - 0.04) in plaice. T o e x ­
tum n and b o tt o m s o u t in e a r l y spring. T h r o u g h o u t the year,
substantial d ifferences in F pw ex is t a m o n g areas. B etw een
crease, F puc was a n a ly s e d in relatio n lo the v in ta g e o f the
plore the c o n trib u tio n o f te c h n o lo g ic a l c r e e p lo this in­
hull, the v in tag e o f the e n g in e , and the tim e that a vessel
N o v e m b e r a n d A pril, the difference m a y e x c e e d a factor
had b een in operation (m o d e l 3). All e o v a ria h le s sh o w e d
o f 2 , r e d u c in g to ab o u t 1.5 in J u n e a n d July. Plaice /•’|Hie
s h o w s a seas o n al pattern in the s o u t h e r n and ea stern areas,
w ith high v a lu e s b e tw e e n N o v e m b e r a n d Ja n u ary an d low
a significant positive c o n tr ib u t io n lo
a n d e xp lain ed
3.9 % and 1.2% o f the v a r i a n c e in sole and p laice, re s p e c ­
valu es b e tw e e n A pril a n d A u g u s t (F ig u re 5 b ). North o f
5 5 ° N , v a lu e s are re l a ti v e ly high th ro u g h o u t th e year. As
ance (1 .8 % and 0.4% , r e s p e c tiv e ly ) cou ld n o t be ascribed
to a single covariable. C o m p a r i n g m odel 3 w ith m o d el I,
in sole, p la i c e F j)UC differs su b stan tially a m o n g areas. Be­
A IC ch a n g e d by 4 4 5 for sole a n d 4 9 6 for plaice.
tively (T a b le 3). A s u b s tan tial part o f the e x p la i n e d vari­
tw een N o v e m b e r a n d F eb ru ary , the v a lu e s m a y differ by
'f h e m e a n v in tag e o f the fleet increased b e tw e e n 1990
a factor o f 1.5, increasing to abo ut 3 d u r i n g su m m er. H ence,
and 200 3 at an an nu al ra te o f 0 . 6 4 (hull) and 0 .76 (engine),
the c o n tr ib u t io n o f a vessel to total F d e p e n d s to a large ex­
tent o n fishing area and se a s o n .
creased by 0,58. C o m b i n i n g th e s e rates with the slopes o f
while the m e a n o p e ra tio n tim e o f the v essels in the fleet in­
A plot o f w e e k ly e stim a te s o f sole F püa ag ain st plaice
the catc h a b ility re la tio n sh ip s g i v e s an e s tim a te o f the o ver­
F V\ic b y a r e a (m o del 1) s h o w s that th e fisheries on the
two species a r e to a large e x te n t u n c o u p l e d (F ig ure 6). D u r­
all increase in F]wc o f 2 . 5 % in s o l e and 1.4% in plaice, close
to the time trend e s tim a te d in m o d e l 1 (T a b le 4). T h e over­
ing s u m m e r , in d icate d b y red, th e r e is a n overall negative
relationship b e tw e e n the tw o s p e c ie s, w h ereas during
all trend is m a in ly due to an in c re a se in efficiency w hen the
vessel unit is in o p e ra tio n ( 4 2 % an d 4 8 % for s o l e and
s u b ito e.V . lic e n se d c u s to m e r c o p y s u p p lie d a n d p rin ted for F la n d e rs M arin e in stitu te Library (S L I05X 00225E )
P a r tia l J is h in g m o rta lity as in dicator o f effective fis h in g effort
(a)
561
S o le
20
Dutch c o a s t 112
S o u th e rn B ight «3
F lu m borough #4
O yster G ro u n d s It5
x
W itddcn c o ast II6
H elg o lan d #7
G erm an Bight 1/10
D ogger B ank I I 13
(b)
P laice
12
IO
—
Dutch c o a s t 1/2
—
S o u th ern Bight #3
—
F lu m h im m g li HA
X
O yster G ro u n d s «5
- — W m ldcn c o a st 116
H elgoland 1/7
--------------G erm an B ight //IO
1
•
D ogger B a n k //1 3
--------------Central N o rth Sen I I I 6
•1
- X
12
16
20
24
28
32
36
40
44
48
-
■ W hile B a n k « 1 1
52
W eek
l ig u re
5. S e a s o n a l p a tte rn s in /■'|UI0 fo r (a ) so le a n d (b ) p la ic e by fishing g ro u n d (of. F igure I) p red icted by m o d e l I fo r a 2 0 0 0 hp beam
I ra w Ier.
