Biology of blue whales in the Gulf of the Farallones and adjacent areas of California

Citation

Calambokidis, J., G.H. Steiger, J.C. Cubbage, and K.C. Balcomb. 1989. Biology of blue whales in the Gulf of the Farallones and adjacent areas of California. Report to Gulf of the Farallones National Marine Sanctuary, San Francisco, Ca. 58pp.

Executive summary

Blue whale populations have been depleted as a result of commercial whaling conducted up to 1966. Blue whales were hunted in the Gulf of the Farallones from whaling stations operating out of San Francisco Bay. There has been little evidence to document the recovery of blue whales in the North Pacific since the end of whaling. Frequent sightings of blue whales in the Gulf of the Farallones, California since the early 1980s, and the results of this study, indicate the Gulf of the Farallones has become one of the major areas of concentration for blue whales on the west coast.

We studied blue whales in the Gulf of the Farallones (including coastal waters north to off Bodega head) from July to November of 1986 to 1988. Levels and type of survey effort were similar among years with a total of 1,457 hours of vessel surveys and 88 hours of aerial surveys conducted over the three-year period. Individual blue whales were identified from photographs using the pigmentation patterns on both sides of the whale near the dorsal fin. Blue whales were also identified outside the Gulf of the Farallones with the help of other researchers and naturalists who contributed photographs to our catalogue. The lengths of 39 blue whales were measured using aerial photogrammetry.

Increasing numbers of blue whales were seen in aerial and vessel surveys from 1986 to 1988. The distribution and timing of blue whale sightings from 1986 to 1988 were somewhat variable among years. Peak numbers were generally present in August to early October. The water depth of blue whale sightings varied significantly by year, with blue whales occurring in deepest water in 1987. Blue whales were significantly associated with concentrations of marine birds, primarily Cassin's Auklets and phalaropes.

There were significant differences in the lengths of blue whales measured in 1987 and 1988, average lengths were 20.5 (67 feet) and 21.8 meters (72 feet), respectively. This difference may reflect a segregation of animals by age class as has been documented in feeding areas of other baleen whales. The majority of the whales measured fell between the anticipated lengths of sexual and physical maturity. Few calves were seen during the research but this may have been as a result of calves having already been weaned prior to their arrival in the Gulf of the Farallones.

A total of 179 individual blue whales were photographically identified in the Gulf of the Farallones from 1986 to 1988. Most individuals were identified in 1988 (101) and 1987 (75). In both years more than 50% of these individuals were seen on only one day. In 1986, when 35 whales were identified, most whales were seen on more than one day. Fifteen percent of the identified whales were seen in more than one year and five individuals (3%) were seen in all three years.

Movement of blue whales between the Gulf of the Farallones and other locations to the north and south was documented through photographic matches of identified whales. In 1987 and 1988, five and three individuals, respectively, were seen in both Monterey Bay and the Gulf of the Farallones; all but one of these whales travelled from Monterey Bay in August to the Gulf of the Farallones in late August and September. A large number of blue whales were seen north of the Gulf of the Farallones near Point Arena in middle to late October 1988 and half of the animals identified (n=17) were seen in the Gulf of the Farallones in September or October.

Blue whales in the Gulf of the Farallones migrate from the west coast of Baja and the Sea of Cortez, Mexico. We identified blue whales from these areas using photographs provided by other researchers and whales catalogued by Mingan Island Cetacean Study. Nine blue whales seen in the Gulf of the Farallones have been matched to these areas. For example, three blue whales identified in March and April of 1988 off Baja, Mexico were seen in August or September 1988 in the Gulf of the Farallones or Monterey Bay.

Estimates of the number of blue whales occurring in the Gulf of the Farallones were possible from individual identification data and from aerial line-transect flights. A single line-transect flight conducted on 26 September 1988 provided area-dependent estimates of 160 and 250 animals in the study area, the highest numbers that we obtained. Mark-recapture estimates for 1988, despite the violation of a number of assumptions, yielded similar estimates of about 200 animals, depending on the time period and sides of the whale used in these calculations. Population estimates for 1986 and 1987 indicated lower numbers of blue whales than in 1988.

The high number of blue whales using the Gulf of the Farallones indicates it is an important area for this endangered species. The dramatic increase in numbers of blue whales that has occurred in the last seven years is extremely encouraging. If this increase continues the Gulf of the Farallones could become the single most important feeding area for the remnant blue whale population in the North Pacific. 

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