Gray Seals and People, why can’t we just get along?
• by Dr. Sarah D. Oktay, Managing Director UMass Boston Nantucket Field Station •
Happy summer 2013! Currently the island is celebrating Memorial Day weekend with parades, BBQ’s, tributes to our war veterans, and cases and cases of Bud Light for the Figawi race/party. Our story today focuses on the biology of a marine mammal whose recovery after the introduction of the Marine Mammal Protection Act and the cessation of bounties and hunting have allowed their numbers to increase to the point where human and seal interactions are becoming numerous. Gray seals have been frustrating some beach goers and fishermen while their increasing numbers herald a success story for a species virtually driven to local extinction in the U.S. These human and animal conflicts are similar to those occurring across America as a result of rebounding gray wolf, grizzly bear, American bison, and mountain lion populations (http://www.nwf.org/wildlife/wildlife-conservation/endangered-species-act.aspx retrieved May 27th, 2013.)
The Latin name for gray seals is Halichoerus grypus, or “hooked-nosed sea pig”—not the nicest of names, 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 word Pinniped comes from the New Latin word pinnipēs which is derived from Latin “pinna” for feather, wing, or fin + “pēs” for foot. Phocids also include hooded, harbor, and harp seals in addition to gray seals. The true seals are sometimes called “crawling seals” to distinguish them from the fur seals and sea lions of the family Otariidae.
True seals are more highly specialized for aquatic life than otariids, although they still must return to dry land or pack ice in order to breed and give birth (this will become important later when we discuss how populations numbers are determined). 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 sometimes will curl up into a U-shape with their tails and heads lifted to cool off.
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 that can scare the beans out of you if you are not expecting it.
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, Gordon Waring, Solange Brault, and Stephanie Wood have documented populations on Monomoy, Nomans and Muskeget Islands in Massachusetts and Green and Seal Islands in Maine as they have slowly increased 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 to a lesser extent, 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 reduced to 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 in a way that changes its behavior. Due to this protection, approximately 4000-6000 seals have been recorded around Nantucket in the past few years with 1892 pups born in 2005 on Muskeget. More data on stock numbers and counts is available at http://www.nmfs.noaa.gov/pr/pdfs/sars/ao2012segr-wn.pdf which states that 2095 seal pups were born on Muskeget in 2008 (the most recent year of completed tallies). The National Marine Fisheries Service states that the total Canadian gray seal population was estimated to be 348,999. The growth of seal populations in Canada is slowing from a high of close to 13% per year to a current estimate of 7% or less. Seal counts are done using planes and boats with volunteers visiting haul out sites to visually count seal pups and adults in order to “ground-truth” the aerial numbers. The most recent 2011 one day count of seals in the area (all nearby island and Monomoy and Great Point combined) counted 15,000 seals hauled out.
There is a great variety in gray seal coat coloration and shading. Males tend to have a dark brown-gray 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. They also look like large Labrador retrievers 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. Each female gray seal will have one pup per year. Females reach sexual maturity at 3-5 years, males at 4-6 years, although males may not attain territorial status until 8-10 years of age. Females usually give birth at the rookery about a day after coming ashore and they nurse the pups for about 17-18 days during which time pups gain between 1.2-2 kg each day. Newborn pups measure about 90-105 cm in length and weigh between 10-18 kg (or approximately three feet in length and weigh between 22-40 pounds). Towards the end of the nursing period, the mother mates with one or more males (which is pretty darn nuts when you think about it, but does insure everyone is all together for this activity). During the nursing period, neither the dominant males nor lactating females feed. 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. Gray seals are physically only able to reproduce at a rate of 12% percent once mortality is factored into the equation. Up to 30-50% of pups will die in the first year. Gray seal numbers are increasing at Muskeget at a higher rate that is closer to 20% because seals are immigrating down from Canadian waters to join our local colony (probably the Nantucket cachet).
True seals like the gray 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 and chemical analysis of fatty acids in the tissue of the seals to gather information on how high up the food chain seals are feeding and the fishing effort expended 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. Research on a tagged female gray seal known as Stephanie presented at the May 20th seal symposium showed that these seals will travel all the way out to the edge of the shelf to the Georges Bank to feed. You can follow this tagged seals travels at http://whale.wheelock.edu/whalenet-stuff/Stop39393/ and follow many more at http://whale.wheelock.edu/whalenet-stuff/stop_cover_archive.
