GROWTH DEPENSATION AND AGGRESSION IN LABORATORY-REARED COHO SALMON - THE EFFECT OF FOOD DISTRIBUTION AND RATION SIZE

Groups of recently emerged coho salmon fry Oncorhynchus kisutch were r eared for 3 months on food that appeared either asynchronously at a si ngle location (localized) or synchronously and spatially dispersed (di spersed). Groups were further subdivided into those receiving low (1%) or high (3% body weight per day) rations, with five replicate groups for each treatment combination. At low ration there was greater growth depensation, i.e. growth variation, in groups receiving localized as compared to dispersed food. At high ration there was no difference. Th ere was no effect of food distribution upon mean fish weight, but grou ps receiving high rations had greater mean fish weights than groups re ceiving low rations. There was no overall difference in the frequency of chasing between any of the treatment combinations. However, in loca lized food groups, dominants defended positions close to where food en tered the tank, giving them greater access than subordinates. In dispe rsed food groups, while dominants also defended particular areas, this did not result in greater access to food. These results demonstrate t hat although feeding methodology may not directly influence the freque ncy of aggressive interactions, Feeding methods which facilitate feud monopolization by dominants can accelerate the growth of these individ uals at the expense of subordinates. In aquacultural applications wher e greater size is desirable, or otherwise selected for, this may resul t in the unintentional selection fur increased aggressiveness. (C) 199 6 The Fisheries Society of the British Isles