CM_1986_H_55.pdf (3.056Mb)

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Thi~ report not to be quoted without prior reference to the Council*

International Council for the Exploration of the Sea

C .M. 1986/H: 55

Pelagic Fish Committee Ref. Fish Capture

Committee

INTERNATIONAL ACOUSTIC SURVEY ON BLUE WHITING IN THE NORWEGIAN SEA DURING SUMMER 1986

ABSTRACT

The fifth ICES-coordinated acoustic survey on blue whiting in the Norwegian Sea and adjacent waters was conducted from the end of July to the beginning of September 1986. All together eight research vessels from USSR 1 Norway 1 Farce Islands, GDR ¹ Iceland and Denmark participated.

The area from Skagerrak/Kattegat to the Barents Sea was surveyed, and blue whiting was recorded over major part of the Norwegian Sea, the highest concentrations found more to the south than to the north.

*

General Secretary ICES, Pal~gade 2-4 DK-1261 Copenhagen Denmark

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lN'TRLLYUCTION

In accordance with an ICES recommendation the first ICES-:-coordinated international acoustic survey on blue whiting in the Norwegian ·Sea took place in August 1982 (Anon. 1982), and similar surveys have been carried out since. (Anon. 1983a, 1984,

l 985a).

The aim of the acoustic surveys in the Norwegian Sea during summer was to :n:lOnitor the total "northern'-' blue whiting stock as a complement to the acoustic estimate·s of the spawning stock.

The fifth ICES-coordinated acoustic survey on blue whiting in the Norwegian Sea and adjacent waters was carried out from 22 July to 5 September 1986 (C.Res. 1985/2:3:21).

MATERIAL AND METHODS

Eight vessels from six countr~es took part· in the survey:

Inst.rument constant Country Research vessel Survey period

cr

USSR "Vilnius" 30 Jul - 18 Aug 1 . 0 9 Norway "G.Oa Sars" 22 Jul

-

1 7 Aug 0.46 N orway "Eldjarn" 29 Jul

-

18 Aug 0.56*

GDR "Eisbaer" 10

-

31 August 4.38 F aroes "Magnus Heinason" 7

-

19 August 1 . 4 9 Iceland "Arni Fridriksson" 7

-

25 August 3. 0 5 Iceland "Bjarni SCEmundsson" 6

-

21 August 2.60

D enmark "Dana" 1 2

-

26 August -43.2**

-·

* Raised to 1. 0 2 from intercalibration with "G .0. sa·rs"

** Expressed in dB

Vertical opening of pel.

trawl (m)

50 40, 20

20 20 40 17 17 18

Names of the institutes and the scientific staff participating in the survey are given in Appendix I~

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SuLvey plans and tentative cruise tracks and area to cover were worked out by correspondance and telephone. The procedures at sea and the conveyance of data were the same as described in the Planning Group Report of 1983 (Anon. 1983b).

The cruise tracks with the trawlstations and the hydrographical stations are shown on Figs 1 and 2. For some of the vessels the survey had several objectives, of which collecting blue whiting data was only of minor importance. This was the case in the area around Iceland, west of Jan Mayen, from Jan Mayen to the Bear Island area and in the Skagerrak/Kattegat area.

All vessels used 38 kHz echo sounder connected to the echo integrator. The instrument constants (C

1) by vessels obtained by calibration against a 60 mm copper sphere standard target (Foote, 1981), are listed in the text-table above.

The spite of standardized integrator values based on standard-target calibration, a ship to ship-calibration between

"G.O. Sars" and "Eldjarn" showed significant differences (Monstad, 1986). It is hard at present to explain this difference, but one reason might be due to the threshold effect which causes most vessels to have problems with single-fish integration at depths deeper than 100 m (Anon. 1985b). Using the blue whiting recordings obtained from the ship to ship-calibration, i.e., the recordings at depths between 200 and 500 m only 1 the relation "G.O.

Sars"/"Eldjarn" = 1.82 was obtained (Monstad, loc. cit.). This figure was then used to raise the "Eldjarn"-va1ues to the

"G.O. Sars"-level. As no other ship to ship-calibration was performed, all other c

1-values were applied as provided.

The acoustic data from all vessels, but "Dana", were combined and treated as a whole. The blue whiting biomass and abundance were estimated using the same method as in previous years (Anon. 1984 and 1985a). The density coeffisient (CF) used for blue whiting is the same as has been used for young cod, corresponding to a target strength of -40,5 dB for a 30 cm fish (Anon. 1982 and 1985b)

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where Lis fish length (Anon. 1985a).

