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Fishing Threatens Top of Ocean Food Chain

TUCSON, Arizona,

August 5, 2002 (ENS)

Industrial fishing poses the biggest threat to sharks,
dolphins and billfish in the tropical and northern Pacific
Ocean, argues a new study forecasting the effects of
commercial fishing on ocean ecosystems.

Though not targeted by the fishing industry, some ocean
species often get caught by nets or lines used to catch
tuna and other valuable fish, concludes the study by
researchers at the University of Wisconsin-Madison.
The study, presented today at the annual meeting of the
Ecological Society of America in Tucson, points to the potential increased risks for these large, slow growing, slow to reproduce animals at the top of the food chain.

"It's the sharks, dolphins and billfishes that are hurt the most," said Jefferson Hinke, a University of Wisconsin-Madison graduate student and study group member.

Hinke, working under the auspices of the National Center for Ecological Analysis and Synthesis in Santa Barbara, California, is part of a group that is developing computer models able to forecast the effects of fishing on major ocean ecosystems. The hope, he said, is to provide fishery managers with a set of tools that can be used to predict change in sensitive systems.

The models can help reveal "what happens when you fish off the top of the food chain," said James Kitchell, a UW-Madison professor of zoology. "You fish in different ways, you have different effects."

For example, populations of many commercially fished species are stable and viable, thanks in part to restrictions on undiscriminating fishing practices such as drift nets and fish aggregation devices, the report says. But any increase in industrial fishing could play havoc with both target and non-target animal populations, Hinke warned.
"In these systems, environmental variability tends to have little effect at the top of the food web," he said. "What's really important is the fishing."

In the Pacific, tuna populations - the intended and preferred catch of commercial fishing outfits from Japan, the U.S. Mexico and other Pacific nations - are generally in good shape, Hinke said. Because these fish tend to mature and reproduce at much earlier ages than the non-target species like sharks and dolphins, their populations are able to withstand fishing pressures.

However, increased fishing pressure could cause declines, particularly in the already heavily fished yellowfin tuna stocks. And more fishing could devastate non-target species.

"Yellowfin tuna have a life span of only five years," Hinke said. "They have really fast growth rates and they can begin to reproduce early in life. A shark, on the other hand, can live 20 or 30 years and may not reproduce until it reaches 10 years of age. Sharks also produce relatively few offspring as opposed to a tuna which will spawn sometimes every day for a year and produce millions and millions of eggs."

The UW-Madison models also are meant to forecast how fish populations of all kinds will respond to different fishing scenarios. The model suggests that increasing fishing pressure may cause as much as a 20 to 50 percent decline in populations of dolphins, sharks and billfishes like marlin, sailfish and swordfish.

"The models are part of the toolbox for managers, and they have proven to be effective," Hinke said. "We can model at a level now that permits us to tell how some of these animals might respond to different levels of pressure."