# Linking aerobic scope to fitness in the wild reveals potential opportunities to help recover imperiled salmon populations

## TL;DR

This study shows that aerobic scope can influence the survival of Chinook salmon in the wild under certain environmental conditions, offering new insights for their conservation.

## Contribution

The study provides empirical evidence linking aerobic scope to fitness in wild salmon populations under specific environmental thresholds.

## Key findings

- Aerobic scope was associated with survival success during critical life stages only under specific environmental conditions.
- Fitness benefits of aerobic scope were limited outside a narrow ecological threshold defined by critical and stable metabolic index values.
- Reduced aerobic scope increased predation risk, potentially linking it to lower fitness in wild salmon.

## Abstract

Aquatic ectotherms are hypothesized to be vulnerable to warming and deoxygenation associated with environmental change because temperature and oxygen (O2) supply can restrict aerobic scope (AS) in captivity. However, evidence of a direct association between AS and fitness in the wild is lacking, inspiring debate about the circumstances under which AS is the primary driver of population fluctuations. Using respirometry data, telemetry studies, long-term population monitoring, and in situ predator-prey experiments, we related AS to two Chinook salmon (Oncorhynchus tshawytscha) population bottlenecks in the wild, juvenile rearing and migration. We found that AS, which we quantified using the metabolic index (ɸ), was associated with success probability for these bottlenecks only under a relatively narrow window of viable environmental conditions, depending on intraspecific metabolic trait diversity and hydrologic conditions. Opportunities for potentially high-impact temperature- and O2-specific conservation and management actions using existing hydraulic engineering infrastructure could therefore exist when AS is between critical (ɸcrit) and stable (ɸstable) values. Outside of this ecological threshold, changes in AS did not yield appreciable fitness benefits because successful rearing and migration were either exceptionally improbable (i.e., AS<ɸcrit), or seemingly independent of AS (i.e., AS>ɸstable). In addition, AS impairments likely increased susceptibility to predation, and this may have been involved in the putative association between AS and fitness in the wild.

A synthesis of respirometry data, telemetry studies, long-term population monitoring, and in situ predator-prey experiments reveals that aerobic scope can circumstantially both limit and facilitate the fitness of imperiled Chinook salmon populations.