THE EFFECTS OF FOOD PELLET DIMENSIONS ON FEEDING RESPONSES BY ATLANTIC SALMON (SALMO-SALAR L) IN A MARINE NET PEN

The effects of the diameter and length of cylindroid food pellets on the feeding responses of one-sea-winter Atlantic salmon in a marine net pen have been investigated. Pellets of different shapes and sizes were dropped into the pen in a random sequence and the responses of salmon were recorded with a video system. Both the diameter and length of pellets affected the time until first capture, with salmon taking longer to capture small pellets. The probability of a pellet being rejected after having been grasped was related to its length, but not its diameter: shorter pellets were rejected least often. Thus, the larger pellets that appeared to be initially most attractive to salmon were not the sizes that they ingested most readily once grasped. Acceptability (as indicated by rejection rate) varied over the range of pellet lengths that occurs within the size class of commercial feed recommended for these fish, and the optimum length (in terms of the number of pellets eaten rapidly) was shorter than the mean length of commercial pellets. The possible interaction of the effects of pellet size and hardness on palatability should be investigated.