# Deficiency of vital organic nutrients in ecosystems limits brain development and fitness in wild fish

## TL;DR

Low levels of omega-3 fatty acids in aquatic ecosystems impair brain development and competitiveness in wild fish, affecting their survival and growth.

## Contribution

This study links dietary omega-3 deficiency to reduced cognitive and competitive abilities in fish under environmental stress.

## Key findings

- Diets low in n-3 LC-PUFA reduced brain fatty acid content but not brain mass or neuron numbers.
- Fish with lower brain n-3 LC-PUFA showed reduced competitiveness and slower growth.
- Deficiency in n-3 LC-PUFA may impair behavioral flexibility and fitness in altered ecosystems.

## Abstract

Animals in aquatic ecosystems impacted by global changes often face reduced availability of vital organic compounds, such as long-chain omega-3 polyunsaturated fatty acids (n-3 LC-PUFA), which are essential for brain development and cognition. Cognitive skills are crucial for buffering the impacts of environmental stress on fitness, yet the link between the quality of diet and fitness-enhancing behaviours of individuals in food webs altered by global change remains unclear. We examined how dietary n-3 LC-PUFA affect brain development, social dominance and growth in territorial juvenile salmonids in a large-scale model of a natural pre-alpine stream. For this assessment, we used wild fish whose diet quality was estimated using stable isotope analysis, and hatchery-reared fish exposed to dietary treatments in a common-garden experiment. In both wild and common-garden experiment fish, diets low in n-3 LC-PUFA led to a decreased content of n-3 LC-PUFA in brain tissue but did not affect brain mass, morphology (i.e. mass of brain regions) or neuron numbers. Fish with lower brain n-3 LC-PUFA content exhibited reduced competitiveness in social interactions and suboptimal habitat use, resulting in slower somatic growth. Our findings indicate that the limited availability of n-3 LC-PUFA in aquatic food webs may impair the behavioural flexibility of top aquatic consumers, possibly with negative impacts on their capacity to maintain high fitness in ecosystems altered by environmental change.

Summary: Omega-3 LC-PUFA deficiency in aquatic ecosystems alters brain fatty acid composition in salmonids, reducing their cognitive capabilities and competitiveness in social interactions, highlighting the role of diet in fitness under environmental change.