Plankton dynamics and larval herring growth, drift and survival in a frontal area

Horizontal and temporal variations in abundance and productivity of phytoand zooplankton were studied in a frontal area in the NW North Sea during September and October 1985. In addition, distributions of several cohorts of herring larvae found in this area were monitored and larval growth and survival estimated. Phytoplankton biomass and productivity as well as copepod productivity peaked in the transitional zone between isothermal and temperature-stratified water. Copepod egg production was observed to increase markedly subsequent to a severe storm. The frontal region, then. was identified as particularly productive in terms of larval herring food (immature copepods), and frequent autumn storms may also stimulate the production of larval food. Six larval cohorts were identified within the study area, all with estimated instantaneous mortalities of <0.05 d-'. Upon hatching in AugusWSeptember, larvae patches dnfted towards the south until the end of the study in late October. Throughout thls period, larvae tended to be retained within the frontal region. Crossfrontal transects revealed peak abundances of larvae in transitional water concurrent with peak copepod production. In spite of this, larval growth was suboptimal and limited by food availability. It is shown that larval feeding was of the same order of magnitude as production of larval food and it is suggested that competition for food in the larval stage contributes to density-dependent growth, survival and, hence, recruitment.