The energetic equivalence of changing salinity and temperature to juvenile salmon

1. The influence of salinity, temperature and physiological development on habitat choice by juvenile salmon is poorly understood. We measured differences in the net energetic cost of habitats that differed in salinity or temperature using behavioural titration of juvenile salmon and correlated these costs with body size and osmoregulatory enzyme activity to quantify how costs change with physiological development. 2. Juvenile salmon showed a strong preference for saline water (27‰ or 15‰ vs 0‰) and for cold water (9 °C vs 14 °C). It was estimated to be 0·15 W and 0·11 W more costly for salmon to forage in fresh water than in 15‰ and 27‰ salt water, respectively, and 0·09 W more costly to forage in warm than in cold water. 3. We expected fish to prefer 15‰ salt water to fresh water regardless of enzyme activity because they are iso-osmotic with this salt concentration. In contrast, preference for higher salt concentrations should increase with enzyme activity. Consistent with our expectations, enzyme activity was not correlated with preference for 15‰ salt water, but was positively correlated with preference for 27‰ salt water. 4. The cost of changing salinity vs changing temperature were very similar, indicating that knowledge of both of these costs are necessary to understand juvenile salmon habitat choice. Key-words: Foraging, habitat choice, ideal free distribution, Na, K-ATPase activity