A SHIFT IN DISTRIBUTION OF HUMPBACK WHALES, MEGAPTERA-NOVAEANGLIAE, IN RESPONSE TO PREY IN THE SOUTHERN GULF OF MAINE

From the mid-1970's to the mid-80's, Stellwagen Bank was an important humpback whale feeding area with sand lance (Ammodytes spp.) as the ma jor prey. Between 1988 and 1994;, however, the number of humpback whal es we identified each year on Stellwagen declined from a high of 258 ( 1990) to 7 (1994), and the mean number of whales identified per day fe ll from 17.7 (1988) to 0.9 (1994). Adult whales decreased steadily aft er 1988; juveniles decreased rapidly after 1991, Echo-sounder data fro m Stellwagen showed Chat prey trace levels declined from 19.1% of the vertical water column in 1990 to 2.8% in 1992 (no readings were taken in 1988-89, or 1993-94). Simultaneously, the number of whales identifi ed on Jeffreys Ledge, north of Stellwagen Bank, increased dramatically beginning in 1992. Sixty-four percent of the whales identified on Jef freys in 1992-94 were seen on Stellwagen Bank in 1988 and 1989. We hyp othesize that humpback whales shift their distribution in order to pre y upon recovering herring populations, their primary source of food.