The R.V. "Dana" which surveyed the Skagerrak/Kattegat area used a different estimation technique from the other vessels (Degnbol and f?angsborg, 1985). Hence the estimate 'from this area was made separately.

Almost all blue whiting samples were obtained by pelagic trawling.

As in previous years the size of the trawl varied very much between the vessels, as shown by the trawl openings listed in the text-table aboye.

The length distributions, weighted by the abundance

based on data from all vessels, were in the different areas, and the stock was divided into year-classes from age-length keys based on the Norwegian samples.

A great number of hydrographical stations were worked, the vessels using either CTD-sonde, bathythermograph or Nansen water-bottles with reversing thermometer.

In addition to ~he acoustic survey, information on blue whiting as . by-catch in bottom trawl were also received from a Norwegian research vessel which made shrimp investigations in the Svalbard area from mid July to mid August.

RESULTS

Hydrogr~hy

Temperature distributions at 0, 200 and 400 m depth are shown in Figs 3, 4 and 5. Compared with the years since 1980 the te111perature in August 1986 were generally somewhat low. At the surface the. sector ec;:tst of Iceland was relat:ively warm, while the temperatures off the coast of Norway and in the area between Jan Mayen and northern Norway were relatively cool.

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At 200 m the temperature in the East Iceland Current had not changed much since 1985, but the front extended relatively far to the east, to about 1°E. Off the coast of Norway the waters were rather cold. There were no temperatures above 7°C north of 65°N, while water with this temperature extended to 68-70°N in the previous years.

At 400 m the year-to-year fluctuations are generally small, and the conditions in the East Iceland Current were not much different from the previous years. Off the coast of Norway the temperatures were relatively low.

The distribution and the density values of blue whiting recorded during the survey are shown in Fig. 6. The over-all recordings were in general weaker than during the previous four years, but the distribution pattern very much the same. Scattered recordings of blue whiting were made over most areas of the Norwegian Sea, with the highest concentrations found in the southern part; in the Norwegian Deep, in an area along the 0-longitude at 64° latitude, in the polarfront-area between the Farces and Iceland and in a small locality west of Iceland.

The area of distribution was well defined to the west, but not to the south and .to the north. North of the fire a covered by the acoustic survey, i11 . the Svalbard area, various numbers of blue whiting were caught in the bottom trawl catches during a Norwegian

s~rirnp investigation from mid July to mid August (Fig. 7), (0ynes, not published). This showed that blue whiting were distributed at bottom along the slope north to 78°N, and in small numbers also at 79°40'N ..

The total biomass of blue whiting recorded during the acoustic survey was estimated to ~_Q_ mi_ll. tonnes, and the abundance to 28.0 x 109

individuals (Table 1) .. The 1981 year-class and older contributed together with 0. 9 mill. tonnes, the 1982 year-class

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with 1. 0 mill. tonnes and the 1983 year-class with 0. 7 mill.

tonnes. The 1984 year-class was represented with 0.2 mill.

to~~es and the -85 and -86 year-classes with 0.1 mill. tonnes each. The assessed biomass by rectangle is shown in Fig. 8, where also the different sub-areas used in the calculations are marked.

The total length and age compositions of the stock weighted by abui)dance are shown on Figs 9 and 10 , and for the different sub-areas on Figs 11 and 12 respectively.

The 1986 year-class with length from 10 to 17 cm, was found in sub-area III and IV where it in numbers contributed with 45% and 88% of the observed blue whiting. In total, the contribution of the 1986 year-class was 20%, while the most numerous one, the , 1982 year-class, contributed with 27% and the 1983 year-class

with 22%.

While in north, in area V and IV,.the year-classes 1981 and 1982 were most numerous, the year-classes 1982 and 1983 dominated in south, i.e. area I, II and VII.

The blue whiting caught in bottom trawl at the slope in the Svalbard region (Fig. 7) was mostly large fish with length between 33 and 40 cm (Fig.. 13).. The peak length of 37 cm was 10 cm more than found in the total distribution from the acoustic surveyed area in the Norwegian Sea (Fig. 9).

The total weight/length relationship of blue whiting is plotted on Fig. 14.

DISCUSSION

The biomass and abundance by year-class estimated during the blue whiting surveys in the Norwegian Sea since 1983 are given in the text-table below.

