CM_1990_H_13.pdf (5.340Mb)

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International Council for the

Exploration of the Sea C.M. 1990/H:13

Pelagic Fish Committee Ref Demersal Fish & Hydrography Committees

Report on the International Young Fish Survey in the North Sea, Skagerrak and Kattegat in 1990

by

The International North Sea, Skagerrak, and Kattegat Bottom Trawl Survey Working Group

This document is a report of a working group of the International Council for the Exploration of the Sea and does not necessarily represent the views of the Council. Therefore it should not be quoted without consultation with the General Secretary.

xGeneral Secretary ICES

Palregade 2-4

DK-1261 Copenhagen K Denmark

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T A B L E 0 F C 0 N T E N T S

Section

2

3

4

5 5. 1 5.2

6

7

8

INTRODUCTION

SURVEY METHODS AND PARTICIPATION

DATA AVAILABLE

STANDARD OUTPUT FROM THE ICES IYFS DATA BASE

RESULTS GOV-TRAWL FOR 1990

North Sea . . . . Skagerrak-Kattegat (Division Ilia)

RESULTS OF IKMT SAMPLING IN 1990

HYDROGRAPHIC DATA . . . .

REFERENCES

Tables 3.1, 5·1 - 5.5, and 6,1

Figures 5.1- 5.30, 6.1 • 6.3, and 7.1 and 7•2

Page

2

2 4

5

7

7 8

15

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This report presents the final results for the International Young Fish survey in February/March 1990. These data comprise as well the GOV catches of the seven standard species (herring, sprat, mackerel, cod, haddock, whiting and Norway pout), as the IKMT catches of late herring and sprat larvae. This report also gives summarized results of temperature and salinity sampling, which were, until the 1988 IYFS, presented as a separate report to the ICES Statutory Meeting.

The results from the International Young Fish survey become available in successive steps. During the survey the numbers of 1-group fish, based on fish below a certain length, are exchanged between the participating research vessels. Immediately after the survey the length compositions of the seven standard species are sent to ICES (in IYFS exchange format), to be used by the differ- ent assessment working groups, together with all age-length keys which are available by then. Final data, including length compositions for all species and age-length data, should be submitted to ICES before 15 April. These data are then checked and stored in the ICES IYFS data base, in order to be able to run the final analysis in time for the May meeting of ACFM. This report is based on the analysis done in May 1990.

2 SURYEY METHODS AND PARTICIPATION

For all matters on survey methodology, the reader is referred to the IYFS Manual (ICES, Doe. C.M. 1986/H:2). Details on the participation in the 1990 survey are given below. As in 1988 and 1989 the weather during the 1990 survey was rather bad, which restricted the total number of stations which could be fished. Espe- cially the coverage of the IKMT stations was rather poor.

The contribution to the 1990 IYFS by the Netherlands was limited due to the replacement of "Tridens" in the course of February.

The 'old' "Tridens" took part in the International Young Fish Surveys since 1969.

Number of hauls

Country Vessel Period GOV IKMT

Denmark "Dana" 30-1/15-2 24 35

France "Thalassa" 02-2/23-2 58 Germany, F.R. "W. Herwig" 01-2/24-2 86

Netherlands "Tridens" 19-2/23-2 14 8

"Isis" 01-2/09-2 9

Norway "Eldjarn" 21-1/10-2 50 31

Sweden "Argos" 05-2/21-2 45 49

UK England "Cirolana" 27-1/18-2 57 60 UK Scotland "Scotia" 01-2/20-2 52 30

3 DATA AvAILABLE

In Table 3.1 is shown which data were available in the ICES IYFS data base at 1 August 1990.

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2

At the time of the final analy~is of the 1990 survey, all final data were available in the data base except for Swedish age/

length keys for cod in Skagerrak/Kattegat. The numbers at age for cod in that area were added to the figures later.

4 STANDARD OUTPUT FROM THE ICES IYFS PATA BASE

For details on the standard analysis of the data the reader is referred to a description by Pedersen (1989). At request, copies of this paper are available at the ICES Secretariat.

