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Volume 38 Issue 6 • June 5 - 11, 2008
now in our 38th season

Offshore Labradors

by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station

The waters surrounding Nantucket, Tuckernuck, and Muskeget support several species of marine mammals from Right Whales to common and bottlenose dolphins to the southernmost colony of Atlantic Gray Seal. The Latin name for gray seals is Halichoerus grypus, or "hooked-nosed sea pig"—not the nicest of monikers, but a somewhat accurate description of our large marine relatives. Gray seals are large seals that are from the family Phocidae or "true seals." The “true seals” or earless seals are one of the three main groups of mammals within the seal suborder, Pinnipedia. The true seals are sometimes called crawling seals to distinguish them from the fur seals and sea lions of family Otariidae. Phocids also include hooded, harbor, and harp seals in addition to gray seals.

True seals are more highly specialized for aquatic life than otariids, although they still return to dry land or pack ice in order to breed and give birth. They lack external ears and have sleek, streamlined bodies. A healthy gray seal will look like a very fat sausage. A smooth and thick layer of blubber lies underneath the skin, and true seals are able to divert blood-flow to this layer to help control their temperature. Their fore-flippers are used primarily for steering, while their hind flippers are bound to the pelvis in such a way that they cannot bring them under their body to walk on them. Phocids swim by sideways movements of their bodies, using their hind-flippers to their fullest effect. Although their streamlined bodies allow true seals to swim more effectively over long distances, they are very clumsy on land, having to wriggle with their front flippers and abdominal muscles. Their tracks on our sandy beaches are very easy to identify due to this undulating motion with a long groove or sled mark in the center and front flipper tracks on either side. You’ll often see them sunning themselves on haul out beaches such as Great Point, Codfish Park, or Surfside. They often will curl up into a U-shape with their tails and heads lifted to cool off.


Female seal and pup on Muskeget in February.
Photo by Dr. Sarah Oktay

Phocid respiratory and circulatory systems are adapted to allow diving to considerable depths, and they can spend a long time underwater between breaths. Air is forced from the lungs during a dive and into the upper respiratory passages, where gases cannot easily be absorbed into the bloodstream. This helps protect the seal from the bends. The middle ear is also lined with blood sinuses that inflate during diving, helping to maintain a constant pressure. True seals do not communicate by "barking" like otariids. Instead, they communicate by slapping the water and grunting. Gray seals when threatened will make an alarming hissing noise.

Gray seals are found on both sides of the North Atlantic Ocean. Each side of the Atlantic hosts a subspecies of the gray seal. Halichoerus grypus grypus (yes, it sounds redundant) from the western North Atlantic east, including Greenland and Iceland, to western Russian, south to the British Isles and France; and Halichoerus grypus macrorhynchus from the Baltic Sea. The western North Atlantic stock of gray seals ranges from Labrador, Canada, to Long Island, N.Y., with occasional strandings as far south as Virginia. The population center is in eastern Canada on Sable Island, Nova Scotia, which accounts for over one-half of the population, and in the southern Gulf of St. Lawrence. Researchers such as Valerie Rough, Clinton Andrews, Jim Gilbert, Steve Katona, and Stephanie Wood have documented populations on Monomoy Island, Muskeget and Penobscot Bay, Maine as they have slowly developed in size over the past 50 years.

Locally, our resident seal populations are located at Monomoy (off Chatham) and Muskeget Island, which are the primary haul-out sites for gray and harbor seals.

Gray seal populations plummeted from the colonial era to the 1950s due to hunting and extermination programs by people who believed that the seals were out-competing them for cod. By 1950, the numbers of gray seals south of the Canadian Maritimes were less than 50 individuals and they were effectively considered extirpated.  A bounty on seals of five dollars per nose caused a decline of all seals in Massachusetts waters.  By the beginning of the 1960s, this had reduced the number of gray seals to about fifteen.  Then, in 1962, the state outlawed the killing of seals, and since 1972 they have been under federal protection under the Marine Mammal Protection Act, which makes it a federal crime to kill, harass, or disturb a marine mammal by approaching within 150 feet or doing any action that changes its behavior. Due to this protection, approximately 4000-6000 seals have been recorded around Nantucket in the past three years with 1892 pups born in 2005 on Muskeget. The entire Atlantic population including those in Canada has reached levels of around 130,000-160,000 individuals.

