Continental shelf nurseries and recruitment variability in American plaice and yellowtail flounder on the Grand Bank: insights into stock resiliency

In 1994, a directed fishing moratorium was declared for Grand Bank American plaice (Hippoglossoides platessoides) and yellowtail flounder (Limanda ferruginea) stocks because both stocks showed severe declines in abundance from heavy exploitation during the mid 1980s and early 1990s. Four years later, the fishery for yellowtail re-opened while the plaice stock has shown little recovery and the moratorium is still in effect. To assess the possible causes of the differences in recovery between species, we examined the spatial structure and environmental characteristics of the continental shelf nursery habitats of plaice and yellowtail, and their relationship to recruitment variability and overall population size. Depth plays a major influential role determining the spatial pattern and the abundance of juveniles of both species and in the case of plaice the spatial structure of the adult population also determines the amount of nursery area utilised by juveniles. Recruitment variability was higher in plaice than in yellowtail. We found year class synchrony in both species indicating that common environmental conditions and/or biological processes are affecting recruitment in a similar manner. Density-dependent regulation appears to be more severe in yellowtail and this should contribute to a more stable population when compared to plaice. These results are discussed in terms of resiliency of both stocks to over-exploitation.     

不同饵料对斜带髭鲷稚鱼生长和存活的影响

采用5种饵产分别投喂斜带髭鲷（Hapalogenys nitens Richardson）33日龄稚鱼，27d后测定其全长和成活率。结果表明，投喂卤虫无节幼体，斜带髭鲷稚鱼生长最快，成活率最高。全长增长达2．22－2．82倍，平均成活率达21．8％－22．5％；投喂枝角类，斜带髭鲷稚鱼生长也较快，全长增长1．99－2．07倍，平均成活率18．2％。投喂轮虫和人 工配合饲料，斜带髭鲷稚鱼的生长和成活率均很低，全长增长分别仅为1．63－1．72倍和1．47－1．56倍，平均成活率分别仅为1．3％－2．0％。投喂卤虫无节幼体和枝角类的试验组，分别有41．2％－100％个体发育至幼鱼阶段，而投喂轮虫和配合饲料的试验组，全部个体仍停留在稚鱼阶段。可以认为，卤虫无节幼体和枝角类是斜带髭鲷稚鱼较适宜的饵料。