Fisheries Research, 8 (1989) 129-140 129 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

The Use of Tagging Studies to Determine the Optimal Time for Est imat ing Acoustic Abundance of Spawning Cod

OLAV RUNE GODO

Institute of Marine Research, P.O. Box 1870, 5024 Bergen (Norway)

(Accepted for publication 12 May 1989)

ABSTRACT

Godo, O.R., 1989. The use of tagging studies to determine the optimal time for estimating acoustic abundance of spawning cod. Fish. Res., 8: 129-140.

Abundance estimates of the spawning stock of northeast Arctic cod have, in recent years, been made on the basis of acoustic surveys. Migration in and out of the surveyed areas during investi- gation has been considered as a possible major source of error in the estimates. This paper evalu- ates the possibility of using recapture results from tagging experiments to illustrate migration to and from the Lofoten spawning area. Further optimal time period and survey strategy for such surveys are discussed.

Tagging-recapture results seem to illustrate the movements of cod around the time of peak spawning reasonably well. Year-to-year variations are demonstrated. It is concluded that the effect of migration on the acoustic estimates of spawning cod in Lofoten is minimized by carrying out the survey during the last 3 weeks of March.

INTRODUCTION

The nursery and feeding areas of northeast Arctic cod (Gadus morhua L. ) are in the Barents Sea, and the main spawning takes place in nearshore areas along the Norwegian coast (Fig. 1A). The stock has a potential annual long- term yield between 0.5 and 1.0 million tons, and the management is partly based on bottom trawl and acoustic survey results. The abundance of the young fish is estimated by surveys on the nursery grounds during winter when the mature part of the stock has partly moved out of the area for spawning (Hylen et al., 1986; Bergstad et al., 1987). During spawning the mature fish are con- centrated in limited areas and are often pelagically distributed. Thus, the con- ditions for acoustic abundance estimation are favourable, and since 1982, sur- veys have been carried out immediately after the young fish survey (Godo et al., 1982, 1983, 1984, 1985; Raknes and Sunnanfi, 1986).

0165-7836/89/$03.50 © 1989 Elsevier Science Publishers B.V.

130

o

~f

z o

o

o o

r ~ o o 0 c,a

2

o

o ° o o ~

o o

o °

o o c-~

~ o o ~

o

°o_ o o o o o° oo_ o

o o ~ c o r ~

• g g g ~ g ~ o ~ ~ ,~ ~o

I,

o

o ~

t ~ 0~

131

Fish migration can strongly affect acoustic abundance estimation (Johan- nesson and Mitson, 1983), and a major source of error in the above estimates might be caused by the considerable migration into and out of the surveyed areas during the period of investigation.

The main objectives of this paper are: (1) to present and evaluate tagging studies as a method for indicating optimal time for conducting an acoustic survey; (2) to determine an optimal period for conducting a survey on mature northeast Arctic cod.

MATERIALS AND METHODS

A total of 32 400 cod were tagged during the spawning season (March-April) in the years 1975-1985 (Table 1 ). Purse seine was the capture gear in all the experiments. The tagging was accomplished with hydrostatic Lea-tags at- tached anterior to the first dorsal fin (Anon., 1953). The main area for release was roughly the same in all the years, and is shown in Fig. lB. This chart also shows the statistical squares according to which the recaptures were recorded.

The weekly number of boats recorded as taking part in the fishery is used as a measurement of fishing effort. The fishing effort in the Lofoten area is re- corded by the Lofoten spawning-season register. This register is designed es- pecially for the Lofoten spawning fishery, and is open during the most intense spawning cod fishery, normally from the beginning of February to the second half of April. In the calculation of recaptures per unit effort (RPUE), 100 boats week- 1 is used as the unit of effort.

