A Multi-Dimensional Examination of Foraging Habitat Use by Gray Whales Using Long Time-Series and Acoustics Data

SIMPLE SUMMARY: Long term data on the number and location of foraging gray whales on the west coast of Vancouver was used to understand the rate of return and residency time of whales to certain areas. From this data, areas of increased use by foraging whales were determined, and patterns in the way the areas were used by the whales were seen. Whale location data showed them using prey patches and moving to other patches to allow the first to recover, before then returning to feed on them again later in the summer. Calves that follow their mother in their first migration were likely to return to the same site in the years after weaning. This suggests a maternal aspect to the use of foraging areas. Little is known about how whales detect prey; the use of acoustics was considered here, with call types differing between times when prey was more abundant and whales were feeding in close proximity, to those when foraging whales were more distant to each other. This suggests there may be a social aspect to the calling by gray whales in foraging areas. ABSTRACT: Gray whales (Eschrichtius robustus) show high site fidelity to feeding and breeding areas. The whales’ annual cycle could be learned or be driven by factors such as prey abundance or ocean conditions. Long-term line transect and photo-identification data were analyzed to consider intra- and inter- annual patterns of habitat use and the underlying drivers for foraging areas in Clayoquot Sound, British Columbia. Time series, autocorrelation and weighted means analysis were used on the 20-years of data (1997–2016). A generalized additive model showed that whale use of the area was most strongly influenced by the maximum number of whales, and the date of its occurrence, recorded the previous year. This maximum, when it occurred in the summer, impacted the whale numbers for up to four subsequent years. The annual average number of whales per transect also influenced the proportion of whales known to return in multiple years to forage. Many of these returning whales first used the site to wean and returned in subsequent years to feed. The transect data was also used to contextualize passive acoustic recordings, comparing call type and rate for periods when the whale number, location and social context was known. Calling patterns appeared to be socially derived, with shorter-range knock calls dominant when whales were closer, and lower-frequency moans when foraging occurred when whales were more distant from each other. This suggests that prey-finding and site use may also be influenced by conspecifics.