Alternative competitive strategies and the cost of food acquisition in juvenile Atlantic salmon (Salmo salar)

Aggressive interactions were recorded for 3 h per day in 11 groups of 10, 1+,juvenile Atlantic salmon given access to a limited food supply. Observations were made over nine-day periods in which the dominant (most aggressive) fish on each day was identified and removed. In all groups at the start of the study, one (usually relatively large) fish performed most (67% overall) of the aggressive acts. Feeding activity was also polarised, though less strongly so, with the two most actively feeding fish taking on average 41% of the food supplied. Behavioural polarisation was also marked at a later stage in the study, when five fishes remained in each group, but rates of aggression per fish increased markedly over successive days. Food intake was positively related to aggression, but not to relative size once the size/aggression relationship was corrected statistically. The relationship between aggression and food intake on the first day of testing was weak (R-2 = 8%); in particular, 30% gained no food in spite of behaving aggressively, while 11% obtained food even though they showed no aggression, The proportion of attacks received was positively, but weakly (R-2 = 5%) related to feeding rates, but not to level of aggression. As a consequence, fishes that engaged in neither feeding nor fighting were attacked less frequently than others in the group. These results, which suggest that the risk of injury may act as a counter-selection against some strategies for food acquisition, are discussed in the context of previous work on resource competition in salmonid fishes. (C) 1998 Elsevier Science B.V. All rights reserved.