Fish length, not location, to be used for Iowa fish consumption advice

Anglers delight in catching big fish, but smaller fish are probably safer to eat because they have lower levels of mercury.

That’s why the Iowa Department of Natural Resources wants to change its system for setting fish consumption guidelines.

New standards, expected to be released later this year, would use the length of fish — not tissue samples collected from fish in specific lakes and rivers — to advise Iowans about how often to eat local fish.

“We would still do some sampling, but our data with thousands of samples establishes a pretty good link between length and mercury levels,” said Ken Krier, environmental specialist senior with the DNR’s Water Quality Monitoring and Assessment section.

Fish are a low-calorie source of protein and the U.S. Food and Drug Administration (FDA) recommends eating 8 ounces of seafood a week for most adults.

Children and pregnant or nursing women should eat less fish, and avoid some varieties, because many types of fish contain mercury, which builds up naturally in fish and also can be absorbed from polluted water.

Most Iowa anglers — 80 percent — eat the fish they catch at least some of the time, the DNR learned in a 2018 and 2019 survey of 1,628 Iowans with fishing licenses.

Among anglers who said they do eat their catch, the median number of meals was six a year. But some low-income residents or immigrants eat locally-caught fish weekly or daily.

For years, Iowa has provided fish consumption advisories based on types of fish in specific water bodies.

For example, the state says sensitive populations should eat no more than one meal a week of any predator fish (such as walleye or bass) from the Cedar River from the Highway 218 bridge at Floyd to the Iowa-Minnesota state line, or from the Iowa River from Swisher to the dam in Iowa Falls.

These advisories are based on annual tissue samples the DNR takes from fish in Iowa lakes and rivers. The samples are analyzed for the presence of mercury and polychlorinated biphenyls (PCBs).

In 2014, the EPA stopped providing states free lab analysis for PCBs and pesticides in fish, causing Iowa to review its sampling and testing program, Krier said.

The DNR has worked with Iowa State University to compare tissue samples of predator fish with fish lengths to come up with length-based standards. Bigger, older fish have had more time to accumulate mercury in their bodies and they also eat smaller fish that have the metal in their tissue.

“Over the last couple of years, (we did) about 800 to 900 surveys a year to come up with length-based standards." Krier said. “Those (standards) will be coming out this fall or winter.”

Using length to advise anglers on fish consumption isn’t new.

Thirty states had advice based on fish length in 2007, according to a paper published in the Journal of Environmental Health Perspectives in 2008.

This shift would save the DNR staff time and money in regular sampling and lab analysis, Krier said.

“We’ve hit pause on other tissue sampling while we get this mercury problem solved,” he said. “Then we’ll get back to sampling the bottom feeders and looking for historic contaminants of concern as well as emerging contaminants.”

The new length-based standards for mercury in fish will mirror the joint guidance from the FDA and EPA, Krier said.

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