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Mannocci, L., Roberts, J. J., Halpin, P. N., Authier, M., Boisseau, O., Bradai, M. N., et al. (2018). Assessing cetacean surveys throughout the Mediterranean Sea: a gap analysis in environmental space. Sci Rep, 8, 3126.
Résumé: Heterogeneous data collection in the marine environment has led to large gaps in our knowledge of marine species distributions. To fill these gaps, models calibrated on existing data may be used to predict species distributions in unsampled areas, given that available data are sufficiently representative. Our objective was to evaluate the feasibility of mapping cetacean densities across the entire Mediterranean Sea using models calibrated on available survey data and various environmental covariates. We aggregated 302,481 km of line transect survey effort conducted in the Mediterranean Sea within the past 20 years by many organisations. Survey coverage was highly heterogeneous geographically and seasonally: large data gaps were present in the eastern and southern Mediterranean and in non-summer months. We mapped the extent of interpolation versus extrapolation and the proportion of data nearby in environmental space when models calibrated on existing survey data were used for prediction across the entire Mediterranean Sea. Using model predictions to map cetacean densities in the eastern and southern Mediterranean, characterised by warmer, less productive waters, and more intense eddy activity, would lead to potentially unreliable extrapolations. We stress the need for systematic surveys of cetaceans in these environmentally unique Mediterranean waters, particularly in non-summer months.
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Queiros, Q., Fromentin, J. - M., Astruc, G., Bauer, R. K., & Saraux, C. (2018). Dolphin predation pressure on pelagic and demersal fish in the northwestern Mediterranean Sea. Mar. Ecol.-Prog. Ser., 603, 13–27.
Résumé: Sardine Sardina pilchardus, anchovy Engraulis encrasicolus and European hake Merluccius merluccius represent a significant part of the commercial landings in the Gulf of Lions (northwestern Mediterranean Sea). However, their stocks have shown severe declines during the last decades due to fishing pressure and/or environmental changes. The aim of this study was to estimate the current predation pressure of bottlenose dolphins Tursiops truncatus and striped dolphins Stenella coeruleoalba – which are abundant in the area-on sardine, anchovy and hake. To do so, we developed an original approach based on several data sets and models (aerial surveys, stomach contents, allometric and stock assessment models) and Monte Carlo simulations to incorporate various sources of uncertainty due to data limitations. Despite the uncertainties, the results showed that dolphin predation pressure on sardine and anchovy was extremely low in the Gulf of Lions (all simulations <0.5 % of the available stock), indicating little impact of dolphins on those populations. However, significant predation pressure on hake (median value: 23 %) was detected, a value which might have doubled in the last 30 yr because of hake overfishing. Overexploitation has thus reinforced the natural mortality of hake due to dolphin predation, but this predation pressure remains 2 to 3 times lower than that exerted by fisheries.
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Viricel, A., Simon-Bouhet, B., Ceyrac, L., Dulau-Drouot, V., Berggren, P., Amir, O. A., et al. (2016). Habitat availability and geographic isolation as potential drivers of population structure in an oceanic dolphin in the Southwest Indian Ocean. Mar. Biol., 163(10), 219.
Résumé: Delphinid populations show highly variable patterns of genetic diversity and population structure. Previous studies indicate that habitat discontinuities and geographic isolation are major drivers of population division in cetaceans. Spinner dolphins (Stenella longirostris) are distributed in all tropical oceans, but they are particularly common around islands and atolls. This species occurs in shallow waters at daytime to rest and socialise, and feeds on offshore mesopelagic prey overnight. Here, we investigated the genetic population structure of spinner dolphins in the Southwest Indian Ocean along a west-east geographic gradient, from eastern Africa to the Mascarene archipelago. We combined analyses of 12 microsatellite loci, mtDNA control region sequences, and sighting data to assess genetic differentiation and characterise habitat preferences of these populations. Significant genetic structure among the three sampled sites (Zanzibar, Mayotte and La Reunion) was observed using both types of molecular markers. Overall, our results indicate that geographic isolation and potentially other factors, such as shallow-water habitats to rest and socialise, may be important drivers of the genetic population structure of insular spinner dolphins in this region.
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