Iz w o
Netherlands Journal of Sea Research
26 (1): 151-160 (1990)
16432
Tel + 32 (0159 - 323? 18 Fax. + 32-(0) 50 - 32 08,S
ON THE SURVIVAL OF PLAICE AND SOLE DISCARDS IN THE OTTER-TRAWL
AND BEAM-TRAWL FISHERIES IN THE NORTH SEA
F.A. VAN BE E K , P.l. VAN LE E U W E N and A.D. R IJN S D O R P
Netherlands Institute for Fishery Investigations, RO. Box 68, 1970 AB
AB STR ACT
T h is p a p e r su m m arize s th e results o f su rvival ex­
p e rim e n ts w ith p laice and so le d iscard s c a u g h t by
co m m e rc ia l beam -traw l vessels and a research
ve ss el fish in g w ith an o tte r tra w l, and w ith so les
e s c a p e d th ro u g h th e m esh es in c o ve re d -c o d -en d
e x p e rim e n ts . In th e com m ercial b eam -tra w l fis h ­
e ry th e su rvival o f both p laice and so le d iscard s
w as e s tim a te d to be less th an 1 0 % . T h e survival
o f s o les es ca p e d th rough th e m e sh es w as e s ti­
m a ted a t 6 0 % . T h e survival o f d iscard s w as n e g a ­
tiv e ly a ffe c te d by th e catching process. D u rin g
th e c a tch in g process m o rta lity w as caused
th ro u g h th e ac tio n o f th e tic k le r ch ain s and th e in­
ju rie s im p o s ed d uring th e stay in th e n e t. U n d er
p re s e n t-d a y co m m ercial p rac tic e th e p ro cessin g
o f th e c a tc h on d eck hardly affe cts th e su rvival o f
d iscard s.
1. IN T R O D U C TIO N
Trawl fis h e rie s fo r fla tfish are g e n e ra lly m ixed fis h e r­
ies w ith a v a rie ty o f targe t spe cie s. In th e N o rth S ea
th e fis h e ry fo r fla tfis h is m a in ly c a rrie d o u t w ith beam
tra w ls, w h ic h are used in m ore tha n 7 5 % o f th e total
in te rn a tio n a l la n d in g s of sole (S olea so le a L.), p la ice
(P le u ro n e cte s p la te ssa L.), tu rb o t (S c o p h th a lm u s
m a x im u s L.) an d brill (S co p h th a lm u s rh o m b u s L).
B e a m -tra w l fis h e ry is cha racterized by a s u b sta n tia l
b yca tch o f v a rio u s spe cie s of fish and in ve rte b ra te s
an d a c o n s id e ra b le catch of dead m aterial such as
ston es, fo ssil she lls, sand, w ood a n d ga rba ge. A
d e c re a s in g p a rt of the total in te rn a tio n a l ca tch is
ta ke n b y o tte r traw ls. Both kin d s of g e a r use tic k le r
ch a in s in fro n t o f th e ground rope to drive th e fla tfis h
fro m th e se a bo tto m so th a t th e y can be c a u g h t by
th e net.
S ole fis h e ry in the N orth S e a has to be co n d u cte d
w ith a le ga l m in im u m m esh size of 80 o r 90 m m . At
th e s e m esh size s th e 5 0 % reten tion le ng th of so le is
26 o r 30 cm , re sp e ctive ly ( R ijn s d o r p e t al., 1981;
van B e e k e t al., 1982) and a s u b sta n tia l p a rt of th e
u n d e rs iz e d so le ( < 2 4 cm ) can e s c a p e th ro u g h th e
m e sh e s. In o th e r c o m m e rcia l fla tfis h spe cie s, such
as p la ice , tu rb o t and brill, w h ich have a m uch low er
IJmuiden, The Netherlands
reten tion le ng th tha n sole, th e n u m b e rs esca p in g
th ro u g h the m esh es are n e g lig ib le . C o n se q u e n tly
d is c a rd rates in p la ice are m uch h ig h e r th a n in sole
( d e V e e n & R o d e n b u r g , 1971; d e V e e n e t a i , 1975).
D ab (Lim and a lim a n d a L.), a c o m m e rc ia lly less at­
tra ctive fla tfish spe cie s, is also d isca rd e d in great
q u a n titie s ; tu rb o t an d b rill, however, are d isca rd e d in
low q u a n titie s as th e se sp e cie s do not re c ru it to the
fis h in g g ro u n d s before re a ch in g th e m in im u m la n d ­
ing size ( d e V e e n e t al., 1975).
To asse ss th e p o ssib le effects o f te ch n ica l
m a n a g e m e n t m ea sure s such as m esh size re g u la ­
tion s, clo sed areas etc., to im p rove th e e x p lo ita tio n of
p la ice and sole, k n o w le d g e on th e su rviva l of plaice
and sole disca rd s is p a ram o unt. As o n ly in sole s u b ­
stan tial nu m be rs e sca p e th ro u g h th e m e sh e s also
th e su rviva l o f th e se so le s is im p o rta n t to evaluate
th e e ffect of an in cre a se in m esh size. In th e past
rese arch on th e s u rviva l of d isca rd s fo cu sse d on
p la ice (G a r s t a n g , 1905; B o r l e y , 1909; d e V e e n et
a i , 1975; K e l l e , 1976, 1977). Inform a tion on the su r­
viva l of soles e sca p in g th ro u g h th e m esh es is la ck­
ing.