p la i c e , resp ec tiv ely ). R e p la c e m e n t o f v essels ranks second in
i m p o r t a n c e , w h e re a s u p g r a d i n g the e n g in e is o f equal im por­
sm alle r than the overall increase in efficiency t (Table 4),
This implies that a vessel b e c o m e s less efficient relative
ta n c e in so le hut less i m p o rt a n t in plaice. T h e p a ra m e te r esti­
m u l e for the effeel o f o p e m l i o n ti m e (tu) on F jnie (m odel 3) is
to th e fleet a s new v essels enter th e fleet.
T h e p aram eter e stim a te s for the effect o f hull vintage and
eng ine vintage can be related to th e n u m b e r o f years after
w hich 50% o f the vessels or e n g in e s h av e b een replaced.
W ith an estim ated survival time o f 2.1 (hull) a n d 16 ( e n ­
gine) years (Figure 3), the rep la cem en t o f a v essel (hull) in ­
creases efficiency b y 2 5% (21 x 1.17) in sole a n d by 19%
(21 x 0,91) in plaice. R e p la c e m e n t o f a n e n g in e increases
efficiency b y 15% ( 1 6 x 0 . 9 1 )
(16 x 0.19) in plaice.
in
so le
and
by
3%
C u m u la tiv e p a rtia l F b y vessel
T he partial F exerted b y a vessel d e p e n d s on th e area an d
seaso n chosen in w h ich lo operate. R estrictions o n the n u m ­
ber o f fishing days, therefore, m i g h t n o t tra nslate directly
into a proportional re d u c tio n in F. T h e re latio n sh ip between
the n u m b e r o f fishing d a y s and fishing m o rta lity was e x ­
p lored for e a c h vessel by plotting th e c u m u l a ti v e predicted
2
3
4
5
6
7
8
9
10
11
12
P L A I C E (FpucXlO*6)
F i g u r e b. S e a s o n a l p a tte rn s in th e r e la tio n s h ip b e tw e e n F pw for
stri e a n d p la ic e in tliH eroni a rc u s : D u tc h C o a s t (D C , fishing area
2 ): S o u th e r n H ig h i (S, II3 ); F lu m b o ro u g h (F , //4); O y s te r g ro u n d s
values o f F p(l0 against cu m u lativ e effort, afte r sorting the
w e e k s in d e sc e n d in g o rd e r of F puo o r r e v e n u e (Figure 7).
T his a ssu m es that fishers will re stric t th e ir effort pred o m i­
nantly in th o s e w ee ks a n d areas for w h ic h th e y e xp ect
a lo w catch rate. T h e m o r e convex this relatio n is, the better
( ( j, II 1(1); D o g g e r B a n k (D B , t t \ 3 ). C o lo u r s in d ic a te the se aso n
a fisher can select an inefficient w eek.
T h e relationship w as slightly c o n v e x f o r the (target) s p e ­
( d e e p b lu e
cies for w h ich the F vu0 w a s sorted ( F ig u re 7a, d), whereas
( O , II5)\ W a d d e n const (W , //6 ); H e lg o la n d (1-1, U7)\ G erm an B ight
D e c e m b e r--J a n u a ry ; d e e p re d = Ju n e —Ju ly ).
su b ito e.V . lic e n se d c u s to m e r c o p y s u p p lie d a n d p rin te d for F la n d e rs M arine Institute L ibrary (S L I0 5 X 0 0 2 2 5 E )
A. D . R ijnsdorp e t al.
562
T a b le 3. P ercentage o f th e v arian ce in F pua e x p la in e d by m odel I (ln (P ) + area x ( £ 3sin<p + £ 3c o s (p)), h u ll vintage (vh in year), engina
v in tag e (v e in year), an d tim e that th e u n it has b een in operation (tu in decim al y e a r). T h e p e rc e n ta g e explained by vh, v e, and lu w as
c a lcu lated a g a in st the full m o d e l (ty p e 3 analysis).