TABLE 1

Numbers of tagged cod (N) released in the Lofoten spawning area in the period 1975-1983

N Period of release

1975 2800 6-26 March 1976 4000 10 March-2 April 1977 2000 14 March-2 April 1978 2500 1-7 April 1979 2800 21 March-5 April 1980 1500 21 March-1 April 1981 3800 16 March-9 April 1982 3800 22 March-5 April 1983 3900 15-29 March 1984 2000 26 March-6 April 1985 3300 20 March-2 April

132

(A) March

1 0 0 0 1 2 0 0 7 1 o o i

30-

7 0 o o -

30.

6 9 0 0

6 8 0 0

0o

6 7 0 0

1 4 o o 1 6 o 0 1 8 0 o 2 0 0 0 I I I

169

' 4 ' 4 1

(B) A p r i l

1 000 1 2 0 0 7 1 0 0 I

I I 5

7 0 0 0

3 0 - 4

6 9 0 0 -

6 8 0 0 -

6 7 0 0

1 4 0 0 1 6 0 0 1 8 0 0 2 0 0 0 I I I

2

9

4

7 36 12

15,t'33

t ~

(c) May

t o o # 1 2 o o 7 1 a o

57 l o

7 o * * -

1 o -

6 0 o o -

l a .

6 8 o o .

2

1 1 ,

m

2

I

'4 1o f

2118" 1

,o¢, 'r"

Fig. 2. Geographical distribution of recaptured fish during March, April and May in the years of tagging• Numbers in upper left corner indicate the number of recaptures taken north of Area B (see Fig. 1 ).

RESULTS

Geographic distribution of recaptured cod

The geographical distributions of recaptures for March, April and May are given for the year of tagging (Fig. 2A-C) and those for January-April in sub-

133

(A) January

1000 120o 140o 16oo 18o0 2000 71oo i r I l

1o 42

7000

I 6

30

6900 2 9 ~, ~.~k~ kf%~ 2..). ,

1 1 1 6

6800 l 2 5 3 .

6700 ,a ,

(B)

I Ooo 7 1 o o

30

7000

30

6 OO

3o

6 oo

5o

6 oo

Febr luory 120o I 400

J 1605 18o3 2000

l J

21

5 5

Y3

5 9

t 1 35 2

f3 15,

r - - - , , ,

(c)

7110

30-

70oo-

30.

69oo-

30-

68oo-

3o.

6700

Ma,ch oo 12oo 14oo 16oo 18oo 200o

r r i

19 I

2 4

! 5

I 7

(D) p r i l

71010 0o 12ooj 14oo 16oo 1Boo 2000 I J t

30

7000

30

6900

30

68o0

3o

6700

10

2 ~I6 6

Fig. 3. Geographical distribution of recaptured fish during January-April in the years following tagging. Numbers in the upper left corner indicate the number of recaptures taken north of Area B (see Fig. 1 ).

sequent years (Fig. 3A-D ). Figure 2A indicates that few fish have moved out of Lofoten in March. When migration starts, the main route seems to be south- west along the Lofoten Islands and subsequently northwards along the coast: The cod start to leave the area of investigation in April. In the years subsequent to tagging (Fig. 3) it is impossible to distinguish between fish migrating into and out of the spawning area at the time of recapture. However, both the num-

134

25-

20-

~n 1 5 .

g ~_ lo. < u

~ 5-

O-

." ~.

/ : ".

M E A N

I

i I I I I ~ i r 2 4 6 8 1 0 12 14 1 18 JAN F'EBR MARCH APRIL MAY

Fig. 4. Mean s tandardized recapture rates week-1 wi th s tandard deviat ion from Lofoten in Jan- ua ry -May in the years following tagging.

