Biologgers as a tool to investigate the environmental tolerances of fishes

To date, a variety of methods have been used to determine the upper thermal tolerance of fishes, and knowledge of the physiology and behaviour of non-polar fishes at cold temperatures is limited. Thus, in this thesis, I used data storage tags (DSTs) to: (1) examine how measures of upper temperature and hypoxia tolerance differ in free-swimming Atlantic salmon as compared to those obtained from fish in respirometers and using a ‘rapid screening tool’ (performed on anesthetized fish); and (2) understand the impacts of winter temperatures on the behaviour, distribution and physiology of sea-caged salmon. Resting heart rates (ƒHs) were significantly lower in free-swimming fish, and this resulted in greater values for the scope for ƒH and for their critical thermal maximum (CTmax). There were large differences between the fish’s core and water temperatures using the ‘rapid screening tool’, and this technique greatly underestimated thermal tolerance. These results highlight the limitations of commonly used methods of determining a fish’s thermal tolerance, and suggests that scope for ƒH is the most reliable predictor of this parameter. In Atlantic Canada, sea-caged Atlantic salmon experienced temperatures < 5°C for 5 months during the fall / winter, with a decline of 0.35 – 0.57°C week⁻¹ from November (~ 8 – 10°C) to March (~1.1°C). The salmon’s ƒH closely followed water temperature, and while the fish mainly occupied the upper 5 m of the 30 m cage, there were frequent excursions to deeper depths. This novel study validates the use of data loggers for monitoring fish activity and physiology at cold winter temperatures.