Thi paper not
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bc eitcd \\ ithout prior rcfcrcnec to thc
author~.
I ER ATIO
LCOU CILFOR
THE EXPLORATIO OF THE SE
OBSERVATIO
0 HERRI G L RV E C
D RI GTHE I
C.M. 19 'ltH : IY
Pelagie Fish ommillee
GHT BY ISAACS-KIDD MIDWATER TRAWL
G FISH S RVEYS I THE ORTH SEA,
ITEGATI 1977 - '81
by
A. Cortcn and G. van dc Kamp
Ncthcrlands Institute for Fishcry Invcstigations
PO Box 68.1970 B IJmuiden
Thc cthcrlands.
I
I
t
I
I
r
I
~
This paper
not to be cited without prior reference to the author.
C.M. 1981/H : 19.
Pelagic Fish Committee.
International Council for
the Exploration of the Sea
OBSERVATIONS ON HERRING LARVAE CAUGHT BY
ISAACS-KIDD MIDWATER TRAWL DURING THE
INTERNATIONAL YOliNG FISH SURVEYS IN THE
NORTH SEA, SKAGERRAK AND KATTEGAT IN
1977 - 81.
by
A. Corten
and
G. Van der Kamp
Netherlands Institute for Fishery Investigations,
Har ingkade 1,
1976 CP IJMUIDEN
The Netherlands.
Abstract
•
Sampling of pre-metamorphosis herring larvae by IKMT has shown great
variations in abundance and geographical distribution from year to year.
However, 4 out of 5 year-classes sampled by IKMT have now been assessed
one year later by bottom trawl, and they all appear to be of equally
poor strength. It is supposed that it is of vital importance for the
larvae to reach the lnshore waters of the central and southern North Sea
before the spring plankton bloom. According to this hypothesis, prospects
for the last year-class sampled (1980) should be better than for any of
the preceeding ones.
Introduction
In recent years, sampling of pre-metamorphosis herring larvae has
become a routine part of the International Young Fish Surveys (IYFS) in the
North Sea~ Skagerrak and Kattegat. These surveys, which are conducted
each year in February, are primarily aimed at sampling juvenile herring
and gadoids by means of a standard bottom trawl. In addition to the bot tom
trawling during daytime, most participants use an Isaacs-Kidd Midwater
Trawl (IKMT) to fish for pre-metamorphosis herring Iarvae at night •
. The objective of the IKMT-sampling is to find out whether the abundance
of pre-metamorphosis herring Iarvae (about 5 months old in February)
can be used as a first estimate of the final strength of the year-class
concerned. In addition, the distribution pattern of the larvae in
February might provide information on drift routes; variations in drift
pattern~ from year to year, and differences in survival rate between
Iarvae from various spawning grounds.
- 2 -
Sampling by IKMT was first undertaken by Swedish workers in the,
Skagerrak, and later extended to adjacent parts of the North Sea
(Ackefors, 1974, 1975). Other participants in the IYFS started IKMTsampling in 1975, but during the first 2 years of this programme, a
variety of gears was used, and the results are hard to compare both .
between countries and between years. A standard version of the IKMT
and its fishing method was proposed in 1976 (Anon 1977) and starting
from 1977 all countries have gradually adopted this standard gear •
I
•
Preliminary results of the IKMT-sampling have been reportedby'
0
the Herring Assessment Working Group for the Area South of 62 N
(e.g. Anon 1981), and in the annual reports of the IYFS (e.g. Corten
and Kuiter 1981). These results consisted mainly of numbers/haul
for the various statistical rectangles. The present paper, in addition
to presenting summarised abundance data, also looks into the length
distribution of the larvae. This in order to see whether differences
exist between areas and/or years; differences which might be related
to the chances of survival for the larvae.
i
,
i
Material and methods
This paper only presents data for the years 1977 -'81. As mentioned
in the introduction, some countries did already some IKMT-sampling
in earlier years, but the sampling methods and coverage were not
consistent enough to use these data for a comparison with the post'1977 period.
The table below shows which countries have participated in the IKMTsampling since 1977.
•
Sweden
England
Norway
Netherlands
Scotland
France
1977
x
x
x
x
x
1978
1979
1980
1981
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
During all these cruises, the standard "Swedish" IKMT was used, except
for the English cruise in 1977, on which a "Boothbay net" of 1.83 x 2.74 m
mouth opening was used.
