North Atlantic shark populations have declined by more than 50 percent in the past 15 years, with some species approaching the point of no return due to relentless fishing pressure and scant international efforts to protect the toothy ocean predator, according to a new study.

Published in Friday’s issue of the journal Science, the study found that longline fishermen harvesting tuna and swordfish from the Atlantic and adjacent waters are killing huge numbers of hammerhead, great white, tiger and thresher sharks. This is consistent with other studies suggesting a decline in shark numbers in all of the world’s oceans.

A team of researchers at Dalhousie University, in Halifax, Canada, analyzed the logbooks of longline fishing fleets from 1986 to 2000 and found a sharp drop in the number of sharks killed while harvesting tuna and swordfish.

“We estimate that all recorded shark species, with the exception of makos, have declined by more than 50 percent in the past eight to 15 years,” the researchers found.

“This is a worldwide phenomenon,” said Ransom Myers, a professor of biology at Dalhousie and a co-author of the study. “There are only a few areas in the world where we have good data, but wherever we do, they show the same thing - the shark is in serious decline.”

PERCENTAGES NOT GOOD

The study found that hammerhead sharks declined by 89 percent in the Atlantic, while tiger sharks were reduced by 65 percent, blue sharks by 60 percent and threshers by about 80 percent. The trend for great white sharks, the famed predator in the movie “Jaws,” is a decline by about 79 percent. The study found that in at least two fishing areas, no great white has been recorded since the early 1990s.

Myers said the shark is particularly threatened by intense fishing pressure because the animal cannot quickly replace its lost numbers.

“They are like humans,” he said. “They take a long time to mature and have relatively few babies. The bigger sharks have only about four pups a year. That makes them more vulnerable than other fish species.”

Although sharks in some parts of the world are targets of fishermen, in the North Atlantic they fall victim to fleets seeking other types of catch.

“The hammerheads concentrate in exactly the same places where the fleets fish for tuna and swordfish so they are hit because they are at the wrong place at the wrong time,” said Myers. The sharks routinely feed on the herring and squid commonly used for bait by the longliner fishermen, he said, so catching sharks is just a routine part of fishing for the other species.

• Global climate change
  Should Earth continue its warming pattern, scientists fear shore and habitat erosion, increased salinity of estuaries and freshwater aquifers, altered tidal ranges in rivers and bays, changes in sediments and nutrient transport, a change in patterns of chemical and microbial contamination in coastal areas, and increased coastal flooding. Ecosystems particularly at risk include saltwater marshes, mangrove ecosystems, coastal wetlands, coral reefs, coral atolls, and river deltas.
  
  
• Invasion of exotic species
  Both man and nature sometimes add a species to an area where it’s not native. The danger is that this can bring new disease organisms that the native species are not equipped to defend against. Man’s impact often comes via the exchange of ballast water in ships, which can dump marine organisms into new areas. This has been implicated in outbreaks of red-tide in Australia; the invasion of the Black Sea by the American comb jellyfish with disastrous effects on plankton biomass and the anchovy fishery; and the invasion of the Great Lakes by Eurasian zebra and quagga mussels that have caused great economic damage in inland waterways.
  
  
• Alteration of physical habitat
  According to the United Nations, more than half of the world’s population lives within 40 miles of the shoreline and this could rise to 75 percent by the year 2020. And as more people live close to shorelines, that means more erosion, destruction and pollution of habitat used by many ocean species. In the United States and worldwide, coastal salt marshes have been destroyed by dredging and filling, mangroves have been removed for shrimp aquaculture, coastal development has altered natural patterns of erosion and sedimentation, and mining and dredging have directly altered habitats for marine species.
  
  
• Chemical pollution and eutrophication
  More than 75 percent of ocean pollution actually comes from sources on land. These might be factories, farms or even homes hundreds of miles inland, which pollute either into the air or into rivers that run into the sea. The pollution can increase mortality rates, decrease growth, impair reproduction and genetically mutate ocean species. It is also believed to contribute to the increase in certain marine algae that can kill various marine organisms and cause illness and even death in humans who consume contaminated seafood.
  
  
• Fisheries’ operations
  One study found that, in the past two decades, the world’s fishing nations have so excessively increased their efforts that global fishing capacity in the traditional fisheries is estimated to be 30 percent greater than required to take the world catch. In the United States, it has been estimated that about one-third of all the fisheries for which sufficient data exist are overfished.

ADVICE TO POLICYMAKERS

Myers said sharks could be protected by changing the commercial fishing patterns. For instance, some of the sharks migrate along set paths at specific times of the year. Prohibiting fishing during those periods in the migration areas could reduce the by-catch of sharks, he said.

Also, establishing refuges where all fishing was forbidden would give sharks, along with other fish, a safe haven where they could feed and reproduce safely. This eventually would mean bigger catches for the fishing fleets, said Myers, and protection for the sharks.

“The shark is declining now, there is no reason to believe they would not recover if we fished in a responsible manner,” he said.

The United States now forbids harvesting of shark fins for shark fin soup, a favorite in Asia, but longline fleets from Spain and Japan continue to harvest, said Myers. In a recent meeting of the Convention of International Trade in Endangered Species of the United Nations, Japan, a major shark-consuming nation with a vast fishing fleet, led a successful effort to defeat proposals that called for protection of two shark species.

BROADER IMPACTS

David Conover, a professor at the Marine Sciences Research Center at the State University of New York, Stony Brook, and an expert on fisheries, said the Dalhousie study shows “a particularly clear and compelling example where a group of species that are by-catch in commercial fishing are suffering a decline that has gone unrecorded.”

Conover said the shark is at the top of the food chain in the ocean and if that species were to be wiped out by overfishing it could disrupt the entire ecosystem of the oceans.

“We know from many examples that once you start eliminating the predators at the top it has a ripple effect throughout the food web,” said Conover. “It is difficult to predict what would happen, but we know there are major effects in altering the food web.”