5 RESULTS GOY-TRAWL FOR 1990 5.1 North Sea

Preliminary indices based on certain size classes and indices used by the Assessment Working Groups are compared with the final indices and shown in Table 5.1. In the case of the pelagic species herring and sprat the indices are very close. Indices of 1- and 2- group fish of the six standard species from year class 1969 and onwards are given in Table 5.2 The mean age composition of the standard species in the catches within the relevant standard area are shown in Table 5.3.

Figure 5.1 shows the number of hauls used for the analysis of the herring data and the number of hauls used for the other standard species is shown in Figure 5.2. Per species there is a set of figures which gives the distribution of the 1-, 2-, and 3+ group and the mean length of 1-group fish per rectangle. The standard area used to calculate the index of year class strength is indicated in the figures.

HERRING

The indices for age groups 1 and 2 are given in Table 5.2 and the mean number per one hour trawling of 1-, 2- and 3+ group is shown in Figures 5.3 to 5.5. The mean length of the 1-group per rectangle is presented in Figure 5.6.

The 1987 year class was estimated in 1989 to be comparable to the 1983 and 1984 year classes as 1-ringer. The present estimate of the 1987 year class as 2-ringer is 443 which is the lowest estimate since the 1981 year class and about half of the 1983-1984 year classes as 2-ringer. A reduced abundance of North Sea herring is also indicated in Skagerrak-Kattegat (Table 5.4). The distribution of the 2-ringed herring was similar to the distribution in previous years with highest abundance in the northern and western parts of the North Sea (Figure 5.4).

The preliminary index of the 1988 year class is 2224 and the final index 2146. The index is about 60% of the 1987 year class and reflects a trend of reduced recruitment to the North Sea stock. The 1988 year class was distributed in the traditional nursery areas in the southeastern part of the North Sea (Figure 5.3). A reduced abundance of 1-ringed herring is also corrobo- rated by a lower number of North Sea autumn spawners in Division Ilia (Table 5.4). Age group 3+ was found in the northern and northwestern parts of the North Sea (Figure 5.5).

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Sprat indices of 1- and 2-group are given in Table 5.2. The distribution of the same age groups and the 3+ group as well as the mean length of 1-group per rectangle are shown in Figures 5.7-5.10.

The 1988 year class was estimated in the 1989 IYFS as being the highest on record since 1972. It was noted that the high index was totally dominated by an extremely large mean catch in only one rectangle, although this value was based on 5 hauls. There was no obvious reason to reject the high catches but the index was assumed to overestimate the strength of the 1988 year class.

The same year class is now estimated as 2-group in 1989 to be close to the average of the preceding seven year classes and the 2-group index does not support the previous estimate of a very strong 1988 year class.

The index of the 1989 year class is 177 which is far below the historical average and indicates a very poor recruitment to the stock.

MACKEREL

Indices for mackerel are given in Table 5.2 and the distribution of 1-, 2- and 3+ group is shown in Figures 5.11-5.13. The mean length of the 1-group per rectangle is presented in Figure 5.14.

Un unusual high concentration of both juvenile and adult mackerel was found in the western North Sea off the English east coast (Figures 5.11-5.13). The index for 1-group mackerel, 35, is the highest on record since 1975.

Indices are given in Table 5.2, the distribution of 1-, 2- and 3+

group and the mean length of 1-group fish is given in Figures 5. 15-5. 18.

The abundance of the 1989 year class at 1-year-old is far below both the abundance of the 1988 year class and the historical average and at approximately the same level as the 1987 year class which was the second poorest year class in the historical series.

The 2-group index is at the level of the historical average.

The highest catches of 1-group fish were made in the central North Sea and in one rectangle (39E8) off the coast of northern England. In the German Bight almost no 1-group cod was caught.

Two-year-old cod was mainly concentrated in the northwestern North Sea.

HADDOCK

group and the mean length of 1-group fish is given in Figures 5.19-5.22.

