Behavioural thermoregulation in two freshwater fish species

In the presence of a vertical thermal gradient, juvenile three-spined sticklebacks Gasterosteus aculeatus and minnows Phoxinus phoxinus positioned themselves higher in the water column compared with adult conspecifics. This result was consistent regardless of whether age cohorts were tested separately or together. Furthermore, juveniles but not adult fishes positioned themselves higher in water column in the presence of a thermal gradient compared with those in the absence of a thermal gradient. Juvenile G. aculeatus and adult fish of both species did opt to position themselves higher in the water column in the hours immediately following a feeding event relative to their positions in the same gradient when they had not fed.