Though not targeted by the fishing industry, some ocean species often get caught unintentionally in nets or lines used to catch tuna and other commercially valuable fish, says a study presented to scientists today, Aug. 5, at the annual meeting of the Ecological Society of America.
A University of Wisconsin-Madison group's study points to the potential increased risks for the large, slow-growing, slow-to-reproduce animals at the top of food chain.
"It's the sharks, dolphins and billfishes that are hurt the most," says Jefferson Hinke, a University of Wisconsin-Madison graduate student and study group member.
Now, populations of most target species are stable and viable, thanks in part to restrictions on undiscriminating fishing practices such as drift nets and fish aggregation devices. However, any substantial increase in industrial fishing could play havoc with both target and non-target animal populations, Hinke says.
"In these systems, environmental variability tends to have little effect at the top of the food web," he says. "What's really important is the fishing."
Hinke, working under the auspices of the National Center for Ecological Analysis and Synthesis in Santa Barbara, Calif., is part of a group that is developing computer models able to accurately forecast the effects of fishing on major ocean ecosystems. The hope, he says, is to provide fishery managers with a set of tools that can be used to predict change in economically important but ecologically sensitive systems.
The models Hinke and his colleagues are working to develop can help reveal "what happens when you fish off the top of the food chain," says James Kitchell, a UW-Madison professor of zoology. "You fish in different ways, you have different effects."
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 in generally good shape, Hinke notes. 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 relatively heavy fishing pressure. However, increased fishing pressure would very likely cause strong declines, especially for the already very heavily fished yellowfin tuna stocks.
Other animals at that top of the ocean ecosystem heap are a different story. "Yellowfin tuna have a life span of only five years," Hinke says. "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 models being developed by Hinke and his colleagues at the National Center for Ecological Analysis and Synthesis also are meant to forecast how fish populations of all kinds will respond to different fishing scenarios. Ratcheting up fishing pressure, according to model simulations, may cause as much as a 20-50 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 says. "We can model at a level now that permits us to tell how some of these animals might respond to different levels of pressure."
Co-authors of the paper delivered by Hinke include Kitchell, who directs the UW-Madison Center for Limnology, Isaac Kaplan of the UW-Madison Center for Limnology, and George Watters, a scientist at the Pacific Fisheries Environmental Lab in Pacific Grove, Calif. Robert Olson, also a co-author, is a scientist at the Inter-American Tropical Tuna Commission in La Jolla, Calif.