G a r s t a n g (1905) co n c lu d e d th a t th e v ia b ility of
p la ice disca rd s w as go o d d ire c tly a fte r ca tc h in g , but
th a t high m o rta lity o ccu rre d d u rin g th e ir stay on
de ck. B o r l e y (1909) show ed e x p e rim e n ta lly th a t the
s u rviva l of plaice d isca rd s w as m a in ly affected by the
haul d u ra tio n , th e pre sen ce of M e d u sa e an d th e c o n ­
d itio n s on deck. Hot su n s h in e e s p e c ia lly ca u se d a
s u b sta n tia l m ortality. T h e survival of p la ic e d isca rd s
in th e otter-traw l fis h e ry w as estim a te d to be below
10% in m ost cases. In th e beam -traw l fis h e ry the
c o n d itio n o f p la ice d isca rd s w as b e tte r a n d in sho rt
h a uls u n d e r favo urab le c o n d itio n s 50 to 7 5 % of the
ca tch su rvived (B o r l e y , 1909). T he s u rviva l o f both
p la ice and sole d isca rd s in th e s h rim p fishe ry, em ­
p lo yin g a light beam traw l w ith o u t tic k le r ch a in s but
w ith rollers attach ed to th e g ro un d rope, w a s e stim a t­
ed at a b o u t 5 0 -60 % , but was red uce d to 2 0 % in
p la ic e and 3 0 % in so le w h e n sh a k in g sie ves w ere
use d to s o rt th e catch (K e l l e , 1976).
T h e survival of d is c a rd s in m od ern fla tfis h fish e ry
is e xp ected to have ch a n g e d w ith th e in tro d u c tio n of
heavy beam traw lers at th e e a rly 1960’s. In c o m p a ri­
son w ith the otter- and be am -traw l fis h e ry in the be­
152
F.A. VAN BEEK, P.L. VAN LEEUW EN & A.D. RIJNSDORP
g in n in g o f th e century, the fis h in g spe ed had in­
crea sed to a b o u t 5 knots (9 k m -h -1 ) in th e late
1970’s an d to a b o u t 7 knots (13 k m -h -1 ) at present,
an d th e w e ig h t o f th e gear, in clu d in g a large nu m b e r
of tic k le r cha in s, in crea sed to ab o u t 1O103 kg at
p re sen t. A ls o th e tim e th a t disca rd s are exposed on
d e ck w a s re d u ce d by th e in tro d u ctio n o f conveyer
b e lts in th e e a rly 1980’s to im p rove th e w o rkin g c o n ­
d itio n s of th e crew.
T h e p re se n t p a p e r su m m a rize s th e results of the
su rviva l e xp e rim e n ts w ith p la ice and sole d isca rd s
c a rrie d o u t betw een 1972 an d 1982 on board of c o m ­
m ercial beam tra w l ve sse ls and a research vessel In
th e N o rth Sea. A d d itio n a l e xp e rim e n ts w e re carried
o u t to e stim a te th e su rviva l of so le s th a t escaped
th ro u g h th e m eshes.
sea
water
in
2. M E T H O D S
S u rviva l e xp e rim e n ts w ith p la ice and sole d iscard s
w e re ca rrie d o u t on board co m m e rcia l beam traw l
vesse ls o p e ra tin g un de r no rm al c o m m e rcia l c o n d i­
tion s. A d d itio n a l e xp e rim e n ts w e re ca rrie d out w ith
p la ice c a u g h t w ith an otter traw l on R.V. ‘T ride ns’. D is­
ca rd s w ere so rte d from th e catch a n d th e ir co n d itio n
w as c la s s ifie d a c c o rd in g th e sca le given in Table 1.
From ea ch co n d itio n cla ss a random sa m p le was
take n a n d p la ce d in a p la stic ta n k (size: 40x60 cm
and 12 cm high) w ith a m a xim u m of 15 fish per tank.
Stacks of a b o u t 10 tan ks a lm o st fille d w ith seaw ater
w e re p la ce d in a w o od en fram e and su p p lie d w ith
co n tin u o u s flo w o f fre sh seaw ater (Fig. 1). T h e tanks
w e re p la ce d on d e c k in a p o sitio n w h e re ro llin g and
p itc h in g o f th e sh ip w e re m in im a l. On board R.V.
‘T ridens’ th e stacks o f tanks w ere placed in a large
clo se d ba sin o f seawater, w ith co n tin u o u s flow. If c o n ­
d itio n s p e rm itte d , the e xp e rim e n ts w ere che cked ev­
ery 12 h o u rs an d th e dead fish was reco rde d and
rem oved. T h e e x p e rim e n t w as te rm in a te d w h e n all
fish had d ie d o r at th e en d of th e cruise. In a nu m b e r
of ca se s th e e x p e rim e n t had to be stop ped p re m a ­
tu re ly d u e to th e w e a th e r con ditions.
TABLE 1
Classification of the condition of flatfish according to the
damage.
A: Fish lively, no visible signs of loss of scales or mucus.
B: Fish less lively, some scratches and some scales miss­
ing, mucus layer affected up to 20%, some small red
spots on the blind side.
C: Fish lethargic, several scratches and some areas without
scales, mucus layer affected up to 50%, several red
spots on the blind side.
D: Fish lethargic, head reddish, many scratches and areas
without scales, mucus layer affected for more than 50%,
blind side with many red spots and haemorrhages.
water
outlet " =
Fig. 1. Experimental set-up used to estimate the survival of
flatfish discards.
S u rviva l e xp e rim e n ts w ere in itiate d w ith discards
from ca tch e s th a t varied in th e g e a r used (otter or
beam traw l), th e n u m b e r o f c h a in s in fro n t of th e net,
haul d u ra tio n (betw een 15 and 120 m in), fish in g
sp e e d , total vo lu m e of th e catch and th e e n g in e pow ­
e r of th e vesse l. A lso se a c o n d itio n s (w ind force) and
w a te r te m p e ra tu re w ere co n sid e re d . T h e te c h n ic a l in­
fo rm a tio n of th e e xp e rim e n ts is su m m a rize d in Table
2. T h e le n g th range of th e disca rd s used in th e sur­
viva l e xp e rim e n ts was 20 to 30 cm in p la ice and 20
to 28 cm in sole.
In a d d itio n , th e effect of th e in cre a sin g pro cessing
spe ed on th e su rviva l of p la ice d isca rd s w as exa­
m in e d by co m p a rin g d isca rd m o rta lity o f plaice
pro cesse d by th e tra d itio n al m etho d an d o f plaice
d is c a rd s p ro cesse d w ith a so rtin g device. By th is
m e th o d th e ca tch is d u m p e d in a w a te r fille d co n ta in ­
er w h ich s u p p lie s a con veye r belt alon g w h ich the
ca tch is so rte d and p ro cesse d. D isca rd s an d oth er
un w a nted m a te ria l stay on th e conveyer b e lt and are
tra n s p o rte d b a ck to sea im m ediately. In th e sortin g
d e vice used in o u r exp erim e nts, the catch w as kept
w e t w ith s p rin k le rs th ro u g h o u t th e ir stay on the c o n ­
veye r belt.
