HOW DOES COD (GADUS-MORHUA) COPE WITH VARIABILITY IN FEEDING CONDITIONS DURING EARLY LARVAL STAGES

Response of mesocosm-reared cod (Gadus morhua L.) larvae to different feeding conditions was investigated in 1988 in two mesocosms: a large basin and a smaller bag enclosure within the basin. The basin was filled with seawater, and a community of naturally occurring plankton developed. Plankton concentrations were monitored, and cod larvae stocked in the enclosures were sampled for determination of growth, survival, and gut content. In the bag, insufficient amounts of energetically favourable prey, as copepod nauplii, led to non-selective ingestion of plankton from a broad range of sizes, including considerable amounts of protozoans (tintinnid and oligotrich ciliates). Growth of larvae from the bag was low, with daily specific growth rates (SGR) less than 2.8% the first 3 wk post-hatch. This was followed by rapid increase of SGR to 21.7%, which coincided with a large increase in availability of copepod nauplii. In the basin, high nauplii concentrations led to SGR of 13.7 to 21.7% from onset of feeding to 16 d post-hatch, respectively. Under such conditions, the larvae were highly selective feeders. At 3 wk post-hatch, survival was 36.7 and 38.3% in the basin and bag enclosure, respectively. To cope with variations in the feeding conditions, the cod larvae were shown to be opportunists when nauplii were scarce, and included plankton from several trophic levels in their diet, When nauplii were abundant, cod larvae realized their high potential for growth. Both opportunism and realization of a high growth potential may enhance survival of the larvae.