There is a great variety in gray seal coat coloration and shading. Males tend however to have a dark brown-grey coat, sometimes nearly black, with a few light patches, while females are generally light grey-tan colored, lighter on the front, with dark spots and patches. Adult males, and some older adult females to a lesser extent, have a long nose with wide nostrils, giving the species its name "horsehead” seal. As the title of this article implies, they also look like large Labradors when seen in the water. In fact, gray seals are in the Order Carnivora and suborder Caniformia (“dog like” carnivores).

Grey seals are sexually dimorphic in size, which means their size differs greatly between the females and males.  Males grow to 8 feet and weigh about 800 pounds, while females are smaller, growing to 7 feet and weighing about 400 pounds. Female grey seals are estimated to live to at least their mid-30s. Males are estimated to reach only their mid-20s, which may be a result of the fierce competition between males for mating rights with the females. Males are polygynous (“many females”) mates, hence the lack of flowers and sweets on Muskeget on Valentine’s Day. Males do not defend territories or herd females.  They actively compete for access to females using vocalizations, threat gestures, and occasional fighting.

Colonies form a rookery and deliver the pups in January or February. During the nursing period of 21-28 days, neither the dominant males nor lactating females feed. Newborn pups average three feet in length and weigh about 35 pounds. Pups are born with a creamy white coat called lanugo (a remnant of their ancestral association with ice), which is shed shortly before the mother leaves them at about three-five weeks of age. Unlike the harbor seal, grey seal pups are rather helpless, staying on land in the birth area while the mother attends to them. The mother will leave the pup to feed, and upon return will recognize her offspring by its unique smell. Mothers are fiercely protective of their young. Once they wean, the pups live off blubber reserves and go to sea at about five to eight weeks of age. At this time, they are on their own.

Because true seals like the grey seal are such good swimmers, they will forage relatively long distances and the mothers will build up a large fat layer before having their pups. They also produce a very fat-rich milk that will sustain the pups until they are able to forage on their own. Phocid researchers often will evaluate the fat content of seal milk and the fat content of seal tissues in order to evaluate the health of various populations. These figures are combined with diet surveys to determine the fishing effort and prey species available. When fish stocks become depleted or oceanographic conditions push prey such as shad and herring further from shore, each meal requires more energy to acquire.

Marine mammal scientists Valerie Rough and Kristin Ampela, among others, have established gray seal diets by examining and sorting scat (excrement) samples collected on Muskeget and at other shoals and breeding colonies. This is a common practice as is the examination of stomachs from recently deceased seals. Obviously, this type of research is not the number one chosen by graduate students. Scat sampling usually will give better results as most dead seals on land have suffered some amount of starvation prior to their fate. The prey typically consumed by gray seals includes windowpane flounder, silver hake, sand lance, skates, and gadid species such as cod, haddock, and pollock. These prey species are confirmed in research done on the Nova Scotia populations, in which researchers also found capelin, flatfish, Atlantic herring, Atlantic mackerel, and squid on the menu. Seals swallow their meals whole. The size of the fish consumed is 30 cm or less. None of the food habit studies to date have shown that there are lobsters or scallops in stomachs or scat. Occasionally seals will raid bait from lobster pots and mangle gear as they search for their bottom dwelling ground-fish targets.

Seal populations are being impacted by human activities. From boat collisions to entanglements in marine debris, the growing numbers of seals causes an increase in encounters with humans and associated impacts. Visitors to Nantucket Sound and the area around Nantucket are urged to keep their distance from resting seals (stay at least 150 feet away) and to operate their boats safely and slowly when seals are present in the water. 

Members of the Nantucket Marine Mammal Stranding Team are the local experts in seal strandings, disentanglements, and citizen science observation of seals. They respond to calls placed to the police station concerning stranded marine mammals. Within a short amount of time, an incident commander is tasked to start calling volunteers to go out to the stranding site and monitor the stranded animal. Many times, a stranded seal is simply hauling itself out to sun and bask on the shore. Even pups or juveniles will do this frequently, although kind hearted people are not always aware of the fact that the mothers are long gone and that the pups are responsible for feeding themselves. If you see a seal that appears to be entangled in fishing gear or cut, or in trouble, please call the Nantucket Police Station at 508-228-1212 and they will contact the Marine Department and the Marine Mammal Stranding Team. Don’t forget to stay away from the seal and keep your pets and children away from them too. They can inflict a serious bite and although cute, they are wild animals. We are always looking for more volunteers, to join the Stranding, email me at sarah.oktay@umb.edu.
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