This paper not to be cited without prior reference to the author.
INTERNATIONAL COUNCIL FOR TUE
EXPLORATION OF TUE SEA.
C.M. 1978/H : 6
REPORT ON TUE ICES YOUNG UERRING SURVEY IN TUE NORTU SEA IN 1978.
by
(
.,I
A.Corten,
Netherlands Institute for Fishery Investigations,
Haringkade 1, 1976 CP Ymuiden, The Netherlands.
•
.'I
'
~ ':
":J,
This paper not to be cited without prior reference to the author.
C.M. 1978/H: 6.
International Council for
the Exploration of the Sea
REPORT ON THE ICES YOUNG HERRING
SURVEY IN THE NORTH SEA IN 1978.
by
A. Corten
Netherlands Institute for Fishery Investigations,
Haringkade 1,
1976 CF IJMUIDEN
The Netherlands.
Contents
1. Introduction.
2. Material and Methods.
2.1. Participation.
2.2. Methods used in the trawling survey.
•
2.3. Methods used in the IKMT survey.
3. Results.
3.1. Results of the trawling survey.
3.2. Results of the IKMT-survey.
4. Discussion.
4.1. Confidence limits of the I-group estimate.
4.2. Prediction of year-class strength.
5. Acknowledgements.
6. References •
..
1. Introduction.
The ICES Young Herring Survey is an international programme for
obtaining yearly estimates of the abundance of I-group herring in
the North Sea and Skagerak. These estimates are used by the ICES
Herring Assessment Working Group in advising next year's TAC's
for herring in the North Sea.
The abundance of I-group herring is estimated by means of a
bottom trawling survey which covers most of the North Sea,
Skagerak and Kattegat. The surveys were first made in 1960 and
1961. After a break of 3 years they were resumed in 1965 and
from then onward they have been continued without interruption.
At present, 7 countries cooperate in this programme.
In recent years, the surveys have also been used to estimate the'
abundance of O-group herring, which are sampled by means of an
Isaacs-Kidd midwater trawl (IKMT).
Results from the surveys have been reported annually to the ICES
Council Meeting. In addition to these reports, the ICES Working
Group on North Sea Young Herring Surveys has produced several
meeting documents in which the survey methods and results have
been evaluated (most recent report: ICES 1978).
Data on the abundance of other species and hydrography, obtained
from the survey, are presented to the ICES Council Meeting in
separate reports.
2. Material and Methods.
2.1. ~~~~!~!E~~!~~_!~_~~~_l~Z~_~~~~~~~
The following countries took part in the survey:
Ship'
Country
J.C. Svabo
14 Feb
1
Denmark
France
Thalassa
1 - 26 Feb
Germany (Fed. Rep. )
Solea
1
21 Mar
Norway
Johan Hjort
8
28 Feb
Netherlands
Tridens
30 Jan - 2
Argos
6
24 Feb
Sweden
Cirolana
United Kingdom
28 Feb - 20
Uni ted Kingdom
Explorer
21 Feb
2
Clupea
United Kingdom
26 Jan - 9
-
•
-
Mar
Mar
Mar
Feb
2.2. ~~~~~~~_~~~~_~~_~~~_~~~~~~~~_~~~~~~~
The sampling of juvenile herring was concentrated in the standard
area of 57 statistical rectangles, defined in the latest report
of the Working Group (ICES 1978). A stratified sampling pattern
was used, in which most of the sampling effort was directed
towards rectangles which had yielded the highest catches of
juvenile herring in the paste Hauls in the herring standard area
were only made during daytime. Standard duration of the hauls
was 30 minutes •
..
-3Three vessels employed the GOV-trawl which has now been adopted
as standard gear for the Young Herring Survey (lCES 1978).
These vessels were "Thalassa", "Tridens" and "Cirolana". The
other ships used a variety of different bottom trawls, most
of which had also been used during previous surveys. lt was
assumed that the average fishing power of all participating
vessels had remained at the same level as in previous years.
2.3.
