CM_1981_H_42.pdf (760.9Kb)

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International Council for the Exploration of the Sea

Stock size of in the North Sea estimated from an echo

survey in 1981

1981/H: 42 Fish Cormnittee

Svein A Iversen, Aglen and Erling Bakken Institute of Marine Research,

, Norway

ABSTRAC'Jl

The paper presents the results of an echo integrator survey for sprat in the North Sea in January 198lø Integrator readings relating to sprat were determined by continous echo recording combined with frequent trawl sampling, and the biornass was estimated by applying an average acoustic target strength dependent on fish length About 47 000 tonnes of 1-group

sprat (1980 year class) and 49 000 tonnes of older sprat were found within the investigated area The results are discussed in relation to a similar survey in 1980. Attention is drawn to likely sources of error and to the decisive effect of the

target strength value on the biomass estimate

- 2 -

RESUME

...

Le rapport presente les resultats d'une cruisiere sur le sprat avec un integrateur d'echo dans La mer Nord en Janvier 1981.

Des verifications de l'integrateur en relation de sprats ont ete determinees par des enregistrements continuels d'echo, ce qui ont ete combinees avec des peches au chalut frequentes et la biomasse fut estimee en employant la force de mesure acou- stique moyenne qui est dipendante de la longueur du poisson.

Environ 47 000 tonnes du 1-groupe de sprat (la classe de 1980) et 49 000 tonnes des sprats plus vieux ont ete trouve dans le territoire exploree.

Les resultats sont discutes en relation des recherches analogue en 1980. L'attention fut tiree aux sources d'erreur, et

a

l'effet decisif de la valeur de mesure de force acoustique sur la biomasse estimee.

INTRODUCTION

In January - February 1978 and 1979 a series of echo-integrator surveys for sprat was carried out (Edwards and Bailey 1978;

Edwards, Wilson and Bailey 1979; Johnson 1979) ^ø The surveys were restricted to the U.K. coastal waters of the North Sea, and the estimated sprat biomass re1ated to part of the stock only. In January 1980 attempts were made to cover the entire area of sprat distribution in the North Sea by a co-ordination of Scottish, English and Norwegian surveys. The results (Johnsen 1980; Aglen and Iversen 1980; Edwards and Wilson 1980) were

intended to be used for the assessment of the North Sea sprat stock. The assessment working group, however, had reservations in accepting the estimate of total sprat biomass, mainly due to the discrepancies in the value of sprat target strength applied by the three parties (Anon. 1980).

In January 1981 similar co-ordinated surveys were carried out, and the present paper reports on the Norwegian survey in the eastern part of the North Sea

METHODS

A 38 kHz echo sounder connected to a two echo integrator

Sam pl f was

was used for echo

roainly done with trawl. Nansen bottles and CTD sonde were used for hydrographic observations

Average integrated echo intensit given

were n1rn

each fifth deflection nautical mile steamed

per nautical m~le re to the composition of the echo recordings

to 40 dB integrator gain. According trawl catches and the appearance of

values were allocated to three main c

sh recordings containing sprat other

plank·ton

recordings mainly bottom fish

The "echo fraction of '' (Ef ) was calculated from the s

species and size cornposition of the trawl catches as described by Aglen and Iversen (1980)

The biomass (B) of (450 square

nautical mile) was calculated as

B - "M C 450 s

where Ef is the averag·e Ef based on trawl catches considered

s s

representative, ~ is the average integrator value of the

category ¹¹fish recordings containing ^uu and C is the factor used for converting echo intensities (mm1 integrator deflection per nautical mile) to fish densities (kilogram per square

nautical mile) ..

Aglen and Iversen (1980)

<TS> = -10 log L - 15 dB/kg

the target strength function sprat This function was derived from computations by Foote (l 80) which included a normalization of fish t i l t angles To be in line with previous practice the normalization term was

factor C which was then

in calcul on the

the conversion TS

=

-10 log L

- 4

-25 dB/kg. This calculated C was applied in 1980 (Aglen and Iversen 1980) and the same C, adjusted small changes in equipment performance, was adopted the 1981 survey ..

The Planning Group on ICES-coordinated herring and sprat acous- tic surveys (Anon. 198la) recommended sprat an average target strength of -29 dB per kilogram, independent of fish 1ength. The conversion factors corresponding to the two target strength values ca1culated from performance data for January 1980 and January 1981 are shown below:

Average target strength dB/kg.

