Ecology of the Northeast US Continental Shelf
Protected Species: Whales and Dolphins
Marine mammals are a diverse, widespread, and significant component of Northeast U.S. Continental Shelf Large Marine Ecosystem (NES LME). Thirty-three species of cetaceans, including dolphins (Figure 1) and whales regularly inhabit this area. They occupy all pelagic marine environments, from the Gulf Stream to coastal waters, although species-specific ranges exist and their distribution is not uniform. Large-scale and non-random distribution of cetaceans is influenced by oceanographic features and migration patterns, while small-scale distribution is influenced by factors such as the animal's physiology, behavior and ecology.
The physical characteristics of the NES LME and shelf-break ecosystem are important factors that influence marine mammal distribution (Figure 2). Baleen whales are widely distributed off the northeast US coast, although all species exhibit preferences for certain ecosystems. For example, right, humpback, and minke whales prefer continental shelf ecosystems, blue and sei whales are associated with shelf-edge and deeper oceanic waters, while fin whales are found in both the shelf and shelf-edge waters. Large whales, however, are highly mobile and seasonally may occupy a variety of habitats. Baleen whales undergo the most extensive seasonal migrations of all North Atlantic marine mammals, moving between warm, low latitude breeding grounds in winter and cold, high latitude feeding grounds in summer.
Likewise, the toothed whales and dolphins (odontocetes) occupy nearly all marine habitats off the NES LME, although many species exhibit a preference for particular water masses. Continental shelf species found in cool temperate waters are harbor porpoise; Atlantic white-sided and white-beaked dolphins; and killer and long-finned pilot whales. Continental shelf break/pelagic species that are found in temperate to cooler waters include: bottlenose (offshore and coastal forms); common; Risso’s, striped; and Atlantic spotted (coastal form) dolphins; sperm; northern bottlenose; Cuvier’s; Blainville’s; Sowerby’s; and True’s beaked whales. Continental shelf break/pelagic species more often found in warm, temperate to tropical waters are: pantropical spotted; Atlantic spotted (offshore form), long snouted spinner; Clymene; and Fraser’s dolphins; melon-headed; false killer; pygmy killer; short-finned pilot; pygmy sperm; dwarf sperm; and Gervais’ beaked whales. Within these broad water mass habitats, bottom topography and/or frontal boundaries are important characteristics that define cetacean distribution. Unlike baleen whales, only a few species (e.g., sperm and long-finned pilot whales) undergo long-range seasonal migrations.
The baleen whales strain planktonic or micro-nektonic crustaceans and/or relatively small pelagic fish by using sight or a less sophisticated acoustic scanning technique. The toothed whales capture fish, squid, and flesh of other species by hunting using sight in clear water, detecting noises made by their prey, or by using active echolocation. Sound is used by nearly all cetaceans for communication. Some species, such as humpback whales (Figure 3), are known for their songs used during breeding, while other species, such as bottlenose dolphins are known for their complex sound production used in social groups.
Reproduction in baleen whales is tightly linked to their seasonal migratory cycles. Breeding and calving occur in winter in low latitude waters, and ritualistic behaviors are commonly displayed. Gestation period is approximately 12 months, and breeding interval varies from annual (e.g., minke whale) to every 3 to 5 years (e.g., right whales). Seasonal reproductive cycles in odontocetes are not as rigid as the baleen whale model. In the North Atlantic, peak calving is usually in spring-summer. The gestation period (10-17 months) and calving interval (1-5 years) are very variable. Further, some odontocetes (e.g., pilot whales and sperm whales) have a strong social structure that can extend lactation for several years.
Centuries of human activities have adversely affected all North Atlantic cetacean populations with resulting changes in marine ecosystems. Prehistoric people hunted coastal marine mammals for subsistence use. Early subsistence hunting, however, was likely insignificant compared to commercial whaling, which began in Europe about the 10th century. By the beginning of the 18th century, American whalers depleted right whales in coastal waters off the American colonies. Depletion of these stocks initiated pelagic whaling for sperm and humpback whales in other oceans. Sperm whaling in North Atlantic waters ended by the early 1920s, whereas, baleen whaling continued into the mid 20th century. Commercial exploitation of the smaller cetaceans was confined to coastal areas and conducted on a smaller scale.
