March 24, 2022 at 1:37 p.m.
How microbes in the water affect the Friday night fish fry
How microbes in the water affect the Friday night fish fry
However, there are contaminants found in some fish that can be harmful. For those who enjoy catching their own meals, fish consumption advisories let people know "how much is too much" when it comes to eating locally caught fish. Of course, it can still be said that, with locally caught fish, the angler has a better idea of from what conditions the fish came. The state has a fish safe-eating guidelines that are set out for all waters in Wisconsin, and also special consumption advisories for certain lakes and waterbodies.
In a recent seminar, Trina McMahon of UW-Madison spoke about how contaminants such as mercury wind up in fish.
There are two ways in which mercury can accumulate, she said. The first is bioaccumulation. Because fish are in the water 24/7/365, contaminants in the water can, over time, build up in the fat of the fish. This is called bioaccumulation. It happens over the lifetime of the fish. That is why large, older fish have been found to contain more mercury than smaller fish in systems where there is mercury contamination. For instance a musky of legal size to keep, is an older fish. These fish are much more likely to have high levels of mercury in their systems. For that reason, it is not recommended for children and women under the age of 50 to eat musky at all. For women over 50 years of age and men, the limit should be one fish per month.
Biomagnification is another way in which contaminant levels can increase. This has to do with the food chain. If fish eat contaminated invertebrates, for instance, then a seal comes along and eats the fish, and a polar bear comes and eats the seal, the polar bear will have higher levels of contamination due to the amount of its food that is contaminated. Humans, too, can be affected by mercury levels in the same way, as a top predator in the food chain.
Mercury is especially problematic for developing bodies. Fish consumption advisories are important for women of child-bearing age and children under the age of 15. McMahon said normal adults are not seen with mercury poisoning, but it can affect children and women who may become pregnant.
The state also has guidelines for popular commercially sold fish. These include fish found in fish fries across the Northwoods. Atlantic or Pacific salmon, farm-raised catfish, shrimp, pollock and other purchased fish low in mercury have no consumption restrictions for women over 50 and men, but children and women of child-bearing age should limit their intake to two meals per week.
When looking at some of the favorite fish fry fish in the Northwoods, walleye, perch, bluegill and crappie are common table fare. The advisory from the Department of Natural Resources (DNR) states children under 15 and women of child-bearing age should restrict their walleye intake to one serving per month. This moves to one serving per week for women over 50 and men. While men and women over 50 have an unrestricted advisory for yellow perch, bluegill and crappie, children under 15 and women under 50 should limit their consumption to one serving per week. Full guidelines can be found on the DNR website dnr.wi.gov in the publication, "Choose Wisely: A health Guide for Eating Fish in Wisconsin."
McMahon's research focused on how mercury is converted to methyl mercury, which then builds up in the fatty tissue of the fish in our lakes. She said she was interested in learning more about how much methyl mercury is produced, what conditions favor the production of methyl mercury from mercury in the water and how easily it can enter the food web.
Mercury is converted to methyl mercury by microbes in the water, she said. Mercury in its inorganic form cannot build up in the body. Once methylated, when it become methyl mercury, that organic form can build up in the body.
Without that bacteria, there would be no conversion. The issue, especially with methyl mercury, McMahon said, is that it is very hard to measure. There are only a few labs in the country that can do it well, she said. One of those labs happens to be the USGS mercury lab in our state, located in Madison.
She spoke about a recent study in northern Wisconsin that found more methyl mercury production in the water column than in the adjoining wetlands or in the sediment at the bottom of the lake. Perviously, it was assumed the methyl mercury found in the study lakes was coming from the adjoining wetland. However, it was then found that sinking phytoplankton was fueling that methylization. The study found a bulge in the amount of methyl mercury at approximately 13 meters of water. If the contaminant was coming from the sediment, she said, it would have been present in higher concentrations at the bottom of the water column.
She found zooplankton were "hanging out" just above the thermocline, where there was little dissolved oxygen directly below them.
When a bacteria such as cyanobacteria, also known as blue-green algae, dies, it sinks down to this level and gets trapped in the thermocline. