Ecology of the Northeast US Continental Shelf
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Protected Species: Seals
Seal species are mainly distributed in cooler shelf waters of the Northeast U.S. Continental Shelf Large Marine Ecosystem (NES LME), rarely extending south of New Jersey. Harbor seals are the most widely distributed species in coastal waters of the NES LME. A general southward movement from the Bay of Fundy to southern New England waters occurs in autumn and early winter. A northward movement from southern New England to Maine and eastern Canada occurs prior to the pupping season, which takes place from mid-May through June along the Maine coast. Historically, pupping occurred as far south as Cape Cod in the early part of the twentieth century. Seasonally, gray seals (Figure 1) can be found in U.S. coastal waters between Maine and New Jersey, however, the population is centered in eastern Nantucket Sound. The ice seals (harp and hooded), which breed and pup on ice in more northern Canadian waters have extended their non-breeding range south into NES LME waters in recent decades.
Seals are carnivores, eating primarily fish and invertebrates, which are captured by using their well-developed eyes and ears. The pinniped reproductive system is annual, seasonal, and synchronous, which includes delayed implantation. Pinnipeds give birth on land or ice and timing of birth varies among species. Northwest Atlantic gray and harbor seal populations give birth in winter and spring, respectively.
North Atlantic seal populations were similarly over-exploited, but mainly in Canadian and more northern waters. Large scale hunting targeted harp and hooded seals for the fur trade, but gray and harbor seals were also killed. During the 1960’s, killing methods raised public opinion against sealing and initiated management schemes and quota systems.
Long-standing conflicts between humans and marine mammals, particularly seals, relate to marine mammal ecological impacts on economically valuable fishery resources. Most marine mammals are apex predators, removing tons of prey from the ecosystem. Some estimates equate the level of removal to equal that or exceeded by fisheries. Gray, and to a lesser extent harbor seals, also are definitive hosts of the seal worm, which infects many North Atlantic fish species. These issues initiated seal bounty programs in Europe and North America that resulted in regional extirpation (e.g., northeast U.S., Baltic Sea) of gray and harbor seal populations. Although bounty programs have either ended (i.e., US waters) or been greatly reduced, ecological interactions between harbor, gray, and harp seals and fisheries remain an important management issue in the North Atlantic.
In the following, we provide more detailed information on the harbor seal, the most numerous of the seal species in New England.
Harbor Seal (Phoca vitulina)
The harbor seal Phoca vitulina (Figure 3) is a common species of seal that is distributed in the cold temperate waters of the North Atlantic and North Pacific. Males and females are not sexually dimorphic, males are only slightly larger than females, and therefore, it is difficult to tell the sexes apart. The color of harbor seals is variable, light grey to dark brown with various black spots. Males and females are approximately 1.5 m (4.9 ft) and 100 kg (220 lb) and the maximum lifespan is about 35 years.
In U.S. waters, harbor seals are protected under the Marine Mammal Protection Act (MMPA) of 1972. In Canadian waters this species is managed through the Marine Mammal Regulations of 1993.
Distribution and Stock Structure
In the western North Atlantic, harbor seals are distributed from the eastern Canadian Arctic and Greenland south to southern New England and New York, and occasionally to the Carolinas. Current evidence suggests that population or management units are on the scale of a few hundred kilometers. Although the stock structure of the western North Atlantic population is unknown, it is thought that harbor seals found along the eastern U.S. and Canadian coasts represent one population. In U.S. waters breeding and pupping normally occurs in waters north of the New Hampshire/Maine border, although breeding occurred as far south as Cape Cod in the early part of the twentieth century. No pupping areas have been identified in southern New England, but recent anecdotal information suggests that pupping is occurring off Manomet, Massachusetts.
Harbor seals are year-round inhabitants of the coastal waters of eastern Canada and Maine, and occur seasonally along the southern New England and New York coasts from September through late May. In recent years, their seasonal interval along the southern New England to New Jersey coasts has increased. Scattered sightings and strandings have been recorded as far south as North Carolina. A general southward movement from the Bay of Fundy to southern New England waters occurs in autumn and early winter. A northward movement from southern New England to Maine and eastern Canada occurs prior to the pupping season, which takes place from mid-May through June along the Maine Coast. The overall geographic range throughout coastal New England has not changed significantly during the last century.