The standard IK}IT is described in full detail in Anon 1976. The mouth
opening of the net is 2.5 x 4.0 m, and the total length 13.3 m. Mesh
size decreases from 25 mm bar in the front panel to 16 mm and 11 mm
bar in the middle section, and 5 mm bar in the last section.
The cod end has a lining of nylon plankton gauze with rectangular mesh
openings of 1.15 mm.
The net was fished in double oblique tows from the surface to 5 metres
off the bottom and back. Maximum fishing depth was 100 m in the North
Sea and 50 m in the Skagerrak. Towing speed was 3 knots. According to
the standard instructions, the fishing warp was paid out at a rate of
25 m per minute, and retrieved at 12~ m per minute. Hauls were made
only during the hours of darkness; this in order to minimize net avoidance
by the larvae.
- 3 -
The duration of the hauls depended on the depth of the water. Figure 1
shows the relationship between the two parameters as measured during the
Dutch survey in 1981. It is seen that the relationship remains approximately linear over the whole range of water depths.
Making the tows according to this standard procedure ensures that all
catches refer to the same surfaee area, irrespective of water depth.
The exact size of thissurface area is unknown, but its upper limit may
be calculated as folIows:
The relationship between water depth and tow duration 1n figure 1 can
be approximated by the formula
y=2.78:x:.
in which y = bottom depth 1n meters
~= tow duration 1n minutes
2
Taking into aeeount a net opening of 10 m and a towing speed ~f 3 knots
(92.6 m/min), the volume of water filtered per minute is 926 m •
,
For a given depth y', the duration of the tow is ~ minutes, and the
volume filtered is
1
2.78
3
• 926 m
= y'.333
3
m
The quantity of larvae taken in that haul eorrespond to a surfaee area
of
y'
•
.333 = 333 m2
y'
2
As mentioned above, the figure of 333 m is the upper limit for the
standard surface area that all tows refer to. This figure is based
on the assumption that all larvae present in the volume filtered, are
retained in the net. In praetise, there may be avoidanee of larvae in
front of the net, and escapement through the large meshes in the first
parts of the net. The aetual number of larvae eaught in a standard tow
will therefore refer to a surfaee area of less than 333 m2 , but still
this (unknown) surfaee area will be the same for all hauls.
In this paper, all eatches have therefore been expressed simply in
numbers per haul, assuming t~at all participants have followed the
standard fishing procedure, and also assuming that the relationship
between bottom depth and tow Quration has been the same for all vessels.
Results
Mean numbers per haul and mean lengths per statistical reet angle are
shown in figures 2a - e for eaeh survey in the period 1977 -'81.
In order to present length frequeney distributions, a subdivision
of the North Sea into somewhat larger areas had to be devised, as it
would be unpraetieal to present LFD-s on a rectangle basis. The area
subdivision used for this purpose is shown in figure 3, and the corresponding LFD's are given in table 1 and figure 4 for each ~ear separately.
Table 1 also gives the mean number o~ larvae per haul ~or each area.
It is seen that during the first 2 years of observations (1977 -'78),
low densities of larvae were found in all parts of the North Sea.
Larvae were partieularly scarce in the eastern part of the North Sea,
which was rather surprising as it is known that the Skagerrak and German
Bight are the main nursery areas for the North Sea herring.
.....- - - - - - - - - - - - - - - -
~
I
-
- 4-
In 1979 high densities of larvae were found along the Seottish east
eoast, and to a lesser extent in the offshore waters of the northern
North Sea. It was assumed that these larvae originated from spawning
grounds to the west of Seotland. Larvae were still virtually absent
from the eoastal waters of the eentral and southern Nort Sea.
This pieture ehanged drastieally in 1980, when high numbers of larvae
were found all over the North Sea. For the first time sinee the start
of the programme, substantieal numbers of larvae were found in the
Skagerrak, eentral North Sea, and inshore waters along the English,
Danish, German and Duteh eoasts. SampIes taken in the German Bight
contained a mueh.higher proportion of relatively big larvae (> 35 mm)
than sampIes taken in other areas.
•
In 1981 this trend eontinued, and the distribution of the larvae
shifted even fUrther to the south and east. Very high densities were
found in the Skagerrak and along the Danish, German and Dutch coasts .
In addition, the larvae found in these waters were bigger than they
had been on any previous occasion; both along the Danish and German
coast the mean length of the larvae was weIl over 35 mm.
