Cetaceans and threats of the Ninety East Ridge and high seas of the Indian Ocean.

The Ninety East Ridge (NER) is one of the longest mid-ocean ridges in the world - running approximately 5600 km along the 90° Meridional line - in the eastern Indian Ocean. The northernmost area of the Ninety East Ridge is within 500 km of land; however, the majority extends far into the high seas and there are few surveys that detail marine mammal diversity in the region. Here we conducted a visual-acoustic survey for cetaceans using the SY Rainbow Warrior for 10 days from the February 22 to the March 2, 2024 covering 2843 km of survey effort, with 2100 km across the NER. We report a total of 101 acoustic detections of cetacean groups, with all but one attributed to groups of oceanic delphinids. Based on concurrent visual sightings, 18 detections were assigned to species level: spinner dolphins (Stenella longirostris, nine detections); spotted dolphins (Stenella attenuata, six detections); striped dolphins (Stenella coeruleoalba, two detections) and a bottlenose dolphin (Tursiops spp., one detection) that could not be distinguished to species level despite sightings and acoustic recordings. Further, based on acoustic characteristics we report an acoustic detection of a group of Blainville's beaked whale (Mesoplodon densirostris). The remaining 82 delphinid detections could not be classified to species level. We discuss these results in the context of potential threats to species, including fisheries, shipping and pollution. Our data suggest that the region is a habitat used by cetaceans and other pelagic species. Given the lack of information on the NER ecosystem and threats, we suggest that more sustained, systematic surveys conducted through multilateral partnerships could yield important information on how to manage human activities as they develop in future.