Ontogenetic changes in the larval condition of Downs herring: use of a multi-index approach at an individual scale

Evaluating fish larval condition in terms of nutrition and growth is essential as it will influence their development and survival capacity. The present study aims to investigate larval condition of Downs herring (Clupea harengus L.) during winter in the Eastern English Channel and Southern Bight of the North Sea. Four condition indices including ingestion rate based on gut fluorescence, instantaneous growth based on RNA/DNA, DNA/C ratios, and otolith microstructure were combined at an individual scale on herring larvae collected during the 2015 International Bottom Trawl Survey—MIK sampling. The four indices demonstrated a clear shift in the larval condition occurring at a larval size of 13 mm. While smaller larvae were shown to feed and grow, larger larvae exhibited a slower growth rate though actively feeding. This suggests that 13 mm could be a critical size for Downs herring larvae. This ontogenetic shift in the larval condition is discussed regarding environmental conditions, diet shift, and growth strategies. It is concluded that the shift from an omnivorous to a carnivorous diet constitutes an additional critical step besides such as the shift from endogenous to exogenous nutrition.