Relationship between abundance of pre-metamorphosis larvae and
subseguent year-class strength
Of the 5 year-classes sampled by IKMT during their pre-metamorphosis
stage, 4 have been assessed one year later by bottom trawl. The
abundance estimates found at this age (1~ year) are shown below:
•
year-class
abundanee as 1~ year-olds
in NO/hour for standard area
1976
1977
1978
1979
575
139
535
551
For background information, it should be mentioned that the average
abundance index for year-classes of normal strength in the North Sea
(such as 1968 -'73) is about 1500 herring per hour.
The year-classes listed above have therefore all been relatively weak,
and of approximately equal strength. Year-class 1977, which in the
above table is lower than the other ones, has probably been underestimated during the trawl survey~ and should be more in line with the
others (Anon 1981).
So all we can conclude for the moment·is that the large increase in
larval abundance in 1980 has not resulted in a year-class of appreciably greater strength than the preceeding 3 ones. This observation
might cast same doubt on the usefulness of the IKMT-programme. If large
variations in larval abundance are not reflected in the size cf the
ccrresponding year-classes, recruitement strength might be determined
after the time cf the IKMT-sampling, and the sampling cf pre-metamcrphcsis herring larvae may be cf little use in predicting year-class
strength. However, it could very weIl be that the abundance af herring
larvae, required for a really strong year-class, is still much higher
than the level reached in 1980. This quest ion can only be answered
after such astrang year-class (as canfirmed by battam trawling at
age 1!) has passed through the IKMT-stage.
- 5 -
Apart from the abundance of larvae in absolute terms, the geographical
distribution might also be of importance. lt. is known that herring need
coastal waters to pass through metamorphosis and to grow up during
their first summer. The main nursery areas for North Sea herring are
the coastal waters of the German Bight, the Skagerrak, and the inshore
waters along the English east coast. Probably the larvae have to reach
these waters within a given period of time, or else they will starve
in the open sea or get eaten by predators. In the years 1977-'79,
very few larvae were found in the coastal waters of the central and
southern North Sea, and there could be very little hope, therefore,
for a succesful recruitement of the corresponding year-classes.
In 1980 the picture was considerably more favourable, but a comparison
with 1981 shows that the larvae were still distributed relatively
offshore and were of a relatively small size. It could be assumed that,
although the larvae had travelled a lang way in the right direction,
they still didnot make it in time to catch up with the spring plankton
bloom in the inshore waters. Another possibility of course is that the
numbers of larvae were decimeted in the early spring by same unusually
abundant predator.
The picture for 1981 (year-class 1980 which had not yet been sampled
at age 1~) is still more favourable than in the preceeding year.
The larvae had penetrated even furt her into the Skagerrak und the inshore
waters of the German Bight
and their length, especially in the coastal
zone is relatively very great. If the IKMT-results are in any way
related to subsequent recruitment, year-class 1980 should become
stronger than any of the other year-classes sampled so far.
Acknowledgements
•
This paper is based on the results of a joint international survey
and the present authors have contributed only a small proportion of
all the basic data. Credit for this work (if any) should therefore
equally go to all participants in the IYFS that have contributed to
the IKMT-programme •
References
•
•
Ackefors, H. 1914
On the occurrence of herring larvae in
March - April, 1912 -'74, in the Skagerrak.
lCES C.M. 1974/H : 34.
Ackefors, H. 1915
On theoccurrence of herring larvae in
February 1975 in the Kattegat, Skagerrak,
and North Sea in conjunction with the
North Sea Young.Fish Survey.
lCES C.M. 1975/H : 31
Anon. 1911
Report of the Working Group on North Sea
Herring Larval Surveys.
lCES Coop. Res. Rep. 68.
Anon. 1981
Report of the Herring Assessment Working
Group for the area south of 62 oN.
lCES C.M. 1981/H :8
Corten, A. and C.J. Kuiter,
1981.