T h e overall su rviva l of th e d iscard s (S¡) at ex­
p e rim e n ta l tim e i w as c a lcu la te d from the p ro po rtion
(Pj) a n d the su rviva l (S,j) m ea sure d in each co n d itio n
cla ss (j = 1 .. 4) of the total d isca rd catch a cco rding
to:
s i = £FY s ¡j
PLAICE AND SOLE DISCARDS
153
TABLE 2
Technical information on the survival experiments.
total catch x 40kg
Exp.
numb.
horse
power
haul
duration
depth
(m)
water
temp.
air
temp.
weather
cond.
6
6
6
6
6
6
6
6
25-30
25-30
25-30
25-30
25-30
25-30
20-30
20-30
8
8
8
8
8
8
6
6
8
8
8
8
8
8
5-6
5-6
7
5-7
7
8
7
8
8
15
24
24
24
8
24
24
15
24
20-30
30-50
20-30
30-50
20-25
20-25
20-30
20-30
30-40
30-40
9-10
9-10
9-10
9-10
12-13
12-13
18
18
10
10
num her
cod-end
cover o f chains
date
plaice: otter trawl (fishing speed: 3.5 knots)
1
2
3
4
5
6
7
8
600
600
600
600
600
600
600
600
20
60
60
60
100
100
105
105
5.0
5.5
5.5
5.8
5.8
8.8
8.0
8.0
Nov 1972
Feb 1975
-
plaice: beam trawl (fishing speed: 5.0-5.5 knots)
9
10
11
12
13
14
15
16
17
18
1200
1200
1200
1200
1015
1015
1235
1235
1235
1235
60
60
120
120
110
60
120
120
60
120
5
5
10
17
4
1
22
22
11
23
-
-
SW 1-4
SW 1-4
SW 1-4
SW 1-4
W 1-2
S2
Var 2
Var 2
3-8
3-8
Nov-Dee 79
SW 1-4
SW 1-4
SW 1-4
SW 1-4
7-4
7-4
7-4
3-8
3-8
Nov-Dee 79
S 1- 2
S 1- 2
Var 2
Var 2
Var 2
W 2-3
W 2-3
W 2-3
W 2-3
W 2-3
W 2-3
Aug 1980
May-Jun 81
Sep 1982
Dec 1982
plaice: beam trawl, deck-processing with conveyer belt (fishing speed: 5.0-5.5 knots)
19
20
21
22
23
24
25
26
27
1200
1200
1200
1200
1235
1235
1235
1235
1235
60
60
120
120
60
120
120
60
120
5
5
10
17
12
22
22
11
23
-
15
15
15
15
15
24
15
24
15
20-30
30-50
20-30
30-50
20-30
20-30
20-30
30-40
30-40
13
13
12
12
12
16
16
16
16
16
16
-3 0
-3 0
15-20
15-20
15-20
20-30
20-30
20-30
20-30
20-30
20-30
9-10
9-10
9-10
9-10
16
16
16
10
10
-
Oct 1982
Dec 1982
sole: beam trawl (fishing speed: 5.0-5.5 knots)
28)1
29)1
30
31
32
33
34
35
36
37
38
1015
1015
1015
1015
1015
1310
1310
1310
1310
1310
1310
120
120
15
60
120
30
60
90
60
120
15
7.5
7.5
2.0
4.0
7.0
3.0
4.0
6.0
3.5
7.0
0.5
5.5
5.5
0.5
1.0
4.0
0.5
1.5
2.5
3.5
7.0
0.5
-
17
17
17
16-17
16-17
16-17
16-17
16-17
16-17
Aug 1981
Aug-Sep 1981
)1 Irish Sea
T h e s u rviva l o f so le s escaped th ro u g h th e m esh es
w a s e s tim a te d in c o n ju n c tio n w ith m esh s e le c tio n ex­
p e rim e n ts on co m m e rc ia l beam tra w le rs u s in g the
c o v e re d cod -en d te c h n iq u e ( R ijn s d o r p e t a i , 1981;
v a n B ee k e t al., 1982). T h e s u rviva l e x p e rim e n ts
w e re se t up in p a irs, e xp e rim e n t A w ith so le s from
th e cod -en d an d e x p e rim e n t B w ith so le s th a t es­
ca p e d th ro u g h th e m e sh e s o f th e cod -en d an d re ­
ta in e d in th e cod -en d cove r. T h e le n g th ran ge o f th e
fish w a s 18 to 25 cm . T h e fish w e re not c la s s ifie d in
co n d itio n ca te gories. F u rth e r e x p e rim e n ta l p ro c e ­
d u re s w e re id e n tica l to th o se a p p lie d in plaice.
F.A. VAN BEEK, P.L. VAN LEEUW EN & A.D. RIJNSDORP
154
3. R E S U LT S
3.1. DISCARDS
T h e fre q u e n c y o f o ccu rre n ce o f the d iffe re n t c o n d i­
tio n cla sse s of th e d iscard s in the ca tch is sho w n in
Fig. 2. T h e co n d itio n o f plaice disca rd s w as p o o re r in
th e b e a m -tra w l e x p e rim e n ts th a n in th e o tte r tra w l. In
bo th p la ice and sole th e co n d itio n d e crea sed w ith in ­
c re a sin g h a u l d u ra tio n . S urvival of the 4 co n d itio n
cla sse s (Fig. 3) c o rre s p o n d e d to th e su b je ctive c la s ­
sific a tio n o f c o n d itio n , but the su rviva l c u rve s o f the
sa m e c o n d itio n cla ss d iffe re d betw een o tte r and
b eam tra w l a n d betw een d iffe re n t haul d u ra tio n s.
