CM_1985_G_17.pdf (1.768Mb)

(1)

International Council for the Exploration of the Sea

C.M. 1985/G:17

Demersal Fish Committee Ref. B

AN ACOUSTIC SURVEY ON 0-GROUP GADOIDS IN THE NORTHERN NORTH SEA AND THE SHETLAND-ORKNEY AREA, JULY 1984

ABSTRACT

by

A. Aglen, J. Lahn-Johannessen and O.M. Smedstad Institute of Marine Research

Bergen, Norway

An acoustic survey was carried out by two research vessels in July 1984 to assess the distribution and abundance of 0-group gadoids.

The 0-groups were distributed in scattering layers by night and in schools by day. Observed vertical migrations, most predomi- nant with Norway pout, indicated fish ascending towards the surface by night. Rather little mixing between 0-group and demersal fish were observed during daytime.

Acoustic estimates of the abundance of 0-group gadoids were based on the target strength/ length (L cm) relationship TS ⁼

21. 8 logL -7 4. 0. Total estimates for the surveyed area were 124 · 109

Norway pout, 69 · 109

whiting, 42 • 109

haddock and 0.8 · 10 9 cod.

The timing of the survey proved satisfactory in the sense that 0-groups were recorded almost entirely in their pelagic phase and the catches could be considered fairly representative.

(2)

An acoustic survey on 0-group gadoids in the northern North Sea during July-August 1978 gave promising results (Nakken and

R~rvik 1979). Subsequent surveys on herring in the Shetland- Orkney area during July in the years 1979-1983 also·indicated suitable pelagic distribution of 0-group gadoids for acoustic abundance estimation.

This paper presents the results concerning 0-group gadoids from a joint survey in July 1984. Results concerning herring and sai the are reported by Simmonds et al. ( 19 8 5) and Smedstad

(1985), respectively.

MATERIAL AND METHODS

In July 1984 the Norwegian acoustic herring survey in the Shetland-Orkney area was combined with an expanded coverage of the northern North Sea. R/V "G.O. Sars" and R/V "Eldjarn" made a joint survey of the northern North Sea (Fig. 1 and 2). The coverage of the Shetland-Orkney area is shown in Simmonds et al. (1985).

Observations on distribution and abundance of fish were made by echo integration combined with trawling. Technical data of acoustic instruments and trawls are presented in Table 1 and 2.

Average integrator values were printed out each five nautical mile. Echo traces were identified through trawl catches. Such identifications formed the basis for allocating integrator values to 0-group fish.

Average integrator values of 0-group fish were calculated within ICES statistical rect~ngles. The average species composition by numbers, of all catches taken to be representative for the 0-group recordings in each rectangle, where then used to calculate the average integrator value (M) for each species. The number (N) of each species were calculated by the equation

(3)

N =M • 0.1 • 10- 0 • 1 TS • A

where A is the area of the rectangle and TS is the average target strength of the species. TS

=

21.8 logL -74.9 where

E

is the average fish length in cm, was used for all 0-group species.

The echo integrator outputs were standardized for both vessels according to calibrations made with a standard sphere so that equal fish densities should give equal values on both vessels.

To get a check on the results some rectangles were covered by both vessels within short intervals.

R/V "G.O. Sars" repeated a part of the survey track to make comparisons of results obtained during day and night.

RESULTS

The estimated number of each 0-group species within statistical rectangles are shown in Figure 3 and 4. The total estimates for the area covered were

124

.

109

Norway pout 69

.

109

whiting 42

.

109

haddock 0.8

.

109

cod

0-group Norway pout was distributed to the south and southeast of Shetland. It formed pure and rather dense, irregular schools 5-20 m above the bottom during day and scattering layers mixed with other 0-groups in the upper 70 m during night.

0-group whiting and haddock were distributed over most of the area investigated. Whiting dominated in the southern and haddock in the northern part. A slight admixture of 0-group cod were observed in the northern region. These species tended to form small schools in the upper 70 m during daytime which

(4)

scattered and raised slightly towards the surface by night.

The duration of the night situation was 4-5 hours.

Except for a few of the largest haddock and Norway pout no 0-group gadoids were recorded in demersal trawl catches.

The overall length composition of each species are presented in Table 3. In general the average length increased from North- east to Southwest.

Table 4 compares integrator values obtained onboard R/V "G.O.

Sars" and R/V "Eldjarn". This gives no evidence of systematic differences between the vessels. The correlation was low, however, due to great differences in two of the rectangles.

Figure 5 compares values obtained along the same survey track during day and night. The night values were significantly lower than the day values. The average of these observations gave 10 percent lower values during night. The most likely reason for this is that some of the 0-group stayed above the transducer depth during night, while this did not seem to occur during day. If 10 percent reduction is assumed to be representative for all nights, this would lead to an overall underestimation of about 2 percent when day and night observations are combined.

