An understanding of a species’ thermal niche is important for predicting how populations may respond to an altered thermal regime; whether from climate change, riparian zone alterations or thermal pollution. Unfortunately, the thermal ecology of Australian freshwater fishes is poorly understood. The objective of this PhD is to improve our understanding of this issue by researching the thermal ecology of two congeneric species; the river blackfish (Gadopsis marmoratus) and the two-spined blackfish (G. bispinosus). Anecdotal reports based on their distribution suggest that G. bispinosus may be cold-adapted while G. marmoratus may be adapted to warmer water. This PhD will attempt to provide a mechanistic explanation for blackfish distribution by determining the thermal niche of these two species.

Sampling of blackfish communities along an altitudinal gradient in two streams revealed that G. bispinosus are the only blackfish species at higher elevations, with both species coexisting in a significant zone of overlap before G. marmoratus become the dominant blackfish species at lower elevations. To test for evidence of thermal adaptation the swimming and respiratory performance of both species was determined at different temperatures using a Ucrit protocol. No significant species-temperature interactions were detected, with maximal performance of both species occurring at 22 degrees. If the two species are adapted to different temperatures, these adaptations do not appear to affect their aerobic swimming capabilities. Additional research on the effect of temperature on other aspects of fitness is needed to determine if thermal adaptation is responsible for the distribution patterns of blackfish species.