Physical and environmental drivers of oceanic manta ray Mobula birostris sightings at an aggregation site in Bahia de Banderas, Mexico

Oceanic manta rays Mobula birostris are distributed in tropical and subtropical oceans with aggregation sites typically at oceanic islands and seamounts. However, in some regions the species is found in coastal habitats close to highly productive areas. Recently, an important aggregation site for oceanic manta rays was identified along the southern coast of Bahia de Banderas, Mexico. We conducted weekly monitoring trips to the aggregation hotspots over a period of 41/2 yr, recording manta sightings and collecting zooplankton samples, to investigate relationships between sightings and prey availability. We evaluated relationships between manta sightings and physical, biological, and environmental variables, finding a seasonal signal of manta occurrence with a peak in sightings around April, although there were substantial deviations from this trend in some years. Our results show effects of the El Nino Southern Oscillation on sighting rates, with more mantas during La Nina phases. We found significant relationships between manta sightings and sea surface temperature, moon phase, and tidal range. Manta sightings were negatively related to fish egg densities, positively related to copepod and cladoceran densities, and showed no relationship to euphausiid density. Mantas may be feeding on mesopelagic prey in the submarine canyon adjacent to the southern coast and basking in shallow waters through the day during thermal recovery periods. Identifying the seasonal patterns of occurrence and the environmental drivers of sightings will support the development of strategies to mitigate anthropogenic threats, which are common in this coastal population.