The July 30 shark attack at Ballston Beach in Truro has focused national attention on the seasonal occurrence of white sharks in waters close to Cape Cod and Island beaches. White sharks are no strangers to residents here; I certainly won’t forget kayaking with friends to see a female white shark trapped in a coastal pond on Naushon island in September 2004. There seems little doubt that we have witnessed more white sharks in areas frequented by swimmers along the eastern shore of the Cape over the past few years. However, the available evidence points to worrying declines in almost all our coastal shark species. It turns out that sharks have much more to fear from humans that we have to fear from them — even the magnificent white shark.

Historically, a number of shark species — including basking sharks, dusky sharks, sandbar sharks and white sharks — were common in coastal New England waters. Farther offshore, oceanic waters were home to oceanic whitetips, makos, porbeagles and threshers. However, all these species have been fished at unsustainable levels for decades. Populations of dusky sharks, oceanic whitetip sharks and porbeagles have declined by at least 90 per cent since the early 1960s. Many other species are likely in similarly bad shape, but we simply don’t have enough data to draw formal conclusions. Of course, these declines also need to be seen in context of a global fishery that currently removes approximately 70 million sharks from the ocean every year. We are losing the “large cats of the ocean” at a remarkable pace, faster than we can understand the impacts of their loss on ocean food webs.

Today there is no population assessment available for white sharks in the northwest Atlantic Ocean, although a 2003 study documented the capture of over 6,000 white sharks in the region by the U.S. long-line fleet between 1986 and 2000. During this period the population declined by approximately 80 per cent, and white sharks were virtually eliminated from long-line fisheries north of Cape Hatteras by 1990. I suspect that there may be more northern right whales, highly endangered in their own right, than mature white sharks along the U.S. East Coast.

White sharks are currently listed as a prohibited species in the Atlantic, and if caught, must be immediately released with a minimum of injury and without taking them out of the water. While it’s possible that this protection might be generating a white shark renaissance in our area, their low reproductive potential means it will take a very long time before white shark populations along the U.S. Atlantic coast recover to the levels of the 1950s.

If we accept that white shark populations in the northwest Atlantic are very low, why are we seeing more white sharks off Chatham? The answer appears to lie with their prey, and seals in particular. Gray seals have been protected by the Marine Mammal Protection Act since 1972, and their populations have increased significantly since this protection has been in place. Seals and other marine mammals are the preferred prey of large white sharks. It is a safe bet, then, that the white sharks have come to the beaches of Chatham and Truro because of the growing numbers of gray seals at places like Jeremy Point in Wellfleet, Monomoy and Nantucket’s Muskeget islands. (An increase in gray seal populations has also been noted off the Vineyard and Nantucket.) Indeed, Cape Cod may now be home to as many as 10,000 gray seals that must look like a particularly appetizing smorgasbord for a hungry white shark. It seems likely that the seals, and therefore the white sharks, are here to stay.

We may never know if a white shark was responsible for the attack last Monday at Ballston Beach (or if two recent reported sightings off the Vineyard were actually great whites). Nonetheless, there are lessons to be learned that may help prevent attacks in the future:

• Humans can reduce the likelihood of attack with a little common sense: Don’t swim alone or in proximity to seals, stay close to the shore and limit water activity to between the hours of 9 a.m. and 5 p.m. Many sharks are particularly active at dawn and dusk or at night, although not necessarily nearer to beaches. And of course you are much less likely to see a shark before it attacks at night.

• Shark spotters in planes or towers on the beach in areas frequented by white sharks could warn swimmers when white sharks are in the area.

• Shark nets have been used in some countries with limited success, but it is important to note that the nets are designed to catch and kill sharks rather than fence off a beach. These nets also present a threat to other marine life in the area, including endangered whales, dolphins and harmless shark species.

• Technological developments may help with early detection of white sharks at popular beaches. Authorities in western Australia are currently deploying acoustic listening stations (acoustic receivers) that monitor adjacent waters for sharks that have been tagged with acoustic transmitters. When a shark is detected by the acoustic receiver, a message is immediately sent via cell or satellite phone to alert public safety officials. Greg Skomal of the Massachusetts Division of Marine Fisheries has already tagged a number of white sharks in Chatham waters with acoustic tags that could make use of such a system.

Ultimately, none of these measures, alone or in combination, can be guaranteed to be 100 per cent effective. Swimming on outer Cape beaches will continue to have some element of risk. But if we want to maintain what is left of our wild oceans, including a healthy complement of apex predators like white sharks, we will have to learn to live with what remains a very low probability of attack by a shark. With any luck, Massachusetts will wait at least another 75 years before another white shark attack in its waters.

Simon Thorrold is a fish ecologist at Woods Hole Oceanographic Institution and director of its Ocean Life Institute.