Sharks and Seals: A Success Story on Cape Cod
CAPE COD TIMES - While some worry about increase in numbers on Cape Cod, others celebrate the restoration of these species
Eighteen years ago, charter boat captain Joseph Fitzback and his customers held on tight as a 14-foot great white stripped a striped bass off a fishing line, then rocked the boat with a couple of exploratory bumps, 2 miles off Chatham’s Lighthouse Beach.
Television crews and reporters lined up to interview Fitzback, but as the numbers of seals, and the sharks pursuing them, have increased, such interactions are almost commonplace. In a relatively short time the Cape has evolved from ocean playground to wilderness experience, and today Fitzback’s story might get little more than a few hits on social media.
By now, the first of perhaps hundreds of great whites, the largest such aggregation on the East Coast, have returned to the Cape for the summer from their winter grounds to the south. They are hunting a gray seal population that has exploded from almost zero in the 1970s to nearly 30,000, possibly as many as 50,000, today, depending on the science you choose to believe.
With five shark attacks on people since 2012 — including swimmers, kayakers, a paddleboarder and a fatal attack on a bodyboarder last year — surfers, swimmers, public safety officials and business owners now worry that the ocean is suddenly not safe. Some see an imbalance that requires corrective measures, like a cull of seals and maybe sharks to reduce risk and bolster fish harvests.
But others marvel at a pair of conservation success stories in an era where species extinction is more often the news.
“We need these species. They’ve been gone too long and their restoration should be celebrated,” said Joseph Roman, a conservation biologist and an associate professor and fellow at the Gund Institute at the University of Vermont.
A gloomy United Nations report last month predicted that 1 million species are faced with extinction in the coming decades and that 66 percent of the ocean had been significantly altered by humans. A 2014 study by University of British Columbia researcher Villy Christensen showed that the global predatory fish population dropped by two-thirds over the past century, with the trend accelerating over the past 40 years. Many suggest that the government can, and has, played a big role in forestalling or even reversing those trends. NOAA’s 1997 ban on landing great whites, and the Marine Mammal Protection Act of 1972, were largely successful in reversing the eradication of seals, whales and the great white.
“The recovery of the Atlantic white shark population is a conservation success not only for white sharks themselves but also for the overall health of our coastal ecosystems,” said Tobey Curtis, a shark expert and fishery policy analyst with the National Oceanic and Atmospheric Administration in Gloucester.
Curtis co-authored, with state Division of Marine Fisheries shark researcher Gregory Skomal and others, a 2014 paper that reviewed over 200 years of historical fishing data and concluded that there were signs the white shark population in the Northwest Atlantic was recovering from historic low population numbers in the 1970s and ’80s brought on by heavy commercial and recreational fishing.
The goal of marine conservation is to restore populations and their ecological functions, Roman said, but discovering the true extent of the ecosystem role of predatory fish and marine mammals is complicated by the ocean environment, with its complex food web and impenetrability.
Roman places the restoration of the seal population and an improved outlook for great whites alongside other wildlife achievements of the 20th century, like a 500-percent increase in grizzly bears in Yellowstone National Park and the restoration of the American alligator in Florida. Marine mammals in particular have benefited from protective legislation. In a 2015 paper, Roman co-authored a study that found that 42 percent of 92 marine mammal populations worldwide showed increases, with only 10 percent declining in number. The remaining populations were considered stable.
“Too much of my work is looking back at what happens when a species disappears,” Roman said. “Gray seals, humpbacks here (in the Northeast), were gone 50 years ago, and now they’re back. There are concerns, but we need to look at what the ecological impacts are (of their restored populations).”
Some of Roman’s research has been focused on the contribution of whale and seal excrement to oceanic food web. Their feces and urine are rich in the nitrogen that fuels plankton growth, much as fertilizer would on land. Roman said whales alone contribute nearly 53 million pounds of nitrogen a year to the Gulf of Maine, the equivalent of what flows into that basin from all the region’s rivers. Seals excrete almost 90 percent of the nitrogen they ingest, Roman noted in a 2010 research paper, although their overall contribution is still unknown, he said.
Much of the nitrogen in the water is headed for the bottom of the ocean as waste from plankton or dead plants and animals that ultimately sink. But whale and seal feces and urine float in the sunny upper layers where the phytoplankton gets the energy for photosynthesis. Roman said whales timed their arrival for the peak plankton bloom, before they declined after exhausting the supply of nutrients in the upper layers of the sea. Roman believed that the nitrogen in whale excrement may help in extending the bloom.