O b v io u s ly not a ll in ju rie s th a t m ay cause m o rta lity
w e re visib le .
p la ic e
o tte r
tr a w l
In e a ch e x p e rim e n t th e d isca rd su rviva l w a s e s ti­
m a te d fro m th e m o rta lity rate in each co n d itio n class
an d th e p ro p o rtio n o f th is co n d itio n cla ss in th e total
ca tch . In g e n e ra l th e m o rta lity rate w a s h ig h e st in the
firs t 24 o r 48 h an d le ve lle d o ff a fte rw a rd s (T ables 3
an d 4). H o w eve r, in s o m e e x p e rim e n ts s u b sta n tia l
m o rta lity sta rte d o n ly a fte r 24 h.
T h e o ve ra ll s u rviva l w a s e stim a te d as th e survival
a fte r an e x p e rim e n ta l tim e o f 84 h. A fte r th is tim e
m o rta lity a p p a re n tly c e a s e d or w as v e ry lo w (Fig. 4).
In th e fe w e x p e rim e n ts sto p p e d p re m a tu re ly after 60
o r 72 h, the p e rc e n ta g e o f s u rvivin g d is c a rd s w as ex­
tra po la te d a ssu m in g a s im ila r m o rta lity rate betw een
60-72 h and 72-84 h as in the o th e r e xp e rim e n ts.
O ve ra ll d is c a rd s u rv iv a l w as e s tim a te d to be be­
tw e e n 0 an d 5 0 % in p la ice and be tw ee n 4 an d 4 0 %
in sole. In p la ice the s u rviva l w a s lo w e r in be am -traw l
d is c a rd s tha n in o tte r-tra w l d isca rd s (M a n n -W h itn e y,
p < 0 .0 1 ) . D isca rd s u rviva l in the beam tra w l was
p la ic e
o tte r
tra w l
100
to
>
co n d A
60 -
co n d B
co n d C
tn
co n d D
p la ic e
beam
tra w l
0
1 2
2 4
3 6
p la ic e
4 8
6 0
beam
tra w l
4 8
6 0
7 2
8 4
100
>
>
60 -
=5
10
20 -
s o le
beam
0
tra w l
1 2
2 4
3 6
7 2
8 4
1 00
s o le
80
beam
tra w l
100
60
40
to
>
20
>
0
40 -
haul
d u r a tio n
0
Fig. 2. Frequency distributions of the condition (A, B, C or
D) of plaice discards in otter- and beam-trawl catches of 60and 120-min haul duration and of sole discards in beamtrawl catches of 60- and 120-min haul duration. Damage in­
creases from condition A to D (compare Table 1).
1 2
2 4
h o u rs
3 6
4 8
6 0
7 2
8 4
m a in t e n a n c e
Fig. 3. Survival (%) of the different condition classes (A-D)
in plaice and sole discards during maintenance in the hold­
ing tanks.
PLAICE AND SOLE DISCARDS
155
TABLE 3
Plaice: survival in percentage of discards (18-27 cm) and the number of fish at the start of each experiment (N). The bold
figures are extrapolated values estimated under the assumption that the mortality between 60 and 72 and between 72 and
84 hours was equal to the average mortality at this time in the other experiments. The figures in italics are interpolations
by eye between the last observation at 36 hours and the final at 96 hours. The experiment number corresponds to the infor­
mation given in Table 2.
Experiment
Number
Time interval (hours) after start o f experiment
N
0
12
24
36
48
60
72
84
100.0
100.0
87.8
62.6
35.2
32.5
22.8
22.8
100.0
100.0
100.0
81.6
20.5
95.3
64.2
0.0
64.2
50.2
0.0
8.8
47.6
0.0
0.0
44.4
0.0
0.0
40.7
0.0
0.0
31.6
0.0
0.0
100.0
100.0
100.0
100.0
71.6
82.2
100.0
82.5
66.9
64.4
96.6
74.9
52.3
53.5
90.3
57.6
37.4
45.4
86.9
47.2
31.8
41.2
79.0
35.1
16.9
35.0
67.7
33.0
15.7
32.2
54.1
30.8
100.0
100.0
100.0
100.0
75.6
93.7
54.4
100.0
56.7
87.3
45.6
93.2
37.8
68.4
36.5
56.8
31.1
49.4
21.5
38.0
22.2
36.7
16.2
28.0
17.8
29.9
11.6
20.0
14.3
24.0
5.7
15.0
100.0
100.0
100.0
100.0
100.0
100.0
85.4
65.8
59.1
21.3
25.9
87.5
74.0
31.5
44.0
13.0
20.0
65.9
62.5
17.8
21.5
7.0
16.3
52.3
43.8
11.0
4.8
5.4
14.9
40.0
31.3
6.8
3.4
4.6
12.6
32.0
27.1
5.5
2.9
4.6 9.6 25.0
21.8
4.4
2.1
4.6
9.6
21.0
77.6
76.1
8.5
40.0
57.1
73.1
8.0
25.0
46.9
59.6
6.1
19.0
37.7
47.9
4.9
14.0
38.0
16.7
6.1
5.3
26.0
30.0
15.3
5.8
5.0
20.0
26.0
12.5
4.4
3.8
16.0
20.9
10.0
3.5
3.0
14.0
96
Otter trawl: 20 min
1
40
Otter trawl: 60 min
2
3
4
46
17
15
0.0
0.0
Otter trawl: 3 100 min
5
6
7
8
30
28
63
41
-
-
Beam trawl: 60 min
9
10
14
17
45
79
112
44
5.7
11.4
Beam trawl: 120 min
11
12
13
15
16
18
48
73
111
131
135
88
-
19.3
Beam trawl: 60 min and deck-processing with a conveyer belt
19
20
23
26
49
67
270
43
100.0
100.0
100.0
100.0
98.0
99.3
45.0
100.0
91.9
98.5
16.6
90.7
85.7
83.6
11.5
58.1
-
11.6
Beam trawl:: 120 min and deck-processing with a conveyer belt
21
22
24
25
27
50
72
270
270
94
100.0
100.0
100.0
100.0
100.0
78.0
70.9
24.6
24.0
89.4
61.0
41.7
12.1
11.1
55.3
44.0
27.8
8.5
7.7
35.1
12.8
F.A. VAN BEEK, P.L. VAN LEEUW EN & A.D. RIJNSDORP
156
TABLE 4
Sole: survival expressed in percentage of discards (20-28 cm). N indicates and the number of soles at the start of each
experiment. The bold figures are extrapolated values estimated under the assumption that the mortality between 72 and
84 hours was equal to the average mortality at this time in the other experiments. The experiment number corresponds
to the information given in Table 2.
