Abstract
To date, most habitat models of cetaceans have relied on static and oceanographic covariates, and very few have related cetaceans directly to the distribution of their prey, as a result of the limited availability of prey data. By simulating the distribution of six functional micronekton groups between the surface and ≃1,000 m deep, the SEAPODYM model provides valuable insights into prey distributions. We used SEAPODYM outputs to investigate the habitat of three cetacean guilds with increasing energy requirements: sperm and beaked whales, Globicephalinae and Delphininae. We expected High Energy Requirements cetaceans to preferentially forage in habitats of high prey biomass and/or production, where they might easily meet their high energetic needs, and Low Energy Requirements cetaceans to forage in habitats of either high or low prey biomass and/or production. Cetacean sightings were collected from dedicated aerial surveys in the South West Indian Ocean (SWIO) and French Polynesia (FP). We examined cetacean densities in relation to simulated distributions of their potential prey using Generalised Additive Models and predicted their habitats in both regions. Results supported their known diving abilities, with Delphininae mostly related to prey present in the upper layers of the water column, and Globicephalinae and sperm and beaked whales also related to prey present in deeper layers. Explained deviances ranged from 9% for sperm and beaked whales in the SWIO to 47% for Globicephalinae in FP. Delphininae and Globicephalinae appeared to select areas where high prey biomass and/or production were available at shallow depths. In contrast, sperm and beaked whales showed less clear habitat selection. Using simulated prey distributions as predictors in cetacean habitat models is crucial to understand their strategies of habitat selection in the three dimensions of the ocean.
Highlights
Since many marine top predators, especially cetaceans, are currently in decline [1], establishing models that correctly describe and predict their preferred habitats is critical to develop appropriate conservation strategies
We developed habitat models of cetaceans based on the same sightings as Mannocci and colleagues [11] in the South West Indian Ocean (SWIO) and Mannocci and colleagues [12] in French Polynesia (FP) for the same three energetic guilds of cetaceans
In the SWIO, the distribution of Delphininae was best predicted by a model containing, in order of decreasing contributions: euphotic depth, non-migrant mesopelagic biomass, epipelagic biomass and migrant bathypelagic production (Table 1)
Summary
Since many marine top predators, especially cetaceans, are currently in decline [1], establishing models that correctly describe and predict their preferred habitats is critical to develop appropriate conservation strategies. Cetacean habitats have mostly been modelled using oceanographic features and primary production as predictors [3,4,5,6]. Cetaceans were shown to be related to bottom depth or slope, sea surface temperature, nutrient concentrations, as well as primary production. Such predictors are assumed to be good indicators of the distribution of lower trophic levels and subsequently of the entire food web. To ensure that models explore direct biological relationships and avoid these lags, habitat modelling should be performed with predictors as close as possible to driving variables [7], namely prey distribution, since cetaceans are very sensitive to variations in prey abundance or quality [8]. Data on prey distributions are not available at large spatiotemporal scales, explaining the scarcity of studies incorporating prey abundance in habitat models
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