Behavioural tactics across thermal gradients align with partial morphological divergence in brook charr

Area of Science:

• Ecology and Behavioral Biology
• Climate Change Adaptation
• Aquatic Ecosystems

Background:

• Predicting species persistence under climate change requires understanding how animals balance environmental constraints.
• Cold-water fish, like brook charr (Salvelinus fontinalis), face challenges in thermally stratified lakes, navigating temperature and oxygen gradients.
• Behavioral thermoregulation is crucial for optimizing foraging and avoiding physiological stress in variable aquatic environments.

Purpose of the Study:

• To investigate brook charr's behavioral tactics in response to thermal gradients and rising surface temperatures.
• To determine if behavioral thermoregulation aligns with morphological differences, suggesting potential ecotypic divergence.
• To assess the trade-offs between accessing profitable prey in warm surface waters and avoiding thermal stress.

Main Methods:

• Employed high-resolution acoustic telemetry to track fine-scale thermal habitat use, vertical movement, and diel timing in wild brook charr.
• Quantified patterns of epilimnetic and hypolimnetic forays across the summer stratification period.
• Utilized principal component analysis and linear discriminant analysis to link behavioral tactics with morphological traits.

Main Results:

• Brook charr reduced epilimnetic use and vertical forays as surface temperatures increased, with increased hypolimnion use.
• Foraging behaviors showed distinct diel patterns, with epilimnetic forays peaking at dusk and hypolimnetic use at dawn/dusk, influenced by season and moonlight.
• Identified two distinct behavioral tactics ('warm' and 'cool') correlating with thermal exposure and partial morphological divergence between sexes.

Conclusions:

• Brook charr exhibit repeatable habitat use and consistent thermal tactics reflecting a trade-off between foraging and thermal stress.
• Vertical thermal and resource gradients promote individual specialization and phenotypic divergence in cold-water species facing lake warming.
• Behavioral thermoregulation is a key mechanism for brook charr adaptation to changing thermal regimes in stratified lakes.