Report on the lCES Young Fish Survey in
the North Sea, Skagerrak and Kattegat in
1981 : Herring data.
lCES C.M. 1981/H :17
»
-
s a
02
es
Table 1Length-distribution herring larvae by sub-area.
ll,ean
]=cer n=ber
cf
of larvae
per haul
YEAR hauls
AREA
Sccttish East Coast 1979
1980
1981
13
32
20
76.23
33.38
20.60
English East Coast
1978
1979
1980
1981
12
15
12
22
5.83
2.00
25.50
14.01
1977
1978
1979
1980
1981
11
19
9
12
15
1.18
2.05
0.33
12·75
53.42
1977
1978
1980
1981
11
16
10
12
0.45
0.81
13.10
39.06
1978
1979
1980
1981
21
9
15
17
0.29
0.44
25.07
38.60
1977
1978
1980
1981
42
51
42
55
0.24
0.84
17.64
39.25
1977
1978
1979
1980
1981
57
75
74
84
102
1. 14
2.99
0.64
42.25
25.46
1977
1978
1979
1980
1981
27
68
74
111
43
3.74
4.07
11.97
20.87
4.75
Dutch Ccast
-
German Coast
Danish Coast
Skagerrak
Central North Sea
Northern North Sea
Ler.gth distribution in mean n=bers per haul x 10
~20
21
22
3
2
2
3
3
7
1
'.
2
23 24
12
10
4
1
3
2
2
5
1 11
1
1
6
6
3
35
36
38
39
40
41
63 72
22 21
20 22
66 68
19 10
13
9
69
10
5
41 34 19
6
2 2
2
1 2
5
1
1
1
2
2
1
7
2
1
17 24
6 9
1
17
1
10
1
12
2
8
1
7
5
1
1
2
1
3
3
1
4
2
1
3
1
4
1
2
1
2
11
28
13
57
8
55
6
59
10
57
6
54
2 2
1 1
2
3
47 38
1
6
5
28 41
2
10
40
1
3
6 13
43 28
26
27
28
29
30
31
15 25
18 29
7 8
43
35
13
52
40
22
56
36
24
50
28
23
63
34
25
3 3 9
1 1 2
14 25 25
25 12 19
6
3
27
20
7
3
19
4
3
3
19
10
2
2
1
1
2
1
11 18
19 39
1
17
7
1
1
1
'1
1
7
3
2
5
11
6
11
14
1
2
1
1
34
25
1
3
3
2
6
7
4
32
33
1
1
1
1
4 6
4
3
12 22 31 31
1
1
1
1
2
1
2
1
6
1
5 13
1 3
1
1
2
6
2
3
3
2
3
1
5
1
3
2
7
4
10
7
1
13
10
1
1
1
2
1
1
13 29 39
4 11
3
2
3
8
4
2
6
2 5
13 24
4 7
1
5
1
1
3
42
43
1
1
44
45
1
1
1
11
3
15
5
13
14
22
1
1
11 10
17 13
5
1
3
2
1
1
7
18
6
12
10
8
4
3
9
25
7 7
21 21
12
3
8
1
4
11
4
15 26 31
8 14
7
33
22
1
1 2
14 13
42 36
16
22
9
39
9
35
5
13
1
5
18
9
40
8
23
2
1
3
13
4
1
6
1
5
1
3
1
1
4
9
1
24
27
18
49
2
4
4
44
11
1 3
1
47 48
14 14
39
11
37
17
27
14
2
3
3
1
25 17
28 25
5
9
26
5
1 3
3
4
5 6
13 15 14
24 29 24
6
4
4
3
3
11
19
3
5
3
13
13
2
6
2
12
8
2
2
3
30
10
38
1 1
1
8 7
7
28 22 26
18
28
14
19
17
6
1
1
7
4
>45
1
1
1
2
6
2
37
3
1
10
4
1
2
10
1
1
1
2 2
1 1
12
8
19 23
7
14
1
4
14
3
2
1
5
2
1
1
1
7
1
5
2
1
2
14
1
1
1
10
1
1
7
1
5
2
2
2
1
1
WA'TER
DEP'TH
Iml
12
•
11
10
7
2
5
10
15
20
25
30
3S
40
45
DURATION lOW
Imin)
•
Figure 1 •
. Relationship between water depth and tow duration, as measured
during the Dutch survey in 1981. Regression li ne drawn by eye.
E5
E6
E7
E8
E9
FO
F2
F1
F3
F4
. F5
F6
F8
F7
GO
F9
G1
G2
52
H - - - f - - - j c - - - j - - - j - - - -+----+----f-- -
51
,fJf~
50
--+----+---1I----+---I---'l~;:.
1- Q
t7
49
o
60 H----+---i---t--t:j-
48
-1 2-
1- ~
35.0
§.
34.1
1-
1
47
33.0
2- 22
46
30.8
-
1- J§.
8- Q
2- 44
3- Q.
58
45
f
o
~1-.!§.
