Resilience in a Hypoxic World: Fish Respond Through Plasticity in Their Behaviour, Whereas Adaptation and Adaptation of Plasticity in the Behaviour and Metabolism Occur

ABSTRACT The occurrence of climate change‐induced hypoxia, that is, low dissolved oxygen levels in water, is increasing at an unprecedented rate. When organisms cannot escape, they must cope through plasticity, within or across generations, or even locally adapt. Documenting all these responses is essential to better understand the populations' capacity to persist in changing environments over generations. Therefore, two populations of sticklebacks ( Gasterosteus aculeatus ), one exposed to frequent hypoxia in the wild and one not, were bred for two generations, exposing offspring to either normoxia or daily fluctuating hypoxia (35% DO at night). When exposed to hypoxia within a generation, fish were less social and took fewer risks. However, fish from the population previously exposed to hypoxia in the wild were, on the contrary, more social and took more risks while also decreasing standard metabolic rate and growth, showing signs of adaptation. Fish also showed adaptation of their plasticity by losing plasticity for their hypoxia tolerance thresholds. No intergenerational plasticity was revealed. Overall, our study revealed that fish were able to cope with hypoxia within and across generations mainly through within‐generation plasticity on behaviour, potentially giving time before adaptation could take place.