------1

INTERNATIONAL COUNCIL FOR THE CM. 1973/F: 37. EXPLORATION OF THE SEA. Demersal Fish (Northem) Committee

• RESULTS OF DUTCH COD EGG SURVEYS IN THE NORTH SEA IN 1973

by

Niels Daan

Netherlands Institute for Fishery Investigations, Haringkade 1, Ymuiden (The Netherlands)

• , . This paper not to be cited without prior reference to the author.

International Council for the C.M. 1973/F : 37 Exploration of the Sea Demersal Fish (Northern) Committee

RESULTS OF DUTCH COD EGG SURVEYS IN THE NORTH SEA IN 1973. ======by Niels Daan Netherlands Institute for Fishery Investigations, Haringkade 1, Ymuiden, The Netherlands.

Introduction. Since 1970 yearly cod egg und larvae surveys have been carried out in the southern North Sea with the emphasis on the Southern Bight in order to estimate spawning "ock size (cf. DAAN, 1972). In 1973 three surveys were performed during the months ~uary, February und March, which relate roughly to the beginning, the peak and the end of the spawning season. In addition the Channel area was surveyed in Junuary und in February un extension of the survey into the central North Sea has been made in order to get indications of the relative importance of the different spawning areas.

Results. Fig. 1, 2a, 3a present the numbers of cod eggs caught under 1 m2 for each station during the subsequent surveys and fig. 2b, 3b give the same information for larvae. During the January surveys hardly any larvae were caught und no figure for these has been produced. In J~~uary and March spawning in the Southern Bight appears to have taken place more to the north thun observed in former years (cf. DAAN, 1971, 1972, 1973). Significunt egg concentrations in this area ,however, were only found in February. At that time dispersed eggs were widely spread over the central North Sea waters,with the exception of the shallow waters of the Doggerbank. Sea water tempe~'atures were relatively high in comparison with former years and these might have been responsible for the more northerly distribution of the spawning activities. From the numbers under 1 m2 and the surface area investigated total numbers of eggs ~l larvae present in different areas (shown in fig. 4) during the subsequent plankton surveys in 1970-1973 have been estimated (Table I). The interpretation of these results in respect of relative 'spawning intensity appears to be difficult because of differences in timing between the surveys. Still the suggestion is given that about one third of the total spawning of the southern stocks of cod takesplace in the Southern Bight. In comparison with 1972 an increase in abundance of eggs is observed in this area, which is probably related to the maturation of the strong 1969 yearclass. It is, however, still weIl below the level in 1970, whereas the larval abundance was extremely low in 1973. The total egg abundunce in 1973 in the rest of the southcrn North Sea agrees very weIl with the values obtained in 1971 and this is also true for the Guter Rough area.

References. DAAN, N., 1971. Dutch cod egg and larvae surveys in 1970. Ann. BioI., 27 (1970): 88.

DAAN, N., 1972. Dutch data on cod egg surveys in the southern North Sea in 1972. ICES C.M. 1972/F:7 (mimeo). DAAN, N., 1973. Dutch cod egg and larvae surveys in 1971. Ann. BioI., 28 (1971): 98.

ds TABLE I. - Estimated numbers (in thousands of millions)of cod eggs and larvae present in different areas (cf. fig. 4) during the plankton surveys in 1970 - 1973.

YEAR SHIP DATE CHANNEL SOUTHERN BIGHT SOUTHERN NORTH SEA OUTER ROUGH Eggs Larvae Eggs Larvae Eggs Larvae Eggs Larvae

1970 "w. Beukelsz. " 2-12 Jan. 36 0.3 ------"w. Beukelsz." 14-28 Jan'. - - 866 4 - - - - "w. Beukelsz." 23 Feb.-6 Mar. - - 1577 441 - - - - 1971 "w. Beukelsz. " 4-13 Jan. 21 0.5 ------"Corella" 4-14 Jan. - - 140 0 - - -- "W. Beukelsz. " 13-22 Jan. - - 339 0 - - - - "Corella" 20-30 Jan. - - 500 1 - - - - "Tridens" 25 Jan.- 20 Feb - - - - 1056 11 103 1 "W. Beukelsz. " 16-30 Mar. -- - - 653 243 391 243 1972 "W. Beukelsz. " 2-9 Jan. 2 0 ------"W. Beukelsz. " 11 Jan.- 11 Feb - - 165 15 -- - - "Tridens" 23 Feb. ------81 0 "W. Beukelsz. " 6-18 Mar. - - 69 3 - - - - 1973 "W. Beukelsz. " 2-7 Jan. 6 0.5 ------"W. Beukelsz. " 7-19 Jan. - - 79 1 - - - - "W. Beukelsz. " 5-16 Feb. - - 387 15 - - - - "Tridens" 19 Feb.- 2 Mar. - - - - 1086 74 194 0 "W. Beukelsz. " 5-14 Mar. - - 80 5 -- - - o o o o o o o o I WL o 1 OL 2 3 4 5 8 o 57