A: soles collected from the cod-end
Experiment
Number
Time interval (hours) after start o f experiment
N
0
12
24
36
48
60
72
84
100.0
100.0
100.0
69.9
89.9
95.6
60.0
69.6
82.2
42.1
54.3
75.6
35.9
48.3
73.3
22.2
40.2
66.7
19.4
39.4
66.7
19.4
36.9
66.7
100.0
100.0
100.0
100.0
60.6
86.7
85.4
62.7
50.1
60.9
54.7
27.2
31.5
42.3
35.7
24.5
27.8
38.7
32.5
18.9
16.1
33.3
28.6
18.4
15.1
32.3
27.7
18.0
15.1
29.3
25.3
15.2
100.0
100.0
100.0
100.0
8.4
34.5
53.8
55.7
6.7
28.2
43.6
17.3
5.8
18.2
24.0
14.6
5.3
8.2
22.5
9.4
5.3
5.8
12.0
9.2
5.2
5.8
11.6
9.2
4.3
4.9
11.6
6.5
96
Beam trawl: 15-30 min
43
109
45
30
33
38
_
-
-
Beam trawl: 60-90 min
43
109
109
90
31
34
35
36
_
-
15.2
Beam trawl: 120 min
72
25
43
90
28
29
32
37
B: soles escaped through the meshes and collected in the cod-end cover
.
t
,
Time interval (hours) after start o f experiment
Exoeriment
N
Number
0
12
24
36
48
60
72
3.0
-
6.5
84
96
-
Beam trawl: 15-30 min
30
33
38
72
95
75
100.0
100.0
100.0
87.0
73.5
92.0
67.8
58.8
82.7
56.4
49.3
78.7
50.8
46.0
77.3
40.2
43.0
76.0
40.2
43.0
73.3
40.2
40.7
73.3
100.0
100.0
100.0
100.0
82.6
67.9
63.4
66.5
63.0
50.8
44.5
33.0
51.0
41.4
34.9
30.4
44.6
38.3
32.7
25.0
30.6
35.2
29.3
24.0
30.6
35.2
29.3
23.4
30.6
33.5
28.5
20.9
100.0
100.0
100.0
100.0
19.6
43.1
76.4
54.5
15.9
36.9
56.0
15.8
15.0
27.8
41.5
13.0
12.6
18.7
32.2
7.8
12.1
16.2
13.0
7.7
12.1
8.2
13.0
7.7
9.7
6.7
13.0
5.0
-
Beam trawl: 60-90 min
31
34
35
36
i
72
95
95
90
-
20.9
Beam trawl: 120 min
28
29
32
37
104
69
72
90
9.7
-
5.0
PLAICE AND SOLE DISCARDS
1 0 0 -1
p la ic e
beam
157
tra w l
100 i
80 -
p la ic e
d is c a rd s
80
60 60
40 40
20
-
20
— i—
*—
0
i—
*—
12
i—
»—
24
i—
1—
36
100 i
i—
»—
48
so le
i—
«—
60
i—
'—
72
beam
i—
0
84
tra w l
20
100
40
60
80
100
120
140
-
so le
d is c a rd s
80 80 -
60 -
60 -
40 -
40 -
20 -
20 i
i
-I
0
1—
I
12
1—
I
1—
24
h o u rs
I
1—
36
I
48
1—
I
Ï
1—
60
I
72
I
1—
I—
84
m a in te n a n c e
20
40
haul
80
100
d u ra tio n
60
(m in )
120
140
Fig. 4. Survival (%) of plaice and sole discards from 2-hour
hauls in the beam trawl experiments during maintenance.
The vertical bars indicate the standard deviations.
Fig. 5. Survival (%) in plaice and sole discards after 84
hours in experimental holding tanks in relation with haul
duration (min.).
h ig h e r in h a u ls of 60 m in tha n in h a u ls o f 120 m in
(F ig. 5), th e relatio n be in g s ig n ific a n t in so le (S p e a r­
m an r s = - 0 . 8 2 9 , n = 11, p < 0 .0 5 ) and m a rg in a lly
s ig n ific a n t in p la ice (S pe arm a n r s = - 0 . 4 3 8 , n = 19,
0 . 0 5 < p < 0 . 1 0 ) . In o tte r tra w l d is c a rd s o f p la ic e the
o p p o s ite w a s ob serve d, but th e n u m b e r o f d is c a rd s
o f 60 m in h a u ls w a s ra th e r sm a ll. W a te r te m p e ra tu re
had a s lig h t b u t s ig n ific a n t e ffe c t on the s u rviva l of
p la ice d is c a rd s in th e 60 -m in -b e a m -tra w l hauls
(S p e a rm a n rs = - 0.764, n = 8, p < 0 .0 5 ) a n d a m a rg i­
n a lly s ig n ific a n t in the 120-m in h a uls (S pe arm a n
r s = - 0 . 5 9 3 , n = 11, 0 . 0 5 < p < 0 .1 0 ) . S u rviva l in ­
cre a se d at lo w e r w a te r te m p e ra tu re s. T he m e th o d of
p ro c e s s in g th e ca tch on board did not a ffe c t th e s u r­
viva l o f p la ice disca rd s in 120-m in h a uls in a p a ir­
w ise c o m p a ris o n o f the 3 e x p e rim e n ts . In th e 3 p a ir­
w is e e x p e rim e n ts of 60 -m in ha ul d u ra tio n , no d iffe r­
e n ce o c c u rre d be tw ee n c o n v e y e r b e lt and tra d itio n a l
pro ce ssin g in on e e xp e rim e n t (14 .0 vs 15.0) b u t a
s u b s ta n tia lly h ig h e r su rviva l o ccu rre d w ith con veye rbe lt p ro ce ssin g in th e tw o o th e r e x p e rim e n ts
(37 .7 -14 .3 and 47.9-15.0). No s ig n ific a n t e ffe c t of the
m a xim u m w in d fo rc e on the s u rviva l o f p la ice d is­
ca rd s co u ld be d e m o n stra te d in 60- o r 120-m in h a uls
(S p e a rm a n
rs = 0.166,
n = 8,
p > 0 .0 5 ;
r s = - 0 . 2 8 3 , n = 11, p > 0 .0 5 ).
