Identifying a growth and survival bottleneck: oceanic zooplankton abundance and Faroe shelf primary production jointly influence the survival of Faroe Plateau cod larvae.

Abstract

The survival of Faroe Plateau cod larva to the juvenile stage varies significantly between years, impacting recruitment. Zooplankton abundance is likely a key driver in this variability, but to what extent specific prey species are key to survival of the larvae has been unclear. This study explores a seasonally resolved zooplankton dataset from a high-recruitment year (2017) and two following low-recruitment years (2018 and 2019) collected along a hydrographic/biological transect extending from the central shelf to the open ocean west of the Faroes; a key region, with strong influxes of oceanic waters to the shelf. The aim was to identify zooplankton species crucial for cod larval growth and survival. Analyses suggest a positive relationship between pelagic juvenile indices and primary production on the central shelf as well as the oceanic abundance of the copepod Calanus finmarchicus. Using partial least square regression on previously unpublished longer term data on copepod abundances in the upper layer in the Faroe Bank Channel and a primary production index for the Faroe shelf, we found that the shelf primary production combined with the abundance of late-stage C. finmarchicus in oceanic waters in May account for half of the interannual variability in juvenile cod abundance.