I
I
1- 44
2- 46
~
r 1-
1-
57
1-
22
.l
2-
Q
44
34.0
33.5
1- 4
)
o
6
43
29.0
J
1- .l2.
1-
o~
1-
§.
1-
6-
4
.L
1-
2- Q
0
1.
42
41
56
55°
o
7
NY1
\t1
38
4
1
--I-4_1~_-Q-l--+--~~
\.,..\+-_-+-_--+_1_-Q-+-1_-Q-+----I----+-;-1-.8-
54 H----+---B'
~)
olO~
531cn
- 1--
2
,
32.0
/j
_~';3.~ ~
-~~
::
Fig. 2a
'r
~
I~IT-sampling
37
34
February 1977
33
Herring larvae year class 1976
first figure top row: number of hauls
second (underlined) figure top row:
mean number of larvae per haul
32
31
bottom figure: mean length in mm
30
r-
i)'--)
a
29
V
28
~ \'---...-v-=
Q
27
,
4
0
0
2
0
0
0
2
4
0
6
0
0
8
0
10
0
12
E5
E6
E7
E8
FO
E9
F2
F1
F3
F5
F4
F6
F8
F7
F9
GO
G1
G2
52
2-
<>~
4 2- 0
26.1
-
51
.f"'i
-
6t H---+---+---+---+----l----+--+---+--+--Q!""'rf
4- §.
1- Q
1-.?
27. 4
-
31. 0
d
1-
30 . 3
~~
2- Q
-
:
50
;\)
49
48
47
46
1-
1
8-
o
Q 2-
26.0
37.0
.?
45
35.0
58
44
43
o
57
38
37
o
36
35
v
~
2- Q
'\
52
~Y'--J
V
0
51
~
~
lJ .
~
~@;
~~~
Fig. 2b
llOlT-sampling February 1978
34
Herring larvae year class 1977
33
first figure top row: number of hauls
32
second (underlined) figure top row:
mean number of larvae per haul
31
bot tom figure: mean length in mm
,-/
y~~ll.7'~_---...~
~
31.7
~~~
~-(
'-"I
33.4
7
~
-
3- lQ. 2-
-+---+---------hl-
%
I~
fr"
-
/'
30
1---+---+----+--+---+~
o~rJ
r--+ )
50 f-t-----+---+----+I')--'I---+---+I-
H--+-----+----;a~c::-H\
4
o
29
V- /
28
\'----- ~
oo
27
o
2
4
o
o
8
o
10
o
12
E5
E6
E7
E8
E9
Fa
F2
F1
F3
F5
F4
F6
F7
F8
F9
G1
GO
G2
52
~---+--1---j----+---j--+---+---­
1- Q
2- 0
2-
<>~
Q
51
6t I--I---t---t---t---t---t----+----+----+---+-~.t.'k~~.~~:::;;::;
~L...""-.[
2-
~
2- Q
1
2-
~~
1
~~
50
.rl1
28.0
29.0
o H---+--1--_+_~~~;
1~14t7--+---+----j-2---_0---+-2--_-o-f---'l,~~~r ~
60
2-
2.
tj
1
2-
1-
1
2-
49
"~
1
~~1-
48
rJ<f ~r
47
28.0
28.2
31.0
29.0
H----+---+-.---]2---+-2---2-3-+-2---1+-- _11f.--+-2---2.-\-2---1+---+~~h~
n
o
59 H - - + - - - '
~
2
2S.6
28.8
31.3
30.2
:'1",'c..-
1-.1~.tj
1- 88
3- 46
n
2- 33
30.3"
29.9
29.9
29.1
30.2
111
4- 64
2- 22
2-.11
2-.11
2- 0
32.7
29 . 7
27. 1
29 . 2
31. 7
-
2-
~
26.6
--+---1
/
'2-102
~ 3~
2-.!±.I
3- ]2-
31.4
28.9
32.9
E.
1- 4
26.0
28.3
1-
J
o
57
2-
1-lQL
1- 80
34.5
29.3
1-
2-
2-
4
46
'I "'"
3_4._.7-+-_ _>--_,,\._
1
4- 1
~
2.
2-
1
"
~"-
2.
