2020 |
Albouy, C., et al. "Global vulnerability of marine mammals to global warming." Sci Rep. 10.1 (2020): 548.
Résumé: Although extinctions due to climate change are still uncommon, they might surpass those caused by habitat loss or overexploitation over the next few decades. Among marine megafauna, mammals fulfill key and irreplaceable ecological roles in the ocean, and the collapse of their populations may therefore have irreversible consequences for ecosystem functioning and services. Using a trait-based approach, we assessed the vulnerability of all marine mammals to global warming under high and low greenhouse gas emission scenarios for the middle and the end of the 21st century. We showed that the North Pacific Ocean, the Greenland Sea and the Barents Sea host the species that are most vulnerable to global warming. Future conservation plans should therefore focus on these regions, where there are long histories of overexploitation and there are high levels of current threats to marine mammals. Among the most vulnerable marine mammals were several threatened species, such as the North Pacific right whale (Eubalaena japonica) and the dugong (Dugong dugon), that displayed unique combinations of functional traits. Beyond species loss, we showed that the potential extinctions of the marine mammals that were most vulnerable to global warming might induce a disproportionate loss of functional diversity, which may have profound impacts on the future functioning of marine ecosystems worldwide.
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Mannocci, L., et al. "Geographical differences in habitat relationships of cetaceans across an ocean basin." Ecography (2020).
Résumé: The distributions of highly mobile marine species such as cetaceans are increasingly modeled at basin scale by combining data from multiple regions. However, these basin-wide models often overlook geographical variations in species habitat relationships between regions. We tested for geographical variations in habitat relationships for a suite of cetacean taxa between the two sides of the North Atlantic basin. Using cetacean visual survey data and remote sensing data from the western and eastern basin in summer, we related the probability of presence of twelve cetacean taxa from three guilds to seafloor depth, sea surface temperature and primary productivity. In a generalized additive model framework, we fitted 1) basin-wide (BW) models, assuming a single global relationship, 2) region-specific intercepts (RI) models, assuming relationships with the same shape in both regions, but allowing a region-specific intercept and 3) region-specific shape (RS) models, assuming relationships with different shapes between regions. RS models mostly yielded significantly better fits than BW models, indicating cetacean occurrences were better modeled with region-specific than with global relationships. The better fits of RS models over RI models further provided statistical evidence for differences in the shapes of region-specific relationships. Baleen whales showed striking differences in both the shapes of relationships and their mean presence probabilities between regions. Deep diving whales and delphinoids showed contrasting relationships between regions with few exceptions (e.g. non-statistically different shapes of region-specific relationships for harbor porpoise and beaked whales with depth). Our findings stress the need to account for geographical differences in habitat relationships between regions when modeling species distributions from combined data at the basin scale. Our proposed hypotheses offer a roadmap for understanding why habitat relationships may geographically vary in cetaceans and other highly mobile marine species.
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Marsac, F., et al. "Seamount effect on circulation and distribution of ocean taxa in the vicinity of La Perouse, a shallow seamount in the southwestern Indian Ocean." Deep-Sea Res. Part II-Top. Stud. Oceanogr.. 176 (2020): 104806.
Résumé: The La Perouse seamount (60 m depth) has so far been poorly studied despite it being a short distance (160 km) from Reunion Island. As part of the MADRidge project, a multidisciplinary cruise was conducted to evaluate the effect of this shallow seamount on the local hydrology and ecology. Current measurements, temperature and chlorophyll-a profiles, and mesozooplankton and micronekton samples were collected between the summit and 35 km away. Micronekton data were supplemented with stomach content of pelagic top predators as well as fisheries statistics from the domestic longline fleet operating from Reunion. Vertical current profiles revealed distinct patterns between the offshore and seamount-flanked stations, giving evidence of topographical induced flow instabilities, notably on its leeward side (west) relative to the east flank. Distinct patterns in temperature and chlorophyll-a vertical profiles suggest the formation of convergent and divergent circulation cells as a result of the irregular and crescent-like summit topography. Spatial differences in zooplankton abundance were detected with higher biovolumes on the leeward flank. The overall acoustic backscatter for micronekton over the summit was weaker than offshore, but highly concentrated in the upper layer. Albacore tuna and swordfish dominate the longline catch west of Reunion, seemingly in association with a deep (900 m) topographic feature. Yet the largest catch is not directly associated with La Perouse which would be too shallow for top predators to aggregate around in the long term. Enhanced levels of phytoplankton or zooplankton enrichment at La Perouse were not demonstrated in this study, nor was there notable diversity of micronekton species. This might explain the relatively limited importance of this seamount to the tuna fisheries in this region.
