Abstract
The giant devil ray Mobula mobular, the only Mediterranean mobulid, is subject to mortality caused by directed and accidental captures in fisheries throughout the region. Whilst the combination of human impacts, limited range and a low reproductive potential is not inconsistent with its endangered listing, there are insufficient data to enable a quantitative assessment of trends. Without this, it is difficult to assess and prioritise threats and develop effective conservation actions. Using results from aerial surveys conducted between 2009 and 2014 over the Ligurian, Corsican, Sardinian, northern and central Tyrrhenian seas (626,228 km2), this study provides the first quantitative information on giant devil ray abundance and habitat choice in the western Mediterranean. Devil rays were observed in all seasons except winter, with their estimated abundance in the study area peaking in summer. The overall uncorrected mean density in the study area during summer was estimated at 0.0257 individuals km-2 (range: 0.017–0.044), resulting in a total abundance estimate of 6,092 (12.7%CV) individuals at the surface; once corrected for availability bias, this estimate indicates a summer presence of >12,700 devil rays in the study area. Rays were mostly observed alone even if occasionally, larger aggregations up to a maximum of 18 individuals were observed. Although observed throughout the study area, spatial modelling identified their preferred habitat to be over a broad strip connecting the Tuscan Archipelago to Eastern Sardinia, over a wide range of water depths ranging from 10 to 2000m. The observed seasonal changes in giant devil ray distribution in this study, combined with similar evidence from other areas in the Mediterranean, support the hypothesis that the species undertakes latitudinal migrations across the region, taking advantage of highly productive waters in the north during summer, and warmer southern waters during winter.
Highlights
The giant devil ray Mobula mobular is one of the largest elasmobranchs, with a maximum disc width greater than 4.5m and a weight that can exceed 1.5 tonnes [1]
Using results from aerial surveys conducted between 2009 and 2014 over the Ligurian, Corsican, Sardinian, northern and central Tyrrhenian seas (626,228 km2), this study provides the first quantitative information on giant devil ray abundance and habitat choice in the western Mediterranean
The present paper presents results on giant devil rays obtained from a series of aerial surveys, as in the case of [25], primarily aimed at investigating cetacean ecology, providing new insights into giant devil ray distribution, densities, and seasonality
Summary
The giant devil ray Mobula mobular is one of the largest elasmobranchs, with a maximum disc width greater than 4.5m and a weight that can exceed 1.5 tonnes [1]. Small (3–4 animals annually) catches were known to be taken in a few specific locations (e.g. Algeria [6] and Sicily [7]). This picture substantially changed after the discovery of a directed seasonal fishery in the Levantine Sea off Gaza. The species is explicitly protected by Croatia and Malta, as well as being listed in Annex II (“List of endangered and threatened species”) to the Barcelona Convention Protocol on Specially Protected Areas and Biological Diversity in the Mediterranean (SPA/BD), in Appendix II (“Strictly protected fauna species”) to the Bern Convention on the Conservation of European Wildlife and Natural Habitats, and in both Appendices I and II to the Convention on the Conservation Migratory Species of Wild Animals
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