The spawning behaviour of escaped farmed and wild adult Atlantic salmon (Salmo salar L.) in a northern Scottish river

Abstract

As a result of an accident in February 1989, 184 000 growing salmon escaped from a seawater farm located in Loch Eriboll in northern Scotland. Later the same year, the behaviour of wild adults and fish of farmed origin was compared in the nearby River Polla. Wild salmon and farmed escapees were distinguished by their morphology, supported by tissue carotenoid analysis. Radio-tagged salmon of both groups and of both sexes were monitored before and during spawning. Bankside observations were made on tagged and untagged wild and farmed fish over the same period. Carotenoid pigments were determined in ova or alevins sampled later from spawning sites along the river's length. Wild and farmed fish of both sexes spawned throughout the river's length. Overall however, wild fish tended to spawn in the river's higher reaches while farmed escapees tended to spawn lower down. Farmed males were more widely distributed at spawning than farmed females. Farmed fish, particularly females, spawned later than wild females. The extent to which these findings may be generalised to other rivers is discussed in relation to the unique character of the River Polla and particularly the presence of a smolt unit on its lower reaches.