Understanding animal movement is essential for interpreting ecological and evolutionary processes, especially as climate change reshapes coastal ecosystems. In temperate coastal lagoons, productive yet thermally sensitive habitats, marine fishes such as the gilthead seabream rely on seasonal migrations to balance foraging, spawning, and refuge needs. Because temperature-driven migrations can be interpreted alongside laboratory measures of physiological responses to warming, cardiac performance offers a powerful lens for understanding how fishes cope with environmental stress. The heart’s thermal sensitivity is a core determinant of survival, making cardiac thermal performance especially valuable for linking behaviour to environmental conditions. By combining acoustic telemetry, long‑term lagoon temperature records, and cardiac thermal tolerance data, our research reveals how seabreams use lagoons during favourable summer conditions and undertake rapid refuge migrations to sea during heatwaves. These insights highlight the tight coupling between thermal physiology and habitat use, offering new predictive power for understanding fish resilience in a warming Mediterranean.

Heart‑rate logging reveals seabream responses to thermal stress

Researchers from France and Canada implanted 16 wild seabreams with Star‑Oddi DST milli‑HRT ACT loggers to record heart rate at high resolution, 200 Hz for 4 s every two hours overnight and every five minutes during the day. Two groups of four fish were then exposed to controlled thermal trials, one undergoing progressive warming and the other cooling. During the warming trial, water temperature increased by 2 °C every 30 minutes from 25, 29 °C, then by 1 °C increments thereafter. In the cooling trial, temperature was lowered stepwise until fish reached their critical thermal minimum. Individuals were euthanized upon clear loss of equilibrium, specifically, complete loss of dorso‑ventral orientation, marking their CT_max and CT_min. These experiments provide essential insight into the thermal limits and cardiac resilience of seabream under rapidly changing environmental conditions.

Thermal threshold reveals cardiac limits – driving seabream migrations

Laboratory measurements (see table 1. below) of cardiac thermal performance showed that mean heart rate rose sharply from 25 to 29 °C before reaching an upper asymptote, indicating a physiological ceiling beyond which seabream could no longer increase cardiac output. During cooling, heart rate declined to an inflection at 13 °C, below which cardiac activity was broadly depressed. The inability of wild seabream to elevate heart rate beyond 29 °C suggests a critical thermal threshold: if metabolic demand continues to rise above this point, oxygen supply may become limiting, risking fatigue or anaerobic metabolism. In the wild, where movement and migration add energetic cost, this threshold may be even lower, making 29 °C an important “alarm temperature” that could drive refuge migrations during summer heatwaves in warming coastal lagoons.

Table. 1. Showing heart rate response to warming or cooling. Undisturbed is in normoxia at 25°C measured prior to challenges.

You find further results in the article published in Marine Ecology Progress Series, the same group has previously published data on seabream in the Journal of Experimental Biology.