Feeding strategy of Downs herring larvae (Clupea harengus L.) in the English Channel and North Sea

• Downs larvae are not more limited by winter trophic conditions than other species.
• Downs herring larvae are omnivorous and feed on a high diversity of prey.
• Very small protist highly contribute to the diet of 8–12 mm Downs larvae.
• The ontogenetic dietary shift observed at 13 mm constitutes the critical period.
• Scanning Electron Microscopy is essential to study the diet of first feeding larvae.

This study aims to characterize the larval feeding strategy of the Downs sub-population of North Sea herring (Clupea harengus L.). Diet composition, vacuity rate and prey selectivity of larvae from 8 to 15 mm collected during the International Bottom Trawl Survey (IBTS) - MIK sampling from 2008 to 2013 were assessed by direct observation of their gut contents using Scanning Electron Microscopy (SEM). The high contribution of protists and small zooplanktonic prey observed in the gut contents proved the relevance of SEM to study the diet of first feeding larvae. The relatively low vacuity rate of 45% suggests that food may not be a limiting factor for Downs herring larvae in winter. These larvae appeared to be omnivorous and there was a clear shift in term of prey composition at a size of 13 mm. Smaller larvae (8–12 mm) fed on a higher diversity of small prey, mainly small copepods (Oncaea spp. and Euterpina acutifrons), invertebrate eggs, diatoms (Psammodicthyon panduriforme and Coscinodiscus spp.) and dinoflagellates (Dinophysis acuminate and Prorocentrum micans) whereas bigger larvae (13–15 mm) fed on a lower diversity of larger prey, mainly copepods (Temora longicornis and Paracalanus parvus) and dinoflagellates (Gonyaulax spp.). Downs herring larvae had clear prey preferences as some dinoflagellates (Pyrophacus spp., Gonyaulax spp., P. micans and Porocentrum lima), invertebrate eggs, copepods (Oncaea spp. and nauplii) and diatoms (Thalassiosira curviseriata) were positively selected and other diatoms (Nitzschia spp., Thalassiosira tenera, Thalassiosira spp. and Chaetoceros spp.) and copepods (Pseudocalanus elongatus, T. longicornis and Unidentified calanoid) were negatively selected. We argue that this shift in term of prey preferences occurring at a size of 13 mm constitutes the critical period for Downs herring larvae.