Features: July-August 2017

Gray Seals and White Sharks Meet Anew With the return of nearly-extinct Gray Seals to New England waters come rarely-sighted White Sharks.

Robert S. Michelson

Photographs by Robert S. Michelson

From shell middens on Martha’s Vineyard Island, Massachusetts, the remains of 16 gray seals (Halichoerus grypus) have been identified, dating from approximately 2290 BCE to 1460 CE. And from a 5,000-year-old archaeological site on the island of North Haven, Penobscot Bay, Maine, gray seal remains date from approximately 2500 BCE through 1600 CE. Historically, gray seals were distributed along the Northeast coast of the United States and Canada through the seventeenth century. Both harbor (Phoca vitulina) and gray seals were hunted throughout this period. The number of seals hunted, including pups and immatures, increased over time, and seal hunting went from a seasonal to a year-round activity by the 1500s. From the late nineteenth century into the early twentieth century, the Commonwealth of Massachusetts and the State of Maine paid bounties on seals. Records show 15,690 seal bounties paid in Massachusetts and 24,831 paid in Maine during the time of each state’s bounty program. However, there are multiple accounts of fraudulent bounty claims from hunters who would divide one seal pelt into multiple counterfeit parts. Although the bounty records do not give specific, reliable information on the number and species of seals killed, they demonstrate that there was hunting pressure on seals in the Northeast U.S. into the middle of the twentieth century.

Five decades (1920-1970) of recorded gray seal observations on Cape Cod, Martha’s Vineyard, and Nantucket Island show that pups were found throughout this period, but often were limited to four or fewer observed in a year. Maine records contain no pup observations, but do show that during the 1960s and 1970s gray seals were observed along the Maine coast, mostly in Penobscot and Blue Hill Bays. These records indicate that, although a residual gray seal population existed in this area during the twentieth century, gray seals were extinct as a breeding population in the U.S. In 1965, Massachusetts enacted a law to protect the gray seal, and in 1972 the U.S. Federal government passed the Marine Mammal Protection Act, which provided a blanket protection in all states. These two laws acted to protect gray seals in the U.S.

Robert S. Michelson

, though much larger than that in the U. S., has had a similar history of exploitation and recovery. Accounts by early European explorers indicate that gray seals were abundant and widely distributed along the Canadian east coast and the Gulf of St. Lawrence. These accounts describe hunting by First Nations and by European settlers from the 1500s into the early 1800s. By the mid- to late-1800s, gray seals were rare and remained so until the mid-1900s. The total Canadian gray seal population was estimated to be 5,600 animals in 1966. With the protection of marine mammals over the last 40 years, the western North Atlantic gray seal population has rebounded. In the 1980s, naturally re-established breeding colonies were observed off the northern U. S. coast. Since 1991, three small, but increasing, breeding sites have been identified: Muskeget Island in Massachusetts and Green and Seal Islands in Maine. From 1991–2008, aerial surveys were flown over these three areas during the breeding season (December-February) to estimate the number of pups born at each site. Monomoy Island off Cape Cod was also surveyed intermittently during this period. Data for the three colonies that were regularly surveyed show that a minimum of 2,620 pups were born in 2008.

Some of the local breeders were observed with brands and tags indicating they had been born on Sable Island, Nova Scotia, 175 kilometers southeast of the mainland. Because of this discovery, immigration as a source of the U. S. population increase had to be considered. Two large populations in Canada—Gulf of St. Lawrence and Sable Island—were both possible sources. To determine the source and its importance to the U.S. recovery and to assess the stock structure of gray seals in the Northwest Atlantic, tissue samples were collected for genetic analyses from both Canadian and U.S. populations.

The degree of recovery in an exploited species is influenced by species characteristics and by available habitat. Three factors are key in recolonization: the ability of the species to disperse; life history parameters, including age at first reproduction, number of offspring produced per reproductive event, frequency of reproductive events, and lifespan; and environmental conditions that effect the species including hunting pressure and habitat quality.

The gray seal is a large, sexually dimorphic species. Males reach a size of up to 2.3 meters and 300–350 kilograms; females reach a maximum size of 2.0 meters and 150-200 kilograms. Based on cranial differences and on mitochondrial DNA, the species is generally divided into three distinct populations—those in the Baltic Sea, the Northeast Atlantic (U. K.), and in the Northwest Atlantic (Canada & U. S.). The time of breeding varies geographically: seals in the Baltic Sea puppy in March, those in the Northeast Atlantic in September-November, and in the Northwest Atlantic in December-February. Gray seals are gregarious and gather in large groups during the pupping/breeding and molting seasons. They are unique in that they can breed on sandy beaches, rocky ledges, ice, or in caves and show varying degrees of polygyny, depending on the breeding substrate and the amount of crowding on the rookery. On average, females become sexually mature at 4-5 years of age, males at 6 years.

Robert S. Michelson

In 2009, colonies were genetically very closely related and could not be considered independent sources. And when these two datasets were pooled and compared to gray seals born in the U.S., no population differentiation was found. Gray seals in the Northwest Atlantic make up one large, interbreeding population. They are moving between the various pupping colonies. Continuous immigration from Canada has fueled the gray seal population increase in the United States. The establishment of U.S. pupping sites, therefore, represents not only a recent range expansion but also a recolonization event. Although accurate gray seal population estimates are lacking, the Northwest Atlantic population is thought to be more than 400,000 animals, and is increasing in the United States, according to the National Marine Fisheries Service.