~~~~~~~_~~~~_~~_~~~_!~~~_~~~~~l~
Sampling of herring larvae by means of an lKMT was carried out
by the research vessels Johan Hjort, Tridens, Argos, Cirolana
, and Explorer. Each vessel made two night-hauls in rectangles
which were sanpled by bottom trawl during daytime. The IKMT
used was of the Swedish design, recommended by the ICES larval surveys
working g~oup (lCES 1977). Hauls were made according to a
standard procedure described in the above report.
3. Results.
3.1.
.
~~~~!~~_~~_~~~_~~~~!~~~_~~~~~l~
. A total of 218 valid herring hauls (i.e. hauls during daylight
and without gear'damage) were made in the herring standard area,
and another 133 valid hauls were made in the remaining rectangles.
Mean catches per rectangle of I-group herring (yearclass 1976
autumn spawners) are shown in figure 1. The overall abundance of'
year-class 1976 was quite low, at a level only slightly below that
of the two previous year-classes. The highest catches were made
as usual in the southern and eastern part of the North Sea. In
addition, some large catches,of I-group herring were made in the
Moray Firth, probably consisting of juvenile herring that originated from the west of Scotland. The exceptionally high catches
in the Kattegat probably consisted mainly of local O-group spring
spawners, which cannot be distinguished from I-group autumn spawners.
•
The mean abundance of I-group herring for the standard area
(calculated as the mean of all the rectangle means) was 575
herring/hour. Table 1 shows that this value is slightly above
the figure for the two previous surveys, but still only 40 %
of the average for the surveys in the years 1970-75. Catches of
II-group herring (year-class 1975) were extremely low throughout
the North Sea, confirming the low estimate of this year-class
obtained,from the previous survey (figure 3).
3.2. ~~~~!~~_~~_~~~_!~~~:~~~~~l~
The coverage of the North Sea by IKMT hauls was improved again
in 1978. Except for a gap in the sampling area between 57°N and
58°N, the North Sea was almost completely covered up till 61°N.
..
Fig. 4 presents the mean number of larvae caught per'haul in each
statistical rectangle. Relatively high catches were made in the
central NorthSea along the English coast between 53°N and 56°N,
and also over a large a~ea in the northern North Sea. The density
of larvae in the eastern North Sea and Skagerak was low, although
the numbers per tow were 'slightly above those from last years.
Unfortunately, no sampling was carried out in the Moray Firth
where high numbers of larvae had been caught in 1977.
'-
-4Comparing the results from 1978 to those of 1977, the abundance
of O-group herring has increased slightly in most areas that
have been fished_in both years.
A remarkable feature of the O-group herring is their extremely
wide distribution. Relatively high catches were made as far
north as the Shetland Islands.
A great, advantage in sampling O-group herring is their very
even distribution. Because of the low variance between individual hauls within a square, a sampling frequency of 2 hauls
per rectangle seems to be sufficient to obtain a good estimate
of O-group abundance.
4. Discussion.
4.1. 22~~!~~~~~_~!~!~~_~f_~~~_!:~~~~E_~~~f~~~~~
In calculating the mean abundance of I-group herring for the whole
standard area, the average is taken of all individual rectangle
means. This procedure, although providing the most accurate
estimate of the mean abundance, does not allow us to calculate
the variance and confidence limits of this mean. The reason for
this is that several rectangles contain only one or two hauls,
and it is impossible to calculate a variance for the mean of
.each rectangle.
\
In order to calculate a variance and confidence limits, the hauls
have to be grouped into larger uni ts~ e .g .. the 3 strata used for
optimum allocation of sampling effort (lCES 1978). For each stratum,
the mean and variance are calculated. Next, the stratified mean
and its variance are calculated for the whole standard area. -
,
/
•
stratum
number of rectangles'
number of hauls
stratum mean
stratum variance
variance stratum mean
1
37
77
275
1 098 680
14 269
stratified mean for whole area
variance 'stratified mean
standard error
90 % confidence limits
2
15
88
1 160
8 849 402
100 561
3
5
53
869
2 743 301
51 760·
560
13 375
116
560 + 191
The stratified mean for the whole area, calculated according to
the above procedure (560) is slightly different from the mean
that is obtained by averagllgall rectangle means (575). Still,
the 90 % confidence limits calculated above will give an indication
of the precision that can be ascribed to both estimates of the
mean abundance.
