Ocean warming and acidification degrade shoaling performance and lateralisation of novel tropical–temperate fish shoals

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Gregarious behaviours are common in
animals and provide various benefits such as food acquisition and protection
against predators. Many gregarious tropical species are shifting poleward under
current ocean warming, creating novel species and social interactions with
local temperate taxa. However, how the dynamics of these novel shoals might be
altered by future ocean warming and acidification remains untested. Here we
evaluate how novel species interactions, ocean acidification, and warming affect
shoaling dynamics, motor lateralisation, and boldness of range-extending
tropical and co-shoaling temperate fishes under controlled laboratory
conditions. Fishes were exposed to one of twelve treatments (combinations of three
temperature levels, two _pCO₂ levels, and two shoal type levels:
mixed-species or temperate-only) for 38 days. Lateralisation (a measure of asymmetric
expression of cognitive function in group coordination and predator escape) of
tropical and temperate species was right-side biased under present-day
conditions, but side bias significantly diminished in tropical and temperate
fish under ocean acidification. Ocean acidification also decreased shoal
cohesion irrespective of shoaling type, with mixed-species shoals showing significantly
lower cohesion than temperate-only shoals irrespective of climate stressors.
Tropical fish became bolder under ocean acidification (after four weeks), and
temperate fish became bolder with increasing temperature, while ocean
acidification dampened temperate fish. Our findings highlight the direct effect
of climate stressors on fish behaviour and the interplay with the indirect
effects of novel species interactions. Because strong shoal cohesion and lateralisation
are key determinants of species fitness, their degradation under ocean warming
and acidification could adversely affect species performance in novel
assemblages in a future ocean, and might slow down tropical species range
extensions.