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Roberts, M. J., et al. "The MADRidge project: Bio-physical coupling around three shallow seamounts in the South West Indian Ocean." Deep-Sea Res. Part II-Top. Stud. Oceanogr.. 176 (2020): 104813.
Résumé: Compared with other ocean basins, little is known scientifically about the seamounts in the Indian Ocean. Nonetheless, fishers have plundered these fragile ecosystems for decades, and now mining is becoming a reality. We introduce a multidisciplinary project referred to as MAD-Ridge that recently focused on three shallow seamounts in the South West Indian Ocean between 19 degrees S and 34 degrees S. The larger Walters Shoal (summit at 18 m) discovered in 1963 occupies the southern part of the Madagascar Ridge and has long received attention from the fishing industry, and only recently by scientists. In contrast, nothing is known of the northern region of the ridge, which is characterised by a prominent, steep-sided seamount that has a flat circular summit at 240 m and width of similar to 20 km. This seamount is some 200 km south of Madagascar and unnamed; it is referred to here as the MAD-Ridge seamount. MAD-Ridge is the shallowest of a constellation of five deeper (>1200 m) seamounts on that part of the ridge, all within the EEZ of Madagascar. It lies in a highly dynamic region at the end of the East Madagascar Current, where mesoscale eddies are produced continuously, typically as dipoles. The Madagascar Ridge appears to be an area of great productivity, as suggested by the foraging behaviour of some tropical seabirds during chick-rearing and a longline fishery that operates there. The third seamount, La Perouse, is located between Reunion Island and Madagascar. With a summit 60 m below the sea surface, La Perouse is distinct from MAD-Ridge and Walters Shoal; it is a solitary pinnacle surrounded by deep abyssal plains and positioned in an oligotrophic region with low mesoscale activities. The overall aim of the MAD-Ridge project was to examine the flow structures induced by the abrupt topographies, and to evaluate whether biological responses could be detected that better explain the observed increased in fish and top predator biomasses. The MAD-Ridge project comprised a multidisciplinary team of senior and early career scientists, along with postgraduate students from France, South Africa, Mauritius and Madagascar. The investigation was based around three cruises using the French vessels RV Antea (35 m) and RV Marion Dufresne (120 m) in September 2016 (La Perouse), November-December 2016 (MAD-Ridge) and May 2017 (Walters Shoal). This manuscript presents the rationale for the MAD-Ridge project, the background, a description of the research approach including the cruises, and a synopsis of the results gathered in the papers published in this Special Issue.
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2019 |
Virgili, A., et al. "Combining multiple visual surveys to model the habitat of deep-diving cetaceans at the basin scale." Global Ecology and Biogeography. 28.3 (2019): 300–314.
Résumé: Aim Deep-diving cetaceans are oceanic species exposed to multiple anthropogenic pressures including high intensity underwater noise, and knowledge of their distribution is crucial to manage their conservation. Due to intrinsic low densities, wide distribution ranges and limited presence at the sea surface, these species are rarely sighted. Pooling data from multiple visual surveys sharing a common line-transect methodology can increase sightings but requires accounting for heterogeneity in protocols and platforms. Location North Atlantic Ocean and Mediterranean Sea. Time period 1998 to 2015. Major taxa Ziphiidae; Physeteriidae; Kogiidae. Methods About 1,240,000 km of pooled effort provided 630 sightings of ziphiids, 836 of physeteriids and 106 of kogiids. For each taxon, we built a hierarchical model to estimate the effective strip width depending on observation conditions and survey types. We then modelled relative densities in a generalized additive modelling framework. Geographical predictions were limited to interpolations identified with a gap analysis of environmental space coverage. Results Deeper areas of the North Atlantic gyre were mostly environmental extrapolation in the predictions, thereby highlighting gaps in sampling across the different surveys. For the three species groups, the highest relative densities were predicted along continental slopes, particularly in the western North Atlantic Ocean where the Gulf Stream creates dynamic frontal zones and eddies. Main conclusions Pooling a large number of surveys provided the first basin-wide models of distribution for deep-diving cetaceans, including several data-deficient taxa, across the North Atlantic Ocean and the Mediterranean Sea. These models can help the conservation of elusive and poorly known marine megafauna.
