POPULATION ASSESSMENT OF 



THE GRAY WHALE, ESCHRICHTIUS ROBUSTUS, FROM 



CALIFORNIA SHORE CENSUSES, 1967-80 



Stephen B. Reilly, 1 ' 2 Dale W. Rice,' and Allen A. Wolman' 



ABSTRACT 



Estimates of abundance by year were developed for the California-Chukotski stock of gray whales, from a 13- 

 year consecutive series of shore censuses, conducted near Monterey, Calif. Annual estimates of population size 

 range from a low of 10,414 for 1971-72 to a high of 17,577 for 1979-80. Standard errors are about 10% of pop- 

 ulation estimates. During the 13 years censused, the population increased annually by 2.57c, concurrent with a 

 1.2% harvest in the Soviet subsistence fishery, indicating a 3.7% net annual productivity. 

 Seasonal migratory timing was relatively constant during the study period. Gamma probability density func- 

 tion models of the annual migrations past Monterey had an overall mean day of 9 January, with a range from 8 to 

 19 January. A slight depression in mean hourly count for0070-0800 h, during 1978-79 and 1979-80, contrast- 

 ed with a constant mean hourly count through 10 daylight hours during the previous 1 1 years. Aerial surveys of 

 the offshore distribution of southward migrating whales during 1979-80 agreed closely with those reported for 

 1978-79, indicating that 40% pass within 1 mile (1.6 km) of shore and 907c within 2 miles (3.2 km). In the shore 

 censuses, about 20% of the passing whales were missed due to their distance offshore. 



The estimation of population size for large whales 

 has traditionally been based upon information de- 

 rived from exploitation, e.g., catch per unit effort, 

 mark-recapture, or related data (Allen 1980). Be- 

 cause of the recent decline in exploitation of marine 

 mammals, assessment techniques based upon sight- 

 ing surveys are increasing in importance (Eberhardt 

 et al. 1979). The annual migration of the California 

 stock of gray whales, Eschrichtius robustus (Lill- 

 jeborg 1861), makes it especially well suited to 

 assessment by means of sighting surveys. Assess- 

 ment studies on this stock can potentially aid in the 

 development of sighting survey field and analysis 

 techniques, especially those in which the observer is 

 stationary and the population mobile. This paper 

 presents some recent developments in the use of 

 shore-based census data for whale population as- 

 sessment, and the results of the 1979-80 gray whale 

 census. Revised population estimates for the pre- 

 vious 12 annual censuses are also reported, along 

 with a consideration of change in population siz e dur- 

 ing the period 1967-80. 



Each year during the northern winter the California 

 stock of gray whales migrates from feeding waters in 



'National Marine Mammal Laboratory, Northwest and Alaska 

 Fisheries Center, National Marine Fisheries Service, NOAA, 7600 

 Sand Point Way NE., Seattle, WA 981 15. 



2 National Marine Mammal Laboratory, Northwest and Alaska 

 Fisheries Center, National Marine Fisheries Service, NOAA, 7600 

 Sand Point Way NE., Seattle, Wash.; present address: Southwest 

 Fisheries Center La Jolla Laboratory, National Marine Fisheries 

 Service, NOAA, 8604 La Jolla Shores Drive, La Jolla, CA 92038. 



Manuscript accepted October 1982. 

 FISHERY BULLETIN: VOL. 81, NO. 2, 1983. 



the Bering and Chukchi Seas, south along the west 

 coast of North America, to calving areas in Mexical 

 waters (Fig. 1); the stock returns to the Arctic in the 

 spring (Rice and Wolman 1971). In many places 

 along the route, the whales pass very close to land 

 (Gilmore 1960; Pike 1962; Rice and Wolman 1971; 

 Rugh and Braham 1979). Consequently, it is feasible 

 to census the migrating whales visually from strategic 

 points along the shore. 



Early shore-based censuses were summarized by 

 Reilly et al. (1980). Systematic censuses of south- 

 ward migrating gray whales were initiated during the 

 winter of 1967-68 at both Point Loma (lat. 32°40'N; 

 130 m above sea level) in San Diego, Calif., and at 

 Yankee Point (lat. 36°29'N; 23 m above sea level) 

 near Monterey, Calif. The San Diego count was con- 

 ducted intermittently until 1977-78, for a total of 5 

 yr. The San Diego data were not analyzed in this 

 study because an unverified proportion of the pop- 

 ulation passes far offshore south of Point Conception 

 (Rice 1965) and because the migration route may 

 have been influenced by increased boat traffic (Rice 

 1965; Reeves 1977). The Monterey census was con- 

 ducted each year for 13 yr up to and including 1979- 

 80. Beginning in 1975-76 the counting station was 

 moved 3.7 km south to Granite Canyon (21m above 

 sea level) due to real estate development of the Yan- 

 kee Point site. The Monterey data were used as the 

 basis for this study, because they form a continuous 

 time series and are less complicated by coastal 

 geography and boat traffic than the San Diego data. 



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