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The index of 1-group haddock is at the same level as the index of the 1988 year class and far below the historical average, indi- cating that the 1989 year class is the third consecutive very poor year class.

The index of 2-year-old fish confirms the low abundance of the 1988 year class at 1-year-old.

WHITING

Indices are given in Table 5.2, the distribution of 1-, 2- and 3+

Although the index of abundance of the 1989 year class at 1-year old ~s far below the index of the 1988 year class (the highest one in the series), it remains at the historical average level and at a higher level than the 1987 year class. The number of 2- group fish is the higher of the series (2.5 times the average), confirming that the 1988 year class is the strongest one since 1970.

The largest catches of 1-year-old fish were widely distributed over the North Sea. The 2-year-old fish were concentrated in the western half of the North Sea.

NORWAY POUT

Catches of 1-group Norway pout were below the historical average and about two third of those of 1989. The 2-group index confirms that the 1988 year class is approximately at the average level.

Concentrations of 1-group Norway pout were found more northerly than last year.

5.2 Skaqerrak-Katteqat (Division IIIal

The number of hauls per rectangle for herring and gadoids is shown in Figure 5.1 and 5.2.

The final indices for 1- and 2-group herring and 1- and 2+ group of sprat are given in Table 5.4. The herring indices are calcu- lated as the mean catches in four depth starta, covering the depth range of 10 - 150 m, and weighted by the surface area of each stratum. Details of the analysis and separation of spring- and autumn spawners are given in the 1989 report (Anon., 1989).

Age/length keys for cod are now available from the 1981 IYFS and onwards. The stocks of cod in Skagerrak and Kattegat are assessed separately and indices are given for each area. Final and preliminary indices for whiting and haddock are given in Table 5.5.

The distribution of herring, sprat, and cod is included in the figures in Section 5.1.

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HERRING

The indices are given in Table 5.4 and the distribution is shown in Figures 5.3-5.5. The mean length of 1-group is shown in Figure 5.6.

The final index for 1-group herring was 3544 which is the lowest value since 1983. Meristic characters show that the 1-group is dominated by North Sea autumn spawners. The 1-group herring was mainly distributed in shallow waters in the southern part of Kattegat and around the Skaw.

The total index of 2-group herring, 3876, is above the historical average. Most of the 2-group herring could be assigned to spring spawners with an index of 3192 compared to the index for autumn spawners of 684.

The index of 1- and 2+ group sprat is given in Table 5.4 and the distribution of 1-, 2- and 3+ group is shown in Figures 5.7-5.9.

The mean length of 1-group sprat per rectangle is presented in Figure 5.10.

The index of the 1989 year class is 503 which is the second lowest since 1974 and about the same level as the very poor 1988 year class. The index of the 2+ group, 471, is the lowest on record.

group is shown in Figures 5.15-5.17.

The 1988 year class is estimated as 2-group in 1990 to be the weakest since year class 1979 in both Skagerrak and Kattegat.

This year class was estimated as 1-group in the 1989 IYFS to be above average in Skagerrak and very weak in Kattegat.

The 1989 year class is estimated to be close to the average in Skagerrak and strong in Kattegat.

HADDOCK

The index of haddock <20 cm is the second lowest on record and indicates a very weak year class.

WHITING

The preliminary index of 2255 is the second highest since year class 1974 and indicates a strong 1989 year class.

6 RESULTS OF IKMT SAMPLING IN 1990

During the IYFS herring and sprat larvae are sampled using an Isaacs-Kidd Midwater Trawl (IKMT). For details of methodology see the IYFS Manual (ICES, Doe. C.M.1986/H:2).

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6

Series of oblique hauls are made .at night and the abundance of larvae in each haul is expressed by an index value that, to some extent, corrects for differences in haul profiles:

number caught x water depth Cm) haul duration (min)

A mean of index values and of larval mean lengths is calculated for each standard rectangle. The values for the 1990 sampling are shown in Figure 6.1. Finally mean abundance is calculated for larger areas (these areas are given in ICES C.M. 1988/ Assess:17, Figure 2.3.2) and raised by an area factor. An overall index of larval abundance is calculated by adding abundances in the indi- vidual areas (Table 6.1).

