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 mor hua L., in order to examine the effects of initial size distribution o n growth and survival. The initial size distributions significantly af fected 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 rat io 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) am ong graded groups (28.0 vs. 37.5%) resulted in significantly different rates of cannibalism (0.03 vs. 0.39% day-1). Mortality was size-depen dent, 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, an d 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 w as 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 p er day. Final CV (weight) was generally independent of initial CVs ove r 45% and was, therefore, not a good measure of past mortality rates.