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 (m ost aggressive) fish on each day was identified and removed. In all gr oups at the start of the study, one (usually relatively large) fish pe rformed most (67% overall) of the aggressive acts. Feeding activity wa s also polarised, though less strongly so, with the two most actively feeding fish taking on average 41% of the food supplied. Behavioural p olarisation was also marked at a later stage in the study, when five f ishes remained in each group, but rates of aggression per fish increas ed markedly over successive days. Food intake was positively related t o aggression, but not to relative size once the size/aggression relati onship was corrected statistically. The relationship between aggressio n 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, whil e 11% obtained food even though they showed no aggression, The proport ion of attacks received was positively, but weakly (R-2 = 5%) related to feeding rates, but not to level of aggression. As a consequence, fi shes that engaged in neither feeding nor fighting were attacked less f requently than others in the group. These results, which suggest that the risk of injury may act as a counter-selection against some strateg ies for food acquisition, are discussed in the context of previous wor k on resource competition in salmonid fishes. (C) 1998 Elsevier Scienc e B.V. All rights reserved.