HERRING

In February 1990 the unfavourable weather conditions limited the IKMT sampling, and part of the standard rectangles were not sampled. A total of 213 hauls was made. Most catches contained only a few herring larvae. Significant densities were only observed in a restricted area in the central/northern part of the North sea and in the Skagerrak (Figure 6.1). The 'Northwest' and 'Northeast' regions were not sampled during the IYFS, and sampling performed during a cruise in late January by OAFS Aberdeen, is used in estimating indices for these two regions.

The indices and the overall index are given in Table 6.1. The overall IKMT index is very low, and a dramatic reduction since the 1986 year class is observed. This year's index is as low as the lowest figures recorded (the 1976 and 1977 year classes).

The preliminary IYFS recruitment index for 1-ringed herring, year class 1988, was compared with last year's prediction based on the IKMT index (Figure 6.3).

The IKMT index predicted the year class to relatively smaller than the IYFS index, however, they both foresee the strength to be lower than has been attained since year class 1982. The com- parison between IKMT index and IYFS index might be somewhat biased. The IYFS index (as well as the North Sea VPA estimate) do not include juvenile herring in the Skagerrak (IIIa), while the IKMT index includes catches of larvae made here, and also the larvae found off the Danish coast which probably will drift into these areas. These problems are discussed in ICES C.M.1990/

Assess:14, pp. 10-11.

The mean length of herring is shown in Figure 6.2. This year the herring larvae were relatively small compared to previous years.

Samples with larvae of a mean size above 35 mm were seldom whereas normally significant numbers are seen in the southern part of the North Sea.

The number of sprat larvae caught was very low, and does not allow an interpretation of their distribution.

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7 HYDROGRAPHIC DATA

Hydrographic stations were submitted by all participating coun- tries but France. A total of 332 stations were worked, and at 108 of these, nutrients were also collected (by England, Scotland, and Denmark).

The temperature distribution (Figure 7.1), once again reflected the very mild winter, the ~hird in a row. Positive t~mperature

anomalies ranged from 1 c in the northwest to 3.5 c off the German/Danish coast. The salinity distribution (Figure 7.2) showed no marked anomalies except off the German/Dutch coasts where excesses greater than 0.3 occurred. In spite of these higher than usual salinity values, nutrient levels, especially

siJ~cate, ~ere exceptionally high across the North sea between 54 ad 55 N. In this area values have normally been in the range of 3-S~M, but this year's data showed values in excess of 8 over a wide area, with no clear association with the salinity distribution. This points to the possibility of analytical errors but the vigorous resuspension processes that must have occurred at this time, because of extremely severe weather, may have contri- buted to these anomalous nutrient levels, especially silicicate.

Figures of nutrient distributions are not included here, but these as well as temperature and salinity anomaly charts are available from the secretariat.

8 REFERENCES

Anon., 1989. Report of the working Group on International Young Fish surveys. ICES Doe. C.M. 1989/H:2.

Pedersen, L., 1989. International Young Fish survey, computation of aggregated standard tables and charts. ICES secretariat, section computer management.

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Table 3.1 Data available in the ICES IYFS database at 1 August 1990.

Country Denmark France

Germany, Fed.Rep.

Netherlands Norway Sweden

UK (Eng. & Wales) UK (Scotland) U.S.S.R

+ Data available No data available x = No survey made

.ll.2.Q. 19 8 9 .12..M. t i l l t i l l 1.2.ll .ll.a.4. tilU .1.2..a2. t i l l

+ + + + + + + + + ^X

+ + + + + + + +

+ _+* + + + + + + + +

+ + + + + + + +

+ + + + + + + + + +

+ X X X X X X X + +

* No Smalk records for Herring

country _{1 9 8 0}.l.ll.2. ti11l ll1l. .1.ll2. 1.2..Th t i l l til.J. t i l l t i l l

Denmark X + + + + + + + + +

France _X _X _X _X _X _X

Germany, Fed.Rep.

