This papl:r not to be eited \\ ithout prior rcferl:nee to the author.
INTERNATIONAL COUNCIL FOR
THE EXPLORAnON OF THE SEA
C.M. 1981/0: 73
Demersal Fish Committee
FEEDING OF NORTH SEA COD IN ROUNDFISH ARREA 61N 1980
·PRELIMINARY RESULTS
by
N. Daan
Ncthcrlands Institute for Fishery Il1\cstigations
Haringkade 1,1'.0. BOX 6R,
1970 AB IJmuidcn, The Ncthcrlands.
This paper not to be cited without prior reference to the author.
International Council for the
Exploration of the Sea
Demersal
C.M. 1981/G:73
Fish Committee
FEEDING OF NORTH SEA COD IN ROUNDFISH
AREA 6 IN 1980 - PRELlMINARY RESULTS
by
Niels Daan
Netherlands Institute
for Fishery Investigations,
Haringkade 1,
P.O. BOX 68,
1970 AB lJMUlDEN
The Netherlands.
Introduction
The large scale stomach sampling programme initiated in the North Sea
~n 1981 is supposed to yield the necessary information for the calibration
of the food composition of five fish stocks against the year class strength
of the different commercial fish species in that year.
In setting out the requirements for sampling intensity for this programme
it became obvious that such a programme could not be continued on a routine
basis, but at best could be repeated once or twice after a couple of years·.
Still, it would obviously be rather important to test the validity of the
calibration procedure in one particular year against reliable information
about the stomach contents in an independent year.
The information obtained for an earlier period by DAAN (1973) does not
seem adequate in this respect because the sampling intensity was too low
for analysing the data by individual years. Therefore, it was decided to
start a sampling scheme for cod stomachs already in 1980 before the onset
of the international programme. For the particular purpose of investigating
annual variations in food composition and consumption it appeared appropriate
to concentrate the effort in one particular area rather than spreading the
limited effort over the entire North Sea. Therefore, in addition to the
annual lCES Young Fish Survey in February, 3 GOV trawling cruiseswere
carried out by the R.V. Tridens in roundfish area 6 in May, August and
November 1980 respectively. This report deals with the results of these
investigations.
Sampling intensity
I.
Since Scotland and England cooperated in the collection of sampIes both
during the IYFS and during standard trawl surveys in the third quarter
of the year, stomach sampIes came in from a larger North Sea area than
just area 6. Table I presents the number of samples and the number of
cod stomachs investigated in 1980 by quarters, roundfish areas and by
- 2 -
predator size classes. In addition fig. 1 shows the spatial distribution
of the collected stomaehs by statistical rectangles.
Only for area 6 a high sampling intensity has been maintened during all
seasons and this report deals only with the results for that area.
Methods
Sampling methods and the laboratory analysis of the contents were generally
in accordance with the views expressed in the report of the ad hoc WG on
Multispecies Assessment Model Testing (ICES, 1980) and of the-Draft Manual
for the Stomach Sampling Project~), except for minor amendments which were
introduced in due course. Originally up to 25 fish per size class were
opened but this number was later on reduced to 10. iFish showing signs of
regurgitation were excluded from the sampIe and, whenever possible replaced
by other individuals. (In 1981 it was decided to replace fish showing signs
of regurgitation by feeding animals).
•
The stomach contents of all fish within a size class were emptied in jars
and meanwhile inspected for obvious signs of having entered the stomach
during the trawl haul. Such prey were also excluded from the sampIe.
Each trawl haul was further sampled for length distributions of the cod
caught (as weIl as all the other fish species) in order to allow for a
weighting factor for each sampIe duiing additive processes, based on the
average abundance of the fish in the corresponding size class within each
rectangle sampled.
The grouped sampIes were taken back in formalin and processed in the
laboratory. The essential information contained the weights and numbers
of each prey type by prey size category. In addition actual measurements
were made of fish prey whenever possible.
The information was stored on magnetic disc for further analysis. For coding
of the taxonomie units the NODC coding system was extended to include the
relevRnt North Sea fauna.
•
Results
The stomach content data were combined with the relative abundance figures
in each statistical reet angle to produce average figures of food composition
of the total cod population in area 6 by quarter of the year. If N is the
relative measure of density (number per hour fishing) andwriting i for
predator size class, j for prey and k for statistical rectangle, then
the overall average weight (w) of that prey in the stomach of an average
predator·in the area is:
-j
w. =
~
r Ni
k
k
*) Prepared during the stomach sampling meeting in IJmuiden, 13-15
January 1981. A limited number of copies is still available from
the Netherlands Institute for Fishery Investigations, Haringkade 1,
IJmuiden.
