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Residency, feeding ecology, local movements and potential isolation of the Madagascar Omura’s whale (Balaenoptera omurai) population

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Salvatore Cerchio, Boris Andriantenaina, Alex Zerbini Daniel Pendleton, Tahina Rasoloarijao, Danielle Cholewiak

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International Whaling Commission


In 2015 the first detailed description globally of a population of Omura’s whale (Balaenoptera omurai) was published, based on two years of focused field work off northwest Madagascar. Since then a focused, multidisciplinary study of the population was conducted in 2015 and 2016, reported here. Field surveys off Nosy Be, Madagascar were conducted for four weeks in 2015 and six weeks in 2016, tallying 202 encounters with Omura’s whales, and collecting 55 skin biopsies for DNA and 14 fecal samples. Frequent photographic re-sights of individuals were evident within seasons and several noted across seasons, including at least one reproductive female that was sighted in four of six years from 2012 to 2017, once with a calf, suggesting strong site fidelity. Feeding was observed on a near-daily basis, on dense patches of krill identified morphologically as Pseudeuphasia latifrons, which seem to appear in response to dense blooms of a cyanobacteria Trichodesmium sp. Passive acoustic monitoring (PAM) was conducted at four sites spread across 80km for one year. Omura’s whale song was present year-round indicating residency of the Omura’s whale population in this region, with evident spatial and temporal heterogeneity. Four individuals were satellite tagged yielding telemetry tracks ranging from 30 to 58 days. Satellite tagged individuals remained in a restricted range of no more than 405km (mean among individuals of 283km) along the northwest coast of Madagascar, with all individuals moving multiple times throughout their individual ranges. Analysis of movement behavior using behavioral switching state-space model indicated highly localized movement patterns, involving short periods of transiting between specific areas where the whales would then linger for several days displaying primarily localized movements, likely feeding. Habitat suitability modelling indicated favorable conditions for Omura’s whale along the west coast of Madagascar, defined primarily by shallow depth and some undefined influence of primary productivity, with little other predicted suitable habitat throughout the Southwest Indian Ocean. Combination of these data sources indicate that this is a resident, non-migratory population whose distribution is likely determined by local shallow water ecological processes and patchy and ephemeral prey resources. Furthermore, this population of Omura’s whale may be isolated within a fragmented oceanic/global range for the species. Likely threats to the Madagascar population include entanglement in local fisheries, impacts from oil and gas exploration, and most imminent the risk of coastal water contamination from a recently initiated mining operation for Rare Earth Elements. Future work should include a long-term latitudinal study that incorporates multiple methodologies to investigate all aspects of the species biology and conservation threats to the population. Therefore the development of sustained or long-term funding sources is currently a critical requirement for the continued investigation of this population and success of the project.

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