TABLE 2

Standardized weekly recapture for the years following tagging. Mean and s tandard deviat ion ( SD ) are given, as well as the total number recaptured each year ( S U M )

Week 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 Mean SD

]

2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18 19

1.8 0.0 0.0 0.0 0.0 5.4 1.8 0.0 0.0 1.8 0.9 2.7 2.7 7.0 0.0 6.2 1.7 8.1 0.9 7.0 2.7

2.7 0.0 0.0 0.7 0.0 0.0 0.5 0.0 2.1 0.7 0.6 0.9 0.0 0.7 0.0 0.4 2.1 0.0 1.6 6.1 1.4 2.1 0.0 2.5 0.7 0.0 1.2 1.6 1.0 7.4 0.0 1.4 2.1 1.4 2.5 2.0 4.2 2.9 0.0 5.2 3.7 3.0 2.5 1.4 2.8 5.0 3.3 8.5 6.6 5.3 13.4 6.3 0.0 5.1 3.7 1.4 1.3 3.3 5.5 2.9 4.8 7.2 4.8 9.1 4.7 2.6 4.6 2.5 5.2 7.3 1.6 2.6 2.1 8.5 3.0 4.0 2.5

2.7 8.7 5.4 6.0 5.0 5.9 5.5 7.0 4.8 9.3 8.5 6.1 6.2 1.9 4.4 5.2 2.7 7.9 12.5 10.5 7.9 6.1 11.6 9.3 4.2 3.0 7.1 3.3 5.3 6.1 2.7 9.3 23.8 7.2 7.3 12.3 8.5 7.2 10.1 3.0 8.6 5.5

14.2 17.4 8.1 6.5 21.3 14.4 13.9 15.2 16.4 3.1 13.2 9.1 12.7 5.1 15.0 13.0 29.7 15.3 7.5 7.8 15.2 13.9 14.3 9.3 6.9 33.3 15.1 8.3 20.4 12.2 8.1 15.3 8.8 14.4 10.3 5.7 11.1 15.5 4.8 9.1 11.3 4.5 14.2 11.3 8.1 11.1 5.0 2.0 7.3 11.1 7.4 5.2 11.1 9.1 8.6 3.4 5.3 5.2 2.7 6.9 0.0 8.5 1.2 7.0 3.2 7.2 3.7 3.0 4.5 2.6 0.9 2.6 2.7 6.0 0.0 9.8 4.8 2.9 1.1 4.1 3.2 3.0 3.4 2.6 0.9 0.9 0.0 2.3 0.0 3.3 0.6 1.6 3.7 1.0 0.0 0.0 1.2 1.3 0.9 0.0 5.4 0.0 1.3 0.7 0.6 1.6 1.1 0.0 0.0 0.0 1.0 1.5 0.9 0.9 2.7 2.3 1.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.7 1.0

SUM 113 115 37 216 80 153 165 244 189 97 189 33

135

ber of recaptures north of Area B and the geographical spread of the recaptures in Areas A and B, indicate the main concentration is on the spawning grounds in March and April.

Immigration

For the years subsequent to tagging, the pooled weekly number of tagged cod recaptured during 1976-1987 in Lofoten was determined. To make a year-to- year comparison possible, the total number of recaptures each year was stand- ardized to 100, and the corresponding weekly recapture rates were calculated, The mean and the standard deviation of these weekly recaptures rates are shown in Fig. 4. The recapture rate increased steadily, peaking at Weeks 11 and 12, i.e. around 20 March.

The standardized weekly recapture rate for each year (Table 2 ) shows that the maximum varies from Week 10 to Week 13, i.e. from about 10 March to the end of March or the beginning of April. The standard deviation is high, and varies in the period of highest recapture rates from 43 to 70% of the mean,

A main source of variation in the recapture rates is the fishing effort. The available effort measurements (Table 3 ) show that the effort peaks in the pe- riod of highest recapture rates. The effort is relatively stable in the peak period from year to year, except for a considerable decrease in 1985 and 1986. Mean recapture rates (Table 2) per effort unit (RPUE) exhibit two peaks: one con- sisting of the first two observations and the other coinciding with the peak of the recapture rate (Fig. 5).