Netherlands + + + + + + + + + +

Norway Sweden

UK (Eng. & Wales)

UK (Scotland) + + + + + + + X X X

U.S.S.R X + ^X + + + + ^X ^X ^X

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Table 5.1 Preliminary 1990 IYFS indices for 1-group fish based on a split of the length distribution, indices used by assessment working groups and final indices for the North Sea

preliminary working final group

herring 2224 2111 2146

sprat 175 176 177

mackerel 33.6 40.9

*

35.0

cod 4.8 3.4

haddock 180 219

whiting 547 509

NorwaypcJt 1527 1321

*

South of 59° N

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1-'

Table 5.2 IYFS indices for 1- and 2-year-old fish of various species. Indices in mean number per hour within the relevant standard area 0

in the North Sea.

year HERRING SPRAT MACKEREL COD HADDOCK WHITING NORWAY POUT

class age 1 age2* age1 age2 age1 age2 age1 age2 age1 age2 age1 age2 age1 age2

1969 2647 25.9 32 31

1970 1629 98.3 34.5 855 299 274 190

1971 827 90 4.1 10.6 740 971 332 763

1972 1195 123 38.0 9.5 187 110 1156 496

1973 1592 481 0.1 14.7 6.2 1092 385 322 153 2412

1974 452 16.5 0.2 40.3 19.9 1168 670 893 535 4242 385

1975 342 1186 0.4 + ^7.9 ^3.2 177 84 679 219 4599 334

1976 575 136 1.4 + 36.7 29.3 162 108 418 293 4813 1215

1977 139 1474 2.3 + ^12.9 9.3 385 240 513 183 1913 240

1978 535 248 0.2 + 9.9 14.8 480 402 457 391 2690 611

1979 551 1402 + + ^16.9 ^25.5 896 675 692 485 4081 557

1980 1293 106 886 0.1 0.1 2.9 6.7 268 252 227 232 1375 403

1981 1797 149 183 461 0.1 5.2 9.2 16.6 526 400 161 126 4315 663

1982 2663 712 512 335 1.9 0.4 3.9 8.0 307 219 128 179 2331 802

1983 3416 648 347 295 0.1 0.0 15.2 17.6 1057 828 436 359 3925 1423

1984 3667 853 659 101 0.7 2.1 0.9 3.6 229 244 341 261 2109 384

1985 5717 3962 72 71 0.5 + ^17.0 ^28.8 579 326 456 544 2043 469

1986 4192 816 807 1433 8.9 0.1 8.8 6.1 885 688 669 862 3023 760

1987 3468 443 145 442 1.2 1.8 3.6 6.3 92 97 394 542 127 260

1988 2146 4246 557 1.1 1.2 13.1 15.2 210 114 1465 8S7 2079 773

1989 177 35.0 3.4 219 509 1320

* Total North Sea

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Table 5.3 Age composition of the standard species within the relevant standard area in the North Sea

age group 2 3 4 5 6+

herring 2146.3 442.8 93.2 27.6 8.9 sprat 176.8 557.4 146.4 30.0 0.6

mackerel 35.0 1.2 0.2 0.1 0.1 0.0

cod 3.4 15.2 2.0 1.0 1.0 0.8

haddock 219.1 114.1 33.5 52.9 3.4 2.0

whiting 509.1 887.4 202.2 92.4 16.5 3.5 Norway pout 1320.7 773.7 41.1 2.7 0.0 0.0

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Table 5.4

Year

1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990

IYFS indices for herring and sprat in Skagerrak-Kattegat. The herring indices are weighted by the area of four depth strata and the sprat indices are the mean by hour within the total area.

