."

This paper not to be cited without prior reference to the authar

International Gouncil fortheExploration of th? Sea

On the

b'1·:

',- '

,'. G.H. 1969 / R:21Special Heeting

npelugic'Fish (Northern) Committee".' ~. ~.-). . c. ~)~

, ,

Narth Sea. .:.

•

•

K.R. Postumu

Netherlands Institute for Fisher'1 Investigations, IJmuiden, Netherlands

Introduction

In recent '1earo.the development of a Norwegian purse-seine. fishery·in the northern and ea~tern Horth Sea (Revheim und Hamre 1968) resulted:,in a substantial increase of thc total mackcrel landinis fram the North" .Sea, the Skagerak and the Kattegat.· \'Jhereas the total'·landings of mackerelin the North. Sea, the Skagerak and the I~atteGat incre'a'sed onl'1 olowl'1 ;,(between 1945 - 1961), fro~ 20,000 tons to 100,000·tons·and even.droppedin thc years 1962 and 1963 to 73,000 tons, after·t~e introduction of th~purse-seine fishery in 1964 the landings jumped up to 930,000 tons in ,'1968. (table 1, fig. 2) . .

This sudden increase of the totul landings of mackerel from the'North Sea caused some concern in the Dutch fishery •.;Not only the total ."landings of the Dutch mackerel fisheries showed a persisting decrease from1962.onwards (table 1, fig. 2)'but also the eateh per eff~rt (table 2) inthemost profitable mackerel uraas in tho North.Sea declined. The resultsofthe' Dutch mackerel fisheries' thus contrasted 'markedly \;ith the total:international landingsfrom thc North Seu.

In this paper the maeker~i':tish~ry in the'north - eastern and .northern North Sea in April-~~y 0ill be analysed in order to see if the'decline' of the Duteh maekere.l fishery,. operating \'Iith a fairly eonstanteffort, 'resulted either from a redue,tion of the' stock, caused by failingrecruitment or from an inereased mortality, or perhaps from a combinationcf these two faetors.

Naterial

The mackerel population in the North Sea has been analysed withhelp of catch, effort and bioloGic~l data, collected from a trawl-fisheryon mackerel in April - IIay in the northern and north-eastern North Sea.·along the slopes of the Norwegian Dee~ '(fig. 1). Postuma and Zijlstra(1963) pointed out that only' this trawl-fishery can be used for a macke~elstock-analysis, as in all the other North Sea trawl fisheries the mackerelhast? be considered as bycatch. '

Samples were collected from the commercial landings and. in addition,to these, market meusurements from the landings of a considercble numberof ships were taken. The 'catch-effort and biological data were publish~d

annuallY'in the Annales Biologiques from 1959 onwards.

, '

..The age eompositio~

Age was determined by otolith rcading. The otoliths uere imbedded ~n

plastic and read uith a'binocular in direct light. The age determination ~s

dependable up to an'age of six years, after the age of five years the agcdetermination becomes more und more difficult in the greater part of the ,otoliths. Some otoliths however give a very clear picture up to fifteen 'years. The age'composition of the mackerel in the north-eastern ~orth Sea. inthe period 1959 - 1968 in April - Hay is given in tabel 3. In the age-coIllpo­sitions two strong yearclasses i.e. 1955 and 1958 can be followed in the'different years. The possibility to do this, gives some confidence in th~

reliability of the 'method'of age~determinatiön.,As'shown ,by,tllese agecompositions, the mackerel is caught and landed by the traul-fishery fromits second year of life onuards. Recruitment to the fishery seems to be ­completed in the fourth year of life.

Abundanee and reeruitment.

~~' a me~sur~ of ~bundance, for the peri~d 1959 - 1968" of the ,mackerelpopulation fished in the north-eastern North Sea, the mean catch per 100',hour trawling in ueight and numbers is given (table 4). It is clear from :this table, that thc cateh per effort in ueightin the late fifties and ~he

beginning of the sixties ~as higher than in the end of the sixties and dqesshow a steady decrease. The very lou cateh per effort in 1963 is remarkable,and:'riot in aecordance ..-/ith the general trend. It, is possible thaf t'he s'everewinter 1962/1963 influenced natural conditions (temperature) ,and caused a: •very'low availability, Another remarlmble point i5 the very loi'! catch per

'effort in 1968. No suggestions for environmental factorscanbe made in thisease, as the temperatures inthe winter 1967/1968 were normal. ,Thecatch'pereffort, of abundanee, expressed in,numbers of mackerels caught does show thesame ~rend~as ean be observed in catch per effort expresse~ in weight: adecline of the numbers caught per 100 hours fishing.

