Competitive interactions for shelter between invasive Pacific red lionfish and native Nassau grouper

The invasive Pacific red lionfish (Pterois volitans) poses a threat to western Atlantic and Caribbean coral reef systems. Lionfish are small-bodied predators that can reduce the abundance and diversity of native fishes via predation. Additionally, native predators or competitors appear to have a negligible effect on similarly sized lionfish. Nassau grouper (Epinephelus striatus) are a regionally endangered, large predator found throughout lionfish’s invasive range. Because lionfish and Nassau grouper occupy similar habitats and use similar resources, there is potential for competition between these two species. Using large, outdoor in-ground tanks, we investigated how lionfish and Nassau grouper affect each other’s behavior by comparing their distance from and use of shelter when in isolation versus when both species were in the presence of each other with limited shelter. We found that Nassau grouper, which displayed a high affinity for shelter in isolation, avoided lionfish in two distinct ways; (1) groupers positioned closer to and used limited shelter more when paired with similarly sized lionfish and (2) grouper moved much further away from shelter when paired with smaller lionfish. We also found that neither large lionfish nor large Nassau grouper preyed upon smaller individuals of the opposite species suggesting that Nassau grouper do not recognize small lionfish as prey. This study highlights how invasive lionfish may affect native Nassau grouper, and suggests that competition for shelter between these two species may be size dependent.