Vertical distribution of Baltic sprat larvae: Changes in patterns of diel migration?

Abstract

Ontogenetic and diurnal vertical migration patterns of Baltic sprat larvae were investigated for the periods 1989-1990 and 1998-2002. Comparison of the results led to the hypothesis that the diel vertical migration behaviour of sprat larvae >10 mm has changed. In 1989 and 1990, sprat larvae migrated to the surface at night, whereas they stayed 30-50 m deep by day. From 1998 to 2002, sprat larvae showed no signs of diel vertical migration, remaining in warmer, near-surface water by day and night. This behavioural change coincided with a more general change in the Baltic ecosystem, i.e. an increase in near-surface temperature and a general increase in abundance of the major prey organism (Acartia spp.) of Baltic sprat larvae, with more pronounced aggregation in surface waters. © 2007 International Council for the Exploration of the Sea.