Lal S. auteur aut IRD text thesis eng Marine heatwaves (MHWs), defined as discrete, prolonged periods of anomalously high ocean temperatures, have become an increasingly frequent environmental challenge for the global ocean, with significant consequences for certain species, ecosystems, communities and economies dependent on marine resources. This PhD thesis investigates the characteristics, drivers, and ecological impacts on coastal fisheries from historic MHWs in the Southwestern tropical Pacific. Particular emphasis has been placed on characterizing these events within the Exclusive Economic Zones (EEZs) and along the coastlines of 12 Pacific Island Countries and Territories (PICTs), to enhance the usefulness of this study's outputs for natural resource management efforts in these PICTs. Using a range of oceanic temperature products, this research assessed MHWs at the surface and their subsurface extensions. A polygon-based spatial method was developed to quantify MHW spatial extent. Microscale and macroscale MHWs were characterized based on their spatial extents and seasonality. It was shown that distinguishing MHWs by size (spatial extent) and seasonality can greatly affect their characterisation (in terms of areas affected, duration and intensity) and help trace their physical drivers. All 12 PICTs experienced MHWs in the past 30 years with events getting more frequent with greater spatial extents, longer durations, but with less intensity. A significant long-term trend in the annual number of MHW days was also observed along coastlines of Solomon Islands, Vanuatu, Tonga, Fiji and northern New Caledonia. PICTs in the Southwest Pacific (New Caledonia, Vanuatu, Fiji) were more exposed to higher intensity events, with New Caledonia and Vanuatu also experiencing longer duration events. These PICTs were found to experience more macro-scale events, indicating possible links to heat flux changes over a large area in link with large-scale climate modes. This thesis also highlighted that seasonality could play key roles in determining MHW vertical extent due to its effect on mixed layer depths. Seasonality also affects how MHWs impact ecology, since they have more damaging effects in hot season compared to cold season. This thesis also presents an analysis of the main mechanisms and drivers of a case study, the major MHW event which occurred in February 2016 resulting in massive coral bleaching in the region. It was explained by a rare combination of a suppressed phase of the Madden Jullian Oscillation superposing on the strong 2015-2016 El Niño event, resulting in exceptional low winds and clear sky conditions. This MHW dissipated after the passage of a tropical cyclone. Finally, this PhD examined the ecological response of coastal fish growth from otolith records to historic MHWs. Fish life history parameters from five coastal reef fish species; Cephalopholis argus, Naso lituratus, Ctenochaetus striatus, Naso unicornis and Lutjanus fulvus sourced from Tonga and Cook Islands were used along with coastal MHW parameters to explain their growth rates (otolith increment rates), to understand the impact of MHWs. Results revealed, first, the complexity of finding clear relationships between MHWs and marine species in general. We showed that there was species-specific sensitivity, with only two species (Cephalopholis argus and Lutjanus fulvus) showing significant negative growth responses. This research highlights the importance of combining multiple approaches (physical oceanographic, ecological and socio-economic) to understanding environmental challenges as complex and at times controversial (due to choice of definition, baselines, detrending, resolution) as MHWs. This thesis serves as a foundation and guide for future investigations into the physical and ecological dynamics of MHWs in the Pacific and provides insights useful to inform conservation efforts in PICTs. specialized PACIFIQUE 030 040 021 Toulouse Université de Toulouse 3 - Paul Sabatier 2026 203 multigr. 203 multigr. 203 multigr. https://www.documentation.ird.fr/hor/fdi:010098094 [F A010098094] https://www.documentation.ird.fr/hor/fdi:010098094 Accès réservé (Intranet de l'IRD) IRD - Base Horizon / Pleins textes 2026-10-13 2026-10-13 fdi:010098094 fre