Interyear differences in the offshore distribution of eastern North Pacific gray whales

Abstract

Counts of southboundmigrating gray whales (Eschrichtius robustus) have taken place over the last four decades from two shore-based stations: Yankee Point and Granite Canyon on the central California coast near Carmel (Rugh et al. 2008). The selection of these central California sites was based on results from whale marking cruises conducted near Yankee Point (36◦29′30′′N) in the 1960s, when it was determined that few gray whales migrated beyond the visual range of observers on shore (Rice and Wolman 1971). This was confirmed in January 1973, when five flights were conducted near Yankee Point to test the width of the whales’ migration corridor (Sund and O’Connor 1974), with results indicating 96% of the whales passed within 2.6 nautical miles (nmi) (4.8 km) of shore (94% within 1.6 km). This offshore distribution was also documented during aerial surveys near the Granite Canyon station (36◦26′41′′N), where fewer than 2% of the whales migrated beyond the sighting range of shore-based observers (Shelden and Laake 2002). Although most whales migrated within the visual range of shore-based observers, there appeared to be significant differences in the distribution of whales within 3 nmi of shore (Shelden and Laake 2002). It was not clear if these variations were the result of data collection errors during aerial surveys, irregularities in the coastline, or if these 0.2–0.3 nmi shifts were indeed real. In January 1995 and 1996, fixed mounted, 25× binoculars (referred to here as “Big Eye”) were tested at the Granite Canyon site, in part to determine if this technique could be a cost-effective replacement for aerial surveys (Rugh et al. 2002). The probability of detection within 1–3 nmi of shore was fairly high between paired, independent observers (0.97 for group sizes >1 whale and 0.87 for lone whales), making this an effective method for detecting interyear differences within this critical sighting range. We present results from Big Eye studies conducted during eight of the southbound censuses (January 1993–1996, 1998, 2001–2002, and 2007) and compare the offshore distributions to aerial survey results from 1993, 1994, and 1996 (Shelden and Laake 2002). In this paper, we also explore the possibility that there were significant changes in the offshore distribution of whales, thereby affecting sighting rates of counting