1983_G_15.pdf (864.3Kb)

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International Council for the Exploration of the Sea

G.H.

1983/Ge15

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Bemersal Fish Comittee

PRELIMINARY REPORT OF THE NORWEGIAN INVESTIGATIONS ON YOUNG COD AND MADDBCK IN THE BARENTS SEA

DURING THE WSNTER 1983 'OY

John Dalen, Arvid Hylen, Odd Nakken, Kje11 Randa and Odd M, Snieåstad

1n~tit~~t.e of Marine Research P.O. Box 1870, 5011 Bergen-Nordnes,

Norway

ABSTRACT

An acoustic survey and a bottow trawl survey for cod and haddock were carried out in the Barents Sea during the winter 1983. The two surveys gave different results. The a.coustic survey showed a 50 % reduction from 1982 for the Barents Sea stock of young cod. The largest reductlons were found for the 1977-1975 year-classes. For haddoek the reduction in the total stock of young fish was a b o u t 48% witk the largest reductions found for the 1998-1975 year-classes. The bottom t r m E survey showed no such reductions of young fish. In this survey the indices for the younger age-grotaps (2-5) were higher in 1983 than in 1982 for both cod and haddoek,

In both surveys the amount of older fish (6-lo+) was signifi- cantly lower in 1983 tkan in 1982, mainly because the abundant 1975 year-class had reached maturity and had migrated out of the investigated area.

The 1982 y e a r - c l a s s f o r cod i s more numersus t h a n t h e poor 1980 rind 'l‘981 y e s , b u t a t p r e s e n t i t i s n o t possibYe t o t e i t e s i z e . The 1982 year-clA"ss"for haddbbk i s

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probably a s t r o n g 'one a s one y e a r o l d haddock wac caught i n h i g h numbers i n most of t h e surveyed a r e a .

Each y e a r s i n c e 1975 a Norwegian a c o u s t i c survey h a s 'been a a r r i e d o u t d u r l n g t h e wånt:e+ i n t h e B a r e n t s Sea. T h e aim of

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t h e s e surveys have been t o eståmate t h e a b s o l u t e number af cod and haddokk & i t h i n t h e surveyed a r e a . The r e s u l t s 'from t h e s e curveys can be found i n Dalen and Smedstad (1979, 1982) and i n Dalen e t a l .

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Sinte 1981 a s t r a t i ' f i e d random t r a w l survey h a s been c a r r i e d

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o u t i n t h e same a r e a and a t t h e same time o f t h e y e a r a s t h e

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a c o u s t i c survey. P r e l i m i n a r y r e s u l t s from t h e s e surveya a r e r e p o r t e d by Dalen

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MATERIAL A ETHO

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" S t a l l o " c a r r i e d o u t t h e bottom "krak

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l T o t a l l y 357 t r a w l s t take;. ' MT " ~ a & b a i ? k 'E?&ker1'

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worked 135 bottom t r a w l s t a t i o n s , MT " S t a l l o " 131 and RV "G.O.

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The acoustic Lnvestkations

The acoustic investigations were earried out as described in Dalen et al.

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( 1 9 8 2 ) . A new echosounder (SI

replaced the old sne (EK 3 8 S l ; otherwåse the integration system was as En grevious years. The performanse of the system was thus signifåcantly improved, and according to the c a l i b r a -

tion results on the standard target (Foote et al,

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_1981, 1982) the integrator conversåon f a c t o r s , C , were changed. Below are given the values used in 1983 and in previous years:

Cod Baddock

-2*10 1 . 7 2 * 1 0 6 e 1 Previous values of C: 5.25~18 o k - 1 . 6 9

1 9 8 3 values of C : 1 . 8 7 e 1 0 6 1 6.100l0~el - % , 6 9 where l is the length of the fish in cm.

Figs 1 and 2 show the survey trac=ks, hydrographica1 statisns and trawl stations worked by RV " G . 0 . SarsMB.

