THE POTENTIAL FOR INTRACOHORT CANNIBALISM IN AGE-0 WALLEYE POLLOCK, THERAGRA-CHALCOGRAMMA, AS DETERMINED UNDER LABORATORY CONDITIONS

In laboratory experiments, we tested the capability of larger age-0 wa lleye pollock to consume smaller members of their cohort. In separate aquaria, 81 pairs of juveniles covering a wide range of size differenc es (total lengths differing by 12 to 61 mm) were held and monitored ov er a 4 day period. Complete consumption, in which a smaller fish was s wallowed whole by a larger fish, occurred 11% of the time. In 36% of t he pairs, attacks by the larger fish resulted in mortality of the smal ler fish. The mouth width:body depth ratio between the larger and smal ler fish of a pair differed significantly depending on whether the sma ller fish survived, was killed but not consumed, or was ingested whole by the larger fish. Cannibalistic individuals could consume fish clos e to the maximum size physically possible under gape limitation; at th is size the length of the cannibal was approximately 1.7 times the len gth of the prey. Length-frequency distributions of age-0 pollock in fi eld concentrations suggested that, at least in some geographical areas , potential cannibals and prey commonly co-occur. Unsuccessful predato ry attacks by larger individuals may have additional detrimental effec ts on smaller pollock in natural populations.