The Kerguelen Plateau in the south-eastern Indian Ocean is one of the most isolated and understudied regions on earth. As part of the Kerguelen Plateau Drifts project, Marine Mammal Observer (MMO) data were collected during a seismic survey in the austral summer (January–February 2020). Relationships between observation effort, cetacean sightings, seismic operations, and oceanographic variables – including bathymetry (depth and slope), nutrient concentrations, and indices of primary productivity – were investigated using Generalized Additive Models (GAMs). In total, 354 hours and 45 minutes of observation effort resulted in 191 cetaceans (178 adults and at least 13 juveniles) of nine species observed on 48 occasions, over 14 days along the transect line. Marine mammal sightings occurred in water depths of 624–4,699 m, with a hotspot of sightings recorded along the northern flank of the Kerguelen Plateau, in proximity to shelf edges. There was one sighting of a mixed pod of Kerguelan Commerson’s dolphins (Cephalorhynchus commersonii kerguelenensis) and dusky dolphins (Lagenorhynchus obscurus), which to the best of our knowledge, is the first confirmed sighting of dusky dolphins in the Kerguelen Islands. Of the nine cetacean species observed, no niche separation was apparent, and all species were observed throughout the survey area. Dissolved oxygen and chlorophyll-a were the most significant predictors of cetacean occurrence. Systematic MMO data combined with synoptic satellite-derived/empirical oceanographic metadata have great potential to facilitate understanding of behaviour, geographical range, and population-status monitoring of cetaceans. This is especially important for cetacean stock assessment and minimising potential acoustic disturbance in Antarctic ecosystems.

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