The notothenioid fishes, which dominate the upper shelf habitats of the Antarctic Continental Shelf, have evolved in a relatively thermally stable environment for at least 10-15 million years. With the upper lethal limits of these fishes around 4-6°C and the lower limits set by the freezing point of seawater, they are described as extreme stenotherms. As a result, it has been hypothesized that these fishes should exhibit marked responses to acute changes in temperature, but that acclimatory effects may be reduced with prolonged exposure to elevated temperatures is not likely to increase their thermal tolerance.
This study was designed to investigate aspects of the physiological response of Antarctic nototheniid fishes to both acute and prolonged increases in temperature. Changes in haematology, metabolic scope for activity, cardiovascular performance, aerobic swimming ability and the capacity to tolerate additional stressors were investigated in the Antarctic nototheniid Pagothenia borchgrevinki, with some comparative work carried out on the closely related Trematomus bernacchii and the temperate-water notothenioids Notothenia angustata and Bovichtus variegatus. The responses to an acute change in temperature were found to vary between the closely related Antarctic nototheniids, most likely as a consequence of their differing ecotypes. Plasma glucose levels were measured in these fish for the first time and exhibited a delayed rise in response to an acute increase in temperature. Cardiac performance was found to be closely linked to prolonged swimming ability through thermal changes in P. borchgrevinki, indicating a cardiac limitation of aerobic performance. One factor which became apparent was that the acclimation of Antarctic nototheniids to temperatures only ~2°C above their habitat temperature results in a decrease in thermal sensitivity of a variety of physiological parameters. The most notable finding was that P. borchgrevinki possesses sufficient phenotypic plasticity to warm-acclimate prolonged swimming ability, cardiac performance and osmo-regulatory capacity after 4-6 weeks of exposure to 4°C, suggesting that the Antarctic nototheniids may not be as extremely stenothermal as previously assumed and that the consequences of climate change may not be as dire as has been predicted
