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Cod eggs in the Baltic Sea are neutrally buoyant at depths exceeding 55 m. When these eggs hatch the larvae must enter the upper photic portion of the water column to locate and capture sufficient prey to feed and grow. In this study we investigated the time during ontogenetic development at which this vertical migration occurs. The vertical distribution of cod larvae, microzooplankton, light intensity and the physical characteristics of the water column in the Bomholm Basin were investigated during 3 cruises in May, June and July 1994. Larvae designated as pre-feeding were usually located at the depths where they had hatched. After larvae had begun to feed, their distributions moved closer to the water's surface. Since larvae are negatively buoyant relative to the density of water in the upper layers of the Baltic, this migration requires active swimming. Hence the hydrographic structure of the water column in the Baltic likely imposes a modest metabolic cost on larvae. We also investigated factors determining the vertical hstribution of feeding larvae. The distribution of these larvae was poorly correlated with prey abundance (i.e. concentration of copepod stages). However, distributions were correlated with prey availability as estimated by combining measures of light-dependent larval feeding incidence with the measured prey concentrations. Our observations suggest that a vertical migration among Baltic cod larvae is necessary for 2 reasons. This migration enables larvae to obtain suitable feeding conditions, and to avoid mortality that could be induced by exposure to the low oxygen conditions typical for the sub-halocline layer.

Ontogenetic and environmental effects on vertical distribution of cod larvae in the Bornholm Basin, Baltic Sea