International Council for C. M. 1977/H: 26 THE SPAWNING PERIOD FOR MACKEREL IN THE NORTH SEA

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1 ~, Y/iJ-.rE-is?2:~r not to be cited ithout E~ reference to the auj;!.lors.:: nternational Council for C. M. 977/H: 26 the Exploration of the Sea Pelagic Fish (orthern) Committee Re. Pelagic Fish (S) Committee ~~-~:Jy~fir)Co'tc!. tlji THE SPAWG PEROD FOR MACKEREL THE ORTH SEA By Erling Bakken ), Herman Bj!ZSrke ) and Maria Hortense Dias Afons0 2 ) ABSTRACT The intensity and duration of the spaning of mackerel (e.~nl"~~ ~ mb~ L.) as studied by daily plankton sampling from late May to early July 976 at a single locality in the central part of the spaning area in the orth Sea. The main spaning, as indicated by the presence of nely spaned eggs, started in early June and probably ended in early July. Slight maxima in egg numbers ere recorded in early and late June. On an average, about 25 nely spaned eggs per m 2 ere found. The incubation time as estimated to 9-0 days. ely hatched larvae occured from mid June, ith a maximum number in early July. ) nstitute of Marine Research, Bergen, oray 2) nstituto acional de nvestiga~-ao das Pescas, Lisboa, Portugal

2 - 2 - TRODUCTO Estimates of the orth Sea stock of mackerel have been made by tagging methods (HAMRE 975) and cohort analysis (AO 977). Attempts to estimate spaning stock size from egg surveys have also been made (VERSE 973 and 976, JOHSO and DAWSO 975). Such estimates require information on the spaning intensity and the egg development stages in relation to time, information hich is scarce. n the present report some data on the seasonal spaning of the orth Sea mackerel stock are provided. MA TERAL AD ME THO DS Samples of mackerel eggs and larvae ere collected from a stand-by boat at the Ekofish oil field in the orth Sea ( E) from 20 May to 8 July 976. To vertical hauls from 40 m depth to the surface ere taken in succession at 300 GMT daily ith a Juday plankton net, 80 cm diam., 500,.)J. mesh size. Samples ere obtained on 45 days. Three stages of egg development ere distinguished: from spaning until the embryo starts to diverge from yolk from latter stage until the embryo tail curls round to the head from latter stage until hatching. ely hatched larvae, having a large yolk sac, ere recorded together ith total number of mackerel larvae. Mean numbers of the to subsequent hauls are given. RESULTS AD DSCUSSO The sampling position is situated in the central part of the spaning area for the orth Sea mackerel, and high egg densities have been found there (VERSE 977). ",

3 - 3 - Fig. shos the mean number of mackerel eggs and larvae per surface during the sampling period. Eggs ere present in significant numbers from the beginning of June until the sampling ended in early July. Generally, 50-2 ill 50 eggs per m 2 ere recorded during this period, ith a maximum of about 250 in early June. Larvae occured from early June throughout the period. A maximum of about 50 larvae per m 2 as found on the first days of July. Fig. 2 shos the number of eggs in each of the three development stages. The curve of stage eggs indicates that the spaning in the central orth Sea started in the first days of June and continued for at least 30 days. The sampling ceased too early to determine ith certainty the. end of the spaning period. The long period ith presence of nely spaned eggs is likely caused by the mackerel being a serial spaner (MACER 976). From the peaks of the stage and stage curves in early June it seems reasonable to assume that the stage period lasts for 2-3 days. This compares ell ith observations by VERSE (973) and DAELSSE and VERSE (977) ho found that eggs ithout visible embryos ere less than 2 days old. The stage curve in Fig. 2 may be regarded as an indication of the seasonal curve of spaning intensity of the orth Sea mackerel stock. t is orth noting that the observations indicate a shape of the curve hich is rather square" or flat. The general curve shape is of importance hen estimates of stock size are made by egg surveys, since the estimates often incorporate assumptions about the seasonal spaning intensity. Lacking further information, it seems better to assume constant intensity during a period of about 30 days rather than e. g. a normal curve of spaning intensity. The curve for eggs in stage ll, as shon in Fig. 2, has to dorninant peaks. These peaks can be correlated to the to maxima of the stage curve, indicating a time laps of 8 days beteen the to stage s. Daily measurments of temperature at the sampling locality have not been made. Observations from nearby areas, hoever, sho that the ternpe'ature of the surface layer increased rapidly during the sampling period;