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2018 |
Mannocci, L., et al. "Assessing cetacean surveys throughout the Mediterranean Sea: a gap analysis in environmental space." Sci Rep. 8 (2018): 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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Rabearisoa, N., et al. "Toothed whale and shark depredation indicators: A case study from the Reunion Island and Seychelles pelagic longline fisheries." PLOS ONE. 13.8 (2018): e0202037.
Résumé: Depredation in marine ecosystems is defined as the damage or removal of fish or bait from fishing gear by predators. Depredation raises concerns about the conservation of species involved, fisheries yield and profitability, and reference points based on stock assessment of depredated species. Therefore, the development of accurate indicators to assess the impact of depredation is needed. Both the Reunion Island and the Seychelles archipelago pelagic longline fisheries targeting swordfish (Xiphias gladius) and tuna (Thunnus spp.) are affected by depredation from toothed whales and pelagic sharks. In this study, we used fishery data collected between 2004 and 2015 to propose depredation indicators and to assess depredation levels in both fisheries. For both fisheries, the interaction rate (depredation occurrence) was significantly higher for shark compared to toothed whale depredation. However, when depredation occurred, toothed whale depredation impact was significantly higher than shark depredation impact, with higher depredation per unit effort (number of fish depredated per 1000 hooks) and damage rate (proportion of fish depredated per depredated set). The gross depredation rate in the Seychelles was 18.3%. A slight increase of the gross depredation rate was observed for the Reunion Island longline fleet from 2011 (4.1% in 2007–2010 and 4.4% in 2011–2015). Economic losses due to depredation were estimated by using these indicators and published official statistics. A loss of 0.09 EUR/hook due to depredation was estimated for the Reunion Island longline fleet, and 0.86 EUR/hook for the Seychelles. These results suggest a southward decreasing toothed whale and shark depredation gradient in the southwest Indian Ocean. Seychelles depredation levels are among the highest observed in the world revealing this area as a “hotspot” of interaction between pelagic longline fisheries and toothed whales. This study also highlights the need for a set of depredation indicators to allow for a global comparison of depredation rates among various fishing grounds worldwide.
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2017 |
Krause, J., et al. "Injury-mediated decrease in locomotor performance increases predation risk in schooling fish." Philos. Trans. R. Soc. B-Biol. Sci.. 372.1727 (2017): 20160232.
Résumé: The costs and benefits of group living often depend on the spatial position of individuals within groups and the ability of individuals to occupy preferred positions. For example, models of predation events for moving prey groups predict higher mortality risk for individuals at the periphery and front of groups. We investigated these predictions in sardine (Sardinella aurita) schools under attack from group hunting sailfish (Istiophorus platypterus) in the open ocean. Sailfish approached sardine schools about equally often from the front and rear, but prior to attack there was a chasing period in which sardines attempted to swim away from the predator. Consequently, all sailfish attacks were directed at the rear and peripheral positions of the school, resulting in higher predation risk for individuals at these positions. During attacks, sailfish slash at sardines with their bill causing prey injury including scale removal and tissue damage. Sardines injured in previous attacks were more often found in the rear half of the school than in the front half. Moreover, injured fish had lower tail-beat frequencies and lagged behind uninjured fish. Injuries inflicted by sailfish bills may, therefore, hinder prey swimming speed and drive spatial sorting in prey schools through passive self-assortment. We found only partial support for the theoretical predictions from current predator-prey models, highlighting the importance of incorporating more realistic predator-prey dynamics into these models. This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.
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