Sole
Plaice
%
P aram eter
%
Explained
d.f.
P
E x p la in e d
d.f.
P
3 5.7
54
< 0 .0 0 1
4 1 .8
78
< 0 .0 0 1
vh
0.7
I
< 0 .0 0 1
0.4
1
< 0 .0 0 1
ve
0.5
1
< 0 .0 0 1
0.02
1
< 0 .0 0 1
1.0
1
< 0 .0 0 1
0.3
1
< 0 .0 0 1
In(P ) + are a x ( ]T 3sin cp + £ 3cos tp)
tu
M u ltico llin earity
Full m o d el
1.8
39.6
0.4
57
the relationship w a s slightly c o n c a v e fo r the assumed bycatch species (Figure 7b, c), but th e r e w as considerable v ar­
iation a m o n g vessels. This m e a n s that, f o r target species,
a small proportion o f fishing trips contributed more than
p roportionally to the annual p artial F , w h ile fo r by-catch
species the average cum ulative F w as slightly below the
proportional relationship. T he relatio nship s only marginally
differed betw een species, sh ow in g a slightly m ore convex
shape i f p laice was th e target sp ecie s. B e cau se we sorted
the p redicted values f o r the area a n d w e e k fished excluding
the rand om error, fishers should b e able to select their best
trips o n the basis o f personal e x p e rie n c e i f faced with effort
reductions. O n average, therefore, a 4 0 % reduction in fish­
ing effort m ig h t lead to a re d u c tio n in F on the target
species o f 30% for so le and 20 % f o r p laice, whereas a re­
duction in F o n bycatch species w o u ld b e slightly larger
than the effort reduction (45% a n d 50 % , respectively).
H ow ever, because fishers m a k e ind ivid ua l choices, so m e
targeting sole and others targeting p la ic e , the overall effect
should be less than a ss u m in g that a ll fishers target one sp e­
cies o r the other. I f the trips w e r e sorted by descending
observed r e v e n u e 1 p e r d ay at sea (F ig u re 7e, f), the c u m u ­
lative relationships b ec am e m ore lin ear, especially w hen
operating o n sole. T h e shapes o f th e s e relationships suggest
th at i f the total allowable fishing effort is red u ced and fish­
ing vessels redistribute the ir fishin g effort to obtain the
highest revenue, there w ould b e a sligh tly lo w er than p r o ­
portional decrease in total F for s o le , and a substantially
less than proportional decrease in F fo r plaice.
Discussion
Seasonal patterns in F pue an d the d ifferen ces betw een fish­
ing areas (Figure 5) reflect c han ge s i n the availability o f the
tw o species attributable to seasonal m ig ra tio n s o f adult fish
4 3 .0
81
and to recruitm ent. A d u lt plaice migrate seasonally b e ­
tween the s p a w n in g g ro u n d s in the southeastern N orth
Sea in w in ter and the fe e d in g areas in the central N orth
Sea in s u m m e r (D e V ee n , 1978; Rijnsdorp and Pastoors,
1995; H unter el a l., 2 0 0 3 , 2004). Adult sole migrate in
spring from offshore feeding areas to inshore spawning
grounds alon g the con tin ental and English coast (ICES,
1965; De V ee n , 1976; R ijnsdorp e t al., 1992). A s a conse­
quence, adult sole m a y b e c o m e less vulnerable because th e
beam trawl fleet is n o t allo w ed to exploit the spawning
grounds w ithin the 12-mile zone or the plaice box. In a u ­
tumn, a n ew y e a r class leaves the cooling inshore waters
and recruits to the offshore fishing grounds (Beverton an d
Holt, 1957; D e V een, 1978). O ther factors influencing the
observed patterns in F pue m a y relate to variations in the e f­
ficiency o f th e gear cau sed b y changes in fish behaviour.