-10 log L -25 -29 RESULTS

Conversion factor (C) [kg/(nømile) 2

per nun per n .. mile]

1980 1981

34 L 77

39 L 88

Survey routes and trawl stations are shown in Figure l, and the composition of the trawl catches in Table l 107 hydrographic stations were taken in the surveyed areaG

Sprat biomass estimates based on

TS

= -10 log L -25 dB/kg were ca1cu1ated separately for the 1980 year class and for older sprat. This is shown in Figure 2 and 3 expressed as 1000 tonnes per ~⁰x~⁰ rectangle. The total biomass within the covered area was estimated to 47 000 tonnes of the 1980 year class and

49 000 tonnes of older sprat Figure 4 shows the length distri- butions within divisions of the North Sea In the eastern part of ICES Division IVb which cover the main part of the survey area, catches were dominated by 1-group sprat with modal length about 6 cm. In the western part of ICES Division IVb and in Division IVc older sprat, 10-13 cm in length, appeared more dominant. In the individual samples the 1-group was easily separated from the older age groups at a length of 6-8 cm.

The average weight was l 3 grams for the 1980 year class and about 10 grams for the older spratø This gives an estimate of

36 x 109 ind of the l O year clas and 5 x 109 vidua1s of the older

shows the estimates both comparison the e

are a1so shown

DISCUSSION

The surveys tbe occurred in areas not

(Ag1en and Iversen 19 sprat was restricted terized by ini ie 1981 sprat distribution at 20 m

observed in

) G

southern

The sprat occurred

observed. The e timated January 1980 The 1980

·wi thin t.he Tab le 19

o

^surveys~ For

on

J 7 8

l l no o

wa

o 29 dB/kg

tJ1at the sprat mas ses

s·tribution of the Sea water¹¹ charac-

flux of

NOX:'th Sea low dens

was much s·timated

from the iso-lines

water was

were

lower than in to 36 109 individuals in

es·timated to 660 109

1981 whi l 79 year ela s was

V 19

o

The sprat

distribution area wa cover ed during the 1981

surveya However, i Scottish surveys in

the uncovered areas (Anono 19 l b) con that 1980 year c1ass is much weaker than the 1979 year lass This is also supported by the average catch of -group obtained by R/V "Johan ort th ·the trawl in 1980 and 1981. In 1980 the catch was 151 5 f

880 in 1981.

The conditions for echo 1981 as in 1980

small catches of daytime were clo average

same track 1 amount of

u It

los·t c1ose to bottom was

al .mile ·towed to

seerned to be as su i tab1e in haul in the layer gave very

values obtained during s obtained at n.ight both as an

and when along the nable to assume that the

and

- 6 ~

Recordings classified as bottom fish were usually sampled by towing the pelagic trawl close to bot·tom Only two hauls were made with bottom trawl .. The pe1agic trawling close to the bottom will not catch fish nearer than 2 meters off bottom. Sprats occurring close to

ly been classified as "bottom fish^11a

have therefore probab-

The problems and sources of errors associated wi·th t:he estima- tion of the echo fraction . of sprat (Ef ). and separation of s -

contributions from plankton were discussed by Aglen and Iversen

(1980)~ The conclusion was that these errors would not be serious as lang as sprat was the major echo contributor. In 1981 the echo contribution from sprat was estimated to be about 50% of the "fish recordings containing sprat", calculated for areas where sprat was recorded

Bad weather giving air bubbles in the upper layers may have led to some underestimation due to increased sound absorb·tion.

The acoustic instruments and settings in 1981 were nearly identical to those applied in 1980o Calibration by aid of a -29 dB steel sphere in January 1981 gave a sum of source level and voltage respons of 143.1 dB which 0.6 dB 1ower than in January 1980 ..

It may be concluded that equipment, methods and survey condi- tions in January 1981 were quite similar to those in January 1980, and that both surveys gave fairly good es·timates of echo intensities of sprat within the surveyed area Hence>the rela- tive estimates (integrator readings) from the two years should be comparable .. The absolute estimates (biomass) are, however, doubtful due to uncertainties associated with the applied average target strength.