TABLE 3

Fishing effort in number of boats week- 1 taking par t in the Lofoten fishery. R P U E is s tandardized recapture rates {Table 2 ) per un i t mean effort

Week 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 Mean R P U E

5 1579 1560 1906 1307 256 1416 1433 1111 1278 1316 0.35 6 2415 2518 2421 2354 1422 2240 2371 2527 2426 1231 729 2060 0.21 7 3028 3290 3371 3168 2481 2922 3077 3195 3189 1870 1266 2805 0.13 8 3578 3689 3740 3624 2925 3304 3430 3632 3645 2420 1598 3235 0.18 9 3945 4007 4005 4014 3127 3581 3736 4087 3985 2718 1842 3550 0.19

10 4201 4165 4344 4189 3338 3848 4007 4347 4328 3027 2043 3803 0.21 11 4227 4414 4656 4570 3417 4033 4149 4430 4477 3366 2256 4000 0.29 12 4471 4387 4610 4721 3629 4072 4267 4393 4622 3514 2347 4092 0.34 13 4294 4363 4882 4482 3544 4057 3936 4425 3088 2139 3921 0.27 14 3127 3832 4682 4200 3259 1996 1588 4077 2810 2217 2014 3073 0.26 15 2191 2929 3628 3336 1993 1190 2125 1497 1342 1412 2164 0.19 16 914 1704 1543 1793 1175 1187 712 910 1242 0.25 17 1205 591 626 827 0.13

136

35- i'

I 30- I ~ %

l , % %

20- .~f %~ I [~

15- %1 %

~ 10-

5-

0 ~ ) I I I I l l I I I

2 4 6 8 10 12 14 16 18 JA. r~sR MARCH APR,, MAY

Fig. 5. M e a n recaptu re ra tes (so l id l i ne ) and R P U E ( X 100) ( b r o k e n l i ne ) week - I f r o m da ta i n

the years f o l l ow ing tagging.

180- (a)

160-

14o-

120-

l O O -

2 a. 80-

Q: 6 0 -

4 0 -

2 0 - * ~rr-'~ -"~'d'-L'/~'Jl.

0 i l i 1 1 i , 115 r i 3 5 7 9 1 13 17 19 JAN FEBR MARCH A PRI L MAY

700 ( b )

600 / I ~

~: 4 0 0 -

' iJi, 300-

2oo-

0 , ~ ,

3 5 7 9 11 13 15 17 19 21 JAN F E BR MARCH APRI L MAY

Fig. 6. Sum of recaptures 1975-1987 in Lofoten from an outer ( X 10) (solid line) and an inner (broken line) location, (a) in years following the tagging year and (b) in the tagging year.

TABLE 4

Weekly recapture rate for Area B

137

Date in April Weekly recapture rate Weekly recapture rate during during tagging year subsequent years

1-7 11 14 8-14 17 8 15-21 11 8 22-28 17 4 29-30 5 2

The immigration at Lofoten to the spawning area is also studied by analyz- ing recapture rates in Lofoten from an inner location (Location c in Fig. 1B) and an outer location (Location d in Fig. 1B). In the inner location there is a clear maximum of recaptures around Week 12 (Fig. 6a ), as is also observed for the total mean (Fig. 4). In the outer location a minimum is observed at the time of highest recapture rates in the inner location.

Emigration

The recapture rates for the two locations in Area A during the years of tag- ging (Fig. 6b) show the peak in the number of recaptures to be 1 week later for the outer location compared with the inner location, which is situated in the main area of release. Together with Fig. 2A,B, this indicates that the migration out of the release area starts in April. In the years following tagging the recap- ture rate for the inner location peaks 2-3 weeks earlier than in the years of tagging (Fig. 6). A corresponding comparison of the recapture rates of the outer location at the end of March-April, indicates a final drop in the recapture rates to occur simultaneously (Fig. 6a,b). The weekly recapture rate for Area B for the year of tagging exhibits no particular trend, while in subsequent years it decreases during the month of April (Table 4).

DISCUSSION

The northeast Arctic cod stock is annually covered by three acoustic surveys, and the results are combined and used in the management of the stock (Hylen and Nakken, 1985). It is consequently essential that the estimates from the different surveys are comparable and at the same time, are comparable f rom year to year. Taking this into account, information on spawning migration i s important for designing optimal survey strategy.