The suggestion, that the decline of the cateh per, unit effort i~,~

weight as well in numbers may have been caused by a decrease of the recruit­ment to the mackerel stock 1 will be substantiated by the fact that reeruit­ment, as measured by the numbcrs of' four yearold mackerel, has been'verypoor sinee '1963. In fact thc 1955 uhd 1958 yeurclasses, recruiting respec~'tively in 1959 and 1962 as four year old fish to the maekerel stock, are, theonly goo~ year-classes recrui~~ng to the fishery in the period 195~ - 1968(fig. 3). In the percentage age - distributions (table 3) the year-classes1955 and 1958 ean be followed throughout their life, as mention~~,alreadi,und no other'strong year-classes folloued to reinforce the stock, exeeptyear-class 1965 ':in',1967 as two year old mackcrel. 'This yeurclass' 1965 howeverdid 'not reuppear in.:1968 in quantity. "'" .:' ' :'.

, The failure ,of recruitment after yeur-class 1958 had its consequences ttfor 'the length-distributions of.the mackerel stock as shown in the lengthdistributions from 1958 until 1968 (fig.4) It appearsthat since 1959 thcipeak in the length-distributions is moving from the left to right. . .

....

.,.'.,. The shift to the right of the length-df6tribution i6: eo.used by the growth'

of the two strong yearclasses 1955 and1958, and' can befollowed, as re­cruitment of the following yearclasses was failingand the growth of the·younger age groups was not masking the-groV/th of the two abundant year-elasses: . " ... ~

'The 1965 year-class appears in, the':'leng,th-distribution 1967 - 1968as a peuk at 27 em.

2

••

•

Nortality

Mortality estimates of the mackerel population are determined bythe numbers of mackerel older than three.years in the year " a " and thenumbers.of mackerel older than four ye'ars in the year."a.+ 1" (tabel 5).Difficulties in the age determination' of mackereIs older than five years :compelled us to follow this methode The mortality ratei in the different'seasons, ·fluctuate betwecn -0.26 and 1.34, VIi th an overall mean mor,tali tyof 0.38. No definite trend is apparent ~n the set of mortality dita'iritheyea~s~959 -1968 and the only conspicio~s observation'is th~ Vlid~'year foyear variation of the data, presumably caused by the variations::in'ayai-ia­bility which affected the abundance estimates. The low abundance ··~6f·the· :.mackerel population in April - I'lay 1963 was considered to becaused by lowtemperatures. The very high mortality (1.34) in the season 1962/1963 isconsequently no real mortality. In general the mortality in the mackereI·population seems to have been low. The proportion of old mackereIs ·seems·to be rather' high and the mackerel stock had the appearance of an oldaccumulated btock that could bear more fishery than it actually did•.(Postuma 1965) Nevertheless the sudden inerease of the totalcateh in theyears 1965 - 19f8 from a level of 100,000 tons to 900,000 ,tons must have:.had some effect on the mortality and hence on the abundance of the mackerelpopulation. One would have expected an increase of the mortality during;thelast three seasons, but' ~nly in the last season 1967 - 1968 a highmorta~lity (1~34) was recorded. .~

Apparently thewide variations in availability made it difficult to.);.

detect areal sudden inerease of the total mortality.