Since passage of the MMPA in 1972, the observed count of seals along the New England coast has been increasing. Five aerial surveys along the entire Maine coast have been conducted in May/June during the pupping season Uncorrected counts (not adjusted for the proportion of seals not hauled out on land) are provided in Figure 4. A corrected estimate for the 2001 survey based on replicate surveys and radio tagged seals was 99,340 individuals, compared with an estimate of 38,011 individuals based on the unadjusted counts. The average increase in uncorrected counts over the 1993-2001 survey periods (e.g., 1993, 1997 and 2001) has been 3.2%. Since the 1981 and 1986 surveys were conducted in late June, after peak pupping, they were not used in determining the population trend. The 2001 observed count of 38,011 is 28.7% greater than the 1997 count. Increased abundance of seals in the NES LME has also been documented during aerial and boat surveys of overwintering haul-out sites from the Maine/New Hampshire border to eastern Long Island and New Jersey.
Behavior and Life History
Harbor seals use terrestrial habitat “haul-out sites” throughout the year, particularly during the pupping and molting periods. In northern New England, they typically haul-out on tidal ledges. In the eastern portion of Nantucket Sound and on the outer portion of Cape Cod there is a preference for sand beach/shoal haul-out sites. Rock and sand habitats are used south of Cape Cod. Haul-out behavior is strongly influenced by environmental conditions including: tide stage, air temperature, time of day, wind speed, and precipitation. Human disturbance can also affect haul-out behavior although harbor seals appear to acclimate to some human activity (e.g., lobster boats along the coast of Maine).
In eastern Canada, adult females first give birth between the ages of 4-6 years. Newborn pups weigh between 10 to 11 kg. Pupping occurs along the coast of Maine and eastern Canada, principally, from mid May to late June, and pupping sites in Maine are geographically clustered into patches. Newborn pups are capable of swimming and do so in response to tidal changes and disturbance. Pups are weaned at 4 to 6 weeks of age.
Mating occurs in the water after the pups are weaned. Like all pinnipeds, following fertilization the implantation of the embryo is delayed about 2.5 months. The total gestation period is about 10.5 months. Molting (i.e., annual shedding and regeneration of hair) is a common feature of members of all seals. Except new borns, harbor seals molt about 2 to 3 months following the pupping season. During this period, higher numbers of seals are usually hauled-out.
Harbor seals are the most abundant large predator in the NES LME. Seals, perhaps even more so than other marine mammals, are perceived as important predators of economically valuable fishery resources. Harbor seals are opportunistic predators and feed on a wide variety of fish and crustaceans. In New England, regional and seasonal differences in harbor seal diet have been noted based on analysis of scats collected on sandy beaches and stomachs from fishery by-caught animals. In general, the most numerous prey species in these samples were sandlance, silver hake, Atlantic herring, and redfish. Other species included cod, haddock, pollock, flounders, mackerel, and squid. Spatial and temporal differences in harbor seal diet have also been recorded.
Harbor seal populations are impacted by anthropogenic and natural sources. Disease, predation, pollutants, and abandonment (pups) are natural causes of mortality. Virus epizootic events killed thousands of harbor seals in northern Europe in 1988 and again in 2002. While similar mass mortalities have not been recorded in the western North Atlantic, an influenza outbreak in New England waters killed an estimated 500 harbor seals in 1980. Shark predation has been documented as an important source of natural mortality at Sable Island, Nova Scotia. Some stranded seals in New England waters have shown signs of shark predation, but the significance of this source of mortality is unknown. Human sources of mortality include hunting (historical), fishery bycatch, boat strikes, oil spills, illegal shooting, and pollution. Harbor seals were bounty hunted in New England waters until the mid-1960’s, which may have caused the demise of this stock in U.S. waters. Annually, nearly 1,000 animals are incidentally killed in commercial fishing operations in the NES LME. Seals are top predators, therefore they accumulate high levels of pollutants that accumulate in aquatic food chains. Contaminants are known to suppress immune systems and cause reproductive failure.
Current levels of human-caused and natural mortality are likely biologically insignificant, since the population is increasing. The 99,340 seals (or 75,618 non-pups) in New England is one of the largest harbor seal populations. Conflicts between seals and humans, however, will likely increase due to overlapping use of coastal habitats. Seals use terrestrial habitat during resting, pupping, and molting periods. Seals are easily disturbed, and therefore human activity could displace seals from preferred haul-out sites. In the coastal environment seals are subject to injury and/or mortality from boat strikes, entanglement in plastic debris, and oil spills. Intentional shooting of seals has also been documented, but the significance of this activity is unknown.
Waring, G.T and D.L. Palka. 2002. North Atlantic marine mammals. In: Perrin, WE; Wursig, B; Thewissen, JGM, eds. Encyclopedia of Marine Mammals. New York, NY: Academic Press; p. 802-805.
Waring, G.T., E. Josephson, K. Maze-Foley, and P.E. Rosel. 2009. U.S. Atlantic and Gulf of Mexico Marine Mammal Stock Assessments -- 2009. Technical Memorandum NMFS-NE-213