During the s p a w n in g season, m ale plaice are easier to
catch, b e cau se th ey a p p e a r to be more active (Rijnsdorp,
1993; S o lm u n d sso n e t a l., 2003), while females may be
less vulnerable because th e y spend a larger proportion o f
their time in m id w a te r (A rn o ld and Metcalfe, 1996; M et­
calfe and A rnold, 1997). S o le can even be observed n ear
th e surface d u rin g th eir s p a w n in g migrations (De Veen,
1967). C atchability m a y b e affected also by water tempera­
ture (W oo d h e ad , 1964; W in g e r el a l , 1999).
These fa cto rs m ay also h a v e contributed to the observed
differences in th e p red icte d F pue o f models 1 and 2 (Fig­
ure 4). Sea so na l changes in distribution may relate to varia­
tions b etw e en years attributable to local variations in
recruitm ent a n d /o r sp aw n in g -sto c k biomass. Based on the
proportion o f the v ariance e x p la in e d by the week x arca in­
teraction in m o d e l 2, interannual variations in distribution
ap pear to be larg er in sole th a n in plaice (Table 2). Indeed,
so le sh ow larger interannual variation in year-class strength
a s well as a lo w er spatial consistency across years than
plaice (va n B e e k et al., 1989; Rijnsdorp et a l, 1992; Fox
e t a l , 2000). M oreover, s o l e exhibit a distinct evasive re­
p ric e . T h e re la tiv e p ric e o f s o le is fiv e tim e s h ig h e r th a n th a t o f
sponse to low w a te r te m p era tu re during cold winters (e.g.
1963 an d 1996), w h en d e n s e aggregations fonned in the
p la ic e an d cod.
relatively w a r m w ater o f the southwestern North Sea
1 R ev en ue w as c a lc u la te d a s th e su m o f th e lan d in g s x relative
" s u b ite) ¿".V~ )Tœ û 's 5 3 ~ cû sto in er c o p y s u p p lie d a n d p rin te d fo r F la n d e is M arine Institute Library (S L I0 5 X 0 0 2 2 5 E )
P a r tia l fis h in g m o r ta lity as in d ica to r o f effective fis h in g effort
563
T able 4. C o m p a ris o n o f th e o v e ra ll tre n d (/) o f m o d e l 3 w ith the tre n d e s tim a te d by m odel 1. T he c o n trib u tio n o f v in tag e o f h ull (v h ),
vintage o f e n g in e (v u ), u n d th e tim e in o p e ra tio n (tu ) to th e overall tim e tre n d (/) in F IWC a s e s tim a te d b y m odel 3 w a s c a lc u la te d as
the p ro d u c t o f the a n n u a l in crease in th e o o v ario b le (c o lu m n A ) an d th e p a ra m e te r estim ates fo r the c o v a ria b le s in m odel 3 (colum n B ).
Sole
P laice
S lo p e o f lim e
trend (A )
S lo p e from
C o n trib u tio n
C o n trib u tio n
S lo p e fro m
C o n trib u tio n
C o n trib u tio n
m o d e l 3 (B )
to t
(% )
m odel 3 (B )
to l
(% )
vh
0.64(1
0 .0 1 1 7
0.0075
30
0.0091
0.0 0 5 9
42
ve
0.7 6 4
0.0091
0.0070
28
0.0019
0.0015
10
tu
0.5 8 4
0.0181
0.0105
42
0.0115
0.0067
48
in v a ria b le
M odel 3
O v e ra ll Iren d (/)
0.025
0.014
0.028
0.016
M odel 1
O v e ra ll tren d (/)
u.x
2
IIV,
(c)
S o le - so rte d by F
p la ic e
(d )
P la ic e - so rted by F.m p laice
I
k.
