Behavioural changes during the parr–smolt transformation in coho salmon Oncorhynchus kisutch: is it better to be cool?

Behavioural changes that occur during the parr–smolt transformation were investigated in juvenile coho salmon Oncorhynchus kisutch. Fish from two populations were examined from the Fraser River catchment in British Columbia, Canada; a short and a long-distance migrating population. Fish showed a significant decrease in condition factor and significant increase in gill Na⁺K⁺-ATPase activity during the spring indicating that they became competent smolts, but no difference between populations. Temperature preference trials were conducted using a shuttlebox system throughout the spring. Mean temperature preference did not differ between the two populations, but preferred temperature decreased with development from 16.5 ± 0.3°C for parr to 15.5 ± 0.4°C for smolts. Mean swimming velocity was also greater in smolts than parr, but there was no difference between the two populations. The preference for warmer water temperature observed for parr in early spring may be advantageous for stimulating smolt development. Preference for slightly cooler temperatures observed for smolts would sustain elevated seawater tolerance during the smolt window by a short time and may ensure successful transition to the marine environment.