o

f~"'~\ /~;~. '.~'~)

j / 0'·" \ ',, r··· , <.~ .." \ ...... -" r) ! ...'

..,...... ,.. o ....•...,.." 55 ~ . I ...1

.~~...... ) ! \ ) ". .,\..•., ...... -.. \ " ...... , r-_." :...._- .. \, .J 0 ~ "\{\,._~ ...... l ./- _....• • • 0 54

o 53

•

Fig 1 o 2 50 NUViliERS OF COD EGGS UNDER 1m • A: "Wi11em Beuke1sz", 5urvey 2-11 B: "Wi11em Beuke1sz", survey 15-19 january 1973 o o o o o 0 0 0 0 0 1 WL o 1 OL 2 3 4 5 6 "7 8 o ---- 57 ...... 1. Cl ._ '\1 0 ("-•.,1/\\.' Cl \ /.,"'::'J ... t,

•1 ,6°

\!

o 55

• •

o 53

•

.. ·v··' ~

../'"

.... ~ o ...__ \ ..i 2 50 NU~rnERS OF COD EGGS UNDER 1m • A: "Wi11em Beuke1sz", survey 5-9 febr. 1973 B: "Willem Beuke1sz", survey 12-16 febr. 1973 C: "Tridens", survey 19 febr. - 2 march 1973 o o o o o o 1 OL 2 4 7 o 57

o

" ,.....~;' .B .3 o ...... 55 ,; ,." "'\ ,.- .5 5:) \ 5 "" \- • • •

o 53

Fig 2b o 2 NUMBERS OF COD LARVAE UNDER 1m • 50 .•/ .,' .,.. ll A: "T,'lillem Beukelsz", survey 5-9 febr. 1973 B: "Willem Beuke1sz", survey 12-16 febr. 1973 C: "Tridens", survey 19 febr. - 2 march 1973 o o o o o o o o I WL o 1 OL 2 3 4 5 8 o 57 o \J '- o /~~\ r~""" j\} \ I ....>:) .. L

•••• _ •• M ......

(_ !' ...()".(."~':""/:»' . ... /",,) \ } ,....J ..- \ \ ~) \ ./••••••••••h •••••J'···· ! ) {.::.:J {.,...... o (./ 55

·..··..··..M.... j ...... \ ... ~.. ~r·...... _.. \ ...;/ l \ " ..-...... '" . \ -/.' () • ...... •:.~::::.::~~:) • :, ./ ....~..) •6 / . \ > ..'

o 53

...... 1 ', , - , ..

Fig 3a o 50 NUMBERS OF COD EGGS UNDER 1m2 • A: "~ii11em Beuke1sz", survey 5-9 march 1973 B: "Wi11em Beuke1sz", survey 12-1~ march 1973 o o o o o o I WL o 1 OL 2 3 4 o 57

Q

...... 1"...... ·······/ .,...... " o ( ./'.....' .." 55 ./ , ~ ./ I' i i ....-_. ) l / ./ ...... _.... • • 0 54

o 53

b Fig 3 o 2 50 NUMBERS OF COD LARVAE UNDER 1m • .... ,i' l'· ...... _,,,li A: "Wi11em Beuke1sz", survey 5-9 march 1973 ~.•._..._..,.\~ B: "Wi11em Beuke1sz", survey 12-14 march 1973 o o o o o o o I WL o 1 OL 2 4 5 8 o 57

() ..... ~ ('-'_/It~ i) / •...--::) .. i \...<

\ \ ! .J r.::J ,. {.r· ...." /'."."' ...... o ...... ,~.. (/ 55 / !·..·.... I ...... i <. . _ . ...'""- - ' ,-' .: >.::::~:~~=} 'SoiJ'-t ern Nor h • { . ...1 ...... / .._...

o 53

•

o Fig 4 50