0 ,8
.79
a . 0 ,(i
V
0 .4
3
E
3
o
o .:.
0
S o le - s o rte d by o b s e rv e d rev en u e
C u m u la tiv e fish in g e f fo r t
F ig u re 7
(f)
P la ic e - so rte d by observed re v e n u e
C u m u la tiv e fish in g effo rt
R e la tio n s h ip b e tw e e n th e c u m u la tiv e p ro p o rtio n o f the p re d ic te d F pw for (left) so le and (rig h t) p la ic e an d th e c u m u la tiv e effort
fo r in d iv id u a l v e s se ls in 20 0 0 w h e n trip s w ere s o rte d in d escen d in g o rd e r o f F pw o f so le (a, b ), p la ic e (c. d) an d o b se rv e d re v e n u e (c, f).
T h e h e a v y lilu ck lin e sh o w s th e a v e r a g e re la tio n sh ip o v e r all ships.
s u b ito e.V . lic e n se d c u s to m e r c o p y su p p lie d a n d p rin ted fo r F la n d e rs M arin e In stitu te Library (S L I0 5 X 0 0 2 2 5 E )
A. D . R ijn sd o rp c t al.
5 64
( W o o d h e a d , 1 9 6 4 ; H o r w o o d and M illn er, 1998). Such ag­
tec h n o lo g ical i m p r o v e m e n t s an d fish in g skill re q u ire s cur­
g re g a t io n s c o u ld p o s itiv e ly influence th e F puc.
T r a w l i n g a c t i v it ie s m a y also affect the c atc h in g efficiency.
D is tu rb a n c e o n fis h in g g rounds m a y m a k e fish less suscep­
v esse ls and ac tin g c r e w m e m b e r s .
A se c o n d c o n tr ib u t io n to the in c r e a s e in c a tc h efficiency
tible to b e i n g c a u g h t {Ona and G o do , 1990; E ngñs e l a l.,
1995; A lb e r t e t a l., 2 0 0 3 ), n e g a tiv e ly affecting the efficiency
o f tr a w le rs w o r k i n g closely to g eth er in la r g e nu m bers. This
p ro c e s s o f i n t e r f e r e n c e com pe titio n (or c ro w d in g ) is often
a s s u m e d in fleet d y n a m i c studies (G illis and Peterman,
1998; G illis a n d F r a n k , 2001; Gillis, 2 00 3 ) and econom etric
s tu d ies ( P a s c o e e t a l., 2001 ). So far, there is only indirect ev­
id e n c e fo r th is effect (R ijnsdorp e t a l. , 2000a, b; Gillis,
2 00 3). F in a lly , c h a n g e s in the d ire c te d n c s s o f th e fishery
o n e ith e r so le o r p l a i c e w ithin the sp atial units used in our
rently un av aila ble in f o r m a t i o n o n in v es tm en ts o f individual
cou ld be the r e p l a c e m e n t o f old v e s s e ls by n e w o n e s . The
rate o f increase a p p e a r e d lo be s o m e w h a t h ig h e r for sole
(the target species) t h a n for plaice. T h e origin o f this efficiency
in cre ase m a y b e p a r t l y the s a m e techn olog ical innovations
d is c u sse d a b o v e , a s w e ll as i m p r o v e m e n t s in p rop ulsio n.
A 6 % increase in efficiency fo r sole, but n o n e for plaice
w as re p o rte d for t h e pe rio d 1991 — 1998 (M archa i e t a l,
2 0 0 2 a ), w h e re a s no o v e ra ll c h a n g e in the efficiency w a s ob­
served u sing a s t o c h a s ti c frontier a n a l y s i s o f an nu al vessel re­
cords o f e c o n o m ic o u tp u t ( P a s c o e el a l., 20 01 ). T h e last
a n a ly s is m a y e x p la i n variability in catchability.