Migration of cod within the Lofoten spawning area during the spawning season has been previously studied by Dannevig (1955) using tagging-experi-

138

ment results. This method has also been used to map the migration of north- east Arctic cod between the feeding and spawning grounds (Dannevig, 1953; Trout, 1957; Lebed et al., 1983; Godo, 1984).

The general information on the migration pattern fits well with earlier ob- servations (Dannevig, 1955). The acoustic abundance estimation surveys are normally carried out during the first 3 weeks of March. The optimal survey strategy is to cover the spawning stock so that fish migration does not result in either measuring the fish twice or in some of the fish escaping acoustic detection.

Rollefsen (1938) found that first-time spawners reach the spawning grounds later than those having spawned before. When omitting the early peak in the recapture rate per unit effort in Fig. 5, the results suggest that the immigration of multiple-time spawners continues until at least about 20 March, with some year-to-year variation. According to Rollefsen (1938) first-time spawners will still be arriving after that date. Variation in the environment (temperature, salinity, prevailing winds), both between years and within one season, will strongly influence the distribution of the spawning cod (Eggvin, 1938; Ellert- sen et al., 1981 ). Mature cod might feed on main prey species like capelin and herring during the spawning migration, when these species occur along the migration route. Both variable environment and feeding conditions probably cause much of the year-to-year variation in the recapture results. The first peak in Fig. 5 might be an artifact of the establishment of the Lofoten spawn- ing-season register every year in the beginning of February. At this point the fishery has already been underway for nearly a month. However, there is a tendency for boats not to enter the register immediately after the official open- ing, owing to practical causes like distance between office and main area of operation. Furthermore, during the first 2 weeks of February, when the register is established, frequently the fishermen leave tags recovered during the first month of operation, which are often recorded as recovered in early February (E. Larsen, personal communication, 1988).

The limited number of recaptures recovered in Area B in March compared with early April (Fig. 2A,B, Table 4), indicates that the cod start to leave the spawning grounds to a considerable extent at the beginning of April. This is supported by the form of the recapture frequency curves (Figs. 5 and 6). The data in Fig. 6 are not corrected for variation in effort. However, the effort varies little until Week 16, when it reaches half the maximum level {Table 3 ). This indicates that the decrease in the recapture frequencies is real. The time of peak spawning is very stable from year to year and occurs about 31 March + 2 days (Pedersen, 1984). Emigration thus seems to commence about the time of peak spawning or some days later, i.e. as fish complete spawning. The some- what later decrease of tag recaptures in the year of tagging compared with subsequent years {Fig. 6a,b), is probably because tagging is often going on until

139

the beginning of April and thus emigration is to some extent compensated for by new releases.

In conclusion, the tag recapture results seem to give clear and valuable in- formation about cod migration to and from the spawning grounds. The results indicate that the main cod immigration to the Lofoten spawning grounds oc- curs before 20 March, and that the emigration takes place from the beginning of April. To obtain the most reliable acoustic estimates of the cod spawning stock abundance (with respect to influence of migration), survey cruises should be conducted during the last 3 weeks of March. The year-to-year variation ( + 10 days) makes exact timing of the optimal period impossible, and the re- liability of the survey result will be increased by carrying out 2 surveys during this period.

ACKNOWLEDGEMENTS

The fish were tagged by L. Kalvenes and P. Agotnes, who also are thanked for valuable consultation during the presentation of the manuscript. A.M. Skorpen and H. Senneset are thanked for technical assistance, and R. Keeser for valuable comments and correction of the English text.

REFERENCES

Anon., 1953. A guide to fish marks. J. Cons. Int. Explor. Mer, 19: 241-289. Bergstad, O.A., Jorgensen, T. and Dragesund, 0., 1987. Life history and ecology of the gadoid

resources of the Barents Sea. Fish. Res., 5: 119-161. Dannevig, G., 1953. Tagging experiments on cod, Lofoten 1947-1952: some preliminary results.

