Social, stocking density and dietary effects on the failure of farmed cod Gadus morhua

Abstract

A large proportion of cod juveniles fail following transfer to sea cages from the hatchery. Failed cod take and then eject feed and are thin and emaciated fish of low weight compared with “normal” siblings. Failed fish comprised 70% of total mortalities and 10% of fish stocked in the first eight months of production on a cod farm in Scotland. Failed cod comprised from 0 to 8% of fish reared in twelve hatchery tanks over 3 months. These failed cod were graded from larger fish in five consecutive grades at four to six weekly intervals and, after being reared separately, over 90% of these fish were “recovered”. No significant differences were found in food acceptance of six alternative diets and a standard marine commercial diet by failed cod. Smaller pellets or feed moistened in seawater were offered but this did not significantly increase the rate of feed acceptance. The effects of stocking density on the development of failed cod were assessed by comparing performance at densities of 0.2, 2 and 20 kg m − 3 . Aggressive interaction was lowest at an intermediate stocking density of 2 kg m − 3 and highest at a density of 20 kg m − 3 . Fin nipping was concentrated on the first dorsal fin and was not significantly different between stocking densities, but the other dorsal fins, pectoral fins and tail were also attacked more frequently at the highest stocking density. It is concluded that failed cod can be recovered by grading from the main population and feeding these cod separately. Stocking densities in sea cages should initially be similar to hatchery levels and this may be achieved by holding the cod in small enclosures within the main net.