1. Introduction Humpback whales (Megaptera novaeangliae) annually migrate from their summer feeding grounds off southeast Alaska to winter in waters off the Hawaiian Islands, Baja California Sur, Mexico and northern Japan (Baker and Darling). The number of humpback whales in the Hawaiian waters generally peaks from mid-February through mid-March (Baker & Herman, 1984). Calving and breeding is an important function of humpback whales while wintering at lower latitudes (Herman and Herman et al. , 1980). The presence of these whales has spawned a popular and rapidly growing whale-watching industry.
Whale watches are undertaken with a wide assortment of vessels. Because of the ever-growing number of boats involved, concerns are often expressed by those in the whale-watching industry, environmental groups and governmental agencies about the effects of vessel disturbance on the whales (Green, 1998). Humpback whales have been observed to react to approaching boats in a number of different ways ranging from approach to avoidance. On rare occasions, humpback whales have been observed charging towards approaching boats and screaming underwater (Payne, 1978). Bauer (1986) and Bauer and Herman (1986) found that respiration rates, diving, swimming speed, social exchange and aerial behaviors correlated with vessel numbers, proximity, speed and direction changes. They reported that humpback whales generally attempted to avoid vessels and sometimes directed threats towards them.
Increased frequencies of surfacing without blows and dives initiated without raised flukes were some behaviors indicative of avoidance. Green and Green (1990) reported that humpback whales often reduced the proportion of time at the surface, took longer dives, altered direction as th boats approached (horizontal avoidance) and continued to spend more time underwater and decreased swim speed (vertical avoidance) after boats departed. These effects persisted over 20 min after the boats departed. Green (1990) also observed that humpback whales moved from a favored area on days when para sail boats operated. Bauer and Herman (1986) concluded that reactions to vessels probably are stressful to humpback whales but the significance of the stress is unknown. Research performed by Baker and Herman (1989), Baker, Herman, Bays and Stifel (1982), Baker, Herman, Bays and Bauer (1983), and Bauer (1986) in Alaskan waters suggests that humpback whales usually use two main type of avoidance methods.
The first involves a vertical avoidance in which the dive duration increases, with a corresponding decrease in the blow interval and in swim speed. The second method involves a horizontal avoidance in which there is a decrease in the dive duration, longer blow intervals and an increase in swim speed. Baker, Herman, Bays and Stifel (1982) and Baker, Herman, Bays and Bauer (1983) also found that approaching boats often triggered some aerial behaviors such as breaching, flipper and tail slapping. There appears to be little doubt that boat traffic may affect the behavior of humpback whales. Examples of such disturbance by vessels on humpback whales in Hawaii can be found in Tinned (1988) and in the humpback whale recovery team report (HURT, 1991). Consequently, the National Marine Fisheries Service (NMFS), the Federal agency primarily responsible for enforcing the Marine Mammal Protection Act, has imposed a regulation prohibiting boats from approaching within 91 m (100 yards) of any humpback whale in Hawaii (NMFS, 1987).
One issue that has not received much attention is the specific effect of boat noise on the whales. All boats from the smallest motor boat to the largest super-tanker produce underwater noise. However, there is limited information on noise produced by small boats typically used in coastal waters (Richardson, Greene, Male & Thomson, 1995). Considerably more attention has been focused on large ocean-going vessels.
McCauley, Cato and Jeffery (1966) have measured the noise generated by whale-watching vessels in Hervey Bay, Australia. Many of these boats in Hervey Bay operated as ferry's and modify their routine slightly upon encountering a pod of whales. Present regulations in Hervey Bay for approaching whales state that boats must slow within 300 m of whales, which is very different than the 91 m standoff range in Hawaii with no speed limitations. It is also difficult to apply noise measurements from one location to another because underwater acoustic propagation can vary considerably depending on the depth and types of bottom.