Massachusetts Division of Marine Fisheries, Atlantic White Shark Conservancy

The white shark is not new to the western North Atlantic. Its presence is well documented from Newfoundland to the Gulf of Mexico, including the Bahamas and parts of the Caribbean; yet it has not been considered an abundant species. Efforts to study its life history and ecology have been hampered by the inability of researchers to predictably encounter it. Indeed, much of what is known of this species in the North Atlantic comes from the analysis of distribution records, rare behavioral observations, and the occasional opportunity to examine dead specimens.

Massachusetts Division of Marine Fisheries, Atlantic White Shark Conservancy

In their 1985 review of white shark distribution in the western North Atlantic, John Casey and Harold Pratt, Jr., of NOAA National Marine Fisheries Service, compiled observations from numerous sources, including fisheries, confirmed sightings, and published accounts dating back to 1874. They found only 380 records. They also found that, from 1963-1983, white sharks represented only 0.04 percent of the sharks taken by over 2.1 million hooks by pelagic longline commercial fishing from the Grand Banks to the Gulf of Mexico. Nonetheless, Casey and Pratt concluded that white sharks in the western North Atlantic were most abundant on the continental shelf in the Mid-Atlantic Bight, from Cape Hatteras, North Carolina, to Cape Cod, Massachusetts. Moreover, the species is thought to exhibit seasonal movements, mediated by water temperature, into northern latitudes.

In the Pacific and Indian Oceans, the white shark is well studied and known to feed on pinnipeds. The high seasonal abundance of white sharks near seal and sea lion colonies has allowed researchers in those regions to study white shark movements over broad spatial and temporal scales. The only behavioral observations of white sharks in the North Atlantic come from a single acoustic tracking study, conducted in 1982 by Francis Carey and colleagues at the Woods Hole Oceanographic Institution. They acoustically tagged a 4.6 meter (total length) white shark, located 39 kilometers southwest of Montauk Point, N. Y. It was scavenging a fin whale (Balaenoptera physalus) carcass. The researchers tracked the shark for 83 kilometers. The vertical movements of the shark were of interest because it remained largely at a depth of approximately 10–20 meters but made periodic excursions to the bottom. At the time, Carey noted that “the seals, sea lions, and elephant seals, which are common items in the diet of white sharks in other regions, are not available.” He concluded that the observed diving behavior may be associated with searching for dead whales, which are an important food resource for large white sharks.

Dan McKierman/MA Division of Marine Fisheries

It can be argued that the elusive nature of the white shark in the North Atlantic may be the result of the absence of large pinniped colonies. This may not have been the case when gray seals were abundant in southern New England waters—an area Casey and Pratt have identified as having the highest white shark abundance. If this were indeed the case, white sharks may have shifted their diet to other prey, including dead cetaceans, after the demise of the gray seal population in the seventeenth century.

Since its establishment in 1989, the Massachusetts Shark Research Program (MSRP) has been tabulating and investigating reports of white shark sightings in New England waters. In most cases, these reports comprised fisheries gear interactions or observations by fisheries observers, spotter pilots working with commercial fishermen, whalewatch vessels, boaters, beach users, and recreational and commercial fishermen. To confirm species identification, the MSRP has taken into consideration physical evidence (i.e., a dead specimen), photographic/video evidence, eyewitness accounts, and observer experience. In most cases, those species typically confused for white sharks included basking sharks (Cetorhinus maximus), ocean sunfish (Mola mola), and a variety of marine mammals (dolphins, porpoises). Sightings that were clearly not white sharks, based on descriptions and/or photographic evidence, were discarded and not classified.

Massachusetts Division of Marine Fisheries, Atlantic White Shark Conservancy

Massachusetts Division of Marine Fisheries, Atlantic White Shark Conservancy

Off the northeastern United States, regulatory efforts to reduce landings of groundfish—a complex of demersal species, including Atlantic cod (Gadus morhua), haddock (Melanogrammus aeglefinus), and yellowtail flounder (Pleuronectes ferruginea)—have resulted in a dramatic reduction in the number of days that commercial fisherman can fish annually—termed “days at sea” (DAS). From 2001 to 2009, the number of DAS allocated to this fishing activity was reduced 72 percent, and the actual number of DAS used (i.e., fishing effort) declined by 52 percent. Similarly, the commercial bluefin tuna (Thunnus thynnus) fishery off the east coast of the United States has experienced a decline in fishing effort and in landings over the last decade. In short, the amount of fishing effort by those fisheries that typically report white sharks declined by roughly 50 percent during the period when white shark sightings were dramatically increasing.

Robert S. Michelson

Although white sharks will probably continue to scavenge whale carcasses, active predation on gray seals is likely a return to a preexisting trophic scenario, which changed with the demise of the gray seal population several hundred years ago. A similar scenario between white sharks and their pinniped prey has been well documented at the Farallon Islands on the west coast of the United States. Two general phases occurred in that area. As the northern elephant seal (Mirounga angustirostris) population rebounded during the 1970s and early 1980s, increases in individual predation rates by a relatively static number of white sharks caused the attack frequency to increase. It was hypothesized that there were probably six or fewer white sharks in the area at that time, because the number of attacks declined markedly when four of these sharks were removed in 1982. As the seal population stabilized in the late 1980s, individual predation rates stabilized as well, but the number of white sharks attacking and consuming this resource climbed. Given the upward trajectory of the Northwest Atlantic gray seal population, and the relatively low number of attacks on seals so far, the number of individual white sharks utilizing this resource is likely low as well. If the western scenario is applicable to the east, however, we are in the early stages of a renewed relationship. We should anticipate additional white sharks will discover that the Northwest Atlantic gray seal is back on the menu.--GBS & SAW