DISCUSSION AND CONCLUSION

The vertical distribution of 0-group was found to be suitable for echo integration. Only a small proportion seemed to stay above the transducer depth and most of i t occurred separate from larger fish. In some areas i t was difficult to separate recordings of 0-group from the recordings of plankton. In such cases, care was taken to avoid allocating too much of the integrator contribution to 0-group. Therefore the integrator values of 0-group are more likely underestimated than overesti- mated. This does not neccessarily lead to underestimation of the abundance, because the abundance estimates directely depends on the target strength function applied. The function

(5)

appears to give reasonable abundance estimates of larger cod and haddock in the Barents Sea (Dalen et al. 1984). It is not known, however, to which extent i t represents 0-groups in the North Sea.

The conversion factor (C-value) applied during the 0-group survey in 1978 (Nakken and R~rvik 1979) represents a slightly different target strength function.

Acoustic calibrations from that period are too uncertain to make proper adjustments for this difference. Comparisons of the results from 1978 and 1984 must therefore be made with care.

The total estimates for the statistical rectangles covered both in 1978 and 1984 are as follows:

Norway pout whiting haddock cod

Number of fish x 10 -9

August 1978 165

insignificant 23

insignificant

July 1984 57 58 32 0.4

The 1984 survey showed considerably less Norway pout than the 1978 survey when comparing the estimates within the same area.

The estimates for the whole area covered are, however, more similar (124 · 109

in 1984 and 165 · 109

in 1978). In 1978 the main concentrations were found in the central part of the are covered while in 1984 large concentrations near the southern border indicate a possible continuation to the South. On the other hand the 19 7 8 survey did not cover the area between Shetland and the Orkneys, where considerable amounts of 0-group Norway pout have been recorded during the herring surveys in the years 1979-1984.

The English 0-group surveys in August gave a low index for the 1984 year class compared with the 1978 year class, while the

(6)

International Young Fish Survey (IYFS) and the virtual popu- lation analyses for 3.. and 4.. quarter of 1984 indicate that these year classes are of similar size (ANON 1985a) ..

The most striking difference between the results is the insignificant proportion of 0-group whiting recorded in 1978 compared with 1984. This is, however, in good agreement with the pattern of distribution reported by the International 0-group gadoid survey in June-July 1978, whereby rather few whiting were caught within the area covered by the Norwegian acoustic survey, and the main concentrations were found else- where (Benjaminsen et al. 1978).

The estimated abundance of haddock was slightly higher in 1984 then in 1978. Preliminary results from the IYFS indicates, however, that the 1978 year class is larger than the 1984 year class (ANON 1985b).

As previously mentioned the two surveys are not quite compar- able. The areas covered, the timing and the methods applied are slightly different. Even if the estimates are given as absolute numbers of fish, they should be regarded as relative, due to uncertainties concerning the target strength of 0-group gadoids. It is therefore important to repeat the survey in a standarized way for several years to evaluate the quality of the estimates of year classes strength.

REFERENCES

ANON., 1985a. Report of the Industrial Fisheries Working 6-12 March 1985. ICES CJYI 1985/

Group, Copenhagen, Assess:8 (mimeo).

ANON., 1985b. Report of the North Sea Roundfish Working Group, Copenhagen 12-21 March 1985. ICES CM 1985/Assess:9

(mimeo) .

BENJAMINSEN, T., N. DAAN, J.R.G. HISLOP, M.J. HOLDEN, J.

LAHN-JOHANNESSEN and W.G. PARNELL, 1978. The results

(7)

of the International 0-group Gadoid Survey in the North Sea, 1978. ICES CM 1978/G:10 (mimeo).

DALEN, J. , A. HYLEN, T. JAKOBSEN, 0. NAKKEN and A. RAKNES, 1984. Preliminary report of the Norwegia~ investi- gations on young cod and haddock in the Barents Sea during the winter 1984. ICES CM 1984/G:44 (mimeo).

NAKKEN, 0. and C.J. R0RVIK, 1978. Distribution and abundance of 0-group Norway pout and haddock in the North Sea July-August 1978. ICES CM 1979/G:50 (mimeo).

S IMMONDS, E. J. , R. S. BAILEY, A. A GLEN, A. JOHANNESSEN, J.

LAHN-JOHANNESSEN and O.M. SMEDSTAD, 1985. Report on the 1984 herring acoustic survey in the northern North Sea. ICES CM 1985/H:34 (mimeo).

SMEDSTAD, O.M., 1985. Acoustic estimates of saithe in the North Sea in 1984 and 1985. ICES CM 1985/G:14

(mimeo) .