.----------------------------
-54.2. ~~~~~~~~~~_~!_~~~~:~!~~~_~!~~~~~~~
The relationship between YHS mean abundance of I-group
herring and the corresponding estimate from Virtual Population
Analysis is according to the latest analysis (ICES 1978) given
by the equation.
y
= 0.0031
x - 0.21
where y is the VPA stock estimate of I-ringers in billions,
and x is the mean catch of I-group herring per hour for the
standard area of 57 rectangles.
Using this formula, the size of year-class 1976 in February
1978 is estimated at 1.57 billion.
5. Acknowledgements.
Special thanks are due to Dr. J. Hislop and his colleagues of
the Marine Laboratory in Aberdeen, for analysing the raw data
from the survey on their computer.
6. References.
ICES 1978.
Report of the Working Group on North Sea Young
Herring Surveys, 1977. ICES Coop. Res. Rep. No. 81.
ICES 1977.
Report of the Working Group on North Sea Herring
Larval Surveys. ICES Coop. Res. Rep. No. 68 •
•
/ds
I
TABLE I - YHS abundance indices in No/hour for the standard
area of 57 squares, and VPA estimates of yearclass strength.
Survey year
1970
1971
1972
1973
1974
1975
1976
1977
1978
•
'
year-class
No/hour
1968
1969
1970
1971
1972
1973
1974
1975
1976
822
2647
1629
827
1195
1529
452
342
575
VPA-estimate
as I-group
5.79 x 10 9
8.82
1.75
4.39
0.69
,
E5
.
E7
E6
E8
E9
FO
F2
F1
F3
F4
F5
F6
F7
F8
F9
GO
G2
G1
52
51
50
oP; 17
600
0
0
4
~~ /l
0
49
~~~~
1,7J
2
1
1
2
H---t----t----+---'OtJlr'+-0--t-0-+-0-1--0-t--2-t------to~dJ.:.~~ftv
48
I--_-+-__-+-_--11--__+-__+__---+__1_+-_1_+-_--+-ißJ;~r
H
2
2
0
1
0
0
0
2
.2
2
2
2
3
0
12
1
0
2
2
2
2
1 ->+-_2--'1--__ + - __+-__---+__2_1--_
2
~
,...J,>~~
1
1
0
2
1
ruJ
ß~~
47
lJi-re<:j-;,p~
1 :" ':!l1J""b_-+-_---t ._ _-+-_ _f- __-+-_ _+-_---+_ _--+--_~.qxiF">
59°H----I---t...
o ~r
. . ~"'p',r,y....."......--i
~
/
o
58
1
46
_+-_--+_ _-+-_-"'\...
58
1
0
3
308
0
1
1
1
2
1
1
I
0
28
2
2
VL
I
~u.
45
r
1549
44
43
o
57
42
40
39
38
37
36
35
34
33
Figure 1.
Year c1ass 1976 autumn spawners
( l-group)
32
Mean number per hour and number of
hau1s
31
Standard area indicated by broken
1ine
30
IYHS February 1978
29
Catches in Kattegat probab1y contain
a high proportion of O-group springspawners
28
27
4
o
o
2
o
2
4
o
o
10
o
12
E5
E6
E7
E8
E9
FO
F2
F1
F3
F4
F5
F6
F7
F8
F9
GO
G1
G2
52
51
50
600H _ _
-+-_-+-_-jI_~_--{tJ/"H:(7_--If---_f-1_8'_00-l---+--21-25-+-----l<rI~lJ',Ir~rJV
H---t----f----+--+---+----+---\--{------+-----+-t
,....(~d~
59°H---+-_~
I ;
<1
20.25
~~8.50
,
o
'~&r
",-~-~
47
15.75
18.25
18.52
17.10
14.25
(~
46
U,.,
11.42
58
20.25
48
~t'"
b
,,", _ _
ry-
49
.
15.75
11.55 .....11.33
16.25
16.21
19.63
16.77
19.00
i
45
16.29
J
19.70
44
43
57
42
56
o
55
~
~rr
15.73
18.04
38
17.50
16.45
~
Figure 2.
Year c1ass 1976 autumn spawners
( l-group)
32
31
Mean length per square
IYHS February 1978
30
29
28
27
o
2
.