1-
33.7
4
1-.11
34.3
34.0
45
44
30.729.9
43
Q
2-
42
1
30.0
""\ 1- JQ
40
27.6
\
~
1-
~
2-
25.5
2-
.1
35.0
2- Q
~
28.7
2-_
\
2-
53
)
Icn
1-
32.8
2-
4
39
35.3
1
38
2-
Q
1 2- .1 4- 0
~32.5
olv~
4
32.5
Q
\
1-
37.5
2-
1
2- 0
27.5
Fig. 2c
'r
I~IT-sampling
oV
34
February 1979
33
Herring larvae year class 1978
52
first figure top row: number of hauls
~
Second (underlined) figure top row:
rnean number of larvae per haul
32
31
bat tarn figure: rnean length in mm
30
29
28
27
o
.4
o
2
o
o
o
2
.4
u
o
10
o
12
E5
E6
E7
E8
E9
FO
F2
F1
F3
F4
F6
F5
F7
F8
F9
GO
G1
G2
52
51
50
'3- 12 2- ~
25.8
30.5
3- 4
28.5
2- 22. 2- .Q.
27.3
-
_.>."
2- .l§. 2- 22
29.2
27.8
2- 10
27.1
2- 32
27.1
2- .!2. 2- 50
29.1
25.3
49
2- 35
27.1
2-
48
1
47
26.8
46
1- 24
o
58
a
45
33·7
1- 22
44
31.9
43
o
57
2- E..2. 2- 20
26.3
25.8
42
2- .!ll 3- 42
28.0
25.8
2- 11 2- l±1.
27.8
27.3
4- ~ 2- ~ 1- 0
24.8
33.7
3- 1
1-~
29.6
25.7
-
1- .1. 2- 1
41.0
27.3
37
i-..!§. 1- ..!§.
36
~P,7
~,,;p,
2- .2- 2- 14
33.6
32.0
35
1- ..!Q
36-2
34
1- .231.1
33
first figure top row: number of hauls
second (underlined) figure top row:
mean number of larvae per haul
32
31
bottom figure: mean length in rum
30
29
28
27
o
2
4
o
6
o
o
8
o
10
o
12
E5
E6
E7
E8
FO
E9
F2
F1
F3
F4
F6
F5
F7
F8
F9
G1
GO
G2
52
H---+---+----j ---t------- J----+-- - - -
°
2-
51
o
61
50
~: tJ
(
~
Q
H_-+--'--+----+-_I-+~J_ _
j-----__
59
~
0
58
a
,
49
~_+_4---Q-+--1-o---+----+-iflt-
1- Q
r;;r.J
~
-v~
2- ° .rt!
~~.~ ~_u..
~
yg, p
o
1-_0
2-_
1- °
2- J.Q.
1- Q
?
NI ß-;l'1;..
lf<t-~~
-
liV~
~/""
~r ~
2-
i----------+-~
48
~
47
7
46
29.6 ':.1-+----25--+.1_ -+-_---+-----+-_31. 1
2- .2. 4- 4
2- .1 2- Q
~
1- 12. 9-.TI 2- ~
26.3
27.5
26.4
_
30.9
31.4
38.7
/
.L----+---i-----J---+-----l----l----i---+--j----l--~~u""""'..J.-===l_--+_-_+--+--~I
2-.l2. 2- .2.
1- 40
2- 1 2- Q
2- Q
3- .2. 6- E 6-.lQ
.lQI
29.7
28.1
28.9
1-692-.!§.
,/
)
o
24.6
26.0
-
2-.? 2- 1
27.8
27.3
57
2-.!Q. 2- 30
2-
11
2- 28
1-
II
2-
1
4- 23
4-
2
4-
~
25.8
2-
11
2-
1
2-
1
+L~ 2!:i
~
27.6
24.0
4-.l2. 2- 23
2- 1
28.4
2-
4
26.2
31.0
29.6
4- 22
4- 24
3- 74
5- 2
'\j
2A'-"'I-"h-+---,-0{,,,-,-,:1_j-~?<'-"-j
"-----"..?".Lo-Lf
<_~-+-_-4~2g,6L.-!L6 +-<~
<• .L..j"_<~4'7-'44-:-<:l.:LLQ.74-fl-~(JC--\
\
2-
6
~6.4
\
2- .? 2- .? 2- .§. 2- .2. 2-.ll
30.0
28.5
31.3
33.2
35.2
3- 85
35.3
5-.!:1. 6- 79
33.7
36.3
2-.1.2. 2- .? 225.8
26.0
\ 2- 20
~25~
)
°
2
2- 1
28.7
2- 25
27.8
2- 24
25.9
29.5
2-
3- 23
33.5
2- 63
34.0
2- 34
37.3
4- 24
33.2
5- l2.
