This paper not to be cited without prior rcference to the author.
INTERNATIONAL COUNCIL FOR
TUE EXPLORATION OF TUE SEA
C.M. 19861G: 90
Demersal Fish Committee
DUTCU GROUNDFISU SURVEY: DATA ON COD AND WUITING (1980-85)
by
Henk J.L. Heessen.
Netherlands Institute for Fishery Investigations
P.O. Box 68,1970 AB IJmuiden
The Netherlands.
INTERNATIONAL COUNCIL FOR TUE
EXPLORATION OF THE SEA
c.Ml
1986/G:90
\.
Demersal Fish Committee
DUTCH GROUNDFISH SURVEY: data on eod and whiting (1980-1985)
by
Henk J. L. Heessen
•
Netherlands Institute for
Fishery Investigations
P.O. Box 68, 1970 AB IJmuiden
The Netherlands
Summary
Data on distribution and abundanee of young eod and whiting in the
south-eastern North Sea in the period 1980-1985 aie presented. Comparisons
are made between the eatehes of the same ycar elasses in eonseeutive years."
For eod eorrelation diagrams between indices of year elass strength from thc
Duteh Groundfish Survey and the VPA are also given.
Introduetion
•
During 1980-1982, groundfish survcys wcre eonduetcd in all quarters,
following the standard proeedurcs used in the International Young Fish
Survey in February. Data from these survcys on the distribution and
abundanee of young eod and whiting were analysed by Heessen (1983) and a
distinet ycarly migration pattern of youngeod in the Gcrman Bight emcrged •
Partieularly during the first quarter of the ycarj when the IYFS is
eondueted, young eod usually reaeh thc highest degree of eoneentration in
the shallow eoastal zonc. Bceause of this aggregation it is diffieult to
obtain an aeeuratc estimate of thc abundanec of young eod of age groups 1
and 2 at this time of the ycar. Three months carlier, in Oetober/Novcmbcr,
young eod are mueh less eoneentrated and also they are relatively less
abundant in shallow water. Besides, such information might be morc
effieicntly used for management purposes beeausc it would beeome available
earlier in time. Thercfore, a shift in timing of the young fish survcy from
Fcbruary to November, might result in more accurate estimates of year elass
strength of eod. llowever, the IYFS serves other purposcs then only
reeruitment estimates of North Sea eod and for logistie reasons therc is no
possibility to transfer rescareh vessel cffort from February to November
(Anon., 1985).
Still,. beeause of"the problems eneountcred in estimating the strength of
year elasses of young eod from the IYFS (see also Anon., 1983 and 1985) we
deeided in 1983 to eontinue the surveys in Oetober/Novembcr. Although during
thls survey only the south-eastern part of the North Sea 1s f1shed, the ma1n
distribution area of young cod 1s covered.
- 2 Material and methods
The methods used in the Dutch Groundfish Survey are the same as during the
IYFS (see IYFS manual, Anon.,1986a). The gear uscd is a high opening bottom
trawl (GOV-trawl) and the haul duration is 30 minutes.
The area covered in the years 1980-1982 corrcsponded largely with roundfish
area 6 and one haul was made per statistical rectangle. From 1983 onwards
the area was slightly extended. In the castern and for young cod the most
important part two hauls were,made per rectangle and in the western part
only one haul per rectangle. Apart from the standard stations fished by thc
research vessel "Tridens" (in 1980 und 1982 by the chartered commercial
trawler KW 34 "Rose Marie"), in 1984 aud 1985 additional hauls were made in
the coastal zone by research vessel "Isis". In Figure 1 the number of hauls
per statistical rectangle is given for the different years; the total number
of hauls is given in Table 1.
•
As described in the IYFS manual the whole catch is sampled for length
compositions of all species encountered. Otolith sampies are collected for
cod, whiting and haddock.
The "standard-area" (Figure 2) used for the calculation of DGFS-indices
comprises 31 statisticalrectangles, all within roundfish area 6. To correct
for rectangles that were not fished in a certain year, the rectangles in the
standard-area that were fished during all 6 years (18 out of the 31
rectangles) were used as a reference. The coverage of the standard area in
different years is indicated in Table 1.
Results and discussion
In Figure 3-9 the mean number per rectangle for cod (0-, 1- and 2-group) and
whiting (0-, 1-, 2-, and 3-group) is shown for the years 1980-1985. In
Table 2 the indices for the same age groups are given for the standard area.
Comparisons between catches of the same year classes of cod and whiting in
consecutive years are shown in Figure 10 and 11.
There is a high correlation between the indices of O-group cod and the
index of the same year class one year later (Figure 10). In the data for cod
the absence of year class 1984 is very striking, both as 0- and as l-group
the catches were very low. The O-group index for year class 1981 is strongly
influenced by one very large haul. During the first three years of the
survey only one haul was made per statistical rectangle. Over these years in
4 cases over 40% of the total annual catch of a certain age-group was eaught
in a single haul. The effect of eliminating these hauls is especially
noteworthy in the case of O-group cod in 1981, as indicated in Figure 10. A
remarkable feature of the average catch of O-group cod and the catch of the
same year class one year later (Figure 10) is that both are of,the same
order of magnitude, whereas natural mortality is probably still high during
this stage of life. Tbe results of the Multispecies Assessment Working Group
indicated a predation mortality of l-group cod of approximately 40% (Anon.;
1986c). An explanation for this feature might be found in the observation
that in Oetober/November a high proportion of Q-group eod is present in very
shallow waters, where it is hardly possible to fish with a GOV-trawl. Thus,
the largest catch of O-group has been made in 1985 in a shallow gully in
reetangle 36F8 (see Figure 3). At this station only a 15 minutes haul ean be
made. It is likely that in the easternmost part of thc German Bight
comparable concentrations may occur.
