CM_1971_H04.pdf (645.1Kb)

This ;pa£er not to be cited ,v:i.thout Drior reference to the COUl1Cil ) IN'I'ERNATIONAL COUNCIL FOR

'l'HE EXPLORATION OF THE SEA.

C.:rvr.197l/H:4

Pelagic Fish (Northern) COITmdttee

REPORl' OF THE WORKING GROUP ON NORTH SEA YOUNG HERRING 3URVEYS.

================================

I. Introduction and terms of reference

At the 1970 Council meeting it was recommended (C.Res.1970/2:10) that "'I'he Working Group on North Sea Young Herring Surveys" should meet from 19 - 21 April 1971.

The meeting was held at Ymuiden from 19 - 21 April 1971 with the following participants:

Mr. A.F. Lisenko - Uo$.S.R.

Mr. OoJ. ,0stvedt - Norway

Mro K. Popp. Madsen - Danmark Mr. K.H. Postuma (Chairman) - Netherlands

Hr. A. Robles Pineda Netherlands

Mr. A. Saville - Scotland

Mr. D. Schnack - Germany

Dr. K. Schubert - Germany

Dr. A. Schumacher - Germany

Mr. J .R. Wood England

Mr. J.J. Zijlstra - Netherlands

Part of the results of the 1965-1970 surveys were reported on a yearly basis to the Pelagic Fish Northern Committee and were pu- blished from 1968 onwards in Annales Biologiques.

During its first session the Group decided that the meeting should have the following objectives:

1. ~o study the relation between the data on the numbers of herring caught per one hour's trawling during the 1965-1970 surveYSt as an index of abundance of young herring in the North Sea, and the subsequent recruitment to the autumn spawning populations in the North !:lea.

2. £0 investigate changes that may have occurred in the distribu- tion of young herring in the North Sea in the different survey- years and their relation to changes in the composition of the young herring stock.

3. '1'0 study the results of the accoustic surveys made during the 1971 young herring survey as alternative method of estimating the abundance of young herring in the North Sea.

It was agreed that, on the basis of the results of these investi- gations, the group would deciderihether the young herring surveys should be continued. If the surveys were to be continued recommen- dation should be made concerning changes in the timing, in the me- thods applied, and the procedures to be followed.

11. Results

Section I - Abundance estimates of Young Herring compared with Adult Herring recruitment in the North Sea.

Abundance estimates of the year-classes encountered during the sur- veys were obtained by averaging all hauls made in each statistical x) General Secretary,

ICES, Charlottenlund Slot, 2920 Charlottenlund, Denmark.

2 -

square and than averaging the means over all the squares fished.

In this way estimates of year-class strength were obtained for the yearclasses 1958-1959, (1962) and 1963-1969 at an age of the fish of 1,5 year approximately. 'fhe 1962 estimate was based on very few hauls. Estimates for the 1957 and 1960 year-classes were derived from a comparison with the 1958 and 1959 fish at different ages, as explained in Coop. Res. Reprt. Series 14. Annual estimates of re- cruit strength were available for the Northern, Central and Southern North Sea spawning grounds up to the 1965 year-class (North Sea Herring Assessment Group Report)o

New information on the 1966 and 1967 year-classes was added (see table 1). For all three stocks it has become progressively more difficult in recent years, to get reliable estimates of year-class strength of recruits. This is partly because of a severe decline in the fisheries from which the estimates are obtained and partly be- cause of suspected or known efficiency changes in these fisheries.

As a first approach the Young Herring ~urveys estimates were compared with indices of recruitment to the adult stocks in the Northern, Central and Southern North ~ea. These are shown in fig. 1 a-b-c.

with the exception of the Central North Sea, no relationship was ap- parent. A relationship, however, could hardly be expected, as the estimates from the surveys are indicative of the total year-class strength in the North Sea autumn spawning stocks rather than to any of these stocks in particular. In these circumstances it is necessa- ry either to intergrate the recruitment estimates to the adult stocks or to distinguish between the immatures of the three individual

stocks sampled during these surveys.

To achieve an estimate of total recruitment to the North Sea autumn spawning-stocks it is necessary to have estimates of relative stock strength. These were found in the estimates of annual larval produc- tion from each of the three stocks, published up to 1969 in the Re- port of the North Sea Herring Assessment Group to which additional data for the year 1970 was added. It proved nece:3sary to estimate total larval production in one year, both in the Central and Northern North Sea and to make some assumptions regarding the relationship be- tween production in part of the area and total production throughout the entire area for several years in the Central North Sea.