3- 60
3-lQ..!.
26
34.6
33.9
-fq."~,
C
5- 22.
36.2
34.3
34.2
::':"'~l~·-~
4- .§. 3- 2.l"'- ~ '-u
2-
-'1.
, ......
\'2...
1
3-
-
ßS
\1.1"
(f\
30.4
~~
43
32.8
-
41
-
J~~
40
0
~~
I j,
~ ~-{) ~
~
~
~
42
30.5
1- Q
/;1,.,/2- Q
30.0
30.1
25.7
27.4
28.2
29.9
33.9
36.1 ~
""'----.,=---1-4-_-2-3+5---13--+-1---2-+-2--4-+----+---1-2---2-2-+-2--34-+-3---11-9-+-2---2-3+t\---\V
44
31._6-+--_3--,4._0,
3-125 5-!:..2.
7~r t
3.2
27.9
30.1
/
28r-:~~31.5
27.5
3,-0_.1-+_2.:..9..:...64_2.:..:7_.:....9+-2_8_..::..3+_26_._8-+-_2_4._4--l-_2_6•....;:3-+_2_6....:.5-+-....:3:..-0_.1-+....:3:....1..:...24 ....:3:....2_._0
20 2- §.l 2-.2Q 2- .2.
3-.ll 4- 11 4- 6 2- 1 2- 4
-""2-
27.1
5- 2
45
~L-I
r
h(-
'"
39
38
37
36
35
34
IK}IT-sampling February 1981
larvae year class 1980
33
first figure top row: nurr.ber of hauls
32
H~rring
second (underlined) figure top row:
mean number of larvae per haul
31
bottom figure: mean length in mm
30
29
28
27
o
8
o
10
o
12
E5
1:6
E7
E8
E9
FO
F1
F2
F3
F4
F5
F6
F7
F8
F9
GO
G1
G2
52
51
50
60·1-l--..
~>IU
4
--t--t-:{((f,l+--.--+---+----!--+--+----l~~~-,;jJ;OJ
~
49
P'" 1-
H--lf--f--...,--t---t-----t--+--+--t---H\t,'V ~
e
~~~
59°H--'I-4..~r
~y
58·
-1
~~~
48
-
47
~
Northern North Sea
46
.r-s-;\}-_l-.--I~_-+__-+-_-+-_-l-_+-_+-~
Sc 0 t t i
s~ c o~s t
"__
+---I~-+_-+-_-+-_-l-_-I-~_=1ll0bo.L-~r=J- Skagerrak
45
44
43
o
57
38
35
34
33
Figure 3.
Area subdivision for
32
length distributions
31
herring larvae
30
29
1')-'")
a
\~~~
t:::
0
"
Ir-
0
2
0
0
28
_.
2
27
0
"
0
6
0
8
0
10
.0
12
...---------------
~-----
~.
................
J-
1977
northern NS
'
centra1 NS
Skagerrak
German coast
Dutch coast
r=
northern NS
I ~-=====:::::::~=======;===========--t------t--~~~~
1978
centra1 NS
English coast
-
Skagerrak
Danish coast
German coast
.............:.:.....
Dutch coast
2
1979
~
northern NS
::::::':'"
7
6
5
er.
3
2
Scottish coast
centra1 NB
Eng1ish coast
-
Danish coast
Dutch coast
20
25
30
Figure 4a.
Leneth distributions herring 1arvae in 1977 - 79.
Larvae > 35mm are shaded to i11ustrate proportion
of big 1arvae.
35
40
45mm
1980
northern NS
Scottish coast
5
central NS
3
2
i~g;m: ..::::m::::::::::m:::::::::::::::::::::::::::....
Enb1ish coast
3
2
!i~mHHm~ii~l~mm::::::..........
..
Skar;errak
3
Danish coast
2
German coast
:'j~ll!~j~jilll!IIIIIIIIIIIII!III!I!llIIllllllllijiij~jjmlm~mm~m1jljjjjjj1jj!lHl:
2
Dutch coast
20
30
Figure 4b.
Length distributions herring 1arvae in 1930
45mm
1981
northerr~
~~~;
Scottish coast
3
2
•
Enp;lish coast
5
4
3
2
4
3
2
•
4
3
2
6
5
4
3
2
Dutch coast
20
Figure 4c.
Length distributions herring 1arvae in 1981
40
45mm