Steep declines in sightings of manta rays and devilrays (Mobulidae) in southern Mozambique

Abstract

Mobulid rays are one of the most vulnerable chondrichthyan groups due to their low population growth rates and high susceptibility to fisheries. While estimates of human-induced mortality are lacking, sighting trend data can provide an index of their status. We recorded underwater sightings data of Mobula alfredi, M. birostris and M. kuhlii over a 14-year period in southern Mozambique. Generalised linear models were used to standardise sightings and adjust for influences other than time. Standardised sightings of the three species, individually, declined by >90%. Declines in sightings were driven primarily by a rapid decrease between 2003–2007, although the declines continued to 2016. While environmental variables did influence sightings, they did not explain this steep decline over time. Increasing mortality from fisheries is likely to have played a significant role in the declining sightings of these vulnerable species.