172

R app. P.-v. R éun. Cons. int. Explor. M er, 172: 172-174. 1978.

S O M E C O M M E N T S O N H E R R IN G LA R V A L D I S T R IB U T IO N A N D A B U N D A N C EIN T H E N O R T H SEA

A. S a v i l l eM arine Laboratory, P.O . Box 101, Victoria Road, Aberdeen AB9 8DB, Scotland

The growth of herring and the survival of their larvae are perhaps the two aspects of the yield equation of the stocks which are most likely to be influenced by changes in their abiotic and biotic environment. In this paper some comments are made on the results of herring larval surveys, in the North Sea as a whole but with particular emphasis on those from the northwestern area.

The changes in the distribution and abundance of herring larvae in the northwestern North Sea up to 1967 were discussed by Saville (1971) and in the central North Sea by Zijlstra (1970). Postuma and Zijl- stra (1974) have dealt with the relationships between larval abundance, stock, and recruitment in the central and southern North Sea in the period up to 1967 and 1971 respectively. This subject has therefore been fairly well documented, but on the occasion of this symposium it seems appropriate to bring this material together.

Saville ( 1971 ) showed a marked decline in the abundance of small herring larvae in the Buchan area from about 1957, which was to some extent offset by an increase in their abundance in the Orkney area. Zijlstra (1970) has shown a similar reduction of larval production in the Dogger—Well Bank area from 1966 which may have been to some extent compensated by an increase off the Yorkshire coast. Data from more recent International Herring Larval Surveys show that there has been no radical change in this situation to date; the major larval production in the northwestern North Sea continues to be in the Orkney area and in the central North Sea off the Yorkshire coast. The estimates of larval abundance in the three major spawning areas are given in Table 73. Those for the period since 1967 are taken from Saville and McKay(1973) and Wood (1974); for earlier years from Saville (1971), Zijlstra (1970), and Postuma and Zijlstra(1974). The estimates for the Shetland and Buchan areas are the means of all surveys carried out in September, for the central North Sea the means of Sep

tember and October surveys, and for the southern North Sea the means of December and January surveys. In the central North Sea the surveys prior to 1964 covered only the Dogger Bank area; to compensate for this the values from these years have been multiplied by 2-5; a factor given by Zijlstra (1970) as the ratio of larval production in this area to that for all the central spawning areas. In the period prior to 1969, only estimates of total larval abundance are available in the southern North Sea and Channel. In the Buchan and Shetland area the smaller size category of larvae fluctuated fairly widely but without any trend in the period 1958—1965 and then declined to a much lower level in the years 1966—1970. In the three recent years 1971—1973, there has been a recovery to a level comparable with that pertaining prior to 1966. In the central North Sea also, over the period considered here, there has been no obvious trend of declining larval production, the impression being rather of a fairly uniform level with occasional years of higher production at irregular intervals. In the southern North Sea the levels of larval production have been rather low throughout the period considered in Table 73. However, the general impression is of very low levels of abundance in the period 1962—1968, with slightly higher levels in the preceding and succeeding periods.

The major objective of the International Herring Larval Surveys, from which much of this material was derived, was to provide estimates of changes in the size of the spawning stocks which would be independent of catch and effort data from the commercial fisheries. Postuma and Zijlstra (1974) have shown some relationship between annual abundance estimates of the smallest category of herring larvae and estimates of the spawning stock size from catch and effort data for the central North Sea and Downs stocks. Saville (1971) also suggested that there was some relationship between larval abundance in the northwestern North Sea and estimates of spawning stock from catch and

Some comments on herring larval distribution and abundance in the North Sea 173

Table 73. Abundance indices of herring larvae in the North Sea spawning areas, 1958-1973, and estimated spawning stock biomass throughout the North Sea. (Larval abundance x 10-11)

Southern N orth Sea Spawning StockNW N orth Sea Central N orth Sea and Channel Biomass

Year < 10mm 10-15m m < 10mm 10-15m m < 11mm ll-1 5 m m (x 10 '5 tonnes)

1958.................... 30-2 14-1 6-3 1-4 3-1 10-61959.................... 11-6 16-3 2-5 1-6 0-2 17-31960.................... 10-8 16-5 3-5 4-1 1-6 15-91961.................... 53-2 58-3 2-3 0-4 2-0 14-01962.................... 11-2 21-8 1-8 1-0 0-3 10-31963.................... 10-2 63-2 f ? 0-1 17-11964.................... 18-0 43-0 i-2 0-5 0-1 17-41965.................... 22-9 42-7 7-8 0-7 0-1 11-91966.................... 6-9 7-9 1-5 0-2 0-03 9-61967.................... 8-5 8-9 6-0 0-3 0-3 7-41968.................... 1-3 2-0 1-4 0-4 0-2 3-31969.................... 2-9 3-0 0-1 1-0 0-3 0-6 3-31970.................... 3-9 11-7 3-1 1-0 0-2 0-8 2-91971.................... 12-3 7-6 1-8 1-3 0-05 0-03 2-01972.................... 28-4 11-0 1-1 1-5 0-2 0-3 2-41973.................... 13-7 6-6 7-3 2-9 0-2 0-3 ?

effort of the Scottish drift-net fishery. These apparent relationships must be treated with some reserve, however, firstly because they are rather imprecise, and secondly because the estimates of stock size are based on catch per unit of effort data which is of dubious validity in the case of a herring fishery.

