Organisms inhabiting aquatic environments, be they bugs, fish, plants or crabs, are exposed to a range of selective pressures such as variable flow, temperature, biotic interactions and fishing that play a fundamental role in determining whether they live, die, grow, reproduce and by how much. Central to this equation is the fundamental tenet that natural selection operates on the individual, the results of which underpin population and species level demographic metrics such as mortality schedules, ages at maturity and biomass. Understanding linkages among individual-level traits, population ecology and species evolution however requires recognition that individuals and species do not exist in isolation but are part of a broader eco-evolutionary context. Natural and anthropogenic environmental gradients drive the expression of within and among species phenotypic variability in for example thermal tolerance or foraging behaviour, and these in turn alter the ecological outcomes of among-species biotic interactions such as predation and competition. Despite the strong ecological ramifications of trait variation and its obvious importance to species and environmental management and conservation, relatively few studies of Australian aquatic environments have harnessed evolutionary insight to provide context to ecological questions. Here, I will provide an overview of why we should care about understanding trait-based ecology and evolution. I will then present a short case study illustrating how trait variation can be practically used to disentangle the relative importance of oceanic warming and fishing pressure in driving long-term patterns in fish size in SE Australia.