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1983 1984 1985 1986

-

Age

t •10 -6 N •10 - 9 Age

t •10-6 N •10 - 9

Age -6 -9 Age

t•1o-6 N•10-9

years years years t•10 N•10 years

0 0.22 8.5 0 0.05 2.1 0 0.03 2.2 0 0.08 5.0

1 1. 52 22.7 1 1.77 30.6 1 0.47 6.0 1 0.13 2.0

2+ 1. 09 5.3 2 1..56 14.6 2 2.40 24.0 2 0.17 2.1

3+ 0.41 1.9 3 1.58 12.5 3

I

o. 71 5.9

4+ 0.46 2.4 4 1.05 7.9

5+ 0.87 5.1

Total 2.83 36.5 3.79 49.2 4.94 47.1 3.02 28.0

The estimate obtained in 1986 (3.0 mill. tonnes and 28.0 x 10 9 specimen) indicates a reduction both in biomass and in number of approximately 50% compared to the 1985 survey and back to a level close to the level found in 1984.

The Blue Whiting Assessment Working Group has considered all the ICES coordinated blue whiting surveys in the Norwegian Sea as underestimates for a number of reasons (Anon., 1986), and as the procedures and methods have not been changed, the estimates obtained in 1986 is most probably also an underestimate. During a Workshop .in 1985 (Anon. 1 1985b) the problems were discussed in detail. One of the big problems identified by the Workshop was that most vessels were not able to detect or integrate single fish concentrations of blue whiting at the relevant depths. This means that a certain minimum concentration of blue whiting was needed in order to have a proper integration. This minimum concentration varied between vessels.

The threshold effect was clearly demonstrated by the intercalibration between "Eldjarn" and "G .. O .. Sars" in which the standard target corrigated values of "Eldjarn" had to be multiplied by 1 .. 82 in order to be of the same levels as the

"G.O. Sars" values (Monstad, 1986).

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None of the v~sse~s participating in the 1985 and 1986 surveys were able to integrate properly single fish concentration of blue whiting at depths larger than 133 m, and for this reason alone the estimate for ·1arge areas of the Norwegian. Sea must be underestimates. With the present methodology it is not possible to adjust for this effect either.

A change in the migration routes into the Norwegian Sea has been demonstrated due to changes in the hydrographic conditions (Schevchenko, 1984, Schevchenko and Isaev, 198 3 and 198?, and Monstad and Blindheim, 1986). In years with a strong influx from the East Icelandic current the cold water acts as a barrier for the· post-spawners migrating north. in the western part of the Norwegian Sea forcing them to follow a more eastern route. It is hard to quantify this effect on the ass~ssment, if any. However,

bl~e whi~ing mi~rating north in the eastern parts of the Norwegian Sea are · dispe.rs ed over larger areas, · and in this case the bias introduced by the threshold effect may be increased.

As mentioned above the methods use·d during the blue whiting surveys-since 1982 has not been changed, and the cF·value used to convert integrator values 'into biomass has also been the same. It is, however, worth noting that the CF value used for young cod, which previously: was the same as used for blue whiting, has now been changed to a 'value almost 2 dB· larger (Aglen et al., 1985, Hylen et al., 1986). Applying the new cod value on blue whiting would result in an almost 100% increase of the present assessment.

REFERENCES

Aglen, A., Lahn-Johannessen, J. and Smedstad, O_M. 1985. An

acoustic survey on o~group gadoids in the northern North Sea and the Shetland -·Orkney area, July 1984. ICES. C.M.

2..2

8 5/.G : 1 "Z_ , 1 -1 3 [Mime 6 J ~

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Anon. 1982. Report of the International Acoustic Survey on Blue Whiting in the Norwegian Sea, July/August 1982. ICES, C.M. 1982/H: 5, 1-21 [Mimeo].

Anon. 1983, a). Report of the International Acoustic Survey on Blue Whiting in the Norwegian Sea, August 1983. ICES, C.M.

1983/H:5, 1-20 [Mimeo].

Anon, 1983, b). Report of the Blue Whiting Planning Group for the Coordinated Acoustic Survey 1983, Copenhagen, 2-3 March

1983. ICES, C.M. 1983/H:4, 1-12 [Miineo].

Anon~ 1984. International Acoustic Survey on Blue Whiting in the Norwegian Sea, August 1984. ICES, C.M. 1984/H:67, 1-21 [Mimeo].

