Stable isotopes of captive Cetaceans (Killer Whales and Bottlenose dolphins)

There is currently a great deal of interest in using stable-isotope methods to investigate diet, trophic level and migration movement in wild cetaceans. Fundamental to the interpretation of these methods is the need to understand how diet isotopic values are reflected in consumer tissues. In this study, we investigated patterns of isotopic discrimination between diet and blood constituents of two species of cetaceans (killer whale, Orcinus orca and 19 bottlenose dolphin, Tursiops truncutus) fed with controlled diets during 350 days. Diet discrimination factors ( ) for plasma were estimated to 13C =2.3‰ and 1520 N =1.8‰ respectively for both species and to 13C =2.7‰ and 1521 N =0.5‰ for red blood cells (RBC). Delipidation had no significant effect on carbon and nitrogen isotopic values of blood constituents, confirming that cetacean blood no serving as reservoirs of lipids. In contrast, carbon isotopic values are higher in delipidated samples of blubber, liver and muscle of orca tissues. The mean half life for plasma was 32.9 days for killer whales and 27.2 days for bottlenose dolphin and for RBC was greater than 175 days. The potential for conflict between fisheries and cetaceans has heightened the need for trophic information about this taxa within those ecosystems. These results provide the first published stable isotope turnover rates and discrimination factors for cetaceans, which are essential if conclusions are to be drawn on issues concerning trophic structures, carbon sources and diet reconstruction

A Comprehensive Survey of Pelagic Megafauna: Their Distribution, Densities, and Taxonomic Richness in the Tropical Southwest Indian Ocean

The distribution and density of pelagic megafauna (marine mammals, seabirds, elasmobranches, and sea turtles) are important indicators of marine biodiversity, reflecting the condition of the underlying ecosystems. A dedicated aerial survey was conducted in the tropical Southwest Indian Ocean to map their distribution, the taxonomic diversity, and to estimate their densities to serve as a baseline for the area. This large survey across three ecological sub-regions revealed contrasting spatial distributions: maps of taxonomic richness of marine mammals and seabirds revealed different “hotspots” in the area. Densities were estimated for eight cetacean taxa with small and large Delphininae, or small Globicephalinae dominating, and for seven seabird taxa, with terns and noddies dominating. At the community level, the Southwest Indian Ocean megafauna was structured by the marine environment with strong differences between the Mozambique Channel and the Mascarene Islands, or between shelf and slope/oceanic habitats. Our results illustrate how multi-taxa aerial surveys are relevant and cost-effective monitoring tools for marine megafauna, allowing a community-wide approach

Deep ocean drivers better explain habitat preferences of sperm whales Physeter macrocephalus than beaked whales in the Bay of Biscay

Species Distribution Models are commonly used with surface dynamic environmental variables as proxies for prey distribution to characterise marine top predator habitats. For oceanic species that spend lot of time at depth, surface variables might not be relevant to predict deep-dwelling prey distributions. We hypothesised that descriptors of deep-water layers would better predict the deep-diving cetacean distributions than surface variables. We combined static variables and dynamic variables integrated over different depth classes of the water column into Generalised Additive Models to predict the distribution of sperm whales Physeter macrocephalus and beaked whales Ziphiidae in the Bay of Biscay, eastern North Atlantic. We identified which variables best predicted their distribution. Although the highest densities of both taxa were predicted near the continental slope and canyons, the most important variables for beaked whales appeared to be static variables and surface to subsurface dynamic variables, while for sperm whales only surface and deep-water variables were selected. This could suggest differences in foraging strategies and in the prey targeted between the two taxa. Increasing the use of variables describing the deep-water layers would provide a better understanding of the oceanic species distribution and better assist in the planning of human activities in these habitats.Versión del edito

Distribution patterns of marine megafauna density in the Mediterranean Sea assessed through the ACCOBAMS Survey Initiative (ASI)

The ACCOBAMS Survey Initiative (ASI) is the first synoptic large-scale survey of the entire ACCOBAMS Area and as such it plays a key role in filling the current gaps in our biological and ecological knowledge of large vertebrate species occurring in the region. Data gathered during the ASI were analyzed in a distance sampling surface-modelling framework to assess the summer distribution, densities and patterns, as well as to investigate the correlates of these parameters, for large vertebrate species and taxa in the Mediterranean Basin. Static and dynamic explanatory variables, including water depth (m), distance to depth contours (km), distance to canyons and seabed slope (km), sea surface temperature (°C), mixed layer depth (m) and levels of chlorophyll-a (mg/l), were considered to predict density and compute its variance spatially at a resolution of 10x10 km. A strong longitudinal gradient from low densities in the east to high densities in the west is shared by most taxa. In addition, several taxa also showed a less marked latitudinal gradient varying in direction according to species, and finally, a few of them exhibited patchy distributions

Large‐scale modelling of deep‐diving cetacean habitats

Versión del editor5,14

Modelling the habitat suitability of cetaceans:Example of the sperm whale in the northwestern Mediterranean Sea

Cetaceans are mobile and spend long periods underwater. Because of this, modelling their habitat could be subject to a serious problem of false absence. Furthermore, extensive surveys at sea are time and money consuming, and presence–absence data are difficult to apply. This study compares the ability of two presence–absence and two presence-only habitat modelling methods and uses the example of the sperm whale (Physeter macrocephalus) in the northwestern Mediterranean Sea. The data consist of summer visual and acoustical detections of sperm whales, compiled between 1998 and 2005. Habitat maps were computed using topographical and hydrological eco-geographical variables. Four methods were compared: principal component analysis (PCA), ecological niche factor analysis (ENFA), generalized linear model (GLM) and multivariate adaptive regression splines (MARS). The evaluation of the models was achieved by calculating the receiver operating characteristic (ROC) of the models and their respective area under the curve (AUC). Presence–absence methods (GLM, AUC=0.70, and MARS, AUC=0.79) presented better AUC than presence-only methods (PCA, AUC=0.58, and ENFA, AUC=0.66), but this difference was not statistically significant, except between the MARS and the PCA models. The four models showed an influence of both topographical and hydrological factors, but the resulting habitat suitability maps differed. The core habitat on the continental slope was well highlighted by the four models, while GLM and MARS maps also showed a suitable habitat in the offshore waters. Presence–absence methods are therefore recommended for modelling the habitat suitability of cetaceans, as they seem more accurate to highlight complex habitat. However, the use of presence-only techniques, in particular ENFA, could be very useful for a first model of the habitat range or when important surveys at sea are not possible