Data used and modifications applied are given in table 2.

From these estimates of total larval production in each year esti- mates of the proportion produced by 3-year old spawners were derived

from the percenta~e age-composition of the spawning stocks. An addi- tional correction had to be made for the different fecundity of re- cruits and older fish by taking the fecundity of recruits as half that of all older fish. Estimates of recruit strength derived in this way are given in table 2, together with estimates of total re- cruit strength obtained by the summation of those from the individual stocks.

Figure 2 a shows the relationship between year-class-strength, as astimated by the Young Herring Surveys and total recruitment to the North Sea spawning stocks. With exception of the estimate of the 1962 year-class, which was based on very few observations in the surveys, a reasonable good relationship appears.

In figure 2 b the same recruitment data (spawning stocks) are rela- ted to estimates of immature year-class-strength derived from the Blpden fishery (figure 3 of the North Sea Herring ~tisessment Group Report).

The relationship between these two estimates is certainly less con- vincing. From the Young Herring Surveys estimates of year-class- strength are also ~vailable for II group fish from some year-classes

(2,5 years old). The I and II group indices of a year-class were found to be fairly well related. A comparison of the indices of re- cruit strength of II group fish with estimates of overall recruit- ment strength to the adult stocks is shown in fig. 3 and suggests a relationship_

An attempt was made to delimit the distribution of the Downs and Central and Northern Sea immatures. On the basis of length diffe- rences (see sect~on2) with only five year-classes available there was no apparent relationship between the estimates of immature and recruit abundance for either the Downs or the Central and Northern North Sea stocks.

However, in view of the limited time and data available the method was not thoroughly explored.

Table 1 - Recruitment indices to North ~ea stocks.

Year- class

1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969

North-West (Sa,ville) (tenth of a cran per landing)

---

42 71 50 73 17 194 42 22 14 170 70 52 180 51 61 97 128

Central (Zijlstra + Postuma) (hundreds per day f!~!!~~152

___

77 235 43 63 148 373 20 126 7 256 74- 87 2,9 Xl)

27 x 38 x 65 x 70 x

Downs (Burd) (hundreds per shot)

---

218 109 321 243 180 95 80 366 30 180 168 30 100 68 10 330 *)

55 *)

Young Herring .:lurveys estimates

910 1230 337 3410

650 (1)

2797 805 552 273 455 487 1416

x') Before 1963 year-class numbers calculated by season break-down in 3-days periods (restricted season), there after by taking total catch (numbers) divided by total effort. In post 1962 data an increase in efficiency raise of the effort has been corrected, multiplying by 1 •

1.3

*) Dutch catch per effort, converted to English-units using the relationship between Dutch and English cif estimates for the year-classes 1957-1962 and 1965.

- 4 -

Table 2 - Overall recruitment estimates North Sea Herring based on Larval production.

- - - - -

Northern Central

Year Year- North Sea North Sea Downs Total class

a b c a b c a b c c

1960 1957 1080 24 147 2722

) 65 131 16 50 5 183 1961 1958 5320 5 136 1722

) 32 33 56 72 32 201 1962 1959 1120 17 104 1322

), 3 2 29 22 4 110 1963 1960 1020 75 612 1003

)j

^{81 68 7 90 6 686}

1964 1961 1800 40 450 632

) 32 12 6 47 2 464 1965 1962 2290 39 555 4902

) 59 205 5 19 1 761 ^I ,1966 1963' 690 54 255 I 1422

)!

^{69 75 303) 63 141 344}

1967 1964 440 13 31

I

275

!

18 27 40 65 19 77 1968 1965 162 49 53 28! 32 I 5 6 48! 2 60 1969 1966 212 77 133

I

¹¹

!

16 5 87

I

^{94177 215}

,1970 1967 273 81 186

jj

273

I

^{64! 72 95 46}

¹

^{28 286}

j -~--' - - - - - - - - - - -

a b

c

t )

2)

3)

=

Larval abundance (x 109 ).

= %

3-year old spawners.

=

estimated abundance of 3-year old spawners.

estimated.

Only Dogger larvae available, larval production for total area estimated 2 x Dogger Larvae.

No data available, estimated from proceeding and succee- ding years.

Section I I - Distribution of the Young Herring in the North Sea in the years 1965 - 1970.

The distribution of 1-year old herring in the various surveys to- gether with the mean length, V.S. and K2 data available was invest i- gRted.

