Recent Changes in the Central and Southern North Sea Herring Stocks

Abstract

Since 1980 there has been a sudden recovery of recruitment to the central and southern North Sea herring stocks (Bank and Downs, respectively). In contrast, recruitment to the northern North Sea has continued at a low level and spawning stock biomass has even declined since 1979. These events underline the independence of the substocks in the North Sea, each of which has reacted differently to the cessation of the directed herring fishery in the North Sea since 1977. A precondition to the reopening of a herring fishery in the North Sea was that the severely depleted spawning stock biomass should have recovered to 800 000 t, the underlying assumption being that all components would respond in a similar manner and that a reopening of the whole North Sea would be possible. This has not happened and it may well be that management of the North Sea as a single unit is not a viable management procedure. The paper examines the historic changes in sizes of the Bank and Downs stocks, with particular emphasis on the most recent period. Evidence of density-dependent growth change is considered together with the recent data on fecundity and larval herring production. It is concluded that if the separate stocks are to be managed individually for maximum yield, then global North Sea regulations such as a total allowable catch and size regulation are inappropriate. Regulations must also take into consideration the seasonal distributions of the stocks. Indeed, this would lead, in particular, to more restrictive and specific regulation of the industrial fisheries.

The use of vertebral counts to discriminate between North Sea herring stocks

“…Figure 1 illustrates the correlation between mean VS of 0-group herring in July and sea surface temperatures in January of the same year. It can be seen that the observed mean VS for the 1983/84 year class is consistent with other temperature-related values of mean VS in the Downs stock in the years 1973 to 1986, and that the relatively warm sea temperatures in the winter months of 1983/84 resulted in a VS value in the Downs stock that fell within the range found historically in the Bank stock over the period 1948/49 to 1978/79 (Burd, 1985). Consequently, it is concluded that mean VS is significantly influenced by the water temperatures, immediately following spawning, and, therefore, it should not be used as an independent racial character to identify the stock origin of herring in a mixture of Bank and Downs stock fish.…”

Recolonisation of spawning grounds in a recovering fish stock: recent changes in North Sea herring

“…Such relationships have been found in other herring stocks as well, including Gulf of Maine, Norwegian spring spawners, Irish Sea and the Blackwater herring (Hempel and Schnack, 1971;Anthony and Fogerty, 1985;Burd, 1985;Fox 2001). Newly hatched North Sea herring larvae have been surveyed throughout most of the last 40 years.…”

“…Peck and D. Schnack (eds. ) ponent (Burd, 1985;Corten, 1999). A stock is usually viewed as recovered when the biomass has passed a particular threshold (ICES, 1997): B lim as a biomass threshold below which recruitment is reduced and becomes density-dependent, and B pa , introduced in 1997, as a precautionary point to B lim .…”

Long-term changes in the diel vertical migration behaviour of zooplankton

“…The ICES working group reports contain the most comprehensive and definitive estimates for the stock size of the various species in the North Sea (see, for example, previous use of this data by Corten 1990, Corten & Monaghan 1992) and were therefore used here. Due to its socio-economic importance, the herring fishery in the North Sea has been the subject of continued research and assessment over the last century (Burd 1978, Cushing 1980. The collapse of this stock in the 1970s has been well documented and led to the eventual closure of the fishery for a period in the late 1970s (Corten 1990, Corten & van d e Kamp 1992.…”