T h e o b s e r v e d in c r e a s e in catchin g efficiency w ith engine
au th o rs inv estig a ted th e c o n t r i b u t i o n of.several te c h n ica l at­
p o w e r c o r r o b o r a te s th e results o f earlier studies (R ijnsdorp
e t a l. , 2 0 0 0 a ; P a s c o e e t a l., 2001 ). T h e in crease is attributable
to a c o m b i n a t i o n o f th e increase in to w in g speed (R ijnsdorp
e t a l. , 2 0 0 0 b ) a n d the larg er n u m b e r o f tic k le r c h a in s that can
b e to w e d at g r e a t e r eng ine power. Inc re asing the latior
re gim e ou the a n n u a l e c o n o m i c o u t p u t o f individual vessels,
c o m p a r is o n s h o w s th a t t e c h n o l o g y cree p can be estim ated
stro n g ly e n h a n c e s th e catchin g efficiency for so le (Creulz-
in different w a y s w ith different re s u lts . O u r stu d y h as esti­
b e rg e t a l , 1987) b e c a u s e o f their t e n d e n c y lo d ig into the
se d im e n t in r e s p o n s e t o disturbance, w h ereas p laice will bo
ch a n g e s in e n g in e p o w e r w ith i n t h e fleet and c h a n g e s in the
m o r e affected b y to w in g speed, b e c a u s e they a ttem p t to
s w im a w a y ( M a i n a n d Sangstcr, 1981 ; W in g e r el cd., 1999).
se a s o n a l distrib ution o f fish in g effort. Hence, o u r estim ate
c o m e s c lo s e lo the in c re a se in c a t c h i n g efficiency o f a vessel.
In e s tim a tin g F ]Wti w e had to m ake a n u m b e r o f simplified
a s s u m p tio n s. T h e m o s t important is that w e had to ignore
w o u ld g e n e ra te an a v e r a g e fish ing m ortality rate o f about
tribu tes o f the vessel a s well as c h a n g e s in the m an age m en t
but they did n o t c o n s i d e r c h a n g e s in efficiency for individual
lish sp ecies, n o r d id th e y t a k e into a c c o u n t the im po rta nt ef­
fect o f s e a s o n a l c h a n g e s in a v a il a b il it y o f the sp e cies. This
m ated the trend in c a tc h a b ility a fte r taking a c c o u n t of
O u r s tu d y s h o w e d th a t a s t a n d a r d 2 0 0 0 lip b e a m trawler
th e age c o m p o s it io n o f individual la n d in g s, b ecau se these
1.0 x IO 5 (sole) a n d 0.6 x 10 s (p laice) p er d a y al sea,
a re not av aila b le . A ls o ,
was e s t im a te d for individual
w eek s, w h e re a s total F estim ates w e re o n l y available with
q u a rte rly reso lu tio n . It is difficult to j u d g e w h a t effects
but sea s o n a l a n d spatial v a r i a ti o n s w ere large. T h e varia­
tions in / ;’pue in d u c e d o n th e tw o s p e c i e s a p p e a r to a large
th is m a y h a v e had o n the results in te r m s o f bias, but
o f fishing g r o u n d and fishing s e a s o n . It is therefore surpris­
d e g re e to be u n c o u p le d an d fully d e p e n d e n t on the choice
su c h s im p lific a tio n s w ill un d o u b ted ly co ntribu te to un ex­
in g that the re la tio n sh ip b e tw e e n c u m u l a ti v e / ' ' a n d c u m u l a ­
p lain ed v a ria n c e . I-lowever, we d id n o t see c on sistent
ch a n g e s in F pwi a r o u n d the end o f e a c h q uarter, W c further
a s s u m e d that l o g b o o k data acc u rately reco rd the catch o f
tive fishing effort, afte r s o r tin g the fish ing (rips o f individual
in d iv id u a l ve sse ls. T h i s m a y n o t be true, b ec au se th e fishery
is m a n a g e d b y i n d i v i d u a l transferable q u o ta as sh a re s in the
gests that for the ta rg et sp e c ie s o f the b eam traw l fishery,
effort m a n a g e m e n t w o u ld result in a close lo p ro portional
D u tc h portion o f the T A C . S u c h a m a n a g e m e n t sy stem m ay
re s u lt in d is c a r d in g o f over-quota fish or th e c h e a p e r frac­
r e d u c tio n in fishing m o rtality . In c o n tr a s t, the m o r e convex
tio n o f the c a tc h to in c re a se the overall v a lu e o f th e land­
fishing effort w o u ld likely result in a less than p ro portional
d e c re a s e in F in b y-catch sp ecie s.
