The Impact of Marine Heatwaves on Isotherm Displacement and Tuna Distribution in Vanuatu

Marine heatwaves (MHWs) have intensified in frequency, duration, and severity over recent decades. These events, defined by unusually warm sea surface temperatures (SSTs), can cause significant ecological impacts. This is particularly so for Pacific Island countries, such as Vanuatu, where communities rely on marine resources for their food and livelihoods. A common ecological response to MHWs is the movement of oceanic species to cooler waters. Predicting such shifts through monitoring SST isotherms can help identify thermal boundaries that marine species favor. This study explores the connection between MHWs, SST isotherm movement, and tuna abundance in Vanuatu. The displacement of the 28 °C isotherm was analyzed across three major MHW events (2008–2009, 2016, and 2021–2022). It was found that MHWs with longer duration and greater intensity caused more significant isotherm displacement. Additionally, the El Niño–Southern Oscillation had an important influence on MHW formation and isotherm movement. The effects of these displacements on tuna distribution varied between events. The ability to monitor MHWs and SST isotherm movement could be an effective instrument for the prediction of areas of suppressed or abundant tuna activity and can be used to aid in the proactive management of food security and fishery sectors.