Following World War II, technological improvements in fishing gear and vessels led to an expansion of coastal and high seas fisheries. A major by product of these developments was both the incidental mortality of thousands of cetaceans and rapid depletion of their prey resources. By the 1970s, both the collapse of many North Atlantic fish stocks and levels of takes of cetaceans, particularly dolphins in the eastern tropical Pacific tuna purse seine fishery, helped initiate management and conservation measures (e.g., U.S. Marine Mammal Protection Act of 1972) for incidental takes of marine mammals. Over the past three decades, national and international measures have been taken to manage fisheries, (e.g., 1991 U.N. Resolution 46/125- ending high seas driftnet fisheries) and to monitor and reduce fishery related impacts on marine mammals.
Environmental contaminants introduced into aquatic ecosystems pose a serious threat to the health status of cetacean populations. Most cetaceans feed at high trophic levels, therefore accumulate toxins from eating contaminated prey. Numerous studies have documented the presence of organochlorine and heavy metal compounds in tissues of marine mammals. Biological effects of these contaminants include: compromised immune systes; reproductive disorders; and endocrine disruption. For example, organochlorine levels in Baltic and Wadden Sea seals, and St Lawrence Estuary beluga whales were linked to reproductive failures. Presently, the role of contaminants in tissues of the highly endangered North Atlantic right whale is one aspect of a multifaceted study on reproductive declines in that species. Several large-scale epizootic events in northern Europe, the Mediterranean, U.S. east coast, and Gulf of Mexico have killed thousands of seals and dolphins. Although high organochlorine levels were found in tissues of dead marine mammals, direct cause-and-effect has not been demonstrated. High levels of toxic metals (e.g., cadmium, mercury) have not been directly linked to marine mammal mortalities.. Effects due to oil spills are well publicized, whereas effects of acoustic disturbance (e.g., behavioral modification, prey displacement, and mortality) are recent research topics.
North Atlantic right whale (Eubalaena glacialis)
The North Atlantic right whale is the rarest of the large whales in this region. Right whales were considered to be the ‘right’ whale by whalers because of their high quality oil and their propensity to float after being struck. As a result, this species was severely depleted by whaling in the 17th century. It is currently estimated that there are about 400 North Atlantic right whales remaining. Right whales have recently been dubbed the ‘Urban Whale’ because their behavior and migratory pathways place them near coastal urban centers. Ship strikes and entanglement in fishing gear are the principal sources of mortality caused by human activities. Their proximity to both shipping channels (Figure 4) and major fishing grounds places them at great risk to these sources of injury and mortality.
Right whales attain lengths of up to 17m and can weigh over 65 tons. Females first give birth at about 9-10 years. Their calving rate is about half that of southern hemisphere right whales, and the mean interval between births increased from 3.67 years in 1980-1992 to over 5 years in the 1990s although it has since decreased. Steps being taken to reduce ship strikes and entanglement are critically important to reproductive success, but inadequate food resources may also help explain this population’s low reproduction. Right whales need very dense patches of copepods, principally the older life stages of the copepod C. finmarchicus, to feed efficiently. These patches are rare, even on the feeding grounds. Successful reproduction also requires successful weaning and post-weaning survival of the calf, which, in turn, is dependent on adequate food availability, and energy reserves, for the nursing mother. The number of right whale calves observed is strongly related to the number of adult Calanus copepods found in the previous year (Figure 5).
Long-Finned Pilot Whale (Globicephala melas)
There are two species of pilot whales (Figure 6) in the North Atlantic Ocean — the Atlantic or long-finned pilot whale (Globicephala melas) and the short-finned pilot whale (G. macrorhynchus). At sea, these species are difficult to identify to species, therefore some of the descriptive material below refers to Globicephala sp. Off the northeast U.S. coast, the boundary for these two species is likely between Delaware Bay and Chesapeake Bay with sightings north of this area being long-finned pilot whales.
Male and female pilot whales are sexually dimorphic, with males being larger than females, and having a larger, more prominent dorsal fin. Long-finned pilot whales are generally jet black or dark gray in color and are sometimes referred to as “Blackfish” or “Pothead” whales. Adults reach an average length of 6.0 m (19.5 feet). In U.S. waters, all cetaceans are protected under the Marine Mammal Protection Act (MMPA) (1972).
