Does social isolation influence metabolism, copper accumulation, and thermal tolerance in a temperate fish?

Group living, exemplified by shoaling fish, offers individuals physiological benefits including reduced stress responses (social buffering) and metabolism (the calming effect). These physiological benefits may have implications on toxicant accumulation in an aquatic environment. However, limited data exist on whether a social pair is sufficiently social to provide these benefits when compared to social isolation. We sought to investigate whether isolated and paired wild-caught anadromous threespine stickleback (Gasterosteus aculeatus) had different metabolic demands (oxygen uptake rate; ṀO₂) and if ṀO₂ drove accumulation of waterborne Cu, and whether these factors influenced the fish's acute thermal tolerance (CT_max). Social isolation was hypothesized to elicit stress, increasing oxygen uptake and subsequent branchial accumulation of Cu, which in turn was predicted to limit the fish's CT_max. Oxygen demand at the gills was not affected by Cu exposure nor social context. Despite this, isolated fish had on average 87% and 61% more Cu in their gill and intestinal tissue, respectively, compared to paired fish. This suggests that metabolism did not drive the observed socially mediated difference in tissue Cu burden. Rather the elevated tissue Cu may be endogenous, as increased Cu was also present in tissues of isolated fish that were not exposed to Cu. CT_max was marginally but significantly reduced to 32.9 °C (from 33.1° C) in fish that were exposed to Cu, though this is likely not biologically relevant. This study unearths interactions between social context and metal burden post-exposure, paving the way for future work examining sociality and toxicology in fish.