Thermal tolerance varies in tropical and sub-tropical populations of barramundi (Lates calcarifer) consistent with local adaptation

Barramundi (Lates calcarifer) are cultured throughout Australasia and increasingly throughout the northern hemisphere. In Australia, barramundi are found in rivers and estuaries across 16 degrees of latitude (10°S–26°S) exposing fish from northern and southern parts of their distribution to different thermal environments. Australian barramundi show strong genetic structuring and this coupled with exposure to varying thermal environments may have led to thermal tolerance differences among populations of interest to aquaculture. To determine if barramundi are thermally adapted to their local environment we used loss of swimming equilibrium (LOSE) as a predictor of upper thermal tolerance in fish from a tropical (Darwin, Northern Territory) and subtropical (Gladstone, Queensland) population and showed that significant differences exist between these two genetically divergent stocks. The tropical population had significantly higher tolerance to warm water temperatures than sub-tropical fish exhibiting a significantly longer time until loss of swimming equilibrium (LOSE) at 40 °C (ANOVA; F1, 22 = 7.86, P ≤ 0.01). As LOSE challenge tests are not practical to the identification of commercial broodstock animals we also evaluated dissociated caudal fin cells as a sensitive and non-invasive method to determine upper thermal tolerance in this species. Results from dissociated fin cells strongly correlated with those based on LOSE confirming that this method can be used to discriminate populations with different thermal tolerances without having to directly thermally challenge broodstock animals.