The yield-curve. During its first three years of life the mackerel is a fast growerin lengt!J, aI.d weight. After' the age' of three years the growth is decliningconsiderably, as is shown below:

mean values 1956 - 1968age (years) 1 2 3 4 5 6 .7 8: 8length em. 20.0 28.1 32.3 '34.5 34.935.6 36~4 '37~8 "38.7weight gr. 90 134 264 358 389 394 458 478' 500

This gives the yield per recruit curves of the mackerel a particularsteep shapc, as demonstrated in fig. 5a. These yield curves are caleulatedfor maekercl recruiting in the first'year of age (tr = 1), and .fished b~

the fishery from the age of one (t = 1), two (t = 2) and three (t = 3)years onwardR, using·-the followingCstock parametgrs Loo = 37.5 cm. ck = 0.57, t = -1.05, w=oo=411 gr., and M = 0.20.

oIt is clear from these yield curves that the yield per recruit fn:~

a .fishery beginning in thc first year of life (20 cm) is somewhat higher:when the fishing mortality remains under the 0.4 level but results in a .los of the yicld at the higher fishing mortalities when compared withfisheries beginning at the agc of two (28 cm) and three (32 cm) years ofage. 11. fishery beginning during the seeond year of life (28 cm) gives aslightly bctter yield than"a fishery beginning during the third year oflife (32 cm), over the.total mortality range. A more striking differenceof the fisheries attacking at the three different ages is found in themean weight, length, and age of the catch. This is demonstrated in fig.5b, where the mean weight of the fish is decreasing when thefish iscaught far the first time at a younger age and when fishing mortalitiesincrease.This is not important when the fish will be proeessed to meal and oil,but becomes of interest when the fish will be used for human consumpti~n.

3

l•

ITith a total ~ortality of Z = 0.38, as determined for the mackerel popu­lation in the period 1959 - 1968 and a natural ~ortality (H) of 0~20~ :it ~ill be clear fro~ the different catch-curves that the mackerel pop~­

lation ~as underexploited in terms of yield per recruit during the period1959 - 1968 and c'ould very weIl bear an increaGe of fiGhing intensity.'If ~e take on' tho' 'other"hand·t'h'E;valu~o"f'total mortality duririg ,the 'season 1967 ~ 1968 (z ~·~.34) ~s r~~resentative for the mortality in thel~~t two iears,' it iG evident from th~yield curvesthat practically thesame,yield per recruit could be reache'd' tt t a considerable lo~er fishingmortality or fishin~ effort. In other words the present level of' fishin~intensity exerted on the mackerel pop~lation, could ~ell be ~ waste of:effort.

Discupsion

'.", , '. The mackerel stock' fished in April-Hay in the north-eastern' and:northern'North Sea by the Dutch tra~ling fleet was analysed, on age­composition, abundance; recruitment and mortality. In addition, theshape of the yield curve under different fishingconditions was discussed.This attempt was mado to,see whether the impact of the incroaGed total'catch on the mackerel Gtock could be demonstrated to have a substantialeffect, and could ~ell be the cause of the observcd decrea~in abundanceaffecting thecatches of the Dutch trawlerfleet in the North Sea. .For this analysis the mackerel population in the North Sea hasbeen " •considered as a unit stockch~acterisedby the spring-fishery in the'north­eastern North Sea. The fiGhery in the area mentioned is based mostlyon pre-spa~ning fish.In several tagging experiments it ~aG shown, that mackerel tagged duri~g

Gpring in the north-eastern North Sea (Revheim 1951, 1954)migrated:overthe whole North Sea, the Skagerak and the', Kattegat. Thene mackereIs returnedagain to the north-eastern North Sea for hibernation (Postuma 1965 ."1 •. _. and Revheim 1955) and began their npawning migration from thi~'

area in spring. The results of the tagging experimentn of Revheim and:',Postuma were backed by a description of the patternof the buten' trawl<fishery on mackerel in the course of the year (Postuma 1965).

~hen the catchen per unit effort in 'the areals north-east and northof the North Sea are used as an index of abundancc of stock it is evidentthat since 1959 the wcight of thc total stock has decreased significantly.This is also reflected in the total numbers of mackerel caught per effort.(table 4 and fig. 3) Although differences in availability might have 'influenced the abundance estimates in same yearn (1963) thc decrease i~

abundance sincc 1959 has been a rather steady procen, especially so wh~n •considering thc data from the north-eastern North Sea (table 2).Thc agc-analyses of the ntock in the yearn 1959 - 1968 (table 3 and 4).nhowed the following phenomena:

a. A failure of recruitment sincc the 1958 yearclass. The only strangyear-classen entering the fishery at four year of,age were the 1955 and1958 yearclass in 1959 and 1962 renpectively. '.,

: ~ ,

,b. The mortality estimates, an nhown in table 5', showed great fluctu-,ations, cauned presumably by year to year fluctuations in availability.

