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SoMAS Scientists Study the Great White

November 1, 2010

six-gill
Dr. Demian Chapman with a bluntnose six-gill shark,
Hexanchus griseus, off Eleuthera, Bahamas

In 1971, Peter Matthiessen’s Blue Meridian was published, the vivid first-hand account of American filmmaker Peter Gimbel’s global quest to find and be the first to film the great white shark underwater.  Soon after the book was published, came the film version of the expedition, Blue Water. White DeathJaws arrived a few years later and the rest, as they say, is history.

Both Gimbel and Matthiessen were New Yorkers.  They really needn’t have travelled to such far-flung locales as South Africa, Ceylon and Dangerous Reef South Australia to find the most famous shark in the sea.  At one time, the ocean waters off eastern Long Island held one of the largest seasonal aggregations of white sharks to be found anywhere in the world.  Dr. Demian Chapman of the Institute for Ocean Conservation Sciences (IOCS) at SoMAS is an elasmobranch (sharks & rays) specialist.  He’s turning his attention to white sharks, especially the population seasonally present here in the Northeast, and attempting to fill in some knowledge gaps, and perhaps debunk some myths, about this near-mythical animal.

Much more has been written about Carcharodon carcharias than is actually known about it.  One example of our incomplete understanding of this species involves its preferred habitat.  White sharks have historically been described as  a coastal species that inhabits temperate waters.  Many of the established white shark hotspots are indeed in temperate latitudes -- South Australia, South Africa, the North Pacific and northwestern Atlantic.  However, recent tagging research is starting to broaden the picture.   Some tagged whites have traveled thousands of miles, spending considerable amounts of time in the open ocean and diving as deep as 3,000 feet.  Further, many of the tagged animals have ended up in tropical locations, such as Fiji and Hawaii.

One objective of Dr. Chapman’s work is to authenticate or refute claims that the population of white sharks in the U.S. Northeast has declined since the 1950’s and 1960’s, when the big fellows (actually, females are larger than males) were regularly sighted and/or caught off Montauk, at the eastern tip of Long Island.  Why be concerned about white shark populations?  One reason is that the loss of white sharks could have serious ecological implications.  Dr. Chapman explains that mature great whites are apex predators – the top consumers on the marine food chain.  They consume prey such as large tunas, seals and porpoises which typically have no other natural predators.  These prey, in turn, consume a variety of other organisms.  Loss of white sharks can potentially have effects that ripple through the food web, producing what is known as a trophic cascade.

lemon shark
A young lemon shark, Negaprion brevirostris,
captured at Cape Canaveral, Florida

There has been a decline in the sightings and landings of white sharks in the Northeast over the past 40 years, although it is unknown whether this reflects a change in fishing practices, a change in the size of the white shark population or a change in their habitat use and foraging patterns.  Today, sightings of great whites in New York’s local waters in the summer are infrequent. Catching and landing a white shark anywhere along the U.S. East Coast has been prohibited since 1999 under the federal Fishery Management Plan for Atlantic Tuna, Swordfish and Sharks.  Despite this ban,  some are still caught as incidental by-catch in other fisheries.  Like all sharks, great whites are unusually susceptible to the effects of fishing.  They grow slowly, become sexually mature at a relatively  high age, often reproduce only every other year and produce few offspring.

Using genetic tools, Dr. Chapman has seen evidence that the white shark population in the Northeast has either experienced a significant decline and/or it has always been small.  Tests reveal an extremely low level of genetic diversity in the population, similar to other species which are known to be either naturally rare or to have experienced significant declines.  Dr. Chapman hypothesizes that local white sharks have experienced a recent population bottleneck – a term that describes an event where a significant percentage of the population is lost and could result in problems such as inbreeding.  Dr. Chapman and his students are now testing this hypothesis directly by looking at the genetic diversity of white shark vertebral samples archived from the 1960s through to the present. They are also going to analyze stable isotopes in these vertebrae to see if the diet and foraging habitat of white sharks has changed over the same period. It is possible that white sharks have moved offshore as nearshore prey sources have become depleted, which would leave a distinct chemical signature in the vertebral tissues.

Is the atavistic fear that the great white so often generates at all justified?  “Not really,” remark Dr. Chapman and others who study sharks.  They acknowledge that this is a large and potentially dangerous predator and it should be treated with respect and caution.  Seen underwater, the great white is truly majestic .  More fundamentally, its decline as an apex predator in the marine food web may trigger collateral changes in the abundances of other species with negative ecological or economic consequences.   In its interactions with humankind, the great white has been much the loser.  Last summer, the U.S. Coast Guard distributed an advisory alerting the public to the sightings of white sharks in coastal waters of the Northeast.   The advisory was meant as a warning.  Perhaps the warning should have noted not the appearance of the great white, but its disappearance.

 


 

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