J. Cons. Int. Explor. Met, 19: 195-203. Dannevig, G., 1955. Merkeforsok som viser skreiens oppforsel i Lofoten. Fiskeridir. Sm~skr., 2:

3-27. Eggvin, J., 1938. Trekk fra Nord-Norges oceanografi sett i sammenheng med torskefisket. Ars-

beret. Vedkomm. Nor. Fisk., 2: 123-136. Ellertsen, B., Fumes, G.K., Solemdal, P. and Sundby, S., 1981. Influence of wind induced currents

on the distribution of cod eggs and zooplankton in Vestfjorden. In: R. SEetre and M. Mork (Editors), The Norwegian Coastal Current. University of Bergen, Bergen, pp. 604-628.

Godo, O.R., 1984. Migration, mingling and homing of North-east Arctic cod from two separated spawning populations. In: O.R. Godo and S. Tilseth (Editors), Proceedings of Symposium on Reproduction and Recruitment of Cod. Leningrad, September 1983. Institute of Marine Re- search, Bergen, pp. 289-302.

Godo, O.R., Nakken, 0., Raknes, A. and Sunnan~, K., 1982. Acoustic estimates of spawning cod off Lofoten and More in 1982. Counc. Meet. Int. Counc. Explor. Sea 1982(G:62) pp. 1-16 (unpublished).

Godo, O.R., Nakken, 0., Raknes, A. and Sunnan~, K., 1983. Acoustic estimates of spawning cod off Lofoten and More in 1983. Counc. Meet. Int. Counc. Explor. Sea, 1983(G:37) pp. 1-24 (unpublished).

Godo, O.R., Nakken, 0., Raknes, A. and Sunnan~, K., 1984. Acoustic off Lofoten and More in 1984. Counc. Meet. Int. Counc. Explor. (unpublished).

Godo, O.R., Nakken, O., Raknes, A. and Sunnan~, K., 1985. Acoustic off Lofoten and More in 1985. Counc. Meet. Int. Counc. Explor. (unpublished).

estimates of spawning cod Sea, 1983(G:47) pp. 1-24

estimates of spawning cod Sea, 1985(G:66) pp. 1-14

140

Hylen, A. and Nakken, 0., 1985. Stock size of north-east Arctic cod, estimates from survey data in 1984/85. Counc. Meet. Int. Counc. Explor. Sea, 1985(G:67) pp. 1-14 (unpublished).

Hylen, A., Nakken, O. and Sunnan~, K., 1986. The use of acoustic and bottom trawl surveys in the assessment of north-east Arctic cod and haddock. In: M. Alton (Editor), A Workshop on Comparative Biology, Assessment, and Management of Gadoids from the North Pacific and Atlantic Oceans. Proceedings: Part II. Northwest and Alaska Fisheries Center, Seattle, pp. 473-498.

Johannesson, K.A. and Mitson, R.B., 1983. Fisheries acoustics: a practical manual for aquatic biomass estimation. FAO Fish. Tech. Pap., 240: 1-249.

Lebed, N.I., Ponomarenko, I.Y. and Yaragina, N.A., 1983. Some results of cod tagging in the Barents Sea in 1966-1982. Counc. Meet. Int. Counc., Explor. Sea, 1983(G:21) pp. 1-22 (unpublished).

Pedersen, T., 1984. Variation in peak spawning of Arcto-Norwegian cod (Gadus morhua L.) dur- ing the time period 1929-1982 based on indices estimated from fishery statistics. In: E. Dahl, D.S. Danielssen, E. Moksness and P. Solemdal (Editors), The Propagation of Cod Gadus morhua L. Flodevigen rapportser., 1: 301-316.

Raknes, A. and Sunnan~, K., 1986. Acoustic estimates of spawning cod off Lofoten in 1986. Counc. Meet. Int. Counc. Explor. Sea, 1986(G:79) pp. 1-9 (unpublished).

Rollefsen, G., 1938. Aldersundersokelser (Age analyses of the 1937 catch of cod). Fiskeridir. Skr. Set. Havunders., 5: 23-32.

Trout, G.C., 1957. The Bear Island cod: migration and movements. Fish. Invest., Ser. II., 21: 1- 51.