Bottom trawl survey

A total of 279 trawl stations were taken as a part o £ the bottom trawl survey, Anotker 18 stations (8 by RV "G.O. Sars") were taken as part of an experj-ment for controlling the ftshing power of the three vessels. _{T h e} survey design arid all the computatins were carried sut as d e s c r i b e d by Dalen et al. _.-

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( 1 9 8 2 ) . The final i n d i e e s given this year are somewhat differ- ent from those given last year by Dalen et al.

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( 1 9 8 2 ) . Iri that report the stratified mean catch in nuo~ber (Xst) was u s e d . In this report the final i n d e x is colculated os I = X S t e . A * i 0 - ' / ~ ~ where Xst is the stratified mean c a t c h in numbers - f o r a given area A , and SA is _{t h e} area swept by a single standard trawl hau1 of 3 nautiea1 mile, assuming that the trawl has an effec- tive flshing _width of 25 m e t e r s . It: should be nated $hat these figures should be considered as indi.ces o n l y and not as esti- mates of numbers of fisla in _%kle area, The advar~tagc of such an index is that it rna- suppase? to _be proportional to the number

of fish in the area. The csnfidence limits of this index is ca%cuiateå as

S-

where 'st is the standard error of the stratified mean catch in number and t is the 95% guartile of a Student's t-distrfbu-

Fig. 3 shows the trawl srations taken by MT "Hagbart Kramer"

and MT "Stallo" and the bottom trawl stations talcen by RV " G . 0 .

Sars" east of 35'~. Table 1 gives the number of trawl stations in each strata, and Fig. 4 shows the locations of the strata used and the four subareas used for d i v i d h g the total

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area in smaller areas.

RESULTS Hydrography

Fig. 5 shows the distribution of temperature on the surface, I n 100 meters depth and at the bottom. The temperatures were significantly highes than in the pl-evious years, and between 1.0' and 2.0' higher than last winter (Dalen - ^et - ^al. 1 9 8 2 ) in the central parts of the area.

Distribution of co8 and haddock

Fig. 6 shows the dåstribution of eckio abundance for _cod and haddock. As in the prev-ious years the highest concentratåons were found in the western part of the Barents Sea and close to the Norwegian coast. East of 35'~ only one year old cod was found. Figs 7-12 show the distribution _of the age-groilps 2-7t for cod. Except for the two year old fish all age-groups were distributed close to the Norwegian coast. Figs 13-18 show the distribution of age-groups 2-7-k for haddock. As for cod also the main concentrations of kaddock were found in the western part of the area and close to the Norwegian csast.

For both cod and haddock the 1--year old fish is excluded from the computations of the acoustic estinnates. Thås is done since the l-group fish due to small size (low target strength) and scattered deep water dåstribution will contribute very little to the echo abundance. Nowever, since they are caught in h i g h numbers by the trav1 they would have a large infkuence on the

final estirnates of the other year-classes if åncl.uded %n the cornputations.

Acouctic abundance estinnates for cod and haddock Cod

Tables 2 and 3 give the number of cod of the different year- classes in the different subareas and In the total area com- pared to estimates from the previous surveys.

The figures in Table 2 demonstrate clearley the geographica1

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distribution mentioned previously; most of the fish was ob-

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served in the areas elsse to the Nomegåan coast (subareas B

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l and C) while the abundances were less in the offshore parts

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^{(subareas A and D).}

I Except for the 1980 and 1981 year-classes all year-slasses were

l reduced in numbers from 1982 to 1983, resultl.ng in a drarnatic reduction from about 400 million specimens in 1982 to 200

( million specimeas in 1983 of the total Barents Sea ctock (Table 3). For the older age-grsups (7-8) a significant part of this reduction is caused by the fisk b e i n g outside the investigation area undertaking the cgawning migratlon.

Although the 1980 and 1 9 8 1 year-classes were found in higher numbers this year tkan in 1982, the total. nuagibers of these year-classes are low and the areseilts thus confirm previous years findångs indicating that these year-elasces are very poor

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Considering the year-clacses 1977, 1978 and 1979 which made up the bulk of the stock Et seems that the reductåon in numbers

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from 1 9 8 2 to 1 9 8 3 (Table 3 ) are wfthin reasonable limits when taking total mortality into account.

