Estimating the minimum numbers of calling mysticetes using passive acoustic bearings

Monitoring the abundance and distribution of marine mammals is essential for evaluating human and environmental effects. Passive acoustic monitoring offers a non-visual alternative, but estimating animal numbers remains difficult. We introduce Spatial Counting of Animal Numbers (SCAN), a method that counts unique bearings of detected calls to estimate a conservative minimum number of animals present. SCAN is a complementary metric to traditional presence/absence results and calibrated density estimates. The method is demonstrated using sei whale calls recorded by a four-element array on an autonomous glider. SCAN increases the information on marine mammal populations derived from passive acoustic recordings.