Updated at 1:51 p.m., Thursday, December 14, 2006
Study: Pacific tuna stocks falling, but not imperiled
By Courtney Dentch
Bloomberg News Service
Stocks of tuna and other large ocean predators fell between 9 percent and 64 percent from 1950 to 2004, according to the study, published in the journal Science today. Earlier reports suggested populations had fallen as much as 90 percent, said Mark Maunder, study co-author and senior scientist at the Inter-American Tropical Tuna Commission in La Jolla, Calif.
"Some of the stock have declined, but they're not going extinct," he said in a telephone interview.
The fall in the number of larger fish, so-called top-level predators, was due to increased human demand and fisheries going after smaller, younger fish, Maunder said. Aside from preventing development into large predators, targeting younger fish may limit reproduction that replenishes populations, he said.
"You want to catch them before they stop growing, but it may be a real problem because you won't have any reproductive stock," Maunder said.
Fishing practices led to a fall in the average size of tuna to 83 centimeters (33 inches) in 2000 from 93 centimeters in the 1950s, the study said. Tuna measuring more than 175 centimeters now comprise less than 1 percent of the total population, compared with 5 percent in 1950, it said.
"We need to control the fishing efforts to make sure it doesn't increase," Maunder said. "We don't want to increase the fleets any more than they already are."
Researchers said the biomass, or total weight, of tuna populations hadn't fallen catastrophically, even though catches continuously increased in the past half-century. More than 50 million tons of tuna have been taken from the Pacific Ocean since 1950, and it produced more than 2.5 million tons, or 64 percent, of the 2004 global tuna catch.
The biomass of four tuna varieties was more than 74 percent of the level it would have been without fishing, and between 36 percent and 49 percent for four others, the study said. The biomass of some populations may have increased because catching predators allows more fish to survive, it said.
Researchers used data such as catch information from fishing fleets and the size of fish caught to reach the findings, Maunder said. Some fish were tagged to track movements, he said.
The study was funded by the Joint Institute for Marine and Atmospheric Research, the National Oceanic and Atmospheric Administration, and members of the Secretariat of the Pacific Community and Inter-American Tropical Tuna Commission, which is responsible for fishery management and conservation in the eastern Pacific Ocean.