0
2
o
10
o
12
r - - - - - - - - - - - - - - - - - - - --
E5
E7
E6
E8
E9
FO
F1
F2
F3
F4
F5
F6
F7
F8
F9
GO
G1·
G2
52
51
50
49
48
47
46
o
85
45
2496
vl
o
151
~
21.30
44
~
43
42
38
37
5
36
35
34
33
Figure 3.
o
·51
Year class 1975 autumn spawners
( II-group)
32
Mean number per hour and mean 1ength
31
IYHS February 1978
30
29
50 H------1f----+--+---+----f---+-c/
1')-1
H----If----+--,.--H
a Q \
o
2
Ir'
28
\t"---..);:::::"
o
o
27
o
10
o
12
I
I
E5
E6
E7
E8
E9
FO
F1
F2
F3
F4
F5
F6
F7
F8
F9
GO
G1
G2
52
1-+---+--+---1-- -- _..__.. -+---+--- - - -
~
~
L __-+-_-i-_~--\__-~~i~~~rr:-;;
6t H__-+-_--+__-+-_-+
/{i
4
0
0
0
2
3
0
0
16
7
1
~P'"-.s
6
.i 1
H---+---t---+--<rl1~~1.,4
f'?
_ _ -+--_-+-_-+-c..:...'----1_ _ I
-"V~b
1
60
~
~.
4
1
H _ _-+-_-+-_-+-_- + __
5901-+--+-1-.
~~~
~
51
..
+-_~
I
2
50
't.~
I!,
49
~~
~
_~ 3-+_---+-f~~ tJ,
7
8
V<t ~~
~~
)
14
8
__ 1 - -_ -+-_
6
I1
7
l:J
0
6
~
_
\Ji'1!
1
1
6
7
~
~"-n'I'Jr'\Y""""-/~I,I----+--9-+-0 -+---3---+--9---j~- ---2----+--1-!------O>,I...~
48
2
01"
1
Nt
47
\
46
.~,>
+
N-lT
45
./--+---1---1---+--+--+--1 .-. __+-_+-_-f---=~U(F::>ll.-..r:.'--+-_-+-_-+-_-l-~--'t
({;JjJ)
58
~"-~~-""""11---+-_-+-_-+_-+_10
/
T
8
jH---+---+--
57°
-+_+--I~
_-1-_+-_+-_+-_+-_1
1
2
0
~
)2
4-+--0-+--0-+-0-+-1-+-I~l"--
- l - -- l - --+--·
2
4
4
J7
11
15
8
3
0
2
1
1
1
+
,
,
,
,
2
A
2
0
~
(du!
-_:-_-8~_+:~0~::~_1-_·:-_-0~::~_1-_:-_-_2-_:-_-_4-_:-_~6-_-+f-_-_-l~:I....:r-..(J-l,~'"
7J:
1
~i1
12
31
+
2
0
0
o. _.-"I,,-<-_+--_+--_+--_+-_+-~=,-----,.c..:...t7-l,.J
\
P"''-~Il..-.
)
2
0
1
1
--....r
+
44
1\
,~
42
3
~~
41
f
~~~f~
\) ~
-
43
"
lhJ~
2
1
0
0
8
8
0
+
~~\
d1
....,\~--+\-5-+---1-+--0-+--7--1---+---+-1--+--:---+--.a. _
3
6
),., 20
f~
-it-.~ J0' i\
p
\\-.. .
\
~
0
40
39
38
~ ~ 37
~
36
!l
35
/-=:!=--+---+--+-~q.,.Lj
'0
52
,,0
•
0
2~
'0
~,J+----+----+-,I-7
.3
V
~
_
~
5..
_~I
~~
~_~~+--f--<~ [
,.)
v a--'-'
tr :-~~QJ>~'.r&~-v'_"""",,~
/'
34
33
Figure 4.
Year c1ass 1977 autumn spawners
( O-group)
Mean number per IKMT-tow
IYHS February 1978
32
31
30
o~rJ
29
50 H---f---f---f--+--+--+7'"
1')-1
H----+---+-,.,---H
a ~ \
\-___...._;;='
Ir-
28
27
o
10
o
12