(8)

Table 1 Technical data and settings of acoustic equipment

Echo eounder Frequency Receiver gain Time varied gain Pulse 1ez:tgth ilandwidth Transducer

Effective beam angle (10 log 't')

Basic range:

Source level + Voltage response Integrator 1ntegrator gain Integrator threshold Instrument constant (C

1)

fo~ survey settings Date of calibration

R/V "'G.O.Sars"

Simrad EK 400 38 kH:.:

- 10 dB

20 logR + 2·0.008·R 1.0 ms

3.3 kHz 45 )( 48 cm - 23.2 dB

1~0 m 134.4 dB

NORD-10 computer 40 dB

17 millivolts 0.087

25 .July 1904

Table 2 Technical data of trawl equipment

Trawl type

Vertical opening (typical) Mesh 111i:r.e front

(stretched) Mesh t&iz.e

cod end (stretched) Bridle length Door shape Poor weight

IK>ttcm f·oto (Hod.BD) Campelen herring trawl shrimp trawl

15 Ill 5 m

6400 ll1m BD I1D1l

11 lnrn 6 mm

QlO m 40 m__, Circular

7~0 kg 4.6 Ill 2

R/V "Eldjarn ..

Simrad LK 400 30 kUz - 20 dB

20 logR + 2"0,008"R 1.0 IllS

3.3 kHz 30 )( 30 ^C1L - 19.6 dB

150 m/250 m 141.3 dB

IDRD-10 computer 40 dB

28 zi1livolts 0.104

14 .June 1984

R/V .. £ldjarn"

Pe1aQic Bottom Capelin Campelen trawl shrimp trawl

15 IlD 5 1111

200 ITD1l BD lniil

10 mm 6rnm

~

Rectangular

)(jDuring 10-17 July a capcliu tru...-1 ide11ticul to tlw ouc on "Eldjaru" was used.

(9)

Mean Number

Cm-group 2 3 4 5 6 7 8 9 10 11 12 13 length measured

Norway pout 0,2 4,9 19,3 39,6 27,4 6,3 2,3 5' 67 922

Whiting 0,5 6,3 18' 6 28,8 24,0 14,4 4,6 1,7 1,0 0,1 5,83 2293

Haddock 1,5 6,8 17,0 25,3 22,4 16,0 6,5 2,9 1,0 0,5 0,1 7,09 3266

Cod 1,0 33,7 48,0 16,3 1,0 5,33 98

Table 4. ~omparison between average integrator values of 0- group fish per statistical rectangle jointy covered by R/V ¹¹G. 0. Sars" and R /V ¹¹Eldjarn¹¹• Number of 5 nautical miles surveyed in brackets.

IC~Srectangle

I

^48E8 ^48E9 ^48FO ^48F1 48F2 48F3 47E8 47E9 46E8 46E9 45E8 ^ID

R/V ¹¹G. 0. Sars¹¹1 82 101 112 52 39 15 235 88 177 108 41

( 3 7) (33) (6) ( 7) (6) (6) ( 24) ( 19) ( 11) ( 12) ( 15)

R/V ¹¹Eldjarn¹¹ 250 89 85 56 32 21 183 101 170 109 164

\2) ( 13) ( 11) ( 13) ( 12) ( 14) ( 4) (14) ( 10) ( 13) ( 11)

(10)

"' 14 '" '"

^l ^t ^~11---^...

.

J32

9

^JJO

~~t:-9----~

JJJ 331. JJS

338 317

n3 1n

'----o-r----r-0~--or-D--t--_....

JI.P 350 351 JSZ

10-17 JUL. 1984 ST.N0.764 -777 TRAWL ST. N0.330-356 "'G.O. SARs·

Figure 1. Survey track and stations R/V ¹¹G .. O. SarsP 10-17 July 1984.

, ..

271

&.----.:r 2l0

E JAAN

Figure 2. Survey track and stations R/V ¹¹Eldjarn¹¹ 21-31 July 1984-

(11)

53

1 I I I I {\ I {\ ') I I I r

^{1 5 3}

0 0

~

1\ 0 0,3

~

I/"'

~ If'

0 0 0 0

c~

I

0

~~

./"'1,,. ^[]^!:;)^0,8 ⁰ ⁰ ⁰ ⁰ ^~

~

0 ^Q ^6,2~ 2,2 + 3,9 0 0 0

~~

~

I

6,0 2,3 2,2 _• 0 3, 7 2,0 0 0 0

·~

~ ^~111,6^Cl ^10,5 ⁰ ^{0,5 10,3} ^{4,9 0,2}

{;;

⁽⁽

(

~

11 0 0 0 2,3 1,6 5,8 1,1 )

/ 0 0 0 0 19,9 0 46

0,6 ^~

0,2 0,1 0

c~

.I

+A)

{j

^{[} +} 0,7 0,5 0,2 ^I

~

+ _Q +

''lr.~

~~

+

·o

~ 0, 7 1,9 0,6 0,4 0,1 +

~

Q

+ 0,2 _•0,5 4,3 0,5 0,1 1,6 0,9 0,4

{;;

I

.