The mean average mortality measured over the period 1959 - 1968 was ,Z = 0.38.The combination of the low total mortality (0.38), and thun high nurvivalrate, with the rich year-classen 1955 and 1958 and no follo~ up of othergood year-clanses, led to an accumulation of a rather old and accordingto the yield per recruit curves (fig. 5), underfished stock.

4!-

•

~ith the development of a,purse-seine'fishery the uccumula~ed,' ,old stock "JUS fishcd down drastically, resulting in an enormous "incietts~

,of the total cutch from a lo:ve.l of,,;100,QOO tonp to 900,000 tons. Thoughan increasc of the total morta,li tY:'~·as. U', direct consequence of the' öbvIousin total, effor,t (see Recheim, .1968),.',.c<?ul{b~"expected, un increase of thetotal mortulity could only be recorded for the l~st season under conside-ration viz. 196'7 - 1968. (1.34): ,:, ...~; "~'C, ' '." ,"

The data suggest, thut thc impact of thc purse-sciner fishery in the lastthree ~ears, ~ogctherwith a faili~g,.recruitment,rrqsuited iri u,st9c~still relatively old, but weak us compared with the stock p'rcoe'nt' in ' .1959 (see fig. 4). l~;,.; .\ t""';·~·

Ari6·t'n~fl:a"spect:is aiso visible in the fig. 4, a~ 'e'xample ··of"'the dlmgerof a ,v.cry· i,iJ.temH.ve fishery for industrial purposes. In the age compositionand i~~gt~Ldi~tributionof 1967 th~ 1965 yearcluss was very weIl representedThis "Ied'tis':to suppose that in 1968 this, yearclass would re-appeur ugui~ inforce 00' tp:roc yeurold fish (Postumu '''7 Zijlstra ; Annales Biologiques 1967),but the yearclass 1965 failed to ·turn uP. in quuntity in 1968 us three .y~a:r (;lIds •. ,.,':,' '. ,.....-; .. ,., -,;' . . . .: "", 'Revheim and Hamre(1968) report, that the 1965 year-class was caught inquantity by the purse-seine fleet.for reduction,purposes' in 1967. So itseenis 'r,c:asonable::to suppose that th~ purGe~se'ine fishery hus decimutedthe numbers of '1965 year-class.: p'reliniinary da:ta do expect that the macke­rel se'abon' 1969 '\'Jill be' the \"Iorst 'o'n'racor'd "i:n the North Seu and this J

means··thut aguin ,the.recruitment of a yea):'.class·as four year old macker~l

to the consumption fishery has failed, a; y.ear-class that hud been recordedas outstanding ~t;tw~.years of a~~~ '0' .• _ .

The introduction 'of a minimum riize in the mackerel fisheries seems to behardly justifiedby 'the yield per recruit graphs under different t -vulues,but on the other hand u very"in'tensive fishery on small sizcd mackgrel :

(920 cm) can endanger recruitment ta thespu\'Jning stock and possibly endan­gers the re,cruitment potpnti.i!. of.thc;ptock'ultimately. In this respect :much depends on the value of the ~uckercls. natural mortalityin the youngcr stages. In the culculations,of the yield curves the naturalmortality was uccepted to be 0.20 und und~r these premises an effect onrecruitment at' high-fishing'intensities s~ems:unuvoidable. However, theeffect decreases immediitely when thenaturalmortality increases. Depen­dable mortality estimates ofnatural mortal~ty of mackerel ure thercforphighly wunted. - '

Conclusions

•1. The decrcase of thc abundance of mackerel in the North Seu, as measurcd

by thc catch per effort in April - Hay in the north-eastern and nor-'thern North Sea, is caused by paar recruitment, as measured by theubundance of four year old mackereI, by the year-clusses coming to trcfishery after the 1958 year-class.