- 3 -
The output tables are rather voluminous due to the hundreds of prey
types/size categories distinguished. Only global results will be given
here but the basic tables are available upon request.
Fig. 2 provides the %weight composition by major taxa. The weIl
established transition for cod from feeding on crustaceans to feeding
on fish is apparent in all seasons but in detail considerable differences
are exhibited. These variations will be partly caused by changes in absolute
abundance of the prey types but they are also an effect of patterns of
redistribution of the cod population over the area. As an example in fig. 3
the abundance of I-group cod over the area is shown for each of the four
surveys. In winter these fish were particularly abundant in the inshore
area, but they moved out rapidly and spread widely over the area during
the sumrr!er. In autumn they were concentrating again along the continental
coast.
•
Table 11 summarizes the information averaged over the four seasons. The
mean weights of the stomach contents in individual quarters are plotted
against predator length on a double logarithmic scale in fig. 4. For
3
comparison the relationship obtained in an earlier study (w =0.000158 x U ;
DAAN, 1973) is also shown. The general agreement between th~ two data sets
is striking. This is also true for the general trend in the nr of prey items
per stomach with increasing predator size (fig. 5), although the present
level is increased approximately by 1 item per stomach.
The weight %composition by commercial species given in the lower part
of table 11 will be of later interest when comparable data for 1981
become available. In order to study the prey size preference of cod
the frequency distributions of the different prey size classes by weight
are given by predator size class in table 111. In fig. 6 these data have
been plotted, after a correction procedure based.on log prey size class
band width. (This was necessary because the division in size classes
follows only approximately alogarithmic trend). The black part of the
columns represents fish and the rest consists predominantly of crustaceans
(and molluscs) since most annelids were not classified to size classes
and appear at the right hand side of the figure. Because of their elongated
format it did not seem appropriate to include annelids in an analysis of
prey size class spectra. A general upward shift in prey size class is
apparent which is associated with the transition from crustacean feeding
to fish feeding. In fact in the larger size classes abimodaldistribution develops
separating fish from crustaceans.
Fig. 7 provides similar information for the %composition in numbers
in the stomaehs by prey size classes. As could be expected the peaks
have shifted to the left.
An interesting comparison can be made between the prey size preference
of cod and of turbot (WETSTEljN, 1981). The range of prey size classes
within the food of turbot of a particular size is smaller and also the
peaks are much more pronounced than for cod. This suggestthat not only
the cod is a less discriminate fee der in terms of the overall food spectrum but also in relation to the"optimal" prey size.
References
DAAN, N., 1973
- A quantitative analysis of the food intake
of North Sea Cod, Gadus morhua.
Neth. J. Sen Res. 6(4): 479-517.
- 4-
•
lCES, 1980
- Report of the ad hoc Working Group on
multispecies Assessment Model Testing.
lCESC.M. 1980/G:2.
WETSTElJN, B., 1981
- Feeding of North Sea Turbot and Brill.
leES C.M. 1981/G:74 .
Table 1 - S~pling intensity of cod stomaehs in 1960. by quarters, roundfish areas and predator size classes
(n" nr of sampIes; N .. nr of stomachs).
---.
1st Quarter 1980
1
Rnd! area
2
3
4
3·
3
11
9
149
14
Sub-total
4
4
31
405
40
2
3
12
1
4
22
132
12
26 2 29
4 55
3 61
24 414 25 532
1 12
1
2
18
170
31
118
1
64
N
11
13
86
8 21
31
4
3 4
43 4
19
4
1
2 2
14 246 19 311
9 83 10 13
10
5
6
1
25 - 30
20 - 25
n N
15 - 20
n N
10 - 15
n N
1 - 10
N
f'
408 60
.
19
3
4
4
1
13 165
6 20
11
8
68
421
41 334
48 278
8
234
1
3 3
18 563
4
4
18
1\)
11
~)
.)
10 - 100
N
Il
50 - 10
N
11
19 61
4 42
2 2
3 38
.)
.)
)
N
fl
120
63
1
31
2
141
63
21
5
6
5
1
.)
