Several seal species increase in numbers in Cape and Island waters
Four species of seals can be found seasonally in waters around the Cape and Islands. They are harbor seals (Phoca vitulina), gray seals (Halichoerus grypus), harp seals (Pagophilus groenlandicus), and hooded seals (Cystophora cristata). The number of species present and the number of individuals are highest from late autumn through early spring.
Harbor and gray seals are the most common species, numbering in the thousands, while harp and hooded or "ice seals" number less than 100 per year. Ice seals in Nantucket Sound are usually solitary and are considered at the southern part of their range. When talking about seals in Nantucket Sound and adjacent waters, most people are referring to harbor and/or gray seals.
Harbor seals and gray seals can be seen resting or hauled-out on rock piles, ledges, sand bars, and jetties exposed during low tides. They also rest on isolated beaches, in marshes, and on floating docks at all tide stages.
Male and female harbor seals look similar, with a maximum length of about 5 feet and weight of about 220 pounds. Their coat (pelage) ranges from light gray to brown with spots and marks.
Gray seals exhibit a striking sexual difference in coat color and size. The male coat is dark brown, gray or black with lighter blotches, while the female coat is lighter in color with dark spots. Males grow to 7.5 feet and 775 pounds, while females grow to 6.6 feet and 450 pounds. The male's head shape is very pronounced and is commonly referred to as "horse head."
The historical abundance and distribution of seals in New England are largely unknown. Archaeological records reveal that Native Americans hunted both harbor and gray seals along the New England coast. European colonists viewed seals as competitors for fishery resources and hunted seals for the fur trade.
In the late 1800s, Maine and Massachusetts enacted non-species specific seal bounty programs that also contributed to local depletion of seal populations. The Maine bounty program was rescinded in 1905, the Massachusetts bounty program in 1962. Passage of the U.S. Marine Mammal Protection Act in 1972 provided additional protection that allowed seal populations to flourish and expand their range into historic habitats.
Photo courtesy resortaerialphotos.com
Two important differences in the birthing and infancy of harbor and gray seals may help explain the likely differences of the bounty programs on these species. Harbor seals have their pups in late spring on scattered tidal ledges and small isolated treeless islands, particularly along the coast of Maine. Mother/pup pairs may enter the water immediately following birth, and mothers nurse their pups for 25 to 30 days. Harbor seals are extremely skittish and move rapidly into the water when disturbed.
Gray seals pup in winter at a few sites referred to as colonies, resulting in higher pup densities at these locations. Mothers stay ashore and nurse pups for about 16 days, then the pups remain on the colony for two to three weeks until they shed their creamy white birth coat and grow a darker one. Pups are generally loosely associated into small groups during the post-weaning phase. As a result, gray seal mothers and pups are susceptible to human actions during nursing and the post-weaning period.
Photos courtesy of Gordon Waring
The first aerial surveys of seals in the Cape Cod region, from Race Point to Nomans Land and the Elizabeth Islands were conducted in winter and spring in the early 1980s by the Manomet Bird Observatory. These surveys showed an average count of around 1,600 harbor seals, with the largest aggregations at Monomoy National Wildlife Refuge (MNWR). Twenty to thirty gray seals were also recorded in Nantucket Sound between Wasque Shoals and MNWR. The gray seal sightings were surprising, since the seal was considered to be a rare species in New England waters.
In her studies on Muskeget Island, the late naturalist Valerie Rough reported that some of the gray seals were marked by brands indicating they had been born on Sable Island, and that gray seals were again pupping on Muskeget Island. These observations provided the first evidence that "Canadian" gray seals were recolonizing historic Nantucket Sound habitats. Rough's research through the late 1990s documented the rapid increase and year-round presence of gray seals in Nantucket Sound.
Subsequently, researchers and graduate students at the Northeast Fisheries Science Center (NEFSC) headquartered in Woods Hole began more intensive studies of seals in the Cape Cod region. Genetic analyses have shown that gray seals pupping in Nantucket Sound are closely related to the Canadian gray seals.
Population assessments have shown that gray seals are the most numerous species in Nantucket Sound, and that Muskeget Island is the largest U.S. pupping site. Gray seals are also using habitat that was previously used by harbor seals. Diet analyses indicate that both gray seals and harbor seals have a very diverse diet.
Since the late 1990s, NEFSC has been conducting intermittent monthly monitoring surveys from autumn to spring between Plymouth and Nomans Land. Counts of seals on land confirm the increased abundance and expanding distribution of both harbor and gray seals.
Gray seal pup counts on Muskeget Island increased from 0 in 1989 to 1,000 in 2002 with 5,600 gray seals excluding pups counted on Muskeget Island and Monomoy in March 1999. For comparison, in March 1986 and 1999, respectively, 3,400 and 3,900 harbor seals were counted around the Cape and Islands.
Coastal storms alter access to "sandy" haul-out sites, particularly around the outer portion of Cape Cod, and in Nantucket Sound. The resulting seasonal shifts in seal haul out sites may give the impression of a sudden appearance of large numbers of seals at specific sites, such as around Wasque Shoals.
The year-round presence of gray seals and the seasonal presence of harbor seals in Nantucket Sound and adjacent waters have led to increased positive and negative seal/human interactions. While some people worry about competition with commercial/recreational fishing or the possibility that seal populations will attract sharks into coastal areas, other people enjoy "seal watching" and view the return of seals to Nantucket Sound as a positive event
Gordon T. Waring is a research fisheries biologist in the protected species branch at the Northeast Fisheries Science Center, Woods Hole. He has been involved with seal research in New England since the early 1990s.
Stephanie Wood is a doctoral student at the University of Massachusetts in Boston and has worked with the NMFS/NEFSC on gray seals for several years.
For general reference information, Mr. Waring recommends: Seals of the Atlantic Canada and Northeastern United States, by Janice Hannah (1998), International Marine Mammal Association, Inc, and A Field Guide to Whales, Porpoises, and Seals by Katona et al. (1993), Smithsonian Institution Press.