Recent trends in stock-recruitment of Blackwater herring (Clupea harengus L.) in relation to larval production

Abstract

Recruitment at age 2 plotted against spawning stock biomass (SSB) for the Blackwater herring stock shows a dome-shaped pattern. This has been apparent in recent years (1991-1995) when, despite high spawning stock biomass, resulting year classes have been poor. In 1994, 1996 and 1997 plankton surveys were undertaken in the Blackwater Estuary to estimate the production of yolk-sac larvae. The results of production, average cohort growth and mortality were compared with the results from surveys carried out in 1979 when spawning stock biomass (SSB) was close to the minimum historically recorded. In each year of the present study, estimated levels of larval production were higher than in 1979. Following hatching, average cohort growth rates appeared to be similar to those in 1979 but mortality rates appeared to be greater. Levels of larval production increased linearly with spawning stock biomass but there was an inverse relationship between estimated survival from hatching to recruitment at age 2 and the level of larval production. The results suggest that, in the years studied, year-class strength was determined by density dependent and/or environmental processes operating after hatching as opposed to processes operating at the egg stage.