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

Recruitment at age 2 plotted against spawning stock biomass (SSB) for the B lackwater herring stock shows a dome-shaped pattern. This has been apparent in recent years (1991-1995) when, despite high spawning stock biomass, res ulting year classes have been poor. In 1994, 1996 and 1997 plankton surveys were undertaken in the Blackwater Estuary to estimate the production of yo lk-sac larvae. The results of production, average cohort growth and mortali ty were compared with the results from surveys carried out in 1979 when spa wning stock biomass (SSB) was close to the minimum historically recorded. I n each year of the present study, estimated levels of larval production wer e higher than in 1979. Following hatching, average cohort growth rates appe ared to be similar to those in 1979 but mortality rates appeared to be grea ter. Levels of larval production increased linearly with spawning stock bio mass but there was an inverse relationship between estimated survival from hatching re, recruitment at age 2 and the level of larval production. The r esults suggest that, in the years studied, year-class strength was determin ed by density dependent and/or environmental processes operating after hatc hing as opposed to processes operating at the egg stage.