Biology of humpback whales in the Gulf of the Farallones, California

Citation

Calambokidis, J., G.H. Steiger, J.C. Cubbage, K.C. Balcomb, and P. Bloedel. 1989. Biology of humpback whales in the Gulf of the Farallones, CaliforniaReport to Gulf of the Farallones National Marine Sanctuary, San Francisco, California. 93pp.

Executive summary

Under a research contract to the Gulf of the Farallones National Marine Sanctuary and the National Park Service, we studied the abundance, distribution, and behavior of humpback whales in central California from 1986 to 1988. The goal of the research was to gain information helpful to management and protection of humpback whales and other marine mammals present in the Gulf of the Farallones National Marine Sanctuary and the proposed sanctuary around Cordell Bank. In addition, other marine mammals were studied and research on blue whales was conducted concurrently to this study (see Calambokidis et al. 1989).

We conducted 1,457 hours of vessel surveys over the three year period primarily in August through October of each year. The purpose of the surveys was to photographically identify individual humpback whales and gather sighting information on marine mammals. We also flew over 88 hours of aerial surveys to examine the distribution of marine mammals, provide information on whale distribution to vessels, measure the length of whales through photogrammetry, and conduct line-transect estimates of abundance. Additionally, cooperating researchers and naturalists aboard nature trips in the region provided photographs of individual humpback whales and information on sightings.

Over 1,000 sightings of humpback whales were made during the vessel and aerial surveys in the study area. The sighting rates were highest in 1987 compared to 1986 and 1988. An influx of humpback whales occurred in August in all three years. Maximum numbers were seen from the middle of August to the middle of September in 1986 and 1987 and in September and October in 1988. The distribution of sightings in the study area varied both by season and by year. The areas around the Farallon Islands and to north of Fanny Shoal were some of the most consistent areas to find humpback whales. They were episodically abundant at other areas, including near Cordell Bank and north of Bodega Canyon (west of Bodega Bay), during some years and months. Humpback whales were significantly associated with water depths of 200 to 500 feet, though the depth varied significantly among years.

The changes in distribution we observed are likely the result of variations in the distribution and availability of prey, predominantly euphausiids and anchovies (Rice 1977). We examined raw data made available to us by D. Rice (pers. comm.) from humpback whales taken during commercial whaling in the Gulf of the Farallones from 1956 to 1965. In those catches there were dramatic annual and seasonal variation in the locations of capture, as well as annual variations in prey species.

Through fluke markings, we identified 225 different individual humpback whales from 1986 to 1988, with the most seen in 1987 when 141 were identified. Just over half the identified whales were seen in only one year (122), 65 were seen in two years, and 38 were seen across all three years. Three whales first identified by other researchers in 1981 were seen in our study, up to seven years later. On average, identified whales were seen more than five times each year. A "resident" subgroup of whales tended to be seen more often each year and across years. One of the humpback whales identified all three years was Humphrey, the whale that attracted public attention when it swam up the Sacramento River in 1985. During our study in 1986 and 1987, Humphrey was seen in consort with other humpback whales. However, in 1988 it was seen alone on three separate days in the shallow waters of Drakes Bay, Bodega Bay, and Bodega Harbor; all areas where we had not seen humpback whales previously.

Comparison of humpback whale photographs from the Gulf of the Farallones to those in other areas revealed movements of whales to other areas. Along the California coast, a majority of the humpback whales seen from Pt. Sur to Pt. Arena matched with whales in the Gulf of the Farallones, whereas a smaller proportion seen north and south of these points matched with the Farallones group (implying the existence of a central California feeding aggregation). The wintering grounds of central California humpback whales are preponderantly along the coast of Mexico and Central America. Twenty-three humpback whales seen in the Gulf of the Farallones (almost 10%) matched with Mexico and two matched with Costa Rica (provided by Richard Sears and the only two identified from this area). A few individuals (3) from the Farallones also occurred in Hawaii.

Few calves were seen during the study, though sightings increased over the three years. Calves were seen as 0.4%, 1.7%, and 5.3% of the whales sighted in 1986, 1987, and 1988, respectively. The proportion of identified cows with calves was low compared to other areas. The lengths of humpback whales measured from aerial surveys ranged from 10.5 to 13.6 m (n=24, mean=12.0, s.d.=0.76) and were generally smaller than lengths reported from whaling data.

Estimates of the abundance of humpback whales were calculated from line-transect aerial surveys and individual identification data. One aerial survey each year was flown using line-transect methods to provide a single-day abundance estimate. These estimates ranged from 60 humpback whales on 16 October 1986 to 109 on 25 September 1988. The number of individuals identified each year were higher than the line-transect estimates, probably reflecting individuals moving in and out of the study area. Within-year estimates based on mark-recapture yielded from 109 whales in 1986 to 211 in 1988 while between year estimates tended to be slightly higher, 226 for 1986 to 1987 and 253 for 1987 to 1988. However, we found violations of some assumptions of mark-recapture estimates based on the Gulf of the Farallones data.

Our estimates of population size are all lower than those reported for September 1982 by Dohl et al (1983). The population at that time was thought to be increasing based on the survey results from 1980 to 1982 and the reoccurrence of spring sightings of humpback whales near the Farallon Islands (Huber et al. 1980). Our data and that from other sources do not reveal an increase in humpback whale abundance. Instead there appears to be large annual variations in numbers, timing, and distribution of humpback whales in this area. Variations in timing and distribution also were apparent in the whaling data.

Sixteen other species of marine mammals were seen. The most frequently seen marine mammals included three pinniped species (harbor seals, California sea lion, and northern sea lions), three small cetaceans (Dall's porpoise, harbor porpoise, and Pacific white-sided dolphin), and two whales (minke whale and blue whale). Blue whale occurrence in the study area increased dramatically during the three years consistent with an increased abundance that began in the late 1970s (Calambokidis et al. 1989). 

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