Vairao (Tahiti), November 2025 – The MaHeWa project has reached an important milestone with the completion of the first experimental simulation of marine heatwaves applied to the orbicular batfish Platax orbicularis, locally known as Paraha peue. This species is emblematic of Polynesian lagoons and holds aquaculture interest for the fenua. The experimental setup was designed and implemented during the mission of Benjamin Geffroy (Ifremer – UMR MARBEC, Montpellier), who supported the Ifremer team in Vairao in preparing the system and launching the trials.
To reproduce heatwave conditions similar to those occurring in Polynesian lagoons, four scenarios were developed within a controlled experimental system. The reference scenario represents the average lagoon temperature of 28 °C. Two additional scenarios simulate constant heatwaves at 32 °C and 34 °C maintained for twenty-five days. The fourth scenario simulates two successive heatwaves at 32 °C separated by a six-day recovery period, during which temperatures return to lagoon conditions.
This design makes it possible to investigate the effects of different heatwave intensities as well as the influence of event frequency on fish physiology. The last scenario specifically allows comparison between the impact of a single prolonged event and the impact of repeated events, helping determine whether successive heatwaves amplify thermal stress or whether an initial exposure improves the fish’s ability to cope during a second event.
The scientific objectives of this experiment are fully aligned with MaHeWa’s ambitions for WP2. The team aims to determine the thermal sensitivity of Platax, identify critical physiological thresholds, and understand how fish respond to conditions expected to become more frequent under climate change. Responses will be assessed at the individual level (activity, growth) and at the molecular level, notably through measurements of cortisol, the so-called “stress hormone.” Cortisol will be measured in the water to assess group-level stress, in blood to provide individual responses, and in scales to determine long-term accumulation that reflects chronic exposure to elevated temperature.
The first sampling operations were conducted following a rigorous protocol to ensure data quality and comparability across scenarios. The collected samples will feed into the physiological and molecular analyses planned for 2026, with the objective of improving understanding of thermal stress responses in tropical species. This experiment is the result of a close collaboration between UMR SECOPOL (Ifremer Vairao) and UMR MARBEC (Ifremer Montpellier). It was made possible through the technical support of VSC Martin Debatis, analytical support from 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 in this study were provided by the Coopérative des Aquaculteurs de Polynésie Française, ensuring that the biological material was both local and well adapted to the research needs.
Photo credits: Carole Di Poi