2. In the first years after 1962 pOOl' recruitment was probably caused b~

natural causes only, whereus in the years after 1964 a very intensiv~

fishery on two year old mackerel mayaIso have influenced the recruit-ment of muckerel as measured as foul' year old fish. {

3. The stock composition and mortality estimates in thc ycars 1959 - 1968indicate that the mackerel stock before the development of the pursc~seine fishery had probably a rather low explotation rate (Z = 0.38) ,and an increase of fishing intensity would lead to an increase of theyield per recruit.

4. An increase of the total mortality in the years of the increase of thctotal catch (1964 - 1968) could only be measured in the last season1967 / 1968 (Z = 1.34J

5

References

Aker, A

Anbn.

1961 "Untersuchungen über die Biologie der Makrele(scomber scombrus)'in der Nordsee"Ber~Dt~ ~iss. Meeresforsch. 16(2):102-108.

1945-1967 Bulletin Statistiques 1945-1967.

,4

1964 ".Some remarks on the estimates of abundanceof herring and mackerel from data of ·thecatches of the Netherlands Trawler Fleet".Rapports et Procesverbeaux 1964 155:117-221 •

Postuma, K.R. andZijlstra, J.J.1959-1967 Annales Biologiques 1959-1967 vol. 15-23.

Postuma y K.R. andZijlstra, J .J •.

Postuma, K.R. . 1965 " Some remarks on·the Mackerel migration inthe North Sea".mimeo I.C.E.S.1965 C.M. Scombr.Cttee. no. 22.

1954

Postuma, K.R. andPeerear, A.

Revheim, A.

Revheim, A.

Revheim, A.

Revheim, A. andRam~e, J.

1965 " On the decline of the total catch of mackerel·by the Dutch trawlers in the.North Sea".mimeo,I. C.E.S .1965 C. N. Scombr.Cttee. no. 21.

1951.Tagging·experiments on mackerel in Norwegian'waters.Annls. Biol. Copenh., -:114-73.

Tagging experiments onmackerel in Norwegianwaters.Annls. Biol. Copenh~, 11:172-173.

~955Tagging experiments in the Skagerak.Annl. BioL Copenh., 12: 217-218.

1968 A brief statement on the present state of theNorwegian mackerel fishery.mimeo I.C.E.S. C.M. 1968.Pelagic Fish (N) no

.. ~ .

6

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•

•

Table I. Total:international and Dutch mackerel landings (tons) fromthe North Sea, the Skagerak and the Kattegat.(~ulletin Statistique)

Gatch per effort area N.N. and'N.E. -:-'Ndrth Sea, per 100hours, trawler 500 B.P.H~~ in tons."" " "

International

•

•

Year

19451946194719481949195019511952195319541955

, 1956,195719581959

,1960,196119621963

,1964.1965,,196619671968

Table II.

year

19591960196119621963196419651966196719681969

. ~ \ ':

\ .,.

19,65925,45236,16153,69362,96357,25573,00466,86265,187 ' ,68',00980,11065,66397,52087,90093,75191,550

101,36579,65573,419115~329208,786529,971939,194---,---

N.N.

11,916,918,28,03,9

10,21,58,7

12,96,3

--,--

Dutch

571533

1,7955,6147,1426,605

10,73211,68811,23011,83216,33311,23015,48317,97118,86626,42524,85519,65111,84617,08416,97712,2139,9455,322

N.E.

25'~'5

22,518,222,97,6

13,411,615,314,54,8

--,--

- 7 -

Table 3. Percentage agecomposition of mackerel population inApril - Hay in the North Sea.

year/age 2 3 4 5 6 7 8 >8

1959 37 22 -17 7 5 11

1960 3 3 4 56 13 14 4 6

1961 9 25 14 27 16 5 3 31962 1 3 20 1" 40 1 25

1963 2 7 11 23 2 1 32 221964 15 9 13 10 14 8 9 21

1965 2 12 6 11 7 20 6 391966 2 7 5 7 8 13 14 46

1967 32 7 33 9 5 5 9 311968 20 12 5 5 14 7 7 31

•

Table 4. Catch per 100 hours fishing of a standard trawler of500 B.R.P. in tons, and numbers (000).