Some research has shown that these benefits are not confined to the sea. Last month, a paper published in the journal Current Biology showed how large penguin colonies in the Antarctic transfer nutrients from the sea to enrich the land, with the nitrogen from their feces creating an island of life hundreds of times larger than the colony itself in an otherwise barren landscape. A 2003 paper in the Journal of Animal Ecology showed that inland plants benefited from the nutrient-rich feces and urine of the Galapagos sea lion colonies. Locally, a 2016 paper by Michelle Woods of Allegheny College showed a similar effect with nitrogen deposits from gray seals on Cape Cod beaches, although the outcome was limited due to the seasonality of the two Truro seal haul-outs that Woods studied.
These discoveries lead scientists to caution that we don’t yet know enough about the full range of beneficial effects that sharks and seals have on our local ecosystem to take drastic measures like a cull.
“For some species, such as gray seals and spiny dogfish, white sharks are the only natural predators that can help keep their populations in check,” Curtis said. Whether it’s bears, tigers, wolves or sharks, humans have reduced the populations of large predators worldwide. And researchers say that has created destabilized ecosystems where second tier predators, or grazing animals, live without fear and can eat unimpeded as their numbers grow exponentially.
When gray wolves were restored to Yellowstone National Park, it quickly became obvious there were both risks and benefits to the return of a top-of-the-food-chain predator. The risks to people and to their possessions, like livestock, were well known. But the benefits to the local ecosystem and ultimately to ranchers were somewhat of a surprise.
White sharks and wolves can limit the impact that mesopredators, the species below the large apex predators, like raccoons, skunks, small sharks and rays, have on smaller size species. Large predators keep their numbers down by eating a lot of them, usually the weakest or most vulnerable, and that can help to strengthen the general population.
Top predators can also control populations by frightening their prey into altering their behavior to avoid being eaten. When they don’t linger in one spot too long, or they choose areas that may be less rich in food but have fewer predators or better shelter; they eat less, hide more, and tend to reproduce less often. Researchers say their populations are then kept in check well beyond the actual numbers of animals killed by top predators.
An apex predator can affect a food chain or web right down to the plants level, researchers say. One of the unanticipated consequences of the return of wolves to Yellowstone was the reestablishment of healthy aspen groves, which all but disappeared from the park after the last of the wolves disappeared in the 1920′s. When they were reintroduced in 1995, Yellowstone’s elk herds no longer lingered in aspen groves, and the trees showed the first noticeable increase in height and growth in 50 years, according to a 2007 report by William Ripple and Robert Beschta of Oregon State University.
The benefits didn’t end there. Other researchers showed the resurgence of aspens resulted in more beaver dams which helped fish stocks, underground water supplies, even songbirds.
Trophic cascades, impacts that ripple down through the food chain, or across a food web of predator and prey, are harder to see in the marine environment, but researchers at Florida International University and Dalhousie University in a 2008 study that reviewed marine food webs pointed to rising catch levels of large sharks, especially blacktip sharks, leading to a population surge in cownosed rays that may have helped collapse bay scallop stocks.
Known by researchers as mesopredator release, this phenomenon of smaller predators suddenly free of fear with the removal of top predators around the world, is playing havoc with ecosystems. Some believe the rise of the spiny dogfish on the East Coast that plagues fishermen trying to catch commercial species may have had a role in the current cod crisis by eating the young fish before they can reach spawning size.
The Dalhousie and Florida International University study suggested that the impacts of large predators may extend all the way down to the sea grass and other vegetation growing on the ocean bottom. They claim that the element of fear and direct predation by a top predator like the tiger shark can alter the marine food web like it did with wolves, elk and aspens in Yellowstone, land reestablish a balance.
“If I’m concerned about the overgrowth of seal population, the first thing I would want would be the white shark,” Skomal said. “Their removal could be, or has, led to what we have right now.”
Cape Cod’s gray seals are already showing signs that sharks are having an impact. Duke University researcher David Johnston tagged gray seals off Chatham and found that in the winter they left and returned to land any time of the day or night. But as soon as sharks returned in the late spring, researchers have noticed they stuck close to shore or remained on shore during the day, leaving only under cover of darkness to hunt.
But exactly where these two populations are headed is not known. Until recently, both species were largely absent from our coastal ecosystem. There is little evidence what their historic populations looked like.
Marine predator/prey relationships tend to be more complicated than those on land; other predators take the place of the ones that go missing, and the ecosystem adjusts. And our coastal ecosystem isn’t what it was hundreds of years ago, industrial scale fishing reduced fish populations and environmental conditions, like water temperature, have changed in the past centuries. The “historic” numbers of both white sharks and seals may no longer apply.
“Those numbers are definitely challenging. We can probably model it, but it will be wrong,” Roman said.
That gives little comfort to those who want to know what the future ecosystem will look like. Is it possible, they ask, that gray seal and shark populations continue to increase, along with the potential for greater interactions between great whites and humans?
“I can see where that uncertainty makes people uncomfortable,” Roman said. “The numbers are not going to rise forever. But where those numbers will settle, it’s hard to know.”
— Follow Doug Fraser on Twitter: @dougfrasercct.