The -10 log L length dependence is a consequence of a 1ength3 / weight relationship and a length2

/scattering cross section relationship (Nakken and Olsen 1977, Foote 1980). -29 dB/kg is an assurned "field" value taken to be 6 dB the maximurn dorsal aspect value for a 12 cm measured Nakken and

7 ⁼

Olsen (1977). None of the values have verified to be representative for sprat. in ·the field 'I'he

value are, however ,· assumed to gi ve a t ratio be- tween small and large sprat The

and the length dependent 10 log L -25 values are equal for a 2 5 cm , but

factor of 4) for a 10 cm sprat This c need for "in situu measurements of angle distributions

REFERENCES

29 dB/kg) strength 6 dB (a

the and ti1t

AGLEN, A- and IVERSEN, S.A. 1980 Distribution and abundance of sprat in the North Sea in winter 1979 - 1980

determined by acoustic methods Counw Meetu int. Coun ..

Explorø Sea, 1980 (H 41): 1-20 [Mimeo J

ANON., 1980. Report of the Herrin.g Assessment Workin.g Group for the Area South of 62°Ne

Sea, 1980 (H 4).: 1-44 Tabs, Figs IMimeo.]

ANONa, 1981 a. Report of the Plan.ning Group on ICES-coordinated Herring and Sprat surveys Coun. Meet. int. Coun. Explor.

Sea, 1981 (H 5) .. [Mimeo@]·

ANON .. , 1981 bm Report of the Herring Assessrnent Working Group

for the Area South of 62°N. Coun. Meet. int. Explor. Sea, 1981 (H 8) 1-47, Tabs, Figs. [Mimeo ]

EDWARDS, J .. I. and BAILEY, R S 1978 An echo integration survey for sprats in UK coastal waters from the Humber to the Moray Firth, January 1978 Coun

---~--

int .. Coun .. Explor.. 8 (H 55) l 5 v 'rab, Figs ..

[Mirneo .. ]

EDWARDS, J.I., WILSON, J.P.Eø and BAILEY R@S 1979e An echo- integrator survey for sprats in UK coastal waters from the Tyne to the Moray Firt.h, February 1979 ..

Coun .. Meet ... int .. Coun .. ExE_1or" Sea, 19 9 (H 4 O) : 1-6, Tabs, Figs. [Mimeo ]

EDWARDS, J.I .. and WILSON, J .. P .. F~ 1980.. Echo-in·tegrator surveys for sprats in UK coastal waters from the Firth of Forth to the Moray Firth, January and March 1980.

Coun. Meet. int. Coun. E~plor. Se~, 198 (H 44): 1-4.

[Mimeo .. ]

FOOTE, K.G. 1980. Averaging of fish target strength functions.

J. Acoust .. Soc ..

Am ..

67 (2) Feb .. 1980 .. pp 504-515 ..

JOHNSON, P.O. 1979. The resu1ts of echo-integrator surveys for sprat carried out in the coasta1 waters of north-east England (Farne-Washl in February 1978 and 1979. Coun.

Meet .. int .. Coun .. Exp1or., Sea, 1979 (H 23): 1-8, Tabs, Figs. '[Mimeo.]

JOHNSON, P.O. 1980.. Report on echo-integrator surveys for sprat undertaken in the North Sea during the 1979-80 winter season. Coun. Meet. int .. Co~no Explor .. Sea, 1980 (H 6): 1-9, Tabs, Figs. {Mimeo.]

NAKKEN, O. and OLSEN, K. 1977. Target strength measurements of fish. Rapp. P.-v. Riun. Cons. int. Explor. Mer, 170:

52-69 ..

9

Table l . Composition of trawl catchesQ (number of fish per nautical mile towed).

Table 2 ..

Salps, ctenophores and jellyfish are excluded.