in g s ( A n d e r s o n , 1994; G illis el a l., 1995a, b). A lth ou gh
h ig h g r a d in g d o e s o c c u r from time lo Lime, no quantitative
in f o rm a tio n is a v a il a b le .
vessels in d e s c e n d i n g o r d e r o f r e v e n u e , o n ly w e ak ly d e v ia t­
ed from a lin ear p ro p o rtio n a l r e l a ti o n s h ip for sole. T h is sug­
r ela tio n sh ip o b s e r v e d fo r p la ic e in d ic a te s that a re d u c tio n in
W e c o n c lu d e that Fpw m ig h t be u s e d as a tool in an effort
m a n a g e m e n t re g im e , a l l o w i n g a c a r e f u l design that could
D e sp ite th e s e u n c e rta in tie s and w e a k n e s s e s , o u r study
lake a c c o u n t o f the p red ic tab le effects o f seasonal changes
s e e m s c a p a b le o f e s t im a ti n g the effect o f e n g in e p o w e r on
the c a tc h in g e ff ic i e n c y o f the gear, as w e l l as th e effects
in distribution on c atc h ab ility , as w e l l as o f the grad u a l in­
c re a s e in the c a tc h in g efficiency ( t e c h n o lo g y creep). Intro­
o f the sp a tia l a n d te m p o r a l c o m p o n e n ts o n the fishing m o r­
tality in d u c e d d u r i n g a fishing day. A lso, it is cle a rly shown
b y these a n a ly s e s th a t the efficiency o f th e fleet has in­
c re a s e d o v e r ti m e , irre sp e c tiv e o f e n g in e p ow er.
du ction
o f effort m a n a g e m e n t
m ay
lead to substantial
c h a n g e s in fishing pa ttern s re s u lti n g in cha ng es in calc li­
ability, O u r m e th o d m ay b e useful in m o n ito rin g changes
T h e p o s itiv e effec t o f o pera tion t i m e on F puo m a y reflect
in catc hab ility, p erm itting f i n e - tu n i n g o f m a n a g e m e n t regu­
lations. T'he a p p ro a c h s e e m s p a rt ic u l a rly useful lo r mixed
a g ra d u a l i m p r o v e m e n t o f the skills o f the c re w as well as
te c h n o lo g ic a l a d v a n c e s in auxiliary e q u ip m e n t (D G P S,
d em ersa l fisheries an d can b e a p p l i e d easily to o th e r gear
echosounders,
e tc.).
D isen tan glin g
the
c o ntrib utio n
of
if data o n a trip level are a v a i l a b l e to correct for seasonal
c h a n g e s in a v ailab ility o f the r e s o u rc e .
su b ito e.V . lic e n se d c u s to m e r c o p y s u p p lie d a n d p rin te d fo r F la n d e rs M arine In stitu te Library (S L I05X 00225E )
P a rtia l fis h in g m o rta lity a s in d ica to r o f e ffective fis h in g effo rt
A cknow ledgem ents
This m a n u s c rip t w a s p repa red as part o f D L O research pro­
gram m e 4 1 8 . W e thank Jan W ille m de W ilde (LEI) for his
assistance w ith th e data on the NRV1 datab ase o f individual
fishing vessels, and Sarah K raak, Paul M archai, and an
a n o n y m o u s r e v ie w e r fo r helpful c o m m e n ts on an earlier
draft o f th e paper.
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