year tons all ages 4 years >4 years

1959 .. 25~5 95,0 3;',2 58,9 •1960 22,4 8/+,0 3,4 78,9

1961 18,2 71 , l~ 10,0 38,6

1962 22,9 74,.7 14,9 50,0

1963 6,7 21,3 2,3 17,1

1964 13,5 42,2 5,5 26,2

1965 11,7 31,7 1,9 26,3

1966 15,3 41,2 2,1 36,6

1967 14,5 52,6 1,8 31,1

1968 4,7 13,4 0,7 8· 6,

Table 5. Total instanteneOus mortality (z) of mackerel calculatedfrom)3 years (in year a) to)4 years (in year a+1) inApril - May in the area N.E.

season z seasons z

59/60 0.17 59/60--60/61 0,47

60/61 0.76 60/61--61/62 0.37

61/62 -0.03 61/62--62/63 0.65 •62/63 1,34 62/63--63/64 :0.52

63/64 0.30 63/64--64/65 0.06

64/65 0.18 64/65--65/66 0.04

65/66 0.26 65/66--66/ 67 0.02

66/67 0,,22 66/67--67/68 0.78

67/68 1.34

}iean overall Z = 0.38

- 8

•'.

•

c.

~·liS NN~ • •• •'\\

• • • • •

·1• • •• • • •• • •

'p • • • •}j:"~ o· • • •NW

• • •• •• •0

• •

• •

• •

s

APRIL

mackerel

C/E1961- 1964

Fig. 1. The distribution of mackerel in the North Sea in April1961-1964 as indicated by the catch per effort.

CATCHOO.OOO.TONS

7.

6.

5.

••

3.

1.

.0 0 o.TONS

28

INTERNATIONAL

DUTCH

•

YEARS

24

20.

•16.

12.

8 .

••

1946 1948 1950 1952 1954 1956 1958 1960 1962 1964 1966 1968

Fig. 2. The total International and Dutch catch in tons onmackerel in the North Sea.

YEARS

,

NUMBER.ooo.

4VEAR

•

J2.

24.

16 .

8.

>4VEAR

70.

60.

50 .

• 40.

20.

10.

I•YEARS

1959 1960 1961 1962 1963 1964 1965 1966 1967 1968

YEARS

Fig.3. The catch per effortof a trawler of 500 B.H.P. innumbers of four years old mackerei, and mackerel inApril-May older than four years. in the north-easternNorth Sea. C •

,I

12000_

8000 _

4000 -

8000 _

4000 _

8000 _

4000 _

. .

1959-60

1961-62

1963-64

8000 _

4000 _

8000 _

-f\~

S?J '---.

1965-66

1967-68

/\ )

4000 - •

~;=::a.::I:::_,,:-/_)....-,"~-~""'i_~~_'_"_i-:-----~t:n......,=."---::-20 24 28 32 36 40 44 48 CM

Fig. 4. The length distribution of the mackerel population inApril-May in the north-eastern North Sea in 1959-1968,combinations of two years, expressed in numbers as de­termined by the catch per effort.

,..1

200

160

120

80

.40

.:...--:::t -= " Pli ,.. - _ .

~~·::·.:.:c·::.·~·~c- - -0- .~ ..- --~---D~ •• - L.I - - - - __ -c.

/ ......•I ....

I. ....... o TC,.. TC2

• TC3

- F

GR380

.

~.3.40 '- ----.-'. -.- .- • •

. '" • e_ TC3

300 , .~., .----.."". -.- .- -. •260 -.- TC 2

"'-.220 ~.

""'--.-- 180 -.-- .- TC,

1.40

100

0.2 0.4 0.6 0.8 1.0 1.2 1..4 1.6 1.8 2.0

F

Fig. 5. The yield per recruit curves of mackerel with the cor­responding mean weights of the catch at different fishing­intensities when:

L 00 == 37. 5 - k :c: 0, 57 - t 0 '-, - 1. 05w 00 == 411 - M == 0 20 - t:, 1• r .