Applying distance sampling to estimate densities of fin whale calls recorded by ocean bottom seismometers in the Marianas region

Ocean bottom seismometers (OBSs) are opportunistic instruments that can be used for passive acoustic monitoring of fin whales. To estimate animal population densities from acoustic data using distance sampling, a standard density estimation method, it is essential to estimate ranges to detected calls. Regional OBS networks are too widely spaced to localize fin whale calls through arrival time differences on multiple instruments. This motivates the development of ranging methods using single instruments. We previously developed and published a single-OBS ranging method for fin whale calls that uses the relative timing of multipath arrivals for calls reverberating in the water column to estimate horizontal ranges to fin whales. In this study, we applied multipath ranging and distance sampling methods to fin whale calls recorded on 7 OBSs deployed over a 200 × 300 km² area east of the Marianas Trench from February 2012 to January 2013. We detected and ranged to calls within 40 km at each OBS, then applied point-transect distance sampling methods to fit detection functions and estimate the probability of call detection. We calculated monthly fin whale call densities during the Marianas calling season from December to April, except for February when frequent airgun shots prevented ranging. We discuss sources of uncertainty in call density estimations, and identify encounter rate variance as the largest contributor. Increasing the number of OBSs used for ranging would improve the precision of call density estimates. Looking forward, converting call densities into estimates of animal abundance will require independent estimates of fin whale call rates.