Feeding ecology of Central Baltic sprat Sprattus sprattus larvae in relation to zooplankton dynamics: implications for survival

We investigated the larval feeding ecology of sprat Sprattus sprattus L., an important commercial fish species in the Central Baltic Sea. We collected seasonally resolved larval abundance and gut content as well as zooplankton data, with the goal of identifying mechanisms leading to variability in larval survival. Our results show sprat larvae feed progressively on larger food items as they grow during the season depending on their size and the composition of the available prey. We found first-feeding larvae early in the spawning season to prey exclusively upon microplankton. Small larvae fed mainly upon nauplii of the copepods Acartia spp., Temora longicornis and Centropages hamatus (ca. 65%), whereas larger larvae consumed up to 80% Acartia spp. copepodites and adults, as well as cladocerans. Trends in sprat larval diets were to a large degree explainable by selective feeding. Feeding success and gut fullness increased linearly with larval size. Trophic niche breadth increased linearly until larvae reached a predator size of 16 mm, after which it decreased. We explain the latter decline by a restricted size spectrum of prey available to larger sprat larvae, which points towards the importance of considering the structure of the zooplankton community when evaluating the predator size to niche breadth relationship. Our results suggest first-feeding Baltic sprat larvae to be always food-limited, while larger larvae are not. We hypothesize medium-sized sprat larvae to be the life stage that has the potential to cause most of the interannual variability in sprat larval survival, which is dependent on a match between larval production and the state of the plankton cycle.