Vairao (Tahiti), November 2025 – The MaHeWa project has just reached a decisive milestone with the completion of the first marine heatwave simulation applied to the orbicular batfish Platax orbicularis, or “Paraha peue,” an emblematic species of Polynesian lagoons and of aquaculture interest for the fenua. This experiment was designed and implemented during the mission of Benjamin Geffroy (Ifremer – UMR MARBEC Montpellier), who came to support the Ifremer Vairao team in preparing the system and launching the trials.
To simulate a heatwave period similar to those occurring in Polynesian lagoons, four scenarios were developed in a controlled experimental system. The first reproduces the average lagoon temperature of 28 °C (reference point). The next two correspond to constant heatwaves at 32 °C and 34 °C maintained for twenty-five days. The last scenario reproduces two successive heatwaves at 32 °C separated by a six-day recovery period (return to lagoon temperature).
This design makes it possible to examine both the effects of different intensities of marine heatwaves and the effects of event frequency on fish physiology. The final scenario allows a comparison between the impact of a single prolonged event and that of two repeated events, to determine whether successive heatwaves intensify thermal stress or, on the contrary, whether an initial exposure promotes a better response during the second.
The scientific objectives of this experiment are fully aligned with MaHeWa’s ambitions. The team seeks to determine the thermal sensitivity of the batfish, identify critical physiological thresholds, and understand how the fish respond to conditions that will become more frequent with climate change. Responses will be analyzed at the individual level (activity, growth) as well as at the molecular level, particularly through measuring cortisol, known as the “stress hormone.” It will be analyzed in the water to assess group-level stress, in the blood to better understand each fish’s response to thermal stress, and in the scales to evaluate cortisol accumulation over time, thereby reflecting a chronic temperature response.
The first samples were collected following a precise protocol to ensure data quality and comparability between scenarios. The collected samples will support the physiological and molecular analyses planned for 2026, with the goal of better understanding the mechanisms of thermal stress response specific to tropical species. This experiment is the result of close collaboration between UMR SECOPOL (Ifremer Vairao) and UMR MARBEC (Ifremer Montpellier). It was made possible thanks to the technical support of VSC Martin Debatis, the analytical assistance of VSC Justine Gamet, and the daily involvement of the local scientific team: Guillaume Mitta, Caline Basset, Stéphane Lallement, Thomas Camus, Denis Saulnier, Cristian Monaco, Julien Rouxel, Yann Dorant, and Carole Di Poi.
(!) The fish used come from the Cooperative of Aquaculture Producers of French Polynesia, ensuring local and biologically suitable stock for the study.
Photo credits: Carole DI POI
Cover image: Sampling 3
