| Summary: | Groups of eight parr of hatchery reared Atlantic salmon (Salmo salar), sea trout (Salmo trutta), rainbow trout (Oncorhynchus mykiss) and Arctic charr (Salvelinus alpinus) were investigated in monoculture with two size groups of fish. Aggression was assessed by recording seven behavioural patterns: Approach and body-bending, lateral display, charge, chase, bite, frontal display, and fight. Aggression between fish of different size showed one step in a hierarchy where larger fish were most aggressive in all species. Most aggression occurred between fish of similar size. Atlantic salmon most frequently showed charge, bites and intention movements. Sea trout showed most of intention movements and frontal display. Rainbow trout performed charge and bite most frequently, and intention movements and chasing secondly. Body-bending was typically performed by rainbow trout, and this pattern was often intentionnally showed in advance of charge. Arctic charr performed charge, bite and chase at a similar frequency.
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