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 (CTmax). 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 CTmax.
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. CTmax 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.