REPRODUCTIVE-BEHAVIOR OF SEA-TROUT (SALMO-TRUTTA) - THE CONSEQUENCES OF SEA-RANCHING

Reproductive behaviour and factors associated with male and female mat ing success were studied in sea trout (Salmo trutta) in a stream water aquarium, during three successive years. We compared a sea-ranched an d a wild-produced strain, both of the same genetic origin. In general, dominant males courted females more often, spent more time on the spa wnable areas, and had higher mating success than lower ranked males. B ody mass explained about 18% of the males' position in the dominance h ierarchy. Sea-ranched males, however, achieved on average fewer mating s than wild ones. Wild males courted the nest-preparing females and ch ased away other males more frequently than did the sea-ranched males. Females were observed to be aggressive towards males, especially when the females were preparing a nest. The males that were more frequently the target of female aggression had smaller adipose fins and were mor e often aggressive towards females. In general the females spawned jus t once in each nest. Sea-ranched females defended and tested their red d less frequently than the wild females did. Digging activity by wild females diminished closer to spawning time, but not that of the sea-ra nched females. To what extent the observed differences were due to the environmental conditions or to genetic differences is not known at pr esent. However, the results of previous studies have revealed that beh avioural and morphological differences exist between the strains, desp ite being reared under the same conditions. Thus, the behavioural chan ges noted in the sea-ranched sea-trout might have a genetic element.