Humpback Whale Encounter Rates Offshore of Maui, Hawaii

Abstract

I investigated a suspected decline in Hawaiian humpback whales (Megaptera novaeangliae) in the Four Island area around Maui, Hawaii, from 1981 to 1986. Observations were made from commercial whalewatching boats during the peak of the winter breeding season. Relative encounter rates differed among the years studied, but there was no evidence of a decline in the relative rate of individual humpback, humpback pod, and calf encounters. A gradual increase in relative calf encounter rate was noted. Cows and calves may be deserting traditional resting areas near the Maui shore in favor of waters 3-4 km offshore. J. WILDL. MANAGE. 52(2):301-304 Waters off Maui, Hawaii, referred to as the Four Island area (Fig. 1), constitute the major population center for the Hawaiian breeding stock of the Northeast Pacific humpback whale (Rice 1978). Although humpback whales frequent many areas in Hawaii (Baker and Herman 1981), there are more humpback whales in the Four Island area than in any other calving area in the North Pacific. The prospect of the humpback whale being displaced from its primary Pacific calving ground is alarming because only 10,000 remain from a nineteenth century population estimate of 120,000 (Johnson and Wolman 1984). Scientists, naturalists, and other concerned observers have become vocal about a perceived decline in the numbers of whales seen off Maui since 1984 (B. Wilson, Lahaina News 6[41]. 1985). Another concern is the possible displacement of cow-calf dyads from their favorite resting locations near the Maui coast (Glockner-Ferrari and Ferrari 1985). The alleged decline in whale numbers has been attributed to increased toxic runoffs; increased human boat traffic, particularly whalewatching; and other water activities off Maui (Markrich 1984). Although qualified observers believed the population was diminishing, there was no proof because an accurate population census of humpback whales in Hawaiian waters does not exist. Rice and Wolman (1978) reported a survey estimate of 550-790 whales, including 382 for the Four Island area. Using a capture-recapture technique, Darling and Morowitz (1986) estimated a Hawaii population of 1,000 humpbacks in any 1 year and 2,100 humpbacks over several winters. However, Hammond (1986) indicated that the problems inherent in using the capturerecapture technique make estimates speculative. Because the size of the humpback whale population in Hawaii cannot be determined accur tely, estimates of change must be based on some other index. I contend that the aggregate number of humpback whales or pods encount red/unit time provides a reasonable index of changes in population size. If the methods used for sighting whales remain the same and the sightability of whales remains reasonably constant, comparisons among years should be valid. I examined trends in the number of humpback whales encountered in the Four Island area during the peak of the winter breeding season (15 Feb-15 Mar) from 1981 through 1986 using encounter rates as an index to population size. I thank the owners, operators, captains, and crews of Captain Zodiac, Kaulana Cruises, Maui Cruise Company, Scotch Mist Charters, and Seabird Cruises for making their whalewatching boats available to me. I also thank R. A. Lancia and 2 anonymous reviewers for their invaluable comments and suggestions. Partial fiscal support was from Grant 2-79474 from Southern Illinois University at Edwardsville.