-<-)

en l 2 3 4 5 6 7 8 9 lO l l 12 13 14 15 16 17 18 19 20 21 22 23 24 25

12 06 20 19 17 24 04 DS 12 21 00 14 16 22 lG l3 19 05 l3 02 08 l3 20 01 09

110 75 65

o

20 22 16 40 18 15 14 19 l7 7 lO lO 8 l l

o

16 25 20

o o o

26 16 20 27 20 o

28 04 29 13 30 14 31 22 32 04

o

lO 20

o o

8 33 14 64 34 01 30 35 08 50 36 23 o

37 03 10 38 09 10 39 13 40 17 41 19 42 02

lO 15 25

o

8 4 l 12 72 44 18 40 45 21 o

4-: lO lO

48 12 30

42

700 52 l

2474 136 430 3800 2000 4 5 365 864 30 56 26 16 1802

1000 350 415 2455 318

325

560 202 250

3 246 23 8

-<-) r.l ~

o, Ul o, ::l o

~ tfl l N /\l

2

440 lO 18 200 2 44 3925 136 l l l 775

8 813

5

7

3

40 27 34 40 l

6 27

7

l l 800

2

l 17

2 89 8 35

107 100 50

2

l /\l

59

191 7 lO

5 3 32 47 27 l

l

l 6

l

l

42 12 16

'1J 8

l l l l 3

l l l l

l

l

35 l l l

5 189 100

3 l 56 104 8 12 4 12 l G 104 l

l 2 5 5 2 58

lO 3

lO 3 2 115 4 16 l 6 26 158 15 2 5 4 2 136 20 109 l

15 80 lO 4 18 7 11

l 2 l 4 ' 2 4 80 144

l

5 5

l

Vt

200b Sa

6a Sa

l Ja l7a 38a la lOOa l

2200g 6g l 2000a

Ja l la Sa

2a 43a 2la

7Ce 22e lO 15 l 25e

2h 6 l 9h lh lh l le 20h l l8h

l l 6h l lh 4h

l

...

O.lc l . Od

O. 2c

O.Sr O.H O.Sd O.lc O.Sd O.lc

0.2f

2h O.ld 3h l.Od 694e 0.3e

27a 0.6d

3050h la

4la 0.6d

25a 3la 2a

l

0.1d 0.2d l44b ll2Je O.lc

2 l8e

Jla 2a Sa Sa

17 l

8. bottom trawl. a) Malnly gobies. b) Mainly MOllers pearlside.

c) ~t.ainly squid. d) 11ai:.ly et~~hausiids. c) Mainly haddock.

::1 :·\a :.nly ~~h·imi.linq crab. gl t'1ainly stickle-bdck. h) ~l! i.n1y gurnard.

Biomass estimates (1000 tennes) of 1-group sprat

and older sprat based on two different target strength references. L~ fish length (cm)

~-· ·---~

Average target strength (dB/kg)

-10 log L -25 -29

Sprat biomass (1000 tonnes)

1---~--- - - - - Jan 1980

l-gr

660 300

;:: 2 gr

350 80

Jan 1981 l-gr

47 19

'> 2-gr

49 l l

- 10 ^~

Fig. l . Survey track and positions of trawl hauls.

R/V "Johan Hjort" - January 1981 ..

Fig. 2.

o o o o +

o o o o o

o o

0.6 0.7

• 0.8 2.0

+ 2.3

+ +

Biomass of sprat, 1980 year class, January 1981.

1000 tonnes per statistical rectangle. Isohalines at 20 m depth are showne +: Less than 50 tonnes.

- 11 -

sg o

58°

o o o

l

^o

l

^o

l

^o

l

^o

l

^o

l

^o

l

^{o [o}

l

⁰

1

^{o o o ·~·· +}

^l~

57° ^{o o o}

·(~

o o o o

o o o ^0.1 o 0.2 o

o o + ^0.1 o +

56° 0.2 0.2 0.1 o + o

o 0.4 0.2 o o ₊ o o 0.1 0.2 o o 0.3 0.5

o o 0.2 0.2 o o 1.2 0.2 55°

+ + o 1. 3 0.2 0.3

o + + 0.7 0.7 0.1 1.2

0.6 0.9 + o o

O.L. + + O 1.1

0.3 0.1 9.4 1.1 o

O. 2 2.8 0.8 7.3 5.9 6.3

zo

Fig. 3. Biomass of sprat, pre-1980 year classes, January 1981. 1000 tonnes per statistical rectangle. +: Less than 50 tonnes.

IV b W 2 SAHPLES N • 279

20]

'5

! l IV c

3 SAMPLES N • 383

J

^{IV lO SAY?LES b E}

(

l

l

10° .

Figø 4. Length distribution of sprat in ICES divisions IVb West of 3°E, IVb East of 3°E and IVc of the North Sea.

Lengths measured in half cm intervals.