Undoubtedly the best estimate of variation in spawning stock size is that derived from virtual population analysis, although it does have the limitation that it is available only for the total North Sea population and not for the individual spawning stocks. Estimates of the spawning stock size for the total North Sea population, for each of the years from 1958 to 1973 derived from the virtual population analysis carried out by the North Sea Herring Assessment Working Group, are also given in Table 73.

As pointed out by Zijlstra (1970), even if the estimates of larval abundance in each of the spawning areas do give a precise estimate of variations in spawning stock size from year to year in a given area, they do not necessarily give an estimate of the relative sizes of these stocks because of possible variations between stocks in the duration of the hatching period, the prehatching mortality, and fecundity and larval growth rates. Accordingly, the estimates given for the individual stocks are not necessarily additive to give a measure of annual variation in total larval production for the North Sea population as a whole. However, during most of the period considered here the larval production in the Buchan and Shetland area is so much greater than in the other two areas that even if all these factors act together to cause overestimation of the production in this area, relative to the other two, it would seem likely that the spawning stock in the

northwestern area must predominate in the total North Sea population. Yet it is clear that the larval production estimates in this area, as well as in the North Sea as a whole, are not related to the variations in the adult stock size as measured by virtual population analysis. In particular the relatively high levels of larval abundance in 1971—1973 are difficult to equate with the very low spawning stock biomass in 1971 and the further reduction in it which there is every reason to think took place in 1972 and 1973. Although these surveys may provide some index of variations in spawning stock size, it would appear that they are rather imprecise ones and that much of the year-to-year variation in apparent larval abundance is derived from factors other than variation in the size of the spawning stock.

Various authors have suggested that the indigenous occurrence of larvae in the spawning area around Orkney may be augmented by the drift of larvae from the spawning grounds in division Via. If on a large enough scale, this could result in a discrepancy between the rate of decline in the North Sea spawning stock and the apparent decline in larval abundance. However, the years in which the larval abundance figures are much too high relative to the size of the spawning stock are 1958 and 1965. Both years predate the major increase in the size of the spawning stock in V ia. It would appear, therefore, that if the drift of recently hatched larvae from V ia is a factor in distorting the relationship between larval production and spawning stock size in the North Sea, the extent to which it does so must vary from year to year in relation to factors other than larval production in Via.

One might also hope that it would be possible on the

174 A. Saville

V)D01

■OOJ"oE

48 7-232 40 560-8 1-6 2-4Ratio of abundance of herring larvae

S10mm : abundance 1 0 mm to the abundance of larvae < 10 mm in the September surveys.

The series of observations given in that paper have been extended by the more recent data now available and are shown in Figure 157. The regression is significant at the -001 probability level.

The interpretation of this ratio of larger to smaller larvae was fully discussed in the paper which first showed the relationship and pointed out the problems inherent in considering it as a valid index of larval survival. However, the addition of six more points to the relationship since the appearance of the initial paper has increased the significance of the regression, although no further light has been thrown on how it should be interpreted.

The corresponding ratios for 1970—1973 are 3'52, 0-71, 0-73, and 0-38 respectively. I t is not possible to estimate the strength of these year classes in the same way as for the earlier ones used in the regression because those estimates were based on catch per unit of effort of the Scottish drift-net fishery, which has now ceased to operate. However, it is worth noting that the regression equation using these ratios would suggest that the 1970 year class would be moderately strong, the 1971 year class somewhat below average, and the 1972 and 1973 year classes weak ones. This agrees fairly well with the estimates for the strength of these year classes, for the total North Sea population, from cohort analysis. This gives values of: 1970 — 7T; 1971 — 5-0; 1972 — 2-2; 1973 — 5-9. The estimate given above for the 1973 year class is higher than would have been expected from the larval data; but it should be noted that for this year class there is some evidence to suggest that the value from cohort analysis has been inflated by the catches of juvenile fish derived from the V ia population (Anon., 1977).

R E FE R EN C ES

Anon. 1977. Report of the H erring Assessment W orking Group for the area south o f 62°N. ICES C M 1977/H: 3, 85 pp. (mimeo).

Cushing, D. H . & Bridger, J . P. 1966. T he stock of herring in the N orth Sea and changes due to fishing. Fishery Invest., Lond., Ser. 11, 25, No. 1, 123 pp.

Postuma, K. H . & Zijlstra, J . J . 1974. Larval abundance in relation to stock size, spawning potential and recruitm ent in N orth Sea herring. In T he early life history of fish, pp. 113-128. Ed. by J . H. S. Blaxter. Springer Verlag, Berlin.

Saville, A. 1969. Larval abundance and year-class strength in the Buchan stock of herring. IC ES C M 1969/H: 17, 6 pp. (mimeo).

Saville, A. 1971. T he distribution and abundance o f herring larvae in the northern N orth Sea, changes in recent years. R app. P.-.v. Réun. Cons. perm. int. Explor. M er, 160: 87-93.

Saville, A. & M cK ay, D .W . 1973. R eport on the international surveys o f herring larvae in the N orth Sea and adjacent waters in 1972/73. IC ES Coop. Res. R ep., No. 41: 1-39.

W ood, R . J . 1975. R eport on the international surveys o f herring larvae in the N orth Sea and adjacent waters in 1973/74. ICES Coop. Res. R ep ., No. 48: 1-40.

Zijlstra, J . J . 1970. H erring larvae in the central N orth Sea. Ber. dt. wiss. Kom m n Meeresforsch., 21: 92-115.