Effect of zooplankton on fish larval abundance and distribution: a long-term study on North Sea herring (Clupea harengus)

During the last decade, North Sea autumn spawning herring (Clupea harengus) has gone through consecutive years of low recruitment despite high spawning-stock biomass. Although several mechanisms, such as reduced larval growth and high early larvae mortality, have been identified as co-occurring during these years, the causes behind them have not been identified. In this study, we analyse a long-term dataset of larval distribution, obtained during the International Bottom Trawl Survey, in relation to environmental conditions during winter and zooplankton abundances, obtained from the continuous plankton recorder. These analyses assessed the potential influence of these factors on the reduced survival of larval stages. Generalized additive mixed models on 30 years of data showed the abundance of Pseudocalanus sp. during winter to have a strong relationship with larval distribution and abundance, suggesting that predator-prey processes are behind the low recruitment in recent years. According to our models, the direct effect of temperature on larval abundances was less than the effect of zooplankton abundances.