Accounting for sperm whale population demographics in density estimation using passive acoustic monitoring

Sperm whales Physeter macrocephalus are highly sexually dimorphic, with adult males having larger bodies, more powerful echolocation clicks, and slower echolocation clicking rates compared to females. This study introduces methods for estimating sperm whale population densities in the Gulf of Mexico (GoMex) by accounting for the population demographics using passive acoustic monitoring and reveals that ignoring the differences between demographic segments can introduce bias in density estimates. Weekly densities were estimated per 3 demographic segments: social groups consisting of adult females and their offspring, mid-size animals, and adult males. Analysis revealed that the GoMex sperm whale population is primarily composed of social groups, which account for 92 to 98% of the overall population. Mid-size animals and adult males made up a small proportion of the population and were only intermittently present. Our 7 yr GoMex density estimates, including the 2010 Deepwater Horizon (DWH) oil spill period and subsequent years, revealed demographic-specific trends. Declines found at 2 north-central GoMex sites, coupled with increases at a southeastern site, may indicate population movements and potential impacts from the 2010 DWH oil spill and elevated noise levels from anthropogenic activities in the north-central GoMex.