Declines in sightings and changing visitation patterns of reef manta rays at an important aggregation site in Mozambique

Understanding the spatiotemporal distribution and habitat preferences of marine megafauna is critical for effective conservation. Many large pelagic elasmobranchs are threatened by overfishing, yet fine-scale data on their occurrence and distribution remains limited. The reef manta ray Mobula alfredi aggregates at several locations throughout the coastal waters of the Inhambane Province of Mozambique, including Praia do Tofo, where sightings have declined by up to 99% over the past two decades. However, trends at another aggregation location, Praia de Závora, remain unknown. Here, we analyze reef manta ray sightings in Závora over a 15-year period from 2009 to 2023. Our findings reveal an 82.4% decline in sightings, coupled with a shift from year-round sightings to a seasonal aggregation and a change in site preference from two cleaning stations to a single one. Generalized additive models (GAMs) identified year, day of year, and moon illumination as significant predictors of reef manta ray occurrence at their primary aggregation site, suggesting that temporal factors may play a larger role in their visitations in this area than other environmental variables, including temperature, salinity, chlorophyll-a, and the Indian Ocean Dipole. While the steep declines in sightings along the Mozambican coastline are of serious concern, the large seasonal aggregations in Závora, likely for reproduction, provide hope for this population. Ensuring the survival of manta rays in Mozambique will require fostering collaboration among researchers, local communities, and policymakers, while strengthening enforcement of protective measures and integrating comprehensive monitoring data to mitigate threats to the population.