Top Fish Populations Being Replaced by Rays, Smaller Fish

Shark, tuna, marlin, and other top undersea predator populations inhabiting the middle of the ocean, thousands of miles from the nearest land, are being replaced by smaller, less desirable rays and other fish, according to a new study published in the April issue of Ecology.

Studying an area of more than 6,000 square miles in the middle of the equatorial Pacific Ocean south of Hawaii and north of the Fiji Islands, researchers Peter Ward and Ransom Myers from Dalhousie University in Nova Scotia found that sharks, tuna, and other top-of-the-food-web fish are half the size and their populations 80 percent smaller in numbers than they were 50 years ago.

"The main consequence of many years of industrial fishing in the area is a reordering of the ecosystem, thus increasing the number of small fish that we don’t like to eat," says Professor Myers. "Pelagic stingrays are becoming a major part of the ecosystem, replacing tunas and other desirable fish. Yellowfin tuna, for example, caught in recent years on average weigh half of those caught in the 1950s."

Part of the Pew Global Shark Assessment, the study compared catch-data from longline fishing vessels in the 1950s with more recent observations in 1999-2002. It found that the largest and most abundant predators, such as shark, marlin, and large tuna, suffered the greatest declines in abundance and striking reductions in mean body mass.

The study showed that the average blue shark weighed 115 pounds in the 1950s and weighed only 49 pounds in the 2000 study. During that same time, the number of blue sharks shrank by more than 85 percent. For black marlin, the decline in body size was even greater.

Those observed during the 1950s weighed 275 pounds, and the average black marlin observed 50 years later was less than 100 pounds. “Large predatory fish are slow to mature and are now being caught earlier in their lifecycle, which also explains their dwindling numbers,” says Ward.

By contrast, several small and formerly rare species, such as the pelagic stingray and snake mackerel, increased in abundance in the area. However, the increases in the small species did not balance the reductions in the biomass of large predators.

"This study shows a profound change in world order under the sea," says Ellen Pikitch, executive director of the Pew Institute for Ocean Science, which funded the study. "Years of unabated commercial fishing not only has changed the ecosystem in near-shore fisheries, but has had similar effects thousands of miles away in the open ocean."

Pikitch, also a professor at the University of Miami Rosenstiel School of Marine and Atmospheric Science, adds, "It’s unlikely that we’ll have any encounters between a modern day Santiago and the monster marlin in Hemingway’s The Old Man and the Sea. The huge predator fish of old are gone."

Myers and Boris Worm, also from Dalhousie, recently released a study showing a 90 percent decline in predatory fish populations, notably cod, in North Atlantic Ocean commercial fishing areas between North America and the British Isles.

"We are concerned that projections of current fishing mortalities into the future could lead to the extinction of certain species of sharks and other large predators at the top of the food web," says Professor Myers.