~

~ ^~~^Cl ^{0' 5} ^{0,9 1,9} ^{0,6 0,3} ^0,2 ^0,2 ⁽⁽

[

h 2 0 9,1 3,1 2,7 1,7 1,1 0,3

~ ^'

) """

3,3 5,4 3,1 1,6 2,0 1,3

) /

52

J I I I I

^{u, ~I} ^u,⁶

I

^u,¹

I

^u

I

^~52

61 ^V I I I I I I I I I ll I 6 1°

51 i I I Jfi I I I I I N ~ I 51

5o ~

I

+

Ql

·o ~I o, 7

I

1,

91

o, 6

I o,

4

I

o, 1

I

⁺ ~~

d

5o 6 0 ^V

r

I I I .I I I I ~ }~ 6

o

⁰

4 9

i I I

~

I I I I I I I

~\X

Y

4 9

sn~

I I I I I I I I j.\r1 48

5) ^0,~~·

0,51 0,91 1,91 0,61 0,31 0,2

I

^0,2

I I OO"v ~

⁴⁸

47 1 f ?

nl

o

,1

~

,1

?

71, 71,,

In~ I I ( 147

58° 46

y

0 0 0 24,1 45 58°

V-

^4,2 3,4 1,2 0,5 45

-

A

j

⁰ ⁰

)

I ^j

1,5 2,1

) - B

I

⁴³

57° ⁴⁴ ⁴⁴

57°

43

,... ,,...

h ^,... ⁾ ^,... h " ^n. " " ,,. _h ,... h /""\ n ^,... h /""\

4 3 2 1 0 ¹ 2 3 ₄ 5. 6 4 3 2 1 ⁰ ₁ 2 3 ₄ 5 6

Figure 3. Estimated number in millions of Norway pout (A) and whiting (B) by statistical rectangles.

1-' 1-'

(12)

0 1,5

n

"\_

53 0 0,01

("'

0,7 2,2 0,8 0 ^~

(~

+

Au

^{[] +} 2,2 3,1 2,8 0,7 I

~

Q ,,.~.

? 3 5 5,9 1,6 1,8 0,5 0,1

~{

+ 0 ,

Q :-

61°

0,04 0,24~0,04

0

Au

^{[] +}

^o,o:

0' 15 0,01

Q

'J1r.~

0 Q 0 :- 0 0,02 0,09 0 52

61°

51 50

~G

^I

+ 0,1 0,7 0,7 1,8 0,7 0,8 0,5 0,2

• ^~r

60°

0 0 0,01 0 0,09 0 0

I

60°

49

+ ^I_rb 0' l 0,3 0,8 l', 2 0,2 0,1 0,1 (

-d

~ ^Cl ⁰

z~

⁽

~ ^~ ⁰ ^l ^0,4 ^1,3 0,4 0,3 0,3 0,1 '

j "'"'

) / 0,4 0,4 0,6 0,3 0,4 0,5 59°

58°

I ~ 0

~

^~^C'. ^0,01 ⁰ ⁰ ⁰ ⁰

~ ⁰ ⁰ ⁰ ⁰ ⁰ ⁰

J

0 0 0 0 0

/ 48

59°

47 46

58⁸

~~

^0,3 ^0,~ ^0,1 ⁺ ⁴⁵

y

⁰ ⁰ ⁰ ⁰

-

A

j

^0,1 ^0,2

)

"

-," I

h " ^h " ^h " ^h ^r. ^I

57°

4 3 2 l 0 l 2 3 4 5 6

B

j

⁰ ⁰

)

I

A T A 1'1 ^A h ,.... _1'1

44

57°

43

4 3 2 l 0 l 2

Figure 4. Estimated number in millions of haddock (A) and cod (B) by statistical rectangles.

0

+

0 0

0 0 0

{'\ h

3 4

~

t'\ 7

(~

~

If

I

~

~ _~

(

...

,....

5 6

53

52 51

50 49

48 4 7 f-1

[\.)

46 45 44 43

(13)

0 300

41 N 0

0 0 01

6 02

L'llU

(I)

::>

...

et >

'"'

D

,J,.)

a;

'"'

en C1J ,J,.)

c

lOO

0

Figure 5. Integrator values of 0-group fjsh recorded by RJV "G.O. Sars" 13-14 July on part of the transect along N60° 22¹•

N =night coverage. D 1 and D 2=First and second day coverage.

(14)