Parrotfish demonstrate little behavioural shift during marine heatwaves.

The world is experiencing notable increases in the duration, intensity and frequency of marine heatwaves (MHWs). Documented effects of MHWs on fishes encompass various population- and community-level impacts, ranging from localized mass mortalities to transient or semi-permanent range shifts, while some populations show little measurable change. However, there is limited information on in situ behavioural responses of individuals before, during and after MHWs. In this study, we used acoustic telemetry to examine the activity levels, depths and daily displacement of 32 terminal-phase male parrotfishes belonging to five species across 12 moderate-sized MHWs (2016-2021). The study took place in the northern Red Sea and coincided with a mass mortality of parrotfishes during a rapid warming event. The parrotfishes exhibited only minuscule behavioural changes during or after the MHWs, regardless of how the MHW was defined or characterized. Thus, within the range of MHWs studied, MHWs have little to no impact on parrotfishes' behaviour. Our observations suggest that either the mechanisms by which parrotfishes respond to thermal stress are not behavioural, or that such responses may only occur under more severe MHWs.