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

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

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

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

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

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

Supplementary Information for "Targeting fin whale conservation in the North-Western Mediterranean Sea: insights on movements and behaviour from satellite telemetry and habitat modelling"

Biologging and habitat modelling are key tools supporting the development of conservation measures and mitigating the effects of anthropogenic pressures on marine species. Here, we analysed satellite telemetry data and foraging habitat preferences in relation to chlorophyll-a productivity fronts to understand the movements and behaviour of endangered Mediterranean fin whales (Balaenoptera physalus) during their spring-summer feeding aggregation in the North-Western Mediterranean Sea. Eleven individuals were equipped with Argos satellite transmitters across three years, with transmissions averaging 23.5 ± 11.3 days. Hidden Markov Models were used to identify foraging behaviour, revealing how individuals showed consistency in their use of seasonal core feeding grounds; this was supported by the distribution of potential foraging habitat. Importantly, tracked whales spent most of their time in areas with no explicit protected status within the study region. This highlights the need for enhanced time- and place-based conservation actions to mitigate the effects of anthropogenic impacts for this species, notably ship strike risk and noise disturbance in an area of exceptionally high maritime traffic levels. These findings strengthen the need to further assess critical habitats and Important Marine Mammal Areas that are crucial for focussed conservation, management, and mitigation efforts

DataSheet_2_The ACCOBAMS survey initiative: the first synoptic assessment of cetacean abundance in the Mediterranean Sea through aerial surveys.pdf

The “ACCOBAMS Survey Initiative” (ASI) is a pilot programme aimed at establishing an integrated and coordinated monitoring system for cetaceans across the Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea and contiguous Atlantic (hereafter “ACCOBAMS”) area. Conducted in coordination with Mediterranean coastal countries, it supports the implementation of European and regional policies, in particular the EU Marine Strategy Framework Directive and the Ecosystem Approach process. In summer 2018, a synoptic survey was conducted across the Mediterranean Sea and contiguous Atlantic area, combining visual monitoring from aircrafts with visual and passive acoustic monitoring from vessels. Species density and abundance were estimated through design-based approach in a line-transect sampling framework. Based on data arising from the aerial survey only, uncorrected design-based abundance was obtained for striped (N=426,744, CV=0.13), common (N=65,359, CV=0.4), bottlenose (N=63,333, CV=0.17), and Risso´s dolphins (N=26,006, CV=0.3), Cuvier’s beaked whales (N=2,929, CV=0.4) and long-finned pilot whales (N=5,540 CV=0.4). A merged category of either striped or common dolphins resulted in 212,483 individuals (CV=0.26). Fin whales abundance of 1,749 animals (CV=0.3) was corrected for both availability and perception biases and resulted in 3,282 (CV=0.31). The ASI survey offers an overall picture of the distribution and abundance of cetaceans throughout the Mediterranean basin, providing robust estimates to be considered as a baseline for future regional systematic monitoring programmes. The ASI survey is the first step towards establishing a long-term monitoring program across the entire ACCOBAMS area, and, as such, it sets the basis for further future basin-wide monitoring efforts using systematic, shared, coordinated and comparable methods. The information gathered will further enhance knowledge on cetacean status, facilitating the development of informed conservation and mitigation measures, as well as supporting the implementation of international obligations. Furthermore, the outcomes of this survey will support both place- and threat-based conservation efforts in the ACCOBAMS area, through the identification of Important Marine Mammal Areas and Cetacean Critical Habitats. Here the results of the ASI survey are presented and discussed alongside proposed management and conservation actions aimed at ensuring the persistence of cetacean populations in the region.</p