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Blue marlin in gulf have high mercury levels


As sport fishes go, the blue marlin is a king of sorts – highly prized for its beautiful shape and its ferocious fighting ability when hooked.

That’s the good news. The bad news is that many blue marlin caught in the Gulf of Mexico contain 20 to 30 times the acceptable levels of mercury.

Texas A&M University at Galveston researchers Jay Rooker and Gary Gill are trying to learn why the mercury levels are so high in blue marlin compared to similar fishes and why so many of them are potentially toxic timebombs.

Funded by the McDaniel Charitable Foundation of Santa Fe, Texas, the project aims to find answers to several key questions related to mercury accumulation in the flesh of pelagic fishes (those such as marlin, sharks, cobia, amberjacks and others). Rooker and graduate student Yan Cai sampled pelagic fishes in the Upper Gulf Coast region, from Port Aransas to Venice, La. "We are attempting to get a good coverage area in the northwestern Gulf of marlin and other species in our survey of fish tissue mercury," Rooker says from his Galveston office.

Mercury is a highly toxic substance that is easily absorbed into tissues of aquatic creatures, especially fish. Human consumption of fish that contain mercury can pose severe health problems.

The harmful effects of mercury on living organisms range from kidney damage to reproductive failure and birth defects. The issue of elevated mercury content in seafood has received a great deal of public awareness in Gulf Coast states in recent years and several articles have focused on the possibility that oil and gas platforms in the Gulf of Mexico are the major source of the mercury problem in the Gulf.

To investigate the source question, Rooker, Gil and Cai are examining mercury in pelagic fishes from areas with high concentration of petrochemical industries or production platforms and comparing these with other regions. Also, the team is using unique natural biomarkers (stable isotopes and fatty acids) to determine the feeding history of these fishes, and will use this information to see if feeding patterns influence the level of mercury found in the tissue of top-level predators.

Rooker says one reason marlin may have higher mercury levels deals with the fish’s size and feeding patterns. Although most blue marlin caught in the Gulf are approximately 200 to 400 pounds, some can weigh more than 1,000 pounds and can live up to 20 years, and because they are so large, they tend to eat larger than average fish, ones that likely contain higher levels of mercury. Over time, the amount of fish they consume is considerable, meaning mercury levels in each blue marlin will continue at a sustained rate over a period of years. "In simple terms, marlin may have more mercury because they eat larger prey and live longer than other fishes," Rooker explains. "Still, the story appears to be more complex and we are in the process of examining other large pelagic fishes, and comparing our results to blue marlin."

Rooker says that preliminary data show fish with levels appear to have similar feeding histories. In addition, the team has found several species captured in Texas waters have higher mercury levels than those closer to Louisiana. "This appears to be a source issue," Rooker believes. "But this is something we need to examine more closely."

He says sport fishermen who have caught marlin should not be overly alarmed, but should be informed of what they’ve hooked. "I would recommend that they be aware of the problem, and they may want to limit their consumption of sport fishes caught," he adds.

"We’ve just scratched the surface on this problem," he believes.

"The mercury levels we discovered surprised us and were much higher than we had anticipated. What we need to look at now are the diet patterns of these fishes and the exact locations they’ve been found. Is there an undeniable link to petrochemicals that is causing these higher mercury levels? We need to find the mechanism that is triggering all of this."

Keith Randall | EurekAlert!
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