Haddock

Tables 4 and 5 give the total number of fish of the different year-classes in the different subareas and in the total area.

Table 4 shows that also for haddock most of the fish is Eound in the western and coastal subareas B and C. Thås is true for all age-groups except the 2 years old fish where half of the total estimated stock was found in subarea D. Table 5 showa that there has been a reduction in the total estimate from $ 0

mill. fish in 1 9 8 2 to 5 0 mill. fish in 1 9 8 3 . The reduetion is greatest for the 5-74- year old fish. A part of this reduction i is due to a spawning migration out of the investigated area of the year-classes 1 9 7 7 - 1 9 7 5 . Table 5 aPso shows that the

1 9 7 9 - 1 9 8 1 year-classes are very poor compared to earlier

year-classes of the same age.

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Bottom trawl survey Cod

Table 6 gives the indices from the bottom trawl survep for the different year-classes in each subarea and Table 7 gives the indices for the total area in 1 9 8 1 - 1 9 8 3 . The age distribution is very similar to the one found in the acoustic survey (Table i 2) because mueh the same age and length samples are used in both surveys.

Table 7 shows that there is no consåstensy in the estimates of the same year-classes for the three years. There has been an increase in the abundance indices far 2-5 years old fish for all three years. The total indices for 2 - 1 0 6 years old fish (Table 7) do not show the large reductions experienced in the results from the acoustic surveys, instead they indicate a more or less stable population.

The two surveys als0 show different distribution patterns for fisk 2-104- years old. A s mentioned earlier, most of t:he fish was found in subareas B and C in the acoustic survey. Table 6

skows that nearly two thirds of the fish was found in subareas A and D in the bottom trawl survey.

These large differences in dfstr&bution and abundance indices between the two surveys are difficult to explaan, but a part o f

it may be explained by the fish behavisur in the curvey period and in the fishfng perforrnance of the trawl, During the survey period much cod is found above bottom feeding on capelin. Cwch a pelagic distribution of cod makes snly a portion of the stock available for bottorn trawling, but is favourable for the acoustic method. It is thus quå.te possible tkat the ratio between the trawlable part of the stock and the total stockc may change within the survey area and from year t o year, so that the indices from the bottom trawl survey ds not fulby reflect the variations in stock size. On the other hand the fish which are situated close to the bottom will not be recorded by the acoustic instruments. I f this psrtion varies from area to area and year to year this will cause a varying bfac to the aeoustic estimates which is difficu1.t to assess. Data on the vertical distribution of the echo abunclance for cod and haddock along the ships tracks (Fig. 3) are available, but renlain to be analyzed.

In Dalen

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^al. (1982) the buttom trawl survey indices were corrected using the average proportbon of the t o t a k echo abundance to the echo abundance found between ten m e t e r s above bottom to the surface. The underlying assumption was that the bottom t-rawl snly caught få.sh between b o t t o n i and ten meters above bottom. These correction Eactors were very caarse and if applied on the 1983 results they would have given an estimate of the total stack which was f o u r times higher t h a n the sne given in Table 6 ,

Table 6 gives an index for the _{1 9 8 2} year-class of 44.6. About 90% of the 1 year old fish was founå in the eastern subarea D.

It is difficult to assecs the strength of t h i s year-class from

this single value, but eoqawed to the indices of the 1980 and 1981 year-classes as one year old fish (Table 7 ) the 1982 year-class is far more numerous than the averiage of these two very poor year-classes.

Haddock

Table 8 gives the indices from the bottom trawl survey f o r the different year-classes in each subarea and Table 9 gives the indices for the total area in 1981-1983. As for cod more than 50% of the fish are found in subareas A and D in the bottorn trawl survey. Table 9 shows that also for haddock, the bottom trawl survey does not show the reduction found in the acoustic survey. This is problably caused by similar difficulties as (

for cod in interpreting the results from this survey because only a unknown and varying proportion of the total population is sampled in this survey.

