Circum-island line-transect abundance estimates of spinner dolphins around Oʻahu, Hawaiʻi

Spinner dolphins (Stenella longirostris subsp.) in Hawaiʻi face the highest documented human activity exposure rates of any dolphin species worldwide, raising concerns about long-term impacts. Fundamental to any appropriate management action for a protected species is the availability of reliable information on their abundance and distribution. Currently, abundance estimates are only available for one of the six designated stocks of spinner dolphins in Hawaiʻi. To address this information gap, small-boat line-transect surveys for spinner dolphins were conducted around the island of Oʻahu between October 2020 and May 2022. A total of 73 survey days were achieved, resulting in 3098 km of transects surveyed around the Oʻahu coastline. We used a novel method utilizing opportunistically collected unoccupied aerial system data to assess dolphin availability at the surface. We also investigated the potential for responsive movement of animals, for which no evidence was found. Estimated abundance of spinner dolphins for Oʻahu was 594 animals (95% CI [360, 980], CV = 0.26). Our findings represent the first circum-island abundance estimate of Oʻahu's spinner dolphins, allowing management to explore strategies aimed at minimizing human-dolphin interactions in the region and assess long-term impacts.