Record highest mesophotic fish diversity found in Raja Ampat

Abstract Mesophotic coral ecosystems (MCEs; reefs 30-150 m depth) are poorly studied, with existing research heavily geographically biased away from the most species-rich reef regions. Yet, MCEs are of high interest because of their potential to act as refuges from fisheries impacts. We surveyed MCEs in Raja Ampat, West Papua using baited remote underwater video, representing the largest MCE fish community survey ever conducted in the Coral Triangle region. Here we show that Raja Ampat MCEs are exceptionally fish species rich, with 152 fish species recorded – representing the greatest fish richness of any MCE previously surveyed. We find fish communities are highly depth driven, with declines in fish abundance and biomass at increased depth. In contrast to previous studies elsewhere in the world, we found that planktivores declined as a proportion of MCE community at increased depths. While greater human population density resulted in lower reef fish biomass, we found no evidence that MCEs provide a depth refuge from fisheries impacts. Our results suggest that previously established MCE-depth patterns may not hold true for Raja Ampat and highlight the need for future MCE studies within the Coral Triangle region.