Herring Sprat

Total Spring-spawners Autumn-spawners

1-group 2-group 1-group 2-group 1-group 2-group 1-group 2+group

1325 5339 2069 5713 984 5119 2117 3338 1482

2311 387 1607 307 704 80 4960 3592

3246 1393 966 1318 2250 75 2809 3068

2560 549 1408 445 1152 104 1577 4695

5419 1063 1522 946 3897 117 1173 1685

6035 1947 2793 1419 3242 528 4141 2216

7994 2473 - * 1867 -* 606 2077 2067

21489 2738 -* 1562 -* 1176 684 4834

11733 3671 -* 2921 -* 949 1830 16543

67753 10095 - * 7834 - * 2161 945 8238

17451 4976 -* 0 -* 4976 442 2891

3544 3876 0 3192 3544 684 503 471

*Separation not valid

Total

6697 7236 4820 8558 5877 6272 2858 6357 4744 5518 18373 9183 3333 974

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Table5.5 Indices of 1- and 2-group cod and 1-group whiting and haddock in Skagerrak-Kattegat. The preliminary indices are the mean number per hour for cod <25 cm, whiting and haddock <20cm.

Cod 1-group Cod 1-group Cod2-group Whiting Haddock Year Class Skagerrak Kattegat Skagerrak Kattegat 1-group 1-group

Prel. Final Prel. Final Prel. Prel.

1974 499

1975 236

1976 99

1977 392

1978 561

1979 79 386 93 171 722 40

1980 18 15 42 26 31 63 968 4

1981 36 36 126 104 30 258 690 48

1982 32 28 113 96 19 143 262 34

1983 24 23 49 39 52 106 500 72

1984 18 14 18 9 10 72 940 161

1985 82 78 229 213 113 372 1379 57

1986 15 5 48 11 18 28 2178 251

1987 81 77 76 68 24 48 2978 125

1988 62 56 6 3 8 16 478 20

1989 25 31 131 153 2255 8

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Table 6.1 Calculation of yearly IKMT-indices of abundance of herring larvae.

Area North North Central Central South South Skagerrak/ Channel IKMT

west east west east west east Kattegat INDEX

Areafactor 27 11 28 33 12 30 10 10

Year Class

1976 16.2 4.2 36.5 1.5 2.4 0.7 0.5 4.9 1658

1977 7.1 7.1 15.1 4.4 16.7 3.8 1.8 10.2 1273

1978 52.7 9.3 108.3 6.0 3.0 1.5 22.3 0.0 5061

1979 18.4 58.4 78.7 122.4 57.7 43.0 23.8 16.0 9821

1980 15.6 0.2 43.4 34.6 26.7 101.6 74.5 56.1 7455

1981 59.1 1.1 86.8 59.6 64.4 193.5 32.7 10.7 13016

1982 7.6 3.3 20.4 74.4 87.0 92.6 140.9 42.1 8918

1983 5.7 2.0 34.3 80.4 81.2 142.0 101.7 113.2 11173

1984 25.0 5.7 90.8 77.7 298.7 215.4 83.1 89.5 17617

1985 34.8 17.2 126.3 103.1 139.2 233.2 25.5 25.3 17242

1986 95.1 8.7 218.9 167.0 249.0 273.8 14.3 73.2 26381

1987 23.3 9.3 125.0* 94.3 47.8 185.5 144.9 148.4 16415

1988 10.7 6.1 28.5 28.8 45.2 123.2 54.4 4.8 6935

1989 24.9* 8.9* 11.9 28.1 4.0 10.6 12.3 0.0 2520

* estimated from sampling during other cruises

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Figure 5.1

Herring, CLUP HAR Number of Hauls.

Date of Production: MAY 09, 1990

10 12

52 51

50

49 48 47 46 45 44

35 34 33 32 31

30 29 28 27

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16

International Young Fish Survey 1990

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50

49 48 47 46 45

32 31 30 29 28 Date of Production: MAY 09, 1990 ²⁷

10 12

Figure 5.2 Number of Hauls.

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International Young Fish Survey 1990

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10 12

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Number per Hour per Haul, Age Group 1.

35 34 33 32 31 30 29 28 27

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International Young Fish Survey 1990

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10 12

CM_1990_H_13.pdf (5.340Mb)

Report on the International Young Fish Survey in the North Sea, Skagerrak and Kattegat in 1990

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