3.2. SURVIVAL OF SOLES ESCAPED THROUGH
THE MESHES
A n u m b e r o f e x p e rim e n ts w e re ca rrie d o u t to e sti­
m ate th e su rviva l of soles e s ca p e d th ro u g h the
m esh es an d c o lle c te d in the cod -en d cove r. T h e su r­
viva l o f so le s o rig in a tin g from the co ve r w a s s lig h tly
b u t s ig n ific a n tly h ig h e r tha n in th o s e fro m th e coden d (W ilc o x o n ’s sig n e d ra n ks te s t, P < 0 .0 1 ). T he
F.A. VAN BEEK, P.L. VAN LEEUW EN & A.D. RIJNSDORP
158
resu lts, g iv e n in T a b le 4 and Fig. 6, sh o w th a t su r­
viva l d e cre a se d w ith in c re a s in g haul d u ra tio n and
th a t s u b s ta n tia l su rviva l o ccu rre d at sho rt h a uls o f 15
m in. As th e d e cre a se d s u rviva l at lo n g e r haul d u ra ­
tio n s m ay be ca u se d by th e p ro lo n g e d stay o f the es­
cap ed so le s in th e cod -en d co ve r, th e e x tra p o la te d
s u rviva l a t a ha ul d u ra tio n o f 0 m in is ab o u t 6 0 % and
th is fig u re w o u ld give a c re d ib le e stim a te o f the s u r­
viva l o f s o le th a t e sca p e th ro u g h the m eshes.
80 i
60 -
ra
>
>
40 -
20
-
0
2 0
4 0
haul
6 0
10 0
8 0
d u ra tio n
12 0
14 0
(m in )
Fig. 6. Survival (%) (after 84 hours) of sole escaped through
the meshes and collected in a cod-end cover of a beam
trawl in relation to haul duration. The extrapolated survival
at a haul duration of 0 min is estimated at 60%.
4. D IS C U S S IO N
S everal fa c to rs m ay c o n trib u te to the m o rta lity of d is ­
ca rd s d u rin g ca tc h in g and s o rtin g on board. T he ex­
p e rim e n ts p re se n te d in th is p a p e r a llo w a firs t
e va lu a tio n o f th e m ain fa c to rs in flu e n cin g th e discard
m o rta lity in m od ern be a m traw l fishe ry.
A lth o u g h th e m a in te n a n c e c o n d itio n s o f th e fish in
th e ta n ks on board w e re c a re fu lly m o n ito re d and
kept as o p tim a l as p o ssib le , th e y m ay have cau sed
som e a d d itio n a l m o rta lity. F ish ca n g e t se a sick and
stre sse d ; a c c o rd in g to K e l l e (1977) stre ss is a m a jo r
ca u se o f m o rta lity. T h e re fo re , o u r e x p e rim e n ts m ay
have o v e re s tim a te d th e tru e m o rta lity of p la ice and
sole d is c a rd s . A lth o u g h th e a d d itio n a l m o rta lity
re su ltin g fro m m a in te n a n c e can be in v e stig a te d in
e x p e rim e n ts in itia te d w ith fish not a ffe cte d by the
fis h in g g e a r, o u r e x p e rim e n ta l set-up d id not a llow
th is . T w o o b se rv a tio n s in d ic a te th a t th e ad d itio n a l
m o rta lity is re la tive ly low. F irst, no s ig n ific a n t e ffe ct
of ro llin g an d pitch in g of the sh ip in po or w e a th e r
c o n d itio n s c o u ld be d e m o n s tra te d fo r plaice . Se­
c o n d , in th e o tte r-tra w l e x p e rim e n ts , p la ice o f c o n d i­
tio n cla ss A (no v is ib le d a m a g e ) show ed 100%
s u rviva l; in th e b e a m -tra w l e x p e rim e n ts the su rviva l
of p la ic e a n d s o le of c o n d itio n A w a s a b o ve 7 0 %
(Fig. 3). R e pe ated in sp e ctio n o f e x p e rim e n ta l fish
d u rin g the e x p e rim e n t sh o w e d th a t fish w h ich were
re co rd e d to be u n d a m a g e d at th e s ta rt of th e e xp e ri­
m en t sho w e d s ig n s of d a m a g e la te r on in th e e x p e ri­
m ents.
To e va lu a te th e ca tc h in g pro ce ss itself, survival
e x p e rim e n ts w e re in itia te d w ith d is c a rd s w h ich
d iffe re d in th e n u m b e r o f tic k le r cha in s, to ta l catch
a n d haul d u ra tio n . B eca use th e n u m b e r of e xp e ri­
m e n ts w as re la tiv e ly s m a ll, o n ly a q u a lita tiv e eva lu a ­
tio n is po ssib le. In bo th sole a n d plaice d is c a rd s the
s u rv iv a l w a s n e g a tiv e ly c o rre la te d w ith haul d u ra tio n ,
in d ic a tin g th a t m o rta lity fa c to rs w o rkin g in th e coden d d o m in a te d o v e r th e e ffe cts o f th e tic k le r cha in s.
