The use of passive acoustic monitoring as a census tool of gray whale (Eschrichtius robustus) migration

Passive acoustic monitoring adds to knowledge of whale presence and distribution over space and time. The gray whale (Eschrictius robustus) undertakes an annual migration between remote feeding and breeding locales, with its highly sonorous nature suggesting employment of low-frequency calls to be inherent to travel behaviours, perhaps aiding navigation and social cohesion. We consider whether these calling behaviours can be used to better understand the process of migration, where call presence and relative abundance might represent a means to track the progress of migrating whales. Passive acoustic recordings made on the west coast of Vancouver Island, a mid-way point of gray whale migration, were first analysed for abundance over time to examine the progression of migration. Then the acoustic record was compared to shore-based recordings undertaken from Point Vincente, California to better assess their use as a census tool. The timing in peaks of calling, particularly of far-ranging moan-like calls, and shore counts showed a high degree of congruence when considered with a lag time of 8 days to account for travel time between the two survey locations. This suggests that passive acoustic monitoring could complement the long running and shore-based counts in recording annual population movement, and add to our understanding of gray whales on poorly known segments of their migration route or at times when visual observations are not possible.