Hawaii researchers have solved the puzzle of why different species of fish in the ocean have different levels of mercury, even though they are the same size.

“;It has to do with where they are feeding in the water column and what they're eating,”; said Anela Choy, University of Hawaii-Manoa oceanography graduate student who does consulting work with the seafood industry.

Choy led the study with Brian Popp, a UH Department of Geology and Geophysics professor; Jeff Drazen, Department of Oceanography associate professor; and John Kaneko, project director at Pacific Management Resources, known as PacMar Inc.

Mercury is a natural trace element in the environment that has never been associated with toxicity in Hawaii's ocean fish, said Kaneko, whose research focuses on public health.

But methylmercury, an organic form of the mineral converted by bacteria, can be toxic if eaten at high levels by animals or people.

Kaneko said the new research supports evidence that mercury in open-ocean fish is naturally occurring from deep-ocean processes.

The researchers suspected from other work that deeper-ocean animals and predatory fishes might have more mercury, and studies of almost 200 fish collected from longline and recreational fishermen confirmed that, Choy said.

Their findings were published in the Aug. 18 edition of the journal Proceedings of the National Academy of Sciences.

Choy said they looked at mercury levels in tissues of the fish and what was in their stomach when they were caught.

Large fish such as bigeye tuna and swordfish that feed deeper in the ocean have higher mercury levels than yellowfin tuna and mahimahi, which live in shallower water, the team found. That is because they are feeding on fish, squid and shrimp with higher mercury levels, Choy said.

Popp said what was surprising was the difference between yellowfin and bigeye tuna. “;The significance (of the study) is there is more to it than simply the size and age of the fish,”; he said. “;There's an ecological cause to mercury content and the depth at which they're feeding.”;

Kaneko said there is an assumption that mercury in open-ocean fish is directly related to atmospheric mercury emissions and pollution, but there is no evidence of that.

Studies done in 1971 and again in 1998 on the amount of mercury in Hawaii yellowfin tuna showed no differences despite a 26 percent increase in atmospheric mercury emissions, he said.

The researchers are not sure where mercury enters the food web, Drazen said. “;It makes sense that it enters down in deep waters, but we haven't actually looked at mercury in plankton, for instance, consumed by little fish and shrimp. We have to look at lower levels of the food web.”;

Kaneko said there is substantial scientific evidence that high levels of selenium in ocean fish counteract toxic levels of mercury. Selenium, also a natural element, has antioxidant functions and is known to bind to mercury, he said. “;When those two elements bind together, they're biologically inert.

“;What we're finding, in some research we're just finishing now, is it's the ratios of mercury to selenium that is more important than the amount of mercury in fish,”; Kaneko added.

Choy said the research will be expanded and include chemical techniques to see “;who's eating who in the open ocean. The fish that end up on our dinner table we don't really know too much about now. It's important to understand their ecology and how they're connected.”;