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

## TL;DR

Fish cope with low oxygen levels through behavioral changes within their lifetime, and populations previously exposed to hypoxia show signs of adaptation.

## Contribution

The study reveals how fish populations adapt to hypoxia through within-generation plasticity and partial genetic adaptation.

## Key findings

- Fish exposed to hypoxia became less social and took fewer risks, while those from hypoxia-exposed populations were more social and risk-taking.
- Fish from hypoxia-exposed populations showed reduced metabolism and growth, indicating adaptation.
- Fish adapted plasticity in hypoxia tolerance thresholds but showed no intergenerational plasticity.

## 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.

Our study investigates the processes at play behind hypoxia response by looking at the independent and combined roles of plasticity and evolution, while also considering inter‐generational plasticity. We used two fish populations exposed to different levels of hypoxia in the wild and experimentally exposed to two levels of hypoxia conditions. Our main findings revealed that fish response to hypoxia might rely on within‐generation plasticity in their behaviour until genetic adaptation occurs. Although signs of adaptation were observed, not only in the fish behaviour and metabolism but also on the capacity for plasticity in their anaerobic metabolism, no strong inter‐generational plasticity seemed to take place.