Effects of ocean warming and time of day on the behaviours of invasive lionfish and native keystone prey

Climate change and invasive species threaten global biodiversity and co-occur in nature. Despite their potential to interact, they are often studied singularly. For fishes, climate change-driven ocean warming leads to elevated metabolic rates, which can increase feeding. Warming can also decrease fish anti-predator behaviours. Simultaneously, invasive predators drive major declines in prey abundances. Climate change can therefore exacerbate the predatory effects of invasive species. Here, we examine the effects of ocean warming on the behaviours of a widespread, invasive predator, Indo-Pacific lionfishes ( Pterois spp. ), and native parrotfish ( Scarus iseri and S. taeniopterus ) keystone prey. Since lionfish and parrotfish peak activity periods are different, time of day may also temper behaviours. We hypothesized that warming would increase lionfish predatory behaviours and parrotfish foraging but decrease prey anti-predator behaviours, and that these effects would be modulated by time of day. We used experimental mesocosms with lionfish and parrotfish held at 29°C or 32°C and fish were allowed to visually interact at dawn and at midday. We found that lionfish hunted more and rested less at elevated temperatures, but only at midday. Parrotfish feeding, hovering, and swimming increased at 32°C regardless of time of day, but there were mixed effects of warming on anti-predator behaviours. Our findings demonstrate that climate change may magnify lionfish predation and increase parrotfish exposure to elevated predation risks, and that these effects are influenced by time of day. Because parrotfish are keystone herbivores on Caribbean coral reefs, these effects can potentially cascade down food webs.