The centres of abundance of each year-class at 2 years of age were compared in the following surveys.

The 1963 year-class was not sampled in sufficient squares to justi- fy drawing any conclusions regarding its overall distribution. With subsequent year-classes, however, considerable differences were ap- parent in the main centres of distribution and it was also possible to see considerable differences in the mean lengths of herring which were distributed in various parts of the North Sea.

With the year-classes 1964. 1965, 1966 and 1967 sharp bo~aries were apparent between areas in which the 1-year old herring had low mean lengths (1.e. below either 15 or 16 cms) and areas in which the mean lengths were considerable higher (see charts in fig. 4, 5, 6, 7)0 Herring of the 1968 year-class with mean lengths below 16 ems were generally distributed throughout the area surveyed although they were much more abundant in the south-east part of the area.

- 5 -

Because of these differences in mean lengths the abundance in each year-class of those herring which had low mean lengths and those with high mean lengths was examined. For those year-classes which had already recruited to the adult stocks (1.e. 1964-67 year-clas- ses) i t was first ascertained that the length differences on which the separation was based in the 1-year old herring, were closely related to the L_j differences between southern North Sea and nor- thern and central North Sea spawners. In some instances V.~. and in partic'llar K2 values were also usefull in supporting these sepa- rations.

'rable 3 - Division of catches based on mean length differences.

low mean length herring high mean length herring Year-

class no. of mean no. no. of mean no.

. squares . squares

herr~ng per hour herr~ng per hour

1964 2.316 14 171 23.297 22 2.059

1965 j 6 5 ? 52 14.051

48

293

1966 1.307 13 101 14.891 45 331

1967 287 4 ^{57 11.?45 61 187}

I

1968 27.4j4 2~ 1.143 8.252 41 201

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^r

^---;9-- --~:95;--J

(data in- complete)

, . . _ ' l . . ,. • , _ _ _ _ ~ ______ __~ _ _ _

Section III - Acoustic Methods.

'!'wo attemps to apply acoustic methods were made during the survey of 1971. The Norwegian research vessel "G.O.Sars" did an echo- integrator survey in February and the Dutch n:,rridens" measured echo-abundances during trawl hauls.

1. !~!~S~~!~E:~~E!~~~

The survey covered the main part of the North Sea with special emphasis in the young herring areas south and east of the Dogger.

A ~imrad ~cientific Sounder (B.K. 38) connected with two inte- grators as operated throughout the survey. One integrator was set at "1-125 m. and the other at 0-250 m. or, over the Norwegian Deep at 0-500 meter (Tranducer 5° x 5.50).

The integrator values were recorded per 5 nautical miles and ave- rage values were calculated for each statistical square. A pre- liminary inspection of the results brought forth the following points:

a. Within statistical squares differences were observed between day and night recordings. Over the main part of the North ~ea

surveyed a ratio of 1 : 5 was found, while smaller areas south-east and west of the Dogger only produced small diffe- rences between day and night. In the latter case echo-recor- dings at night indicated ~elagic occurences right to the bot- tom of the Rhip and is reasonable to assume, that an apprecia- ble amount of targets were present above the transducer depth and co~sequently failed to be recorded by the integrator.

The existence of a day and night differential indicates the need for introducing a correction factor, when day and night recordings have to be combined.

- 6 -

b. Direct identification of the echotraces proved to be very difficult and sampling the fish traces was done as fre- quently as possible, using a bottomtrawl in day time. 'rhis excludes identification of fish situated more than 3 - 5

meters above the bottom and in future a more appropriate sampling gear is desir'able.

A tentative allocation of the integrator values was done based upon the catch composition in numbers, flatfish ex- cluded.

c. Comparison between integrator values and the average number of herring caught per square by all participants in the young herring surveys failed to produce a significant corre- lation. fhis is probably due to the fact, that in most cases herring only constituted a minor component of the catch.

A correlation between integrator values and total fish cat- ches could not be undertaken because the anpropriate data were not available.

2. ~~~~:~~~~~~~~=_~~~~~~_~:~!~!

Echotraces within two fathoms from the bottom were measured du- ring each trawl haul and recorded in units of mm2 per nautical mile.

~ignificant correlations were found between echo-abundance and the catch in that haul:

Predominant specjes All species

!~rring

r

=

0,76

r

=

0,70

r

=

O,65

In the case of the herring the significan~e of the correlation depended on one outstanding large catch and it was evident that over the main part of the North ~eat where herring abundance was moderate to low, echotraces did not offer a representative pic- ture of the herring distribution.

