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.
Intraspecific colour polymorphisms have been the focus of numerous studies, yet processes affecting melanism in the marine environment remain poorly understood. Arguably, the most prominent example of melanism in marine species occurs in manta rays ( Mobula birostris and Mobula alfredi ). Here, we use long-term photo identification catalogues to document the frequency variation of melanism across Indo-Pacific manta ray populations and test for evidence of selection by predation acting on colour morph variants. We use mark–recapture modelling to compare survivorship of typical and melanistic colour morphs in three M. alfredi populations and assess the relationship between frequency variation and geographical distance . While there were large differences in melanism frequencies among populations of both species (0–40.70%), apparent survival estimates revealed no difference in survivorship between colour morphs. We found a significant association between phenotypic and geographical distance in M. birostris , but not in M. alfredi . Our results suggest that melanism is not under selection by predation in the tested M. alfredi populations, and that frequency differences across populations of both species are a consequence of neutral genetic processes . As genetic colour polymorphisms are often subjected to complex selection mechanisms, our findings only begin to elucidate the underlying evolutionary processes responsible for the maintenance and frequency variation of melanism in manta ray populations.
A global survey of coral reefs reveals that overfishing is driving resident shark species toward extinction, causing diversity deficits in reef elasmobranch (shark and ray) assemblages. Our species-level analysis revealed global declines of 60 to 73% for five common resident reef shark species and that individual shark species were not detected at 34 to 47% of surveyed reefs. As reefs become more shark-depleted, rays begin to dominate assemblages. Shark-dominated assemblages persist in wealthy nations with strong governance and in highly protected areas, whereas poverty, weak governance, and a lack of shark management are associated with depauperate assemblages mainly composed of rays. Without action to address these diversity deficits, loss of ecological function and ecosystem services will increasingly affect human communities.
Opportunistic in-water and aerial surveys in South Africa and the analysis of contributed citizen science data have extended the known range of reef manta rays Mobula alfredi along the eastern coast of Africa by 140 km (to Mdumbi Beach) and verified the first transboundary movements for the species. Additionally, six new long-range dispersal records have provided evidence of connectivity with the M. alfredi population off the Inhambane coastline of Mozambique. Five of these records captured one-way movements, the longest of which was an individual that travelled 505 km from Praia do Tofo to Sodwana Bay with 301 days between sightings. A single individual made a return trip between Závora, Mozambique and Sodwana Bay, South Africa (a total distance of $870 km). These findings support the Convention on Migratory Species listing for the species, suggesting regional transboundary management units are warranted for this wide-ranging elasmobranch.
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