Modeling Seasonal Distribution of Irrawaddy Dolphins (Orcaella brevirostris) in a Transnational Important Marine Mammal Area

Abstract

Fishing activities continue to decimate populations of marine mammals, fish, and their habitats in the coastal waters of the Kep Archipelago, a cluster of tropical islands on the Cambodia-Vietnam border. In 2019, the area was recognized as an Important Marine Mammal Area, largely owing to the significant presence of Irrawaddy dolphins (Orcaella brevirostris). Understanding habitat preferences and distribution aids in the identification of areas to target for monitoring and conservation, which is particularly challenging in data-limited nations of Southeast Asia. Here, we test the hypothesis that accurate seasonal habitat models, relying on environmental data and species occurrences alone, can be used to describe the ecological processes governing abundance for the resident dolphin population of the Kep Archipelago, Cambodia. Leveraging two years of species and oceanographic data—depth, slope, distance to shore and rivers, sea surface temperature, and chlorophyll-a concentration—we built temporally stratified models to estimate distribution and infer seasonal habitat importance. Overall, Irrawaddy dolphins of Kep displayed habitat preferences similar to other populations, and were predominately encountered in three situations: (1) water depths ranging from 3.0 to 5.3 m, (2) surface water temperatures of 27–32°C, and (3) in close proximity to offshore islands (< 7.5 km). With respect to seasonality, statistical tests detected significant differences for all environment variables considered except seafloor slope. Four predictor sets, each with a unique combination of variables, were used to map seasonal variation in dolphin habitat suitability. Models with highest variable importance scores were water depth, pre- and during monsoon season (61–62%), and sea surface temperature, post-monsoon (71%), which suggests that greater freshwater flow during the wet season may alter primary productivity and dolphin prey abundance. Importantly, findings show the majority of areas with highest habitat suitability are not currently surveyed for dolphins and located outside Kep’s Marine Fisheries Management Area. This research confirms the need to expand monitoring to new areas where high-impact fisheries and other human activities operate. Baseline knowledge on dolphin distribution can guide regional conservation efforts by taking into account the seasonality of the species and support the design of tailored management strategies that address transboundary threats to an Important Marine Mammal Area.

Red‐list status and extinction risk of the world's whales, dolphins, and porpoises

“…This hotspot includes numerous islands, complex coastlines, and extensive shallow‐water habitats that provide niches for a high diversity of coastal cetaceans (Figure 4). This, combined with high human population density, results in a region where many coastal cetacean species are exposed to degraded coastal habitats and high fishing intensity and are therefore threatened (e.g., Hines et al., 2020; Verutes et al., 2021). We did not explore geographic patterns of extinction risk of mysticetes because global range maps do not reflect seasonal movements, and, as such, they obscure large areas of seasonal absence and do not capture important areas of seasonally high density (Williams et al., 2014).…”

Testing spatial transferability of species distribution models reveals differing habitat preferences for an endangered delphinid (Cephalorhynchus hectori) in Aotearoa, New Zealand

“…Heinrichs et al., 2010 ; Purdon et al., 2020 ), the occurrence of rare species or populations outside of surveyed areas (e.g. McCune, 2016 ; Verutes et al., 2021 ), and areas which were previously inhabited, or may be suitable for future use (e.g. Gantchoff et al., 2022 ), and therefore suitable for translocations (e.g.…”

Mitogenomes provide insight into complex evolutionary history of freshwater and coastal Irrawaddy dolphin (Orcaella brevirostris Gray, 1866) in Thailand and Indonesia

“…Water depth, salinity and surface temperature were proposed to influence diversification patterns in Orca (Parsons et al, 2010;Moura et al, 2015) and mammals, as reviewed in Steeman et al (2009), Pinheiro et al (2017) and Sholihah et al (2020). Habitat modelling of the Irrawaddy dolphin supported that these factors are significantly correlated to nutrient-rich effluent and productivity and therefore, to the presence of dolphins and their prey species (Minton et al, 2013;Jackson-Ricketts et al, 2020;Verutes et al, 2021). The "species pump" era, when most freshwater diversification occurred, coincided with divergence time estimates and the Pleistocene environmental fluctuation (de Bruyn et al, 2012).…”