524
N
n
If)
6
2
'8
40 - 50
30 - 40
6
90
24
13
36
2
102
11
tot"l
n
100 - 150
r;
fl
62
18
20
11
21
'>1
19
3
2
4
5
1
2
1
2
2
21
58 218
14
19
3
1
5
4
13
1
5~
430
322
49
244
33
1211
343
372
2632
11
3
78
191
2414
26
le5
2709
95
41
')c
~.J
34
N
31
11
108
2nd Quarter 1960
1
Rnd! area
2
3
4
5
6
7
Sub-total
3rd Quarter 1980
1
Rnd! area
2
3
4
5
6
1
Sub-tot&.1
2
2
1
3
10
1
567
31
630
4
9
4 5 34
11
8 5
10 2 24
11 4 31
51 23 221
4 6 53
1 12
4 36
25 311
19
98
10
12
2
1
5 35
3 23
2 30
5 • 49
26 309
6 60
13
5
6
2
5
21
6
49
42
24
29
46
326
60
12
5
6
2
4
16
6
17
4
6
2
216
6
15
21
24
16
31
91
43
13
4
6
1
2
3
5
65
20
21
2
9
8
16
6
2
3
10
3
5
66
32
37
12
25
140
37
282
187
134
128
210
1340
281
48
508
64
516
51 319
53 301
34
141
12
19
349
2562
2
9
5
38
C.
107
21
·911
61
11
43
113
938
133 748
113
413
1(, 1~.
6Ö~.1
12
28
18
17
21
113
44
1
16
12
139
1
12
1
14
1
96 15
89
1
1
16 156
19
268
1 4
16 201
1 10
11 .51
1
12
15
91
15
89
11
157
19
268
17 205
12
74
726
95 1084
173
1513
1311425
11
80
105 1166 155 1632
14,
102
139
Sub-total
22
20
12
49 '389
1
1
10
22 567
419
16
19 • 94
1
1
8
22 . 242
.30
1
4th Quarter 1980
findf area 1
2
3
.4
5
6
7
Total Years
2
j
27
j
54
:'ac~~
r! - S~~~oarJ ~~~ titc~~~h cont~nt J~ta ir. rounJfish_~re~ 6 in 1~80 (Averaged Qver th", quarters).
(~s
J:.t:9.n wei~ht stomacn contents per fish; :1 5 .: mean nr cf prey items per rish;
wp
= m",an
~eight
per prey item in the stomaeh).
1-10
SIZE CLASS
10-15
.10
19
10.2
8.6
Nr. of samples
Nr. of stomaehs
Percentage empty
MelUl length
0.13
4.01
0.03
WB
>ja
p
'tomacn content composition in
i'haiyta
Par
a
Gni
a
Ctenaphara
Rh ynchacaela
Annellida
Castropoda
Bivalvia
Cephalopoda
Crustacea
Priapulida
Echinodermata
Urochordata
Cephalochordata
Cnathostomata
~eight
%by
42
432
26.1
12.9
0.29
3.18
0.09
15-20
62
861
23.8
18.1
0.65
3.12
0.21
20-25 -l<
82
1048
19.5
23.2
1.23
3.56
0.35
25-30
«
86·
953
12.5
26.9
2.10
4.19
0.50
main taxa.
6.48
80.49
9.66
0.70
22.41
1.14
2.26
65.61
60:.16
0.09
13.04
24.03
12.69
0.00
0.01
28.11
3.19
6.16
0.00
45.01
0.02
0.07
0.87
0.05
15.32
0.05
,0.. 01
30-40
81
969
15.1
33.1
40-50
63
929
11.6
44.1
10-100
100-150
51'
.n
348
11.2
55.8
123
21.7
80.1
7
21
13.g
104.8
139.65
10.21
13.60
t41.9
12.0
11.8
50-10
6.42
4.46
1.44
13.22
5.15
2.51
29.71
6.75
4.40
0.03
0.04
0.01
0.08
. 0.04
0.30
0.01
0.00
15.34
0.12
15.30
0.00
49.11
1.06
0.68
10.12
0.20
1.38
0.03
42.33
18.35
0.19
0.03
0.38
17..80
13.01
1.18
4.09
0.02
51.74
0.01
0.07
0.01
0.11
29.66
0.12
0.00
39.79
80.43
92.51
0.08
0.33
0.70
0.25
4.14
2.92
0.49
4.11
2.03
7.98
26.11
2.64
0.02
0.53
9.91
1.22
2.94
0.42
0.35
3.28
4.65
4.81
0.70
0.38
0.52
0.76
0.74
0.14
3.91
0.16
2.62
0.05
28.61
2.16
5.22
48.11
0.31
0.11
0.03
0.02
14.11
0.03
0.14
6.81
Unkna~n
Stomach cantent composition in
Cad
Haddack
Whiting
Saithe
Plaice
Sole
SlUldeel
Herring
Sprat
Na_Lobster
Brovn shrimp
~eight
% by commercial species
11.76 u
30.06
0.02
1.05
0.85
0.00
0.00
2.42
0.00
3.21
0.07
0.04
2.99
0.49
3.17
0.01
5.5J
0.04
3.21
0.28
0.24
0.10
0.37
6.26
0.20
3.25
33.16
30.28
0.29
12.02
12.14
0.23
5.11
14.44
~) The data far the size class 20-30 cm fram the first quarter have been included in bath size classes for obtaining
an annual average.