--------
- 3 Comparison of 1- and 2-group eod (Figure 10) revea1s a sharp deerease in
numbers which may partly result from a high fishing morta1ity but probab1y
also from emigration of 2 year o1d fish out of the investigated area. Hardly
any f1sh of age group 3 and older are caught, indicating that there are not
many o1der eod 1eft in the area.
A1though the time series is rather short the DGFS indices for cod seem to be
reasonably well correlated with VPA numbers (Figure 12).
•
For whiting year class 1983 was remarkably strong in the investigated area.
High abundance of this year class was found as 0-, 1- and 2-group. From the
camparisan of of the var10us age groups of whiting (Figure 11) it 1s clear
that the correlations between 1- and 2-group and between 2- and 3-group are
much better than for the 0- and l-group. Taking into account the size
distribution of O-group it seems likely that in the fourth quarter not all
O-group whiting have recruited to the gear.
Comparisons of VPA numbers for whiting and DGFS indices have not been
presented because the correlations are very poor.
References
Anonymus, 1983 - Report of the International Gadoid Survey Working Group,
IJmuiden 6-12 July 1983. ICES C.M. 1983/G:62.
Anonymus, 1985 - Report of the Working Group on International Young Fish
Surveys in the North Sea, Skagerrak and Kattegat. ICES C.M. 1985/H:2.
Anonymus, 1986a - Manual for the International Young Fish Surveys in the
North Sea, Skagerrak and Kattegat. ICES C.M. 1986/H:2.
Anonymus, 1986b - Report of the North Sea Roundfish Working Group. ICES C.M.
1986/Assess:16.
Anonymus, 1986c - Report of the ad hoc Multispecies Assessment Working Group.
ICES C.M. 1986/Assess:9.
Heessen, H.J.L., 1983 - Distribution and abundance of young cod and whiting
in the south-eastern North Sea in the period 1980-1982. ICES C.M. 1983/G:30.
- 4 -
Tab1e 1.
surveys.
year
1980
1981
1982
1983
1984
1985
Number of hau1s and number of rectangles covered during subsequent
number of rectangles
covered in standard area
number of hau1s
in standard area
total number
of hau1s
28
24
31
27
31
26
28
24
31
SO
83
79
31
34
31
56
92
87
•
Tab1e 2. Indices of year c1ass strength of young cod and whiting from the
Dutch Groundfish Survey. The indices are the arithmetic mean number per hour
traw1ing per rectangle in the standard area.
COD
•
WHITING
year class
age 0
1978
1979
1980
1981
1982
1983
1984
1985
43.2
176.8
26.9
121.5
1.3
143.6
year c1ass
age 0
1977
1978
1979
1980
1981
1982
1983
1984
1985
166.4
1,393.2
165.9
2,649.1
142.8
859.3
age 1
163.8
46.9
83.0
21.8
121.3
3.6
age 1
329.8
205.1
639.8
430.9
1,330.1
783.0
age 2
4.5
11.2
1.6
2.3
1.6
3.1
age 2
61.5
130.5
105.2
223.5
140.7
893.3
age 3
26.8
9.1
52.1
12.4
90.8
31.6
Number of hauls per statistical rectangle in different years.
Figure 1
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Figure 5
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55
.~
}7
56
•
39
32
.
Fl
65;1
42
K
~,
3
5 6
o
FO
-..r
40
33
f-/1(
l
18~.
54
41
~
3
15
0
o
F9
v
0
j
i
'0
iffi
55
· V)
31
·
9
-\
32
·
"
36
16
•
53
56
•
38
~~
r.
·
55
~
57
·
F3
(ffi
39
~
56 r(81
u
29 1
16
.~.
-~
5'
F2
6"
v
55
•
F1
7·
1985 .
·
·
6
10
Flgure 10
-
Correlatlon between DGFS indices of the same year classes of
cod in consecutlve years.
DGFS
eOD 1
83
•
100o~
so -
81
- - - - - - - - - •
80
•
82
•
8i
..8 l.
•
I
SO
100
•
150
DGFS
eOD 0
DGFS
eoo 2
79
•
10-
•
583
•
81
82
~t.
•
80
•
•
I
I
I
50
100
150
OGFS
eOD 1
Figure 11
-
Correlation between DGFS indices of the same year classes of
whiting in consecutive years.
DGFS
WHI 1
83
•
1000 84
•
81
•
500 -
82
•
80
•
•
8l
I
I
1000
2000
DGFS
WHIO
OGFS
WHI2
1000 -
8~
500 81
•
82
80 79
• • •
•
8{
I
r
500
1000
1500
DGFS
WHI 1
DGFS
WHI3
100-
81
•
79
50-
•
82
•
•
/.00
•
800
DGFS WHI 2
Figure 12
Correlation between DGFS indices for cod and numbers in the
-
VPA (Anon., 1986b)
VPA1r - - - - - - - - - - - - - - - - - - - .
eOD
o (- _ ~3.
~
500 -
82 80
• •
250-
~L.
8f
1
•
I
I
100
SO
150
DGFS eOD 0
VPA
eOD2
79
•
30081
82
8.0
•
100-
r
•
8.3
•
200-
T
100
50
•
150
DGFS eOD 1
VPA
COD3
.79
6078
•
81
•
/.0-
•
80
20-
82
~
•
5
•
10
DGFS
eoo 2