Detection of stereotypic call sequences are now commonly used to locate blue whales (Balaenoptera musculus) in the North Pacific (e.g., Watkins et al., 2000a; Stafford et al., 1998; 2001; McDonald et al., 1995). Offshore hydrophones, such as those of the U.S. Navy Sound Surveillance System (SOSUS), have extended our monitoring capability to unprecedented spatial and temporal scales, providing the foundation for descriptions of basin-wide seasonal call patterns (Stafford et al., 1999; Clark, 1995). Consistent seasonal patterns of blue whale calling have been described for both the Northeast (NE) and Northwest (NW) regions of the Pacific basin (Watkins et al., 2000a), with clear differences in call structure suggestive of separate populations (Stafford et al., 2001). Overall, blue whale calls were two to three times more numerous in the NW region (40°N to 55°N latitude, between 150°E and 180°W longitude) than in other regions of the North Pacific, with consistently high calling rates from August through November (Watkins et al., 2000a: Figure 1). Monthly summaries of call occurrence and seasonal depictions of blue whale call locations (Watkins et al., 2000b) demonstrated the year-round occurrence of blue whales in the NW Pacific, with the fall-winter period showing the strongest signal. This was surprising because it belied the oft-repeated assumption that all blue whales migrate south in fall to winter, at temperate latitudes, and occupy North Pacific waters only in late spring and summer (e.g., Bowen and Siniff, 1999). Clearly, the NW Pacific presents suitable habitat for blue whales year-round, although the actual number of whales producing the calls remains unknown.