Photographic identification and abundance estimates of humpback and blue whales off California in 1991-92

Publications >> Photographic identification and abundance estimates of humpback and blue whales off California in 1991-92

Citation

Calambokidis, J., G.H. Steiger, and J.R. Evenson. 1993. Photographic identification and abundance estimates of humpback and blue whales off California in 1991-92. Report to Southwest Fisheries Center, National Marine Fisheries Service, La Jolla, California. 67pp.

Summary

This report summarizes a two year study completed under contract No. 52 ABNF100137 from Southwest Fisheries Center (SWFC). The primary purpose of the study was to develop abundance estimates using identified individual blue and humpback whales feeding off California and to examine their stock structure.

Fifty-one dedicated vessel surveys were conducted between 16 July and 15 November 1991 and 73 surveys between 27 May and 16 November 1992 to obtain identification photographs of humpback and blue whales. Dedicated survey effort covered 368 hours and 2,714 nautical miles (nmi) in 1991 and 601 hours and 5,051 nmi in 1992. Primary vessels used were 16-18 ft inflatable boats with outboard engines.

Additional identification photographs were obtained by collaborating researchers. Though some of this collaborative effort was opportunistic, 35 surveys in 1991 and 32 surveys in 1992 were conducted with the primary purpose of obtaining identification photographs for this study. SWFC also provided humpback and blue whale identification photographs from their CAMMS vessel surveys conducted along the California coast in 1991 (Hill and Barlow 1992). Photographs from these surveys valuable because they provided a representative sample of whales within 300 nmi of the California coast.

Vessel effort was conducted primarily in areas where humpback and blue whales have been known to occur. Aerial sighting surveys, reports from SWFC and nature trip operators, reports from fishermen (solicited through posters requesting reports to a toll-free number), and boat scouting trips along more remote stretches of the coast were used to identify areas of whale occurrence. Vessel surveys were conducted at numerous locations along the California coast. Surveys crossed into southern Oregon on several occasions and one dedicated trip was conducted off northern Oregon.

From sightings of just under 2,000 humpback whales in 1991 and 1992, we identified 482 different whales. More individuals were identified in 1992 (399) than in 1991 (264). A total of 338 individual blue whales were identified from sightings of just under 1,000 animals. Most of these identifications were made in 1992 when 287 individuals were identified. Only 77 individuals were identified in 1991.

Humpback whales were seen and identified in six primary locations: 1) in the Santa Barbara Channel, 2) between Morro Bay and Pt. Sal off south-central California (1991 only), 3) around Monterey Bay (primarily 1992), 4) Cordell Bank/Gulf of the Farallones off central California, 5) around Fort Bragg (1992 only) and 6) the Pt. St. George area off northern California. Smaller numbers of humpback whales were also photographed in other regions. In 1991, blue whale were identified in coastal waters near Cordell Bank/Gulf of the Farallones and in offshore waters off southern California during SWFC vessel cruises. In 1992, blue whales were primarily identified in three regions: 1) Santa Barbara Channel, 2) Gulf of the Farallones/Cordell Bank area, and 3) Pt. Arena to Fort Bragg.

Both southbound and northbound movement of both humpback and blue whales was documented along the California coast, primarily in 1992. Many of the humpback whale seen in June and July in the Santa Barbara Channel traveled north primarily to Monterey Bay. Of six humpback whales identified in May off Oregon, three were seen later in the Gulf of the Farallones. Similarly, blue whales were documented making frequent southbound and northbound movements.

The humpback whales off California appear to be part of a feeding aggregation that ranges along the coast of California, Oregon, and Washington. Frequent interchange of humpback whales was found within this area, only limited interchange with southern British Columbia, and no interchange with areas farther north in British Columbia and in Alaska. Animals from the California coast primarily breed off Mexico with numerous matches between these areas. Four individuals, including one known female, traveled between California and breeding areas in Hawaii (as far west as Kauai). Some animals also traveled to breeding areas as far south as Costa Rica.

Blue whales feeding along the California coast have been frequently identified along the west coast of Baja and in the Sea of Cortez, Mexico with over 78 matches between these regions. Movement to more distant regions is possible but could not be tested due to the small numbers of blue whale photographs available from other areas in the North Pacific. Based on their known occurrence throughout most of the year, however, it appears likely that the primary range for a substantial portion of the blue whales in the waters off Mexico and California. There was some degree of offshore/inshore segregation in blue whales, with animals seen during SWFC surveys in 1991 far offshore being infrequently seen during the coastal surveys.

The best estimate of humpback whale abundance for the feeding aggregation using the waters of California, Oregon, and Washington, was from the Peterson mark-recapture estimate based on all samples from 1991 and 1992. This estimate was based on the largest samples, both of which covered broad geographic regions, and were the most recently taken. The estimate was 581 with a cv of 0.03.

The best estimate of blue whale abundance came from the Peterson estimate comparing 1991 SWFC identifications to those obtained in the coastal surveys in Mexico and California from 1990 to 1992. This estimate was approximately 1,000 blue whales (904 with a cv of 0.41 for left sides and 1,112 with a cv of 0.34 for right sides) for the waters of Baja, Mexico and California. The small SWFC sample was used because it provided a representative sample of animals in inshore and offshore waters and therefore would not be biased by the offshore/inshore segregation.

The number of humpback whale calves seen within the California-Oregon-Washington feeding aggregation appears low compared to other regions. The proportion of calves seen in humpback sightings along the California coast ranged between 2.3% and 4.0% of sightings and 2.6% to 4.7% of the humpbacks identified were females with calves.