Ill. Conclusions and kecommendations.

A. Conclusions.

1. E.stimrites of the abundance of im'nature herring, derived fron the International Young rierring ciurveys both as I and 11 group fish, show a reasonably ~ood relationship to the over- all recruitment to the total !"orth Sea autumn spawning popu- lation.

It should be noted, however, t:lat the estim;.;.te of recruitment to the adult spawning stocks used in the report is dependent on annual estimates of larval production in each of the stocks and at present no other method is available for obtaining the relative sizes of these stocks. It i~ tnerefore necessary to continue and indeed intensify the International Herring Larval

;Surveys if the material collected on the Young lierring :::,u"!:veys

i~ to be fully evaluated.

2. !he treatment of the data, possible in the time available to the group9 did not allow estimates of abundance of recruits to the individual spawning stocks to be made with sufficient precision to evaluate their use fullness as a tool in forcas- ting recruitment prospects for the stocks. This problem re- quires further consideration.

3. On the basis of the separation of herring with a low mean length and with a high mean length i t is clear that for spawning populations with a low L1 both the 196P and 1969 year-classes are relatively very good, while for spawning populations with a relatively high L1 the 1964 and 1969 year-classes are the strongest.

The remainder of the year-classes are all of fairly similair low strength. In each of the 1964 - 1967 year-classes the spawning populations with a low L1 contributed less than 10~

of the herring sampled.

4. When the distribution of each year-cl~ss at both 1 and 2 years of age is compared i t appears that, .... hile there was a marked shift in a westerly direction by both the 1q63 and 1965 year- class fish during this period there was by contrast a S.E.

movement by herring of the 1967 year-class. No obvious chan- ges in the distributions of the other year-classes could be discerned.

5.

In genera] the herring which had low mean lengths and which were ascribed to spawning groups with a low L1 were distri- buted mainly in the ~.E. nart of the central North Sea at 1 year of aFe. Herring with a high mean length tended to be distributed more in the north and west.

B. Recommendations.

In respect of the discussions and the conclusions reached the group decided to draw up the following reco~mendations:

1. Phe Young derring durveys in the North should be continued in future.

2. The survey pattern of the individual flhips should be recon- sidered for futUre surveys and this problem should be dis- cussed during the forthcoming I.C.~.~.-meeting in Helsinki in October.

3. The overall survey period of all ships in future 8urveY$

should be as short as pocisible and in any event sh6uld be confined to February.

4. ~erious consideration should be given to the possibility of carrying out a discriminant function analysis on the material collected during the 1965 - 1971 surveys; an initial apprai- sal of the data available for this purpose should be made.

5.

Acoustic surveys should be continued in future surveys with special reference to the differences in the day and night recordings in the same areas. If ships time is available two echosurveys should be carried out, one during the Febru- ary survey and another during two weeks in autumn.

6. rhe recordings of data on catch composition per species and the weight of each species in future surveys should be stan- dar<liaed.

7. DUring all hauls echo-recordings should be made separately for echotraces from the bottom to 4 ::1eter above the bottom and the remainder of the water column.

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Figure 1

Belaticn between recruitment of three year old herring to the fisheries and the abunaance estimate of one year old herring in the young herring surreys.

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abundance estimate of ¹ year old fish From the young herring survey

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be • 59 ^57·06 •

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Figure 3

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•

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abundance estimate of 2 year old fish rrom the young herring survey

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"1 - - - - J - - - ~ ^{- - -} 15.67 I 16·9Sl' O. I 17.73 I 17.49 I lB.31 I 16.19 ) 14.33 I 16.15

I I 1 I I I I I I

- "I - - - 1 - - - -4 - - - ~ - - - T - - - + - - - t - - -.., - - - -I - - - -,- ~ I 17.93 1 lB.19 : 17.13: 16.95 I O. ; 17.02' 1~.97 ^{I 14.H I I I>}

I 1 I I I ' I I I ~

~

^{-1;.;0 -J,'}

-1~.;8 1,'-1;;;

