# Mixed Support for the Temperature‐Size Rule in Wild Freshwater Fishes

## TL;DR

This study examines how temperature affects the growth and size of wild freshwater fish, finding mixed support for the temperature-size rule.

## Contribution

The study provides a comprehensive analysis of the temperature-size rule using a large dataset of fish from thousands of lakes.

## Key findings

- No species fully conforms to the temperature-size rule.
- Warmer environments are linked to faster life histories and shorter lifespans.
- Faster growth in warm environments rarely results in smaller maximum body sizes.

## Abstract

The temperature‐size rule states that species living in warmer temperatures will grow faster and mature earlier at smaller sizes. While several studies have documented patterns in average body size consistent with the temperature‐size rule in wild populations, a comprehensive test is lacking. Here, we use age and length data of 1.4 million fish across 7 species from 2704 lakes to quantify temperature‐related variation in growth across ontogeny. Our results show that no species fully conforms to the temperature‐size rule; despite patterns of juvenile growth rate and age at maturity typically aligning with the temperature‐size rule, these changes seldom translate to reduced size at maturity or maximum size. We also found evidence that faster life histories in warmer environments are associated with reduced lifespans. A deeper understanding of how temperature shapes growth in natural systems is needed to accurately predict the effects of global warming on wildlife.

We use age and length data of 1.4 million fish from 2704 lakes to evaluate the temperature‐size rule in wild populations. We found evidence that warmer environments are associated with faster life histories and reduced lifespans, but this seldom translates to smaller maximum body sizes. A deeper understanding of how temperature shapes growth in natural systems is needed to accurately predict the impacts of global warming on wildlife.