4 - 4 - from 9 C at 8 June to about 2 eek of July. on 20 June and up to 7' in the first The spaning apparently commenced at a somehat loer temperature than expected. ORTO (920), JOHASE (925) and DAEVG (9 / -8) all report that the main spaning aait a temperature of about 2 0 C. n 976 the heating of the surface layer started later than normal, and the June temperatures in the central part of the orth Sea ere loer than in the previous years. Fig. 3 shos the occurrence of nely hatched mackerel larvae. These young larvae have large yolk sacs, measure 3-5 mm, and are probably less than 3-4 days old. Higher number of larvae as found on 7 June and 2 July. Assuming that these maxima correspond to the peaks on the curve for eggs in stage, the incubation time has been approximately 9 days. Similar length of time as found experimentally by DAELSSE and VERSE (977) at a temperature of 2 C. The incubation time at higher temperatures is much shorter; only about 2 days at 20 C. The observed peak larvae densities are only 2-5 % of assumed corresponding peak egg densities. This probably reflects a high mortality at the egg stages rather than a drift of egg out of the sampling locality. The results presented here are based on very limited material, and a similar sampling programme as established also in 977. t seems possible that the increasing number of fixed installations in the orth Sea should be utilized more for egg and larvae studies. Continuous sampling from one or several platforms provides a time coverage hich combined ith vessel area surveys ill give better estimates of total egg and larvae abundance.

5 - 5 - REFERECES AO. 977 Report of the Mackerel Working Group. Coun. Meet. into Coun. Explor. Sea, 977 {H:2): -35. [Mimeo] DAEVG, A Spaning and groth of young mackerel on the oregian Skagerak Coast. J. Cons. perm. into Explor. Mer, 5: DAELSSE, D. S. and VERSE, S. A Temperaturens innvirkning pa utviklingen a v naturlig og kunstig befruktete makrellegg (Scomber scombrus L.). Fisken og Havet, Ser. B, [nst. Mar. Res. Bergen, orayll977 (2): -7. HAMRE, J The effect of recent changes in the orth Sea mackerel fishery on stock and yield. The changes in the orth Sea fish stocks and their causes. lnt. Coun. Explor. Sea, Symp. Aarhus, 975 (22): -38. [Mimeo] VERSE, S. A Utbredelse og mengde av makrellegg (Scomber scombrus L.). og zoopankton i Skagerak og nordlige del av ordsjfiien i arene Thesis, Univ. Bergen oray.. 7 pp [Unpub) 977 Spaning, egg production and stock size of mackerel (Scomber scombrus L.) in the orth Sea Coun.Meet.int.Coun.Exp0r. Sea, 977 (H:7): -9. lmimeoj JOHA SE, A. C On the influence of the currents upon the frequency of the mackerel in the Kattegat and adajent parts of the Skagerak. Medd. Komm. Havunders. Ser. Fisk., 7 (8):-26. JOHSO, P. O. and DAWSO, W.A The distribution of eggs and larvae of some peagic fish species in the central and southern orth Sea during June 972. Coun. Meet. into Coun.Exp0r.Sea, 975 (H:3): -0, 2 figs. Mimeo. MACER, C. T Observations of the maturity and fecundity of mackerel (Scomber scombrus L.). Coun. Meet. int. Coun. Expor. Sea, 976 (H 6): -7, tab., 3 figs. [Mime 0) OR TO, J. H Sea-temperature, breeding and distribution in marine animals. Mar. Biol.Ass. J., n. s., 2 (2):

6 EGGS ~ a:: D.. 0 z f)!)!) loo. 50., : -.~...; ~..., ", ~---'.,-... / ~, Mt,y JUE ~ :..,!LARVAE,,,,./"... A..,~:...,,... /., -0! /., J! / ~. / : ; '.,,..,.. ' /,, JULY " v l 50 :::E 40 0:: W D.. o 30 Z 20 ~ 0:: <t -J Fig.l Eggs and larvae of mackerel Ekofisk, central orth Sea, 976.

7 Bo " St.Clge, 49 'i / A /" / / /' / / 0-/ / ' V /, ::E Stage Cl::,no (' f) l!l (.!) W 00 Vfv~ Stage i i, ':' i :=;:---;-:=;>{ i v~-/ MAY JUE JULY 'ig.2' 'umoer of mackerel eggs in the different development stages. Ekofis,k, central orth Sea, 976. ::E 5 0: W a.. 4 o z 3- W 2 <{ :> 0: <t.j.. ~, ".~ ~.. /,~ '''''-t',.~.j...,...., ,~..' :! t : /~, - ' ',. ' ", / J.i' /...-~ 30 5 " MAY JUE,JULY F'lQ 3 umber of nely hatched mackerel larvae. Ekofisk. central orth Sea. 976.