T h e tic k le r ch a in s w ill a ffe ct th e su rviva l o f d iscard s
irre s p e c tiv e o f haul d u ra tio n . A m a xim um e stim a te of
th e c o n trib u tio n o f th e tic k le r ch a in s to th e discard
m o rta lity o f sole is given by th e e xtra p o la te d su rviva l
o f th e d is c a rd s an d o f so le s w h ich e sca p e d throu gh
th e m e sh e s at 0 m in, ab ou t 5 0 % an d 6 0 % re sp e c­
tiv e ly . A c o m p a ris o n w ith the su rviva l of a b o u t 2 5 %
in 60 -m in h a uls an d 1 0 % in 120-m in h a u ls show s
th a t be sid e s th e p o ssib le e ffe c t o f th e tic k le r cha in s
a lso d u ra tio n o f th e sta y In th e cod -en d su b s ta n tia lly
c o n trib u te d to th e m ortality. In o u r e x p e rim e n ts haul
d u ra tio n an d th e to ta l w e ig h t of the ca tch w e re co r­
re la te d and th e re fo re it is not p o ssib le to d ise n ta n g le
th e c o n trib u tio n of haul d u ra tio n and to ta l catch
w e ig h t.
T h e m ain in ju rie s o f d is c a rd s w h ich co u ld be ob­
serve d m a c ro s c o p ic a lly w ere th e loss of s ca le s and
m u cu s, and h a e m o rrh a g e s. T h e la tte r o n ly a p pe ared
as d a rk p a tch e s on th e b lind sid e o f th e b o d y several
d a ys a fte r c a tc h in g . It is a ssu m ed th a t the se in juries
w e re m a in ly c a u s e d by the s c ra p in g and p re ssin g of
th e v a rio u s o b je c ts in the cod -en d such as starfish,
sto n e s, she lls, sa n d and piece s of w ood. T h e nature
an d d e g re e o f the in ju rie s w ill de pe nd on th e c o m p o ­
s itio n o f sp e cie s an d ob je cts in th e catch . T h is c o m ­
p o s itio n v a rie d be tw ee n the e x p e rim e n ts b u t was not
re co rd e d . H ow ever, it is lik e ly th a t the v a ria tio n in
c o m p o s itio n o f th e catch co n trib u te d to the o b serve d
v a ria tio n in s u rviva l in c o m p a ra b le e xp e rim e n ts.
T h e su rviva l o f d is c a rd s cla ssifie d as e ith e r c o n d i­
tio n A o r B (not o r o n ly s lig h tly da m ag ed fish ) varied
g re a tly. T h is v a ria b ility is c e rta in ly in flu e n c e d by the
c o n d itio n c la s s ific a tio n be in g su b je ctive , b u t also by
e x p e rim e n ta l c o n d itio n s. In p a rticu la r, te m p e ra tu re
an d d ire c t s u n lig h t are th o u g h t to play an im p o rta n t
role (K e l l e , 1976). A lso, in v is ib le h a e m o rrh a g e s in
th e b ra in can d e ve lo p from o x y g e n d e ficie n cy. K e l l e
(1977) sho w e d th a t exh au ste d p la ice p re ve n te d from
b re a th in g b e ca u se th e y w ere co m p re sse d in th e code n d d e velope d b le e d in g in th e b ra in w h ich w as, in
m o st case s, le th al. O u r e x p e rim e n ts do n o t a llow a
PLAICE AND SOLE DISCARDS
fu rth e r a n a ly s is o f th e se factors. O th e r fa c to rs th a t
m a y c o n trib u te to th e m o rta lity o f d is c a rd s are
d e p re s s u riz a tio n (F e a t h e r s & K n a b l e , 1983) and
m u s c u la r fa tig u e (P a r k e r e t al., 1959; B e a m i s h ,
1966). T h e ran ge o f w a te r de p th of th e m ain fis h in g
g ro u n d s in th e s o u th e rn North S e a is 10-50 m , so d is­
c a rd s w ill e x p e rie n c e a fall in h yd ro s ta tic p re ssu re of
~ 1 to 5 bar. A s p la ic e and sole do not ha ve a sw im
b la d d e r th e e ffe c t of d e p re ssu riza tio n w ill be re la tiv e ­
ly sm a ll in c o m p a ris o n with e.g. g a d o id s. A s th e se
fa c to rs w ill g e n e ra lly cause a q u ic k de a th (P a r k e r et
al., 1959; B e a m i s h , 1966; F e a t h e r s & K n a b l e ,
1983) th e ir e ffe c t w ill be in clud ed in th e m o rta lity e sti­
m a te s o f o u r e xp e rim e n ts.
T h e h ig h e r s u rviva l o f plaice d is c a rd s in the ottertra w l e x p e rim e n ts an d the re p o rte d h ig h v ita lity of
p la ic e in th e h isto rica l in ve stig a tio n s (G a r s t a n g ,
1905) m ig h t in d ic a te th a t fis h in g spe ed is p ro b a b ly
an im p o rta n t fa c to r w h ich a m p lifie s th e m e c h a n ic a l
d a m a g e in flic te d d u rin g the stay in th e cod-end.
O n c e on d e ck th e d isca rd s are s u b je c te d to
m e c h a n ic a l d a m a g e from so rtin g , and to te m p e ra ­
tu re d iffe re n c e s , sun and d e sicca tio n . T h e in flu e n c e
o f th e s e fa c to rs on th e su rviva l can be su b sta n tia l
(B o r l e y , 1909; K e l l e , 1976) but is not c o n s id e re d in
o u r e x p e rim e n ts . In som e of the p la ice e x p e rim e n ts
th e ca tc h w a s not d u m p e d on d e ck and so rte d by
ha n d , b u t w a s pro ce sse d m ore q u ic k ly on a c o n v e y e r
b e lt w h e re th e ca tc h was kept w et. M ost m odern
b e a m -tra w le rs are eq u ip p e d w ith s u c h a de vice .