Marine mammal scientists such as Dr. Kristin Ampela, who has presented her research at past Nantucket Biodiversity Initiative conferences on island, have established gray seal diets by examining and sorting scat (excrement) samples collected on Muskeget and at other shoals and breeding colonies in North America. This is a common practice as is the examination of stomachs from recently deceased seals caught in fishing gear. Obviously, this type of research is not the number one choice of graduate students. Fish otiliths, essentially fish ear “bones” grow in size and form rings to indicate the number of months and years the fish have been alive. These are found in seal poop and stomachs (and many other fish and shark stomachs) and are the clues that Ampela used to determine dietary sources. Each fish otilith is relatively unique to that type of fish. She also measured any hard parts found such as vertebrate or teeth. These types of surveys are more definitive than looking at the chemical composition of the blubber, although they can miss the occasional partially eaten prey. She sampled across all seasons in order to pick up variations in seal diet. The prey typically consumed by gray seals includes windowpane flounder, silver hake, sand lance, skates, and to a lesser extent, gadid species such as cod, haddock, and pollock. Sandlance (Ammodytes spp.) and red/white hake (Urophycis spp.) together accounted for 65% of prey individuals recovered. Skate (family Rajidae) was recovered most frequently, in 38% of samples. Cod (Gadus morhua) was recovered infrequently and comprised fewer than 2% of total individuals. Significantly more skate was recovered in fall than in other seasons (from her paper found at http://www.ices.dk/products/CMdocs/2006/L/L1406.pdf) . Her entire thesis is available at http://udini.proquest.com/view/the-diet-and-foraging-ecology-of-pqid:1890271261/. Other scientists have analyzed how these hard parts travel through the guts of captive seals (“Robust digestion and passage rate estimates for hard parts of grey seal (Halichoerus grypus) prey” by Kate Grellier, Philip S Hammond; Canadian Journal of Fisheries and Aquatic Sciences, 2006, 63:1982-1998, 10.1139/f06-092), yes, they feed the seals and then watch them poop and see what survives the trip and how long it takes! Good times. 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. These researchers have determine that most of the time, gray seals swallow their meals whole and that the size of the fish consumed is normally 30 cm or less. None of the food and diet studies to date have shown that there are lobsters in stomachs or scat (http://whale.wheelock.edu/whalenet-stuff/seal_factsheet3.html retrieved May 27th, 2013) although anecdotal evidence from swimmers and fishermen indicate some of them have observed gray seals infrequently eating small lobsters whole. Occasionally seals will raid bait from lobster pots and mangle gear as they search for their bottom dwelling ground-fish targets and they have been known to once in a great while eat seabirds.
Seals do not eat nearly as much food as one might think, they are opportunistic feeders that consume between 4% and 6% of their body weight per day (http://www.nmfs.noaa.gov/pr/species/mammals/pinnipeds/grayseal.htm retrieved May 27th, 2013). Seals do not feed every day and females fast during the breeding season. According to the Massachusetts Division of Marine Species and this website http://www.stripers247.com/Massachusetts-Striper-Profile.php, stripers and seals do depend on some of the same food species for their diets like sand lance and menhaden. That site also does a good job of synthesizing the ups and downs of the striper population along the entire Atlantic coast over the past hundred years. Years of extremely low populations of stripers were followed by rebounds and the primary drivers of these occurrences in the Chesapeake Bay area were the presence or absence of a variety of pollutants in spawning grounds, fishing pressure, and feeding and nutritional problems of larvae. The data for these fluctuations goes back over a hundred years to cover times in which the gray seals were in greater abundance. It is important to remember that stripers are migratory and influenced by environmental factors, fishing pressure, and management up and down the East Coast.
Seal populations are being impacted by human activities. From boat collisions to entanglements in marine debris, the growing numbers of seals results in 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.
And last but not least, the growing number of seals in Massachusetts has resulted in increased sightings of great white sharks which prey on the seals. Dr. Greg Skomal has been on island a couple of times this spring and many times in the past few years to discuss his successful great white shark tagging program and describe the seal/shark relationship (http://www.mass.gov/dfwele/dmf/spotlight/white_shark_2010.htm retrieved May 27th, 2013). Hopefully it makes you feel better to know that only three fatal shark attacks have been documented in the past three hundred years in Massachusetts, the last one occurred in 1936 (http://en.wikipedia.org/wiki/List_of_fatal,_unprovoked_shark_attacks_in_the_United_States) retrieved May 27th, 2013). New Smyrna Beach, located in Volusia County Florida has the most shark attacks in the U.S. mainly due to their being more swimmers there than elsewhere, not more sharks.
Stephanie Wood and Gordon Waring of the Northeast Fisheries Science Center (NEFSC) in Woods Hole, Massachusetts are using seal cams on learn more about pupping and pup mortality. Their initial work is described at http://www.nefsc.noaa.gov/news/features/seal_cam/ and you can see them in action (the seals, not Stephanie and Gordon) at http://explore.org/#!/live-cams/player/seal-pups-cam. Enjoy!
Portions of this article were previously published in Yesterday’s island in June 2008, August 2010, and June 2011;
http://www.yesterdaysisland.com/2008/features/labradors.php
http://www.yesterdaysisland.com/2010/science/16.php
http://www.yesterdaysisland.com/2011/science/6.php