Table 8 gives an index of 314.5 for the 1982 year-class. This very high index indicate tkat the 1982 year-elass is a strong one

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REFERENCES

Dalen, J., Rylen, A., Nakken, O., Randa, K. and Smedstad, O.M.

1982. Norwegian investigation on young cod and haddock in the Barents Sea b r i n g the winter 1982. ⁽ Coun. Meet. int. Cotiri

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Sea, 19821G:4): 1-32. -- Mirneo

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Dalen, J. and Smedstad, O.M. 1.982. Abundance estirnation of demersal fish in the Harents Sea by an extended acoustic method. Contr. t o S p p . in Fisheries AcoustTcs, 21-24 June, Bergen, Norway,

Foote, K.G., Knudsen, N.P., Vestnes, G., Brede, R. and Nielsen,

R.L. 1981. Ymproved calibration of hydroacoustic

equipment _{w t t h} copper spherec. Coun. Meet, int.

or. Sea, 198P(Bi20): 1-18. Måmeo.

p - p . - -

Foote, K.G., Knudsen, ?H.P. and Vestnes, G. 11982. Standard calibration of echo sounders and integrators wåth optimal copper spheres. Contr. to Symp. in Fisheries Acoustics, 21-24 June, Bergen, Norway.

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F i g . 1. Survey tracks and hydrographical stations taken by W "G.O. Sars"

26.1.-5.3.1983.

F i g . 2. Survey tracks and trawl ctationa taken by RV "G.O. Sars"

26.1.-5.3.1983. l3 Bottom trawl. A Pelagic trawl.

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Fig. 3 . Bottom t r a w l s t a t i o n t a k e n by MT "Hagbart K r m e r " and MT t tal lo" 26.1.-26.2.1983, and botfom t r a w l s t a t i o n s e a s t of 3 5 ' ~ t a k e n by RV "G.O. Sars".

F i g . 4. The surveyed a r e a witli s u b a r e a s (A,B,C,D) and s t r a t a u s e d i n t h e bottom t r a w l s u r v e y .

Fig. 5. The temperature distribution A) on the surface, B) 100 m depth and C) at the bottom.

Fig. 6. The distribution of echo abundance ( relative valuec) for cod and haddock, 26.1.-5.3.1983.

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Fig. 7. The d"ctribution o f 2 y e a r s old eod 26.1.-5.3.1983.

(1000 per n.m.').

Fig. 8. The d'stribution of 3 years old cod 26.1.-5.3.1983.

(1000 per n.m.

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Fig. 9. The d'stribntion of 4 y e a r s old cod 26.1.-5,3.1983, (1000 per nem.

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Fig. 10. ^The g i s t r i b u t i o n of 5 y e a r s o l d cod 26.1.-5.3.1983.

(1000 p e r n.m. 1.

F i g . 1 2 . The d i s t r i b u t i o n o f 7 y e a r s o l d and o l d e r cod 26.1.-5.3.1983.

(1000 p e r n.m. 2 ) .

F i g . 13. (1000 p e r n.m. The

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i s t r i b u t i o n ) . of 2 years o l d haddock 26.1.-5.3,1983,

Fig. ₍₁₀₀₀ 14. _{per nem.} The istribution of 3 years old

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_{) .} haddock 26.1.-5.3.1983.

Fig. 15. The istribution

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of 4 yearc old Biaddock 26.1.-5.3.1983.

(1000 per n.m. ) .

Fig. 16. The g i s t r i b u t i o n of 5 y e a r s o l d haddock 26.1.-5.3.1983.

(1000 p e r n.m. ) .

F i g . 17. The i s t r i b u k i . o n of 6 y e a r s o l d haddock 26.1.-5.3.1983,

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(1000 p e r n.m.

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F i g . 18. The distribution of 7 years and older haddock 26.1.-5.3.1983.

(1000 per n.mn2).