Hawaiian Spinner Dolphins: Vulnerability to Climate Change and Exposure to Anthropogenic Sound

Abstract

Cetaceans such as the Hawaiian spinner dolphin (Stenella longirostris) use the high-frequency (kHz) channel to communicate, find prey, and avoid predators. Anthropogenic activities, such as navigational sonar and vessel noise may result in acoustic disturbance or injury and impede dolphin behavior or have population-level impacts. Despite vessels producing lower frequency sounds (<400 Hz) that are detected at longer distances, it is still an effective proxy for sound propagation because higher frequency sounds are detected at shorter distances. This could mean overlap between vessel noise and dolphin hearing sensitivity, suggesting interference with their acoustic habitat. Climate change may also present additional threats to the distribution and abundance of the population. I evaluated the climate vulnerability and exposure to sound for two Hawaiian Spinner Dolphin stocks to determine if one stock was more at risk than the other and if there were any factors that contributed to such difference. I used NOAA Fisheries’ stock assessment reports to define clear stock boundaries and identify the pelagic and insular stocks. I conducted a literature review of relevant life histories and used a systematic methodology developed as part of NOAA Fisheries’ Marine Mammal Climate Vulnerability Assessment (MMCVA) project to assess their vulnerability to climate change. I quantified potential shipping exposure using a defined scoring rubric comparing the proportion of Marine Cadastre’s 2019 AIS (Automatic Identification Systems) Vessel Transit Counts data layer overlapping each stock boundary. Despite both stocks having high vulnerability to climate change, the insular stock was more susceptible due to the greater number of cumulative stressors and closer proximity to human activity. This stock also had greater exposure to sound because of its larger overlap with high-density vessel traffic. The life history attributes scored for climate change that have the most influence from sound were site fidelity, home range, and diet specificity. While the relationship between climate change and sound is still understudied, sound exposure may intensify the climate vulnerability of marine species if ocean acidification alters the ocean soundscape. Regardless, these findings offer resource managers additional information to consider in the management of Pacific cetacean stocks.