Stock Structure and Distribution
In the North Atlantic, the long-finned pilot whale is distributed from Cape Hatteras, North Carolina east to North Africa (and the Mediterranean) and north to Iceland, Greenland, and the Barents Sea. The stock structure of this population is uncertain. Recent studies have provided little support for any stock structure across the North Atlantic. However, a possible stock structure has been proposed that is linked to sea surface temperature: 1) a cold-water population west of the Labrador/North Atlantic current, and 2) a warm-water population that extends across the Atlantic in the Gulf Stream.
Pilot whales are distributed principally along the continental shelf edge in the winter and early spring off the northeast U.S. coast. In late spring, pilot whales move onto Georges Bank and into the Gulf of Maine and more northern waters, and remain in these areas through late autumn. In general, pilot whales occupy areas of high relief or submerged banks. They are also associated with the Gulf Stream north wall and thermal fronts along the continental shelf edge.
The best current (2004) estimate of abundance for pilot whales from the Bay of Fundy, New Brunswick, Canada to Florida is just over 31 thousand individuals.
Behavior and Life History
Pilot whales are found in small cohesive family groups, which can number from two or three animals to several hundred. Generally, all the animals in the group are swimming in the same direction, which can be spread out over 5 km (3 miles).
Life history parameters are similar to other large odontocetes. Calving interval is 3.3 years; lactation period is about 21-22 months; gestation period is 12 months; births mainly occur from June to November; length at birth is 1.8m; mean length at sexual maturity is 4.90m for males and 3.6m for females; age at sexual maturity is 12 years for males and 6 years for females; and maximum age is 40 for males and 50 for females.
The food habits of pilot whales of the NES LME have been determined by the analysis of stomach contents from both stranded and fishery bycaught pilot whales. Long-finned squid (Loligo pealei), short-finned squid (Ommastrephidae sp.), and Atlantic mackerel (Scomber scombrus) were the most numerous prey species.
Mortality - Fishery interactions
During 1977-1991, observers recorded 436 pilot whale mortalities in foreign-fishing activities. A total of 391 (90%) were taken in the mackerel fishery, and 41 (9%) occurred during Loligo and Illex squid-fishing operations. Due to temporal fishing restrictions, the bycatch occurred during winter/spring (December to May) in continental shelf and continental shelf edge waters. However, the majority of the takes occurred in late spring along the 100 m depth contour. Two animals were also caught in both the hake and tuna longline fisheries.
Since 1991, bycatch has been documented in a variety of domestic fisheries including: pelagic drift gillnet; pelagic longline; pelagic pair trawl; bluefin tuna purse seine; North Atlantic bottom trawl; Atlantic squid, mackerel, butterfish trawl; and mid-Atlantic coastal gillnet fisheries. In 2005 and 2006, respectively, the NMFS implemented the Atlantic Pelagic Longline and the Atlantic Trawl Take Reduction Teams (http://www.nmfs.noaa.gov/pr/interactions/trt/teams.htm) to develop mechanisms to reduce pilot whale bycatch in those fisheries.
Other Mortality - Strandings
Pilot whales have a propensity to mass strand throughout their range, but the role of human activity in these events is unknown. Between 2 and 120 pilot whales have stranded annually, either individually or in large groups, along the Northeast U.S. coast since 1980. From 1999 to 2003, 185 pilot whales have been reported stranded between Nova Scotia and Florida, including several mass strandings. Four of 6 animals from one live stranding event in Massachusetts in 2000 were rehabilitated and released. In addition, 11 pilot whales that live stranded on Nantucket Island (off Massachusetts) were returned to the water. However, certain studies have shown that it is likely that animals returned to the water swim away and strand somewhere else. Short-finned pilot whales have been reported stranded as far north as Nova Scotia (1990) and Block Island, Rhode Island (2001) while long-finned pilot whales have been reported stranded as far south as South Carolina.
A potential human-caused source of mortality is from polychlorinated biphenyls (PCBs) and chlorinated pesticides (DDT, DDE, dieldrin, etc.). Moderate levels of these substances have been found in pilot whale blubber. It has been reported that bioaccumulation levels were more similar in whales from the same standing group than animals of the same sex or age groups. Also, high levels of toxic metals (mercury, lead, and cadmium), selenium, and PCBs were measured in pilot whales harvested in the Faroe Islands drive fishery. The population effect of the observed levels of such contaminants is unknown.
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