The Devil We Don't Know: Investigating Habitat and Abundance of Endangered Giant Devil Rays in the North-Western Mediterranean Sea

<div><p>The giant devil ray <i>Mobula mobular</i>, 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.</p></div

Density and uncorrected abundance estimates for the three sub-areas, obtained with design-based and model-based methods.

<p><i>Esw</i>: effective strip width.</p

Predicted summer abundance of giant devil rays in the area considered for quantitative and spatial analysis, using geographic covariates (latitude and longitude) and depth as covariates.

<p>Predicted summer abundance of giant devil rays in the area considered for quantitative and spatial analysis, using geographic covariates (latitude and longitude) and depth as covariates.</p

Subareas (letters indicate corresponding sectors in Fig 1), listed in chronological order of survey timing, with subarea surfaces and transect lengths (on average over 90% of the planned transect length was covered).

<p>Rows with bold lettering correspond to summer surveys, which were used for density and spatial analyses.</p

Perpendicular distance distribution (histograms), and fitted detection functions (lines): a) Pelagos Sanctuary, summer, stratified by year, b) Central Tyrrhenian Sea, summer, stratified by year, and c) all summer data pooled (2009, 2010 and 2013) and stratified by area.

<p>Perpendicular distance distribution (histograms), and fitted detection functions (lines): a) Pelagos Sanctuary, summer, stratified by year, b) Central Tyrrhenian Sea, summer, stratified by year, and c) all summer data pooled (2009, 2010 and 2013) and stratified by area.</p

Total sightings (on track and off track) in the different sub-areas (capital letters refer to sub-area locations in Fig 1), and mean aggregation sizes (CV = coefficient of variation; CI = confidence interval; n/a = not applicable).

<p>Total sightings (on track and off track) in the different sub-areas (capital letters refer to sub-area locations in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0141189#pone.0141189.g001" target="_blank">Fig 1</a>), and mean aggregation sizes (CV = coefficient of variation; CI = confidence interval; n/a = not applicable).</p

Details on Materials and Methods from Behavioural responses of spinner dolphins to human interactions

Additional information on study surveys and data standardisation protocol

The study area subdivided into 5 sub-areas: (A) Pelagos Sanctuary; (B) Sardinian Sea; (C) Central Tyrrhenian Sea; (D) Southern Tyrrhenian Sea; and (E) Ionian Sea.

<p>Red dots are the locations of giant devilray sightings. The areas for which abundance estimates were obtained (A, B and C) are highlighted in grey. Numbers correspond to the following locations cited in the text: 1 –Corsica; 2 –Sardinia; 3 –Gulf of Asinara; 4 –Latium; 5 –Campania; 6 –Tuscan Archipelago; 7 –Strait of Messina; 8 –Adriatic Sea.</p

A pair of giant devil rays <i>Mobula mobular</i> photographed from the air.

<p>The general appearance of the body, the paired cephalic fins, the long tail and the dorsal colouration are diagnostic (photo by Elio Filidei, Jr.).</p