~-.)
larvae
\
28.26
0.19
0.23
9.11
Table ITI - Size class distribution of prey itelliS by predator size class (annual average).
Size cl ass code represents lo~er limit in mm.
Size c1ass
7-10
10-15
15-20
20-25
25-30
30-40
40-50
50-70
10-100 1(;0-150
Weight %
o Eggs
Code
1
0.04
2
3
4
5
7
10
15
20
25
30
40
50
70
100
150
200
250
300
9999 Unclassified
•
0.58
0.02
0.11
0.09
6.00
2.98
22.03
24.63
11.02
5.78
13.47
1.11
0.08
1.76
0.58
5.86
2.93
14.32
14.01
28.56
7.51
10.32
0.14
7.92
0.06
0.09
0.14
0.03
0.01
0.01
0.00
0.01
6".22
1.04
2.47
4.34
11.40
7.54
20.93
5.01
20.98
5.52
5.91
2.17
0.32
0.00
0.15
1.24
3.74
10.22
10.15
18.51
6.14
13.22
4.81
16.88
6.84
1.60
0.01
0.07
0.49
0.93
6.27
9.20
16.52
12.69
9.55
5.34
18.37
15.30
1.35
12.14
6.51
3.94
0.20
4.54
0.06
0.85
3.22
12.21
6.19
5.83
9.89
14.10
10.02
8.34
0.47
0.13
3.59
1.91
7.00
6.32
7.42
5.18
15.07
3.24
11.57
3.29
3.38
0.02
0.00
1.64
2.65
7.59
4.93
9.29
9.41
6.96
9.14
9.72
5.27
3.13
1.40
0.33
12.24
28.56
31.89
28.58
0.32
0.02
14.65
0.31
9.70
0.25
0.09
1.74
5.55
6.22
5.57
11.65
7.40
14.05
6.58
9.99
2.59
2.50
0.57
0.03
0.00
1.56
4.89
7.12
10.73
8.34
19.51
5.46
5.96
3.34
9.31
5.84
1.72
0.11
0.36
2.25
3.27
9.43
11.92
16.21
12.46
10.46
6.62
9.52
2.77
0.08
24.63
6.47
9.8
0.00
0.02
0.29
1.44
4.59
8.28
2.45
1.27
1.66
12.73
22.00
31.97
11.37
0.07
1.87
un
0.5':1
0.02
3.15
0.34
2.28
0.81
9.84
33.89
16.01
4.29
19.07
0.83
NumLer %
Code
o
LggS
1
2
3
4
5
7
10
15
20
25
•
30
40
50
70
100
150
200
250
300
9999 Unclassified
0.46
0.31
14.87
13.57
34.85
20.20
6.54
1.45
3.85
4.;38
0.20
9.18
2.69
19.04
4.25
9.12
5.91
12.83
7.21
7.32
0.02
4.27
17.54
1.01
1. 74
4.84
16.49
9.30
6 ..52
11.79
8.38
3.30
1.62
0.09
0.01
10.67
4.15
12.50
3.69
8.08
8.06
7.85
2.57
3.09
0.66
0.33
0.15
0.08
8.07
11.92
9.82
4.60
6.26
9.63
6.52
3.70
3.29
1.18
0.44
0.09
0.01
20.28
28.30
34.25
0.11
0.68
1.92
7.49
15.63
17.84
4.44
2.25
4.76
16.43
10.07
7.17
1. 70
0.01
8.35
16.37
8.54
0.26
7.57
1.99
1';.95
21.31
9.44
9.24
3.92
J.2C
1. 35
2.02
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