Vertically resolved prey selectivity and competition of Baltic herring Clupea harengus and sprat Sprattus sprattus

Prey selectivity of Baltic herring Clupea harengus and sprat Sprattus sprattus was studied in the Bornholm Basin, Baltic Sea, in June 2001. A total of 165 sprat stomachs (10 to 15 cm total length, TL) and 214 herring stomachs (12 to 30 cm TL) were analysed. The diel vertical distribution of zooplankton prey was analysed by multi-net samples; clupeid distributions were estimated by hydroacoustic measurements. These measurements enabled us to describe the diel feeding rhythm and to estimate vertically resolved selectivity indices for the 2 most important zooplanktivores in the Baltic Sea. Diet composition of herring and sprat were similar (mainly copepods and cladocerans), resulting in strong competition. Possibly to reduce this competition, both species were partly specializing on certain prey species (sprat: Podon spp.; herring: Evadne nordmanni and Temora longicornis) and copepodite stages (sprat: adult [C6] males of Pseudocalanus acuspes; herring: C6 females of P. acuspes). Sprat and, to some extent, herring exhibited a marked shift in prey preference between day and night. Sprat mainly selected T. longicornis during the day and Podon spp. during the night, while herring mainly selected T. longicornis during the day and E. nordmanni during parts of the night. A comparison of the field stomach contents with the estimated gastric evacuation predicted by parameters based on laboratory experiments indicated that sprat fed during the night, while herring did not or only to a minor extent. Comparison of our zooplankton sampling scheme with commonly used sampling designs revealed that investigations which consider both time and stage are needed to fully understand the feeding selectivity dynamics of herring and sprat in the Baltic Sea. However, the objective of a selectivity study should determine the most appropriate zooplankton sampling scheme.