Folkvord, A. 1993. "Experimental studies of growth and mortality of larval and juvenile cod (Gadus morhua L.) with special emphasis on cannibalism." Dr. scient thesis, University of Bergen, 153 pp.


The purpose of this thesis was to evaluate the significance of and the mechanisms behind cannibalism of cod (Gadus morhua L.) in Hyltropollen, an extensive rearing unit used for juvenile cod production. Rapid digestion of cod larvae and juveniles, and a large predator population compared to the prey population, made it unfeasible to estimate cannibalism rates by stomach analysis. A series of controlled experiments was undertaken to elucidate factors of importance for growth, survival and cannibalism among 0-group cod in a farming situation.

The degree of cannibalism was strongly dependent on the size of the fish, the size differences within the population, and to what extent the cod were weaned to formulated feed. Survival of 0.2 and 8 g cod juveniles fed in excess with live zooplankton was around 95% after 4 weeks. Cod juveniles of 0.2 g were much more affected by temporary food deprivation (starvation) than 8 g juveniles, and this was seen as elevated mortality and cannibalism rates. Cannibalism remained a problem among 0.2 g cod juveniles starved and then subsequently fed formulated feed, whereas it disappeared completely among those subsequently fed live zooplankton. The mouth size is relatively large in cod of 20 mm (corresponding to 0.08 g), and may increase the cannibalistic potential at this stage. Large size differences in early cod juveniles (predator:prey length ratio greater than 2:1) may result in high cannibalism rates, partly because of the high growth rate after metamorphosis. Grading reduced cannibalism rates from 1.3-5.0% day-1 to 0.03-0.39% day-1. The large cannibals grew better than the smaller siblings eating formulated feed.

An analysis of growth, survival and feeding conditions in Hyltropollen in 1983, revealed that younger larval cohorts disappeared when the abundance of suitable prey for the oldest cohort declined. The potential predator:prey length ratio was larger than 2:1 at this point and it was concluded that the oldest cohort had the ability and motivation to cannibalize the younger cohorts in the pond. Differential access of the cod to the oldest stages of Calanus finmarchicus after metamorphosis may to some extent explain the increasing size variability of cod observed in this period. It is essential that the released numbers of larvae are kept sufficiently low to prevent reduction of the zooplankton standing stock before the juveniles can be readily harvested and weaned.

Updated 24. November 1995 by Arild Folkvord