Anon, 1985, a). Acoustic survey on Blue Whiting in the Norwegian Sea, August/September 1985. ICES, C.M. 1985/H:4, 1-24 [Mimeo].

Anon. 1985, b). Report of the Workshop on the International Acoustic Surveys of Blue Whiting in the Norwegian Sea, Bergen 6-11 May 1985. ICES, C.M. 1985/H:G, 1-27 [Mimeo].

Anon. 1986. Report of the Blue Whiting Assessment Working Group 1985. ICES, c .M. 1986 /Assess: 3, 1-61 [Mimeo].

Degnbol, P. and Bangsborg, L. 1985. A software package for acoustic

~urvey evaluation. ICES, C.M. 1985/B:39, 1-10 [Mimeo].

Foote, K.G. 1981. Echo sounder measurements of backscattering cross sections of elastic spheres. Fisken og Havet, Ser. B, _l981 (6): 1-107.

Hylen, A., Jakobsen, T., Nakken, 0., Nedreaas, K. and Sunnana, K.

1986. Preliminary report of the Norwegian investig~tions

on young cod and haddock in the Barents Sea. ICES, C.M.

12~~LG:76, 1-25 [Mimeo].

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Monstad, T·. 1986. Report of the Norwegian surveys on Blue Whiting during spring 1986. ICES, C.M. 198618~53; 1-20 [Mimeo].

Monstad, T. and Blindheim, _{J. 1986.} Relationship in distribution of blue whi.ting ·and hydro graphic conditions in the

Norwegian Sea during summer, 1980 - 1985. ICES, C.M.

1 9 8 6 I H : 54 , 1 -1 8 [Mime o

1.

Shevchenko, A. V., 1984. Peculiarities of blue whiting feeding migrations in the Norwegian Sea in· summer 1983. ICES, C.M. 1984IH:8, 1-11 [Mimeo].

Shevchenko, A.V. q.nd Isaev, N.A. 1983.. Year-to-year variations of blue whiting distribution in the Norwegian Sea in spring- summer 1978 - 1982 due to hydrographic conditions. ICES, C . M . 1 9 8 3 I 8 : 2 6 , 1 -1 8 [Mime o ] •

Sheychenko₁ A.V. and Isaev, N.A. 1985. Peculiarities of blue

whiting distribution in the Norwegian Sea in spring-summer 1983 and 1984 in relation to hydrographic conditions.

ICES, C.M. 198518:14, 1-11 [Mimeo].

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Table 1. Biomass (tonnes x 10-3

), abundance (N x 10-6

), size of distribution area (n.mile2 ), density (tonnes/n.mile2

and N x 10-3

/n.mile2

)of blue whiting by year-classes and areas in the Norwegian Sea. July/August 1986.

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a ^VII

^I I

Î ÎI ÎII ÎV

I

^V

^I

^I ^VI ^Total

N

I

^Biom.

I .

Biom. _N Biom. N Biom. N Biome N Biom. N B~om. N Biom.

I

1986 - - - - I 0.1 3 42.2 3012 32.2 2003 - -

·-

- 74.5

1985 6.5 72 33.4 627 I 60.1 825 10.4 146 0.1 1 7.2 89 14.8 248 132.5 1984 50.0 658 17.5 262 70.5 833 21.5 214 0.5 4 4.0 42 5.3 68 169.3 1983 99.9 887 80.8 738 290.1 2418 167.9 1290 10.2 68 28.0 197 37.3 278 714.2 1982 1284.0 2104,102.1 I 853 329.7 2594 189.7 1386 19.4 122 54.5 358 67.9 480 1047.3 1981+j250.4 _I 1299 : 77.3 498 175.5 1174 95.6 585 12.9 77 139.1 700 134.4 737 885.2

N

5018 2008 2099 5858 7897 5070 Sum I

J690. 8 5020 i ! 311.1 2978 926.0 7847 527.3 6633 75.3 2275 232.8 1386 259.7 1811 3023.0 27950

Area sizel 18003 35112 110876 58581 2586 69622 49486 344365

I

Density!38.4 _i 279 8.9 85 8.4 71 9.0 113 29.1 880 3.3 20 5.2 37 8.8 84

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45

_!!_,:Jkf

6 6 r--/ 32 19 54 68 33 36 68 120

,.

,•_ 65 44 ~64

.

^~ 40 30 43 27

~

75

72

68

66

65

61.

63

62

..

