Laboratory studies on the effects of thermal change on the behaviour and distribution of juvenile chum salmon in sea water

Schooling chum salmon Oncorhynchus keta were biased towards the water surface (median position <1 m) under isothermal conditions (10degrees C) in a water column simulator (WCS). Thermal stratification (24/10degrees C) inhibited upward movement with fish congregating at the thermocline and displaying a clear avoidance of potentially lethal surface waters. A tri-phase model based on piece-wise nonlinear regression was used to describe the distribution shifts of chum salmon during a change from isothermal to thermally stratified conditions. Fish distribution was consistent with thermoregulatory behaviour and exhibited 'attraction', 'preference' and 'avoidance' phases. The thermal preference of 50% of the fish lay between 12.2 and 20.2degrees C, however, >83.5% of the fish occupied a 'preferred' temperature range of 13.7-17.9degrees C. The mean temperature at which 50% of chum salmon avoided rising temperature by shifting deeper in the water column and using the cooler thermocline was 20.2degrees C, and 90% avoidance occurred at 22.9degrees C. Behavioural responses to thermal stratification were consistent amongst underyearling fish of differing size and age. (C) 2003 The Fisheries Society of the British Isles.