Seasonal Occurrence of Cetaceans along the Washington Coast from Passive Acoustic Monitoring

Seasonal occurrence of cetaceans along the Pacific Coast of Washington has been difficult to characterize because decreased daylight and inclement weather conditions result in a lack of consistent survey effort. Therefore, passive acoustic recorders were deployed at four sites along the Washington coast from 2008 to 2013 to record and detect sound-producing cetaceans. The most frequently detected cetaceans were gray whales, Eschrictius robustus; humpback whales, Megaptera novaengliae; sperm whales, Physeter macrocephalus; and killer whales, Orcinus orca. Unlike the results from previous surveys, year-round acoustic monitoring indicates that migratory species such as gray, humpback, and sperm whales use parts of the Washington coast throughout the year. This information is essential for estimating the potential for human interactions such as entanglement or ship strikes with these species. Seasonal occurrence of the four species was variable between the four sites, even among those sites that were closest in include several stocks listed under the U.S. Endangered Species Act (Carretta et al., 2014b). The abundance and population density of cetaceans in the California Current off the U.S. Pacific Coast, including the Washington coast, has been estimated from summer and fall ship and aerial surveys (Barlow and Forney, 2007; Barlow, 2010; Chandler and Calambokidis1). However, species distribution in this area likely changes between seasons, since many cetacean species undertake long-distance annual migrations (Calambokidis et al., 2001). Forney and Barlow (1998) found that half of the abundance estimates for species surveyed off the California coast exhibited significant differences between the winter and summer surveys indicating that species’ distribution in the California current changes seasonally. Determining cetacean seasonal distribution patterns is difficult and cost 1Chandler, T. , and J. Calambokidis. 2003. 2002 aerial surveys for harbor porpoise and other marine mammals off Oregon, Washington and British Columbia. Unpubl. rep. on file at Marine Mammal Laboratory, Alaska Fish. Sci. Cent. , NMFS, NOAA, 7600 Sand Point Way NE, Seattle, WA 98115. proximity. These results indicate that caution should be used when extrapolating the results from a single site to a larger scale. Combining multiple sites into a loose network of recorders allowed us to characterize the spatial and seasonal occurrence of these cetaceans on the Washington coast during times of the year when aerial and ship surveys are not feasible. Additionally, we provide a baseline for monitoring yearly shifts in occurrence due to changing population demographics, oceanographic conditions, or anthropogenic inputs. prohibitive due to protracted periods of inclement weather, remote access, and short daylight hours. Previous attempts to better describe the year-round occurrence of cetaceans along the Washington coast have been limited temporally and spatially. Aerial surveys were conducted in the late 1980’s and again in the 1990’s (Shelden et al., 2000; Green et al.2). More recently non-systematic ship surveys have been conducted to locate endangered southern resident killer whales, Orcinus orca, in winter and spring months along the Washington and Oregon coasts (Hanson et al., 2010). Acoustic monitoring and associated small boat surveys have been employed to monitor marine mammal occurrence in the U.S. Navy Northwest Training Range Complex (Oleson and Hildebrand, 2012; Trickey et al., 2015), but these have been limited in space to two sites on the central 2Green, G. A. , J. J. Brueggeman, R. A. Grotefendt, C. E. Bowlby, M. L. Bonnell, and K. C. Balcomb. 1992. Cetacean distribution and abundance off Oregon and Washington 1989-1990. In J. J. Brueggerman (Editor), OR and WA marine mammal and seabird surveys. Ebasco Environ. Rep., Bellevue, WA (unpubl.), p. 1–100.