¹⁵ ¹⁹ ^\21 ³⁷ ²⁶ ¹⁹ ⁵⁵ _~~...k

2 1 17 7

-~~

⁵¹ ^~-

:;.r

^L'

52 'fS9::

,..•"'•': '~i-

.

N

^J[

. Jrl- v)' ,. IT w

³⁵

~~._)93< 84~

?

¹⁰³ ^1~8^\

Q~~ ¹

~

I

-·~

.d

- ^r~

~;;~~~ \

·-

. --·

61

60

59

se

57

56

ss ~~ - 3S - J() 25 n ¹⁵ 10

Fig. 8. Biomass of blue whiting (1000 tonnes) splitted on rectangles, August 1986. Markings of sub· ·reas I-VII.

~

\0

(21)

20

10 15 20 25 30 35 40 cm

Fig. 9. Total length distribution of blue whiting weighted by abundance, Norwegian Sea, August 1986. N: 28.0 x 10 9 specimens.

0 2 3 4 5 6 7 8 9 10 11 12 13 14 15+

Fig. 10. Total age composition of blue whiting, Norwegian Sea, August 1986. N: 28.0 x 109

specimens.

(22)

21

20 N

=

1,8

10

20 _{N = 1,4}

10

30

20 _N

=

2,3

10

20 _N= 6,8

10

20

n

^N⁼^{7, 8}

10

20 I N = 3,0

10

~

^I

30

20 N = 5,0

10

'

^'

~~

I

10 15 20 25 30 35 40 cm

Fig. 11. Length distributions of blue whiting ^we~ghtedby abundance in the different sub-areas (see Fig. ^8)~ N: x 10 specimens.

(23)

22

40 20

60

40 y

20

80 60 40 20

0 2 3 4 5 6 7 8 9 10 11 12 13 14 15+

40

m

20

40

1I 20

40

I 20

OoOOoQepcpcpC?

40 20

0 2 3 4 5 6 7 8 9 10 11 12 13 14 15+

Fig. 12. Age compositions of blue whiting weighted by abundance in the different sub-areas (see Fig. 8). N: same as in Fig. 11.

(24)

% 30 20 10

20

23

N=220

cm

Fig. 13. Length distribution of blue whiting from bottom trawl catches in the Svalbard area, 14 July - 15 August 1986 (see Fig. 7

(From 0ynes, P., unpublished).

(25)

24

(•)

450

400

350

(26)

25

Appendix I Blue whiting acoustic survey, Norwegian Sea, 1986.

Survey period 30.07 - 18.08

29.07 - 18.08

23.07 - 17.08

10.08 - 31 . 0 8

07.08 - 25.08 06.08 - 21 . 08

07.08 - 19.08 12.08 - 26.08

Research vessel

"Vilnyus"

"Eldj arn"

"G.O. Sars"

"Eisbar"

Institute

Polar Research Institute of Marine Fisheries and Oceanography - PINRO - Murmansk USSR

Havforskningsinstituttet Bergen, Norway

Institut fur Hochsee- fischerei und Fisch-

verarb~itung, Rostock GDR

11Arni Hafrannsoknastofnunin Fridriksson" Reykjavik, Iceland

"Bjarni Sa:mundsson"

"Magnus Heinason"

"Dana"

Hafrannsoknastofnunin Reykjavik, Iceland

Fiskirannsoknarstovan Torshavn, Faroe Islands Danmarks fiskeri- og

havunders~gelser

Hirtshals, Denmark

Scientific staff

Vanjukhina N.V., Zubov V.I., Lovchikov A.L., Mamylov

v.s.

Pavljuchenko L.L.,

Semochkin A.I., Ustinov S.B. 1

Ushakov N.G., Shabalin B.S.

Blindheim J., Midttun 0.

Tangen 0., Torgersen 0.

Valantine A., 0stensen 0.

Gullaksen

o.,

Monstad T. (co- ordinator), S~len E.,

Wallevik M., Austgulen L., Abrahamsen H,, M~rk T., Totland A.

Vaske B., Gentzen B., Hamann K., Albrecht J., Buck W.

Sveinbj~rnsson

s.,

Helgason V.

Vilhjalmsson H. ,

Magnusson J.V., Malmberg S.A.

Reynisson P., Astthorsson 0.

i Jakup s stovu

s.

H., Thorns en B.

Kristiansen R.

St~hr K.J., Filt Jensen T., Degnbol P., Henriksen W., Jansen C., Hansen A.D., Kirkegaard E., Lewy P., Harr T., Dahlskov J., Palmen L.E.