Energetic costs as potential drivers of cetacean habitats in the southwest Indian OceanSubmitted by editor on 13 February 2014.
Cetaceans are large vertebrates which occupy the highest trophic levels of pelagic food webs. At the individual scale, it has been shown that cetaceans select their food on the basis of their energetic costs. Cetaceans with the highest energetic costs select the highest quality prey, whereas cetaceans with more economical lifestyles also forage on lower quality prey. In this study we hypothesized that, over large spatial scales, cetacean habitats also reflect their energetic costs.
Our goal was to examine cetacean habitats in light of their energetic costs in the Southwest Indian Ocean. As part of a large aerial census co-funded by the French Agency for Marine Protected Areas and the French Ministry for Ecology, Sustainable Development and Energy, we carried out an aerial survey to monitor cetaceans and other megavertebrates across the Exclusive Economic Zone of the five countries of the Indian Ocean Commission. We flew 83 700 km of transects with two high-wing, double-engine aircrafts (Figure 1) equipped with bubble windows, allowing observers to scan the water for cetaceans underneath the aircraft.
Figure 1: High-wing double-engine aircraft flying over Mayotte islets – copyright: UMS PELAGIS.
For this study, we classified cetaceans into guilds based on their general energetic costs from the literature. Delphininae (e.g. the bottlenose dolphin Tursiops truncatus) represented the most energetically costly guild (Figure 2a). In contrast, sperm and beaked whales (e.g. the sperm whale Physeter macrocephalus) constituted the most economical guild (Figure 2b). We then described cetacean habitats using satellite covariates such as net primary production and static covariates such as bottom slope. Finally, we built habitat models for each guild and predicted their relative densities at the regional scale.
Figure 2: Aerial photographs of bottlenose dolphins (a) and a sperm whale (b) – copyright: UMS PELAGIS.
For the three cetacean guilds, higher densities were predicted in the most productive areas (notably the Mozambique Channel and the Seychelles, Figure 3). However, the costly Delphininae appeared to be more dependent on productivity (showing a 1-to-13 ratio between the lowest and highest density areas) than the more economical sperm and beaked whales (showing only a 1-to-3 ratio). Cetaceans with energetically costly lifestyles appeared constrained to select highly productive habitats whereas more economical cetaceans may occupy habitats of both high and low productivity. This is in accordance with our initial hypothesis. This study sheds light on cetacean strategies of habitats utilization at large spatial scale and could be extended to other top predators in other pelagic regions.
Figure 3: Spatial predictions for two cetacean guilds with contrasting energetic costs.