The relationship between feeding conditions and production of cod fry (Gadus morhua L.) in a semi-enclosed marine ecosystem in western Norway, illustrated by use of a consumption model

Feeding conditions (quantitative and qualitative) of cod during the postlarval stage are of great importance for ensuring the successful production of cod fry. Although population survival of cod during the postlarval stage appears to be food-limited, population growth does not. The investigation was carried out in a dammed seawater pond (270 000 m ') in western Norway in 1987 and 1988. The biomass of zooplankton and the growth and survival of cod were estimated from weekly samples during the period from release of cod larvae in the pond to beyond metamorphosis. A model for estimating the cod’s daily consumption of zooplankton is proposed, and modelled consumption and zooplankton biomass were compared. The cod were also offered alternative feed from the age of metamorphosis. In 1987, the biomass of zooplankton was low, especially of macrozooplankton which occurred in densities <20 n r 3. Estimated consumption rates indicated that production of cod fry was dependent mainly on the production of microzooplankton. Inadequate and unfavourable zooplankton conditions from 21 to 68 days after release seemed to have a dramatic effect on cod fry survival. In 1988 the biomass of zooplankton was high. In spite of a rapidly increasing food demand of cod after metamorphosis, zooplankton was abundant enough to suppor t growth and survival of cod 3 to 4 weeks beyond metamorphosis. The application of the consumption model was useful when explaining the final production results of cod fry in 1987 and 1988 (5000 and 240 000, respectively).

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