Olakino / Your health

A new study traces the presence of mercury found in some of Hawai'i's commercially sold fish products to the combination of mercury emissions in China and a large ocean current that pushes eastward across the Pacific. - Photo Liza Simon

Finding has implications for health, aquaculture

By Liza Simon / Ka Wai Ola

If you enjoy eating your share of 'ahi, ono and opah, you may have wondered about warnings regarding the safety of consuming these deep-sea or pelagic, open ocean fish. They contain mercury, a substance that is a serious threat to human health. Scientists have also been perplexed by a seeming disconnect: Mercury concentrations in waters across the world are not significant, yet these same waters provide habitat for fish with elevated levels of the potentially toxic element. Now for the first time, a new study by a consortium of university and government scientists from the United States and Australia documents a process by which mercury emissions are absorbed into the tissues of pelagic fish and can eventually end up in your plate lunch.

The study in the May edition of Global Biochemical Cycles summarizes research conducted in the Pacific Ocean between Hawai'i and Alaska. It says the main source of the mercury emissions is the coal-burning industry in China, where about two plants a week are coming on line. If this explosive rate of industrial growth continues at the same pace, the report predicts that this will cause mercury emissions in waters worldwide to increase by an additional 50 percent over the next 40 years.

"There is an important message especially for Hawai'i in this report," said David Krabbenhoft, a principal investigator for the study with the U.S. Geological Survey. "It's that even in a geographically isolated and pristine-looking island environment, the reality is that you are not immune from global impacts being felt throughout the planet."

Krabbenhoft said the USGS study will be used by the U.S. Environmental Protection Agency to look for ways to reduce the risk of mercury in fish. A written release from the EPA hailed the new report, noting that it underscored the need to work on developing mitigation and enforcement policies at the global level with governments of China and other Asian nations.

The newly published study describes a process in the ocean that converts airborne mercury into methylmercury, a highly toxic form of the element, which accumulates in fish over time. The airborne mercury first enters the ocean through tiny algae that live near the water's sunlit surface. The algae die and settle downward to an ocean depth where microbes break down the so-called "algae rain" into many byproducts, including methylmercury, which is then consumed by small fish. These small foragers are the prey of larger fish, which amass methylmercury that become extremely concentrated in their muscle tissue. The contaminant becomes more harmful as it makes its way up the food chain.

Krabbenhoft noted there is a second ocean process responsible for transporting the fish from the point source of mercury pollution into Hawaiian waters: "From the coasts of Asia, you have a very large ocean gyre that moves water eastward with a powerful counterclockwise current that intersects with eastern Pacific waters near Hawai'i." Krabbenhoft added that the only prior study on the topic suggested that emissions from the ocean floor could be the source of mercury in fish. "But we did not find this to be the case," said Krabbenhoft.

The National Marine Fisheries Service and at least one commercial fishing organization have disputed the USGS scientists' prediction that increasing mercury air pollution from China will necessarily fuel a corresponding increase in ocean mercury or in fish. Krabbenhoft agreed that further work is needed to confirm the relationship, though he has also heard from several ocean experts who say the new findings are consistent with their own local studies. "One scientist in Alaska says the numbers he is seeing from the mid-1970s show there has been a 250 percent increase in methylmercury concentration in halibut from the Bering Sea," said Krabbenhoft.

The U.S. Food and Drug Administration began in 1994 to issue health advisories about mercury in commercially sold pelagic fish. Mercury is a neurotoxin, which can damage the development of brain tissue in fetuses and young children. The Hawai'i state Department of Health has supported the FDA warnings with a public information campaign, which includes a brochure titled Local Guide to Eating Fish Safely, advising pregnant women, nursing mothers and children to stay away from eating deep-sea fish "at the top of the food chain."

At the same time, the brochure emphasizes that fish is part of a healthy diet and it encourages people to continue eating the right kind of fish for nutritional benefits. "The brochure is one of our most popular," said Health Department toxicologist Barbara Brooks. She also said that the department plans to amend the brochure by adding new options for safe fish consumption, such as portion recommendations for pelagic fish, so that people will not be unduly alarmed into abstaining from fish altogether but will have better guidance to reduce their risk of mercury exposure.

Brooks said the new study answers some puzzling scientific questions, though she said further work by the EPA and the FDA would be necessary to determine if the study's findings on the contamination process necessitate any new policy changes regarding safe fish consumption. The state Department of Health and the FDA conduct random testing on mercury levels in fish that are commercially sold in Hawai'i. In addition, the state investigates reports of suspected fish contamination, where mercury could be a possible cause, Brooks said.

Meanwhile, some say the new study will encourage growth in Hawai'i's aquaculture industry, which includes several aquaculture enterprises that plan to expand their production of farmed fish. "We have been working with several companies to develop the food supply of farmed fish, so that we can harvest 'ahi and other pelagics that eat nothing with either mercury or other toxic substances such as PCBs," said Harry Ako, chairman of the Molecular Biosciences and Bioengineering Department at the University of Hawai'i's School of Medicine.

Ako said the study supports "the logical conclusion" that fish on a mercury-free diet will not have mercury in their tissues. He said UH-led research in fish food supply and other areas of aquaculture has aided Kailua-based Hukilau Foods – operators of a moi farm in 'Ewa, which is seeking state approval to lease more ocean water.

Some Native Hawaiian and environmental groups have opposed aquaculture expansion in general. They have concerns that fish farming alters natural fish behavior, leads to harmful accumulation of fish waste and conflicts with Native Hawaiian cultural fishing practices. Ako, who is Native Hawaiian, acknowledged that research on aquaculture enterprises more than 30 years ago showed some adverse environmental impacts. "There were instances in Norway where the cages were too small and too close to shore and did not allow for (ocean tides) to naturally exchange the water and keep conditions healthy," Ako said. "Those companies were not putting enough of a priority on protecting the environment."

"The motivation for responsible aquaculture is to replenish the supply of fish," he added, noting that fish stocks are suffering from massive decline as the result of manmade impacts such as mercury emissions from the Asian industry that hang over the western Pacific "like a huge cloud of contamination."

USGS' Krabbenhoft emphasized that his study indicated that fish do not ingest mercury from the water that passes over their gills, but from eating mercury-contaminated prey. He agrees that this might have promising implications for aquaculture operations that have developed technologies for feeding fish in a way that bypasses the food chain in the wild. "Quite simply we documented a process that showed fish get the contamination from the food they consume – much like humans," he said.

Nonetheless, he added that other factors could change the complex web of relationships stretching from Chinese industrial expansion to Hawai'i dinner tables. For example, climate change could shift ocean currents so that mercury-bearing fish no longer make the same migratory trek near the Hawaiian Islands. "The scientists provide the information so that decision makers can do their job," Krabbenhoft said, noting that this suggests further need for coastal communities to stay informed about the state of their natural resources in order to keep fish stocks healthy.