Habitat utilization of herring larvae in an inshore retention area in the Western Baltic Sea

Abstract

Many commercial pelagic fishes such as Atlantic herring run through a life cycle which is tightly bound to the offshore pelagic zones. Their population dynamics are therefore mainly driven by large scale hydrography or climate conditions and studies on early life-stages are usually focused on outer shelf bank spawning grounds and adjacent areas. However, larval herring of East-Atlantic sub-populations spawned within estuarine inshore systems develop in a quite different suite of environmental conditions than their shelf-spawned conspecifics in the neighboring North Sea. Hypothesizing that herring larvae in shallow brackish lagoons utilize different estuarine habitats along their early ontogenesis we sampled ichthyoplankton at distinct pelagic and littoral sites within a major spawning area of the Western Baltic herring. Additionally, we analyzed the vertical distribution of herring larvae which is generally assumed to reflect the well mixed condition of shallow waters in the lagoon. Samples in the littoral zone unexpectedly contained comparably high numbers of herring larvae as samples taken at the pelagic sites. Furthermore we observed high abundances of older flexion larvae in very shallow areas (<1m) close to the shore line. Despite a well-mixed water column, herring larvae showed a distinct vertical distribution indicating an active depth selection potentially influencing the drift potential along the current regimes. Our study underlines the importance of shallow littoral zones for the reproduction success of pelagic key stone species. Future discussions on management and protection demands should take into account the significant function of these habitats for larval fish