C o m p a ris o n o f th e survival of p la ice d is c a rd s
p ro c e s s e d b y hand and w ith th e c o n v e y e r b e lt
s h o w e d a h ig h e r su rviva l in th e la tte r at h a u l d u ra ­
tio n s o f 60 m in. O th e r th in g s be in g e q u a l, th is in d i­
c a te s th a t th e p ro ce ssin g on d e ck can a c tu a lly a ffe c t
th e m o rta lity of d is c a rd s a n d th a t th e q u ic k p ro c e s s ­
ing u n d e r c o n tin u o u s s u p p ly o f w a te r g iv e s a s lig h tly
b e tte r su rviva l. T h e n e ga tive e ffe ct on th e s u rviva l of
d is c a rd s o f a long h a n d lin g tim e on b o ard w a s also
o b se rve d by G a r s t a n g (1905), B o r l e y (1909) and
N e il s o n e t al. (1989). H ow ever, in m o d e rn beam tra w l fis h e ry w ith haul d u ra tio n s o f 120 m in the
p ro c e s s in g of th e catch on d e ck d o e s not m e a s u ra ­
b ly a ffe c t th e su rviva l o f plaice d isca rd s as th e c o n d i­
tio n o f p la ic e d is c a rd s is a lre a d y b a d ly red uce d
d u rin g th e c a tc h in g p ro cess itself. T h is c o n tra sts
w ith th e s h rim p fish e ry, w h ere the s u rviva l rate of
a b o u t 5 0 % in p la ice an d sole disca rd s is s u b s ta n tia l­
ly re d u c e d w h e n th e ca tch is p ro cesse d w ith a s h a k ­
ing sie ve ( K e l l e , 1976). R o ta tin g s ie ve s and
s p e c ia lly -d e s ig n e d sh a k in g sieves w e re s h o w n to im ­
p ro v e th e s u rviva l rate o f plaice d is c a rd s (B o d d e k e ,
1989).
In p ra c tic e th e haul d u ra tio n of c o m m e rc ia l beam
tra w le rs is in th e o rd e r o f 120 m in un le ss th e g ro u n d
is v e ry rou gh a n d a high am o un t o f b yca tch n e ce ssi­
ta te s a re d u ctio n in haul d u ra tio n (d e V e e n e t al.,
159
1975). T h e re fo re , the su rviva l of both p la ice an d sole
d is c a rd s can be estim a te d at 10 % . T h is p e rce n ta g e
is a m a xim u m estim a te , b e cau se it is lik e ly th a t in the
s u rviva l e x p e rim e n ts not all fish d ie d w ith in th e ex­
pe rim e n ta l period of 84 h o u rs on w h ich th e a b ove
s u rviva l e stim a te is based. T h e su rvivo rs a re p o ssi­
b ly the fish cap tured in the la st m in u te s o f the haul.
T h e su rviva l e x p e rim e n ts w ith so le s co lle cte d from
th e c o d -e n d cove r sho w e d a d e c re a s in g s u rv iv a l rate
w ith in cre a sin g ha ul d u ra tio n . T h e e xtra p o la te d su r­
viva l a t a ha ul d u ra tio n of 0 m in w as e stim a te d at
a b o u t 6 0 % . T he m o rta lity o f so le s e sca p in g th ro u g h
th e net m a y be ca u se d by th e in ju re s in flicte d by the
c a tc h in g p ro cess (tickle r c h a in s, d u ra tio n of sta y in
th e net) an d d u rin g e sca p e th ro u g h th e m eshes.
S ole can esca pe th ro u g h re la tive ly n a rro w m esh es
by b e n d in g in the d o rso -ve n tra l a xis and sq u e e zin g
th ro u g h th e m esh. W h e n th e net is h a u le d in, m any
so le s are seen to be ca u g h t h a lfw a y th ro u g h the
m e sh e s (‘s te k e rs ’). V isu a l in s p e c tio n o f such ‘ ste k­
e rs ’ sho w s som e loss o f scales. In sp e ctio n o f th e ex­
p e rim e n ta l sole tha t d ie d also sh o w loss o f sca le s
a n d ha e m o rrh a g e s. T h e e stim a te d s u rviva l of ab ou t
6 0 % th e re fo re se e m s a rea son able, but n e ce ssa rily
c ru d e estim a te . It im p lie s th a t th e g a in o f an in crea se
in m esh size w ill be lo w e r tha n g e n e ra lly c a lcu la te d
a s s u m in g th a t all soles th a t esca pe th ro u g h the
m e sh e s survive.
5. R E F E R E N C E S
F.W.H., 1966. Muscular fatigue and mortality in
haddock, Melanogrammus aeglefinus, caught by ot­
ter trawl.—J. Fish. Res. Bd Canada 2 3 : 1507-1521.
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B o d d e k e , R ., 1989. Management of the brown shrimp
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Management 2: 60-65.
G a r s t a n g , W., 1905. Vorläufiger Bericht über die Natur­
geschichte der Scholle auf grund der Untersuchun­
gen der Kommision B in der Zeit bis zum 30 Juni
1904.— Rapp. Proc. Verb. Cons. int. Explor. Mer 3,
Appendix H: 1-56.
K e l l e , W ., 1 9 7 6 . Sterblichkeit untermasziger Plattfische im
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B e a m is h ,
2 5 : 7 7 -8 9 .
, 1977. Verletzungen an untermassigen Plattfischen in
der Garnelenfischerei.—Arch. Fisch. Wiss. 28:
157-171.
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160
F.A. VAN BEEK, P.L. VAN LEEUW EN & A.D. RIJNSDORP
caught by longllne and otter trawl gear.— Can. J. Fish.
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P a r k e r , R.P., E.C. B l a c k & P.A. L a r k i n , 1959. Fatigue
and mortality in troll-caught pacific salmon (On­
corhynchus).—J. Fish. Res. Bd Canada 16: 429-447.
R i j n s d o r p , A.D., F.A. v a n B e e k & P.I. v a n L e e u w e n ,
1981. Results of mesh selection experiments on sole
with commercial beam trawler vessels in North Sea
and Irish Sea in 1979 and 1980.— ICES 1981/B:31:
1-19 (mimeo).
J.F. d e & W.F. R o d e n b u r g , 1971. Discarding
in the Dutch sole fisheries in 1969 and 1970.— ICES
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V e e n . J.F. d e , P.H.M. H u w a e & M.S.S. L a v a l e y e , 1975.
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Veen,