EFFECTS OF INITIAL SIZE DISTRIBUTION, DAY LENGTH, AND FEEDING FREQUENCY ON GROWTH, SURVIVAL, AND CANNIBALISM IN JUVENILE ATLANTIC COD (GADUS-MORHUA L)

Two experiments were carried out with juvenile Atlantic cod, Gadus morhua L., in order to examine the effects of initial size distribution on growth and survival. The initial size distributions significantly affected growth and cannibalism rates among 0.6-g cod but not among 10-g cod. Cannibalism occurred among 0.6-g cod when the max:min weight ratio exceeded 3.5:1 and was the main mortality factor when the ratio was above 8.5:1. Total mortality rates ranged from 0.25 to 10.0% per day. Small differences in initial coefficient of variation (CV, weight) among graded groups (28.0 vs. 37.5%) resulted in significantly different rates of cannibalism (0.03 vs. 0.39% day-1). Mortality was size-dependent, with survival of the smallest fish being very poor in the groups with the largest size variation. The largest fish, presumed to be the cannibals, grew faster than smaller cod. Average growth of 0.6-g cod was about 4.0% per day. No cannibalism was observed among 10-g cod, and those fed moist feed grew faster than those fed dry feed (2.8 vs. 2.5% day-1). In a third experiment the growth rate of graded 0.9-g cod was higher with 24 h of light per day than with 16 h of light per day (2.7 vs. 2.5% day-1). No differences were found in growth or mortality rates using feeding frequencies from once every minute to four times per day. Final CV (weight) was generally independent of initial CVs over 45% and was, therefore, not a good measure of past mortality rates.