ONTOGENIC AND ENVIRONMENTAL-EFFECTS ON VERTICAL-DISTRIBUTION OF COD LARVAE IN THE BORNHOLM BASIN, BALTIC SEA

Cod eggs in the Baltic Sea are neutrally buoyant at depths exceeding 5 5 m. When these eggs hatch the larvae must enter the upper photic port ion of the water column to locate and capture sufficient prey to feed and grow. In this study we investigated the time during ontogenetic de velopment at which this vertical migration occurs. The vertical distri bution of cod larvae, microzooplankton, light intensity and the physic al characteristics of the water column in the Bornholm Basin were inve stigated during 3 cruises in May, June and July 1994. Larvae designate d as pre-feeding were usually located at the depths where they had hat ched. 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 migrat ion requires active swimming. Hence the hydrographic structure of the water column in the Baltic Likely imposes a modest metabolic cost on l arvae. We also investigated factors determining the vertical distribut ion of feeding larvae. The distribution of these larvae was poorly cor related with prey abundance (i.e. concentration of copepod stages). Ho wever, distributions were correlated with prey availability as estimat ed by combining measures of Light-dependent larval feeding incidence w ith 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 ox ygen conditions typical for the sub-halocline layer.