Humpback whales (Megaptera novaeangliae) occur in all oceans of the world, generally inhabiting waters over continental shelves, along continental edges and around some oceanic islands . They winter in warm waters in a few specific locations and mate and give birth on wintering grounds where little feeding is thought to take place . For the summer season, they migrate to high-latitude areas where they tend to stay relatively close to shore (although some groups inhabit deeper water) and spend the majority of their time feeding .
Humpback whale populations were greatly depleted by commercial whaling . Prior to whaling, humpback whale numbers are thought to have exceeded 125,000 . American whalers alone killed between 14,164 and 18,212 humpback whales between 1805 and 1909 . Humpback whales first received protection in the North Atlantic in 1955 when the International Whaling Commission placed a prohibition on non-subsistence whaling by member nations . Protection was extended to the North Pacific and southern hemisphere populations following the 1965 hunting season . Although hunting has largely been stopped (some exceptions exist that allow the take of a limited number of whales), and populations appear to be increasing, human impacts such as vessel collisions and entanglements are factors that may be slowing the recovery of the humpback whale population .
The total level of human-caused mortality and serious injury is unknown, but data indicate that it is significant . Humpback whales are also vulnerable to marine pollution . The increasing levels of anthropogenic noise in the world’s oceans, such as that produced by certain types of sonar, may also be problematic for whales, particularly for baleen whales that communicate using low-frequency sound .
GULF OF MAINE, WESTERN NORTH ATLANTIC STOCK
There are likely six stocks within the western North Atlantic humpback population . A feeding aggregation in the Gulf of Maine (considered a single stock) is the only one U.S. waters . Within New England waters, humpbacks are present in spring, summer and autumn . They spend much of their time feeding and their distribution in this region has been largely correlated to prey species and abundance . In winter, humpbacks from the different western Northern Atlantic feeding areas mate and calve primarily in the West Indies, where spatial and genetic mixing among subpopulations occurs . From late December to early April most of the population is found at Silver and Navidad Banks at the end of the Bahamian archipelago, and along the coast of the Dominican Republic . They are also found at much lower densities throughout the remainder of the Antillean arc, from Puerto Rico to the coast of Venezuela . The only U.S.-controlled portions of the breeding range include waters along the Northwest coast of Puerto Rico and the U.S. Virgin Islands . Not all of the stock migrates to the West Indies every winter, however, and significant numbers of animals are found in mid- and high-latitude regions during the winter months . There have recently been a number of wintertime humpback sightings in coastal waters of the southeastern U.S. .
North Atlantic humpback numbers are thought to be slowly increasing. An average increase of 1 percent (SE=0.005) was estimated for the period 1979 to 1993 . The best estimate of the number of North Atlantic humpbacks in 1992 and 1993 was 11,570 (CV=0.069) . Data suggest that the Gulf of Maine humpback whale stock is also steadily increasing in size at a rate consistent with the larger population . The Gulf of Maine minimum population was estimated to be 501 in 1992 and 647 in 1999 . Both of these estimates are likely low due to sampling technique . The best estimate of the actual number of animals is thought to be 902 .
Historic summering range for the North Pacific humpback whales encompassed coastal and inland waters around the Pacific rim from Point Conception, Calif. north to the Gulf of Alaska and the Bering Sea, and west along the Aleutian Islands, Kamchatka Peninsula and into the Sea of Okhotsk . Rough estimates of the pre-whaling population speculate that there were around 15,000 humpbacks in the North Pacific . In 1966, the entire North Pacific humpback population was thought to number only around 1,200 animals . This estimate increased to between 6,000 and 8,000 by 1992 . Although these estimates are uncertain and are based on different methods, the 6 percent to 7 percent growth rate implied is consistent with the observed growth rate of the better-studied eastern North Pacific subpopulation .
Although the International Whaling Commission only considered North Pacific humpbacks to be one stock, there is now good evidence for multiple stocks in the North Pacific . There are at least three relatively separate populations that migrate between their respective summer/fall feeding areas and winter/spring calving and mating areas; the eastern North Pacific stock, the central North Pacific stock and the western North Pacific stock . These divisions are a simplification, however, and are not perfect. In general, interchange occurs (at low levels) between breeding areas, although fidelity is extremely high among the feeding areas .
The eastern North Pacific stock spend much of their lives within U.S. waters . They winter in coastal Central America and Mexico, migate along the U.S. West Coast, and summer in British Columbia . Mark-recapture population estimates increased steadily from 1988/90 to 1997/98 at about 8 percent per year . Surveys of humpback whale abundance in feeding areas in California, Oregon and Washington conducted from 1991 to 2002 show a steady upward trend with the exception of a large decline in 2000 . The 2002 to 2003 population estimate (1,391, CV=0.22) was higher than any previous estimate and may indicate that the lower numbers in 1999 to 2001 exaggerated any real decline that might have occurred . It could also indicate that a real decline was followed by an influx of new whales from another area . This latter view was substantiated by a greater fraction of new whales seen for the first time in 2003 .
The central North Pacific stock, in general, winters around the Hawaiian Islands (some go to Mexico) and migrates to northern British Columbia/Southeast Alaska and Prince William Sound west to Kodiak . Three feeding areas for the Central North Pacific stock have been studied using photo-identification techniques; these include southeastern Alaska, Prince William Sound and Kodiak Island . There has been some exchange of individual whales between these locations, although the aggregation in southeastern Alaska seems to remain relatively isolated from other groups . The current total estimated abundance for this stock is 4,005 individuals . The abundance of the Prince William Sound feeding aggregation is thought to be fewer than 200 whales . In the Kodiak region, 127 individual whales were identified between 1991 and 1994 and abundance was estimated to be 651 (95 percent CI: 356-1,523) . The number of animals in the Southeast Alaska aggregation is thought to have increased . The 2000 estimate of 961 is substantially higher than estimates from the 1980s, which put numbers in the high 300s . In a 2004 report, an annual population rate of increase was calculated to be 10 percent . Another study, based on aerial surveys conducted across the main Hawaiian Islands, and designed specifically to estimate trends in the Central Pacific Stock, found an annual increase of 7 percent from 1993 to 2000 . In 2006, the Southeast Alaska population reached 1,115 .
The western North Pacific stock is the least studied of the Northern Pacific populations . This aggregation winters off Japan and probably migrates to waters west of the Kodiak Archipelago (the Bering Sea and Aleutian Islands) in summer/fall . Recent surveys in the central-eastern and southeastern Bering Sea in 1999 and 2000 resulted in humpback whale sightings suggesting that the Bering Sea is an important feeding area . New information indicates that humpback whales from the western and Central North Pacific stocks mix on summer feeding grounds in the central Gulf of Alaska and perhaps the Bering Sea . A major research effort (the SPLASH project) was initiated in 2004 in order to better delineate stock structure of humpback whales in the North Pacific . There are no reliable estimates for the abundance of humpback whales in the western Pacific stock because surveys of the known feeding areas are incomplete, and not all feeding areas are known .
The 2010 estimate of abundance of humpback whales in the entire North Pacific Basin based on a Chapman-Petersen estimate is 21,808 (CV=0.04) .
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