Depth-dependent effects of culling—do mesophotic lionfish populations undermine current management?

Invasive lionfish (Pterois volitans and P. miles) have spread widely across the western Atlantic and are recognised as a major threat to native marine biodiversity. Although lionfish inhabit both shallow reefs and mesophotic coral ecosystems (MCEs; reefs from 30-150 m depth), the primary management response implemented by many countries has been diver-led culling limited to reefs <30 m. However many reef fish undergo ontogenetic migrations, with the largest and therefore most fecund individuals found at greatest depths. Here we study lionfish density, body size, maturity, and dietary patterns across the depth gradient from the surface down to 85 m on heavily culled reefs around Utila, Honduras. We found lionfish at increased densities, body size and weight on MCEs compared to shallow reefs, with MCEs also containing the greatest proportion of actively spawning females while shallow reefs contained the greatest proportion of immature lionfish. We then compared lionfish behaviour in response to divers on shallow culled and mesophotic un-culled Utilan reefs, and on shallow un-culled reefs in Tela Bay, on the Honduran mainland. We found mesophotic lionfish exhibited high alert distances, consistent with individuals previously exposed to culling despite being below the depth limits of removal. In addition, when examining stomach content we found that fish were the major component of lionfish diets across the depth gradient. Importantly, our results suggest that despite adjacent shallow culling, MCEs retain substantial lionfish populations that may be disproportionately contributing towards continued lionfish recruitment onto the shallow reefs of Utila, potentially undermining current culling-based management.