Comparing the feeding habits of co-occurring sprat ( Sprattus sprattus) and cod ( Gadus morhua) larvae in the Bornholm Basin, Baltic Sea

Abstract

The feeding habits of co-occurring Baltic sprat ( Sprattus sprattus) and cod ( Gadus morhua) larvae have been studied in the Bornholm Basin, Baltic Sea during 12 cruises covering the spawning seasons in 1987 and 1988. The seasonal and size-dependant diet composition is described based on Bongo-net samples. Contrary to investigations from other areas, first-feeding larvae of both species included almost no phytoplankton in their diet. Feeding started on calanoid copepod nauplii which were the dominating food item. Copepodite stages I–V and finally adult copepods were eaten with increasing larval length. Only sprat larvae used cladocerans additionally as food source of considerable importance. Cod larvae included copepodites/copepods in their diet at smaller total lengths than sprat larvae. The trophic niche breadth of both species did not increase with larval length. The feeding selectivity of different sized sprat and cod larvae (Pearre’s C-index) was calculated based on vertical resolving sampling of predator and prey. The results indicate a strong preference of sprat larvae for different developmental stages of Acartia spp., a species showing a pronounced increase in standing stock since the 1980s. Cod larvae selected Pseudocalanus elongatus, a species which decreased in biomass. Possible implications for recruitment levels of both species are discussed.