-:-1;.;"-:-1;';6-

t ~;;1-:-1~.~~ -: - ^~.- - r ---- ~, -I-

t ' . 1 I I I 0

_ }- ___ ~ ____ 1_ .. - _ ; •. ___ I _ - _ L __ .., _ _ _ } ___ ~ _ 0_ ~

C .M.1971/H: 4

I 14.58 I 1$.65 : 18.53 I 15.37 I 14.09 I 13.56 I 14.63 I 16.30 I

I ... ; ~ I ^{I I I I}

I I I

- - - -1- - - -s - - - , - - - -1- - - ~ - - - t- - - - L - - -

j I •. J I I I

15.35 16.77 I 14.11 • 13.98 . . ^I

I ! l t J Q

--- ^~ - - - ^~ - - - ^~ - - --:- - - -!-

^;P

^~

^L

^{~_ <?! -}

^I

I 1 5.38 ^{I I} IfJ

I ! • I I J?I

~-

- - t - - -

~

- .. - .. :- - -

~

L sprl"rJ 1968

O. t /7.1 ~_

yeardass 1f)66 aiumn spawners

mean length per s/:al:;s&ai rectangle

overall m(}tJf1: f6.83cm overall mean

exc/udlf!}

Sk~:

16.83 cm mean

Sk~ak:

17.48

CIIl

( _ of 7^t1 E,

norI:It

^Df 57-f{)

fig. 7

I I , I I t I I • I

I I I ^{I I I • I} ~ , 1 7 . 3 8

- - - :- - - -

~

- - -1- - -

~

^{- - -}

+ --- i - - - ~ - - - ~ - - - - ~ - - - ~ - - - . + --- -.+ ---

11 " I : • 17.92' 20.25 ^{I I :} 18. : 17.38 ^j J6.84

, " I I I ' , I

- _ ^~ ___ L ____ , ____ 1 _ _ _ _ -J _. _ _ _ .! __ :.- _, ____ ^~ ____ , ____ .!.. ____ , ___ ^{-1- __ _}

I ' I I J I I I l I I

1_'~7:

I

L __

I

L'~~S_:_ ~:~ LJ:~5_L~::

I

i

I

~~J:L

I

___ ^{L __}

I

^{L ___}

I

^~

J

r

^{l 1}

I 18.63 I I . 1 18.83 I 18.44 I • : 17.96 I 16.25 1 15.51 I ,

I I I I ) " , I I ^A

( _ -'- ___ t - - - -1- - - - L - - --!.... - -- - ... -- - - ... _ - - J __ - _.J _ _ _ _ I- ___ .. '

} I I I I I I I I I

1r.04 _I 16.35 I I t 18.55 I 18.26 , 17.64 : 1J~08! J5.43 I 16.62 I

I I I I I I I t I I I

.J_ - - - r - - - T - - - - ~ - - - - - - -r- - - -1" - - - - - - - ,

I I I I I I 1 I

17.72 1 I 18.11 I 17.73 ' 16.60 I 17.00: 17.99 I 18.2'" I }7.20 1

I I I I I 1 1 I I I

- J - - - - - - - - J .- - - - I-. - - - I - - - - 1 - - - - , - - - - - - - .- - - - - - J- : ;

~

17.",7 :

1~0'"

: 17.06: 17.80 : 15.79 : 16.97 : :

\3:

, I I ^{I I 1} I Q

- - - - -- L - - - I - - - - - - - - - - - -4 - - - ,. - - - -1- - - -I - - - ...l_~

1 ^{, I I} I

1 118. 06 , 18.94 117.77117. ... '" I 17.56: 17.11 '16.98 ^I, I 0

C .IvI.1971/n: 4

I I I ' I Q

_..I.. ___ -i _ - - .J_ - - J _ _ _ ... _ _ _ !.. _ - .. ^{1- - -} J_ - _.J_ -·-<10

I I I 1 I I I '

: 17.62 I 17.75 ^I 15.38 ^I 18.75 } 18.10 : 18.25 , 16.78 I 18.55 ^I

____ : _ + ___

^~

___ : ____ : ___

^~

____

L ___ : __ _

I I

1

^I J

j '

I 14.85 ^I 17.84 17.80 , ^I ~

I ) I ! , I

oc"f (

^l~

- - r - - - - L .. - -1- - - - I - - - - ' - - - b r t : 1 - - -

I I I I~

I I 13.75 I 16.00 I

tI'

~ - - ~ - - - L - - - : -qt-

I I I 17.8

I I

_,_ - - - I- _

I I

I 13.25 I 19.25 I

_ 2. ___

^{L - _ ..!}

I I

1

.,?

spring 1969

yeardass 1967 auetlTln spawners

meat! length per statistical recI:ang/e overall meat: 11.32 cm

over"! mean

excIudi!J9

Sk~:

1150 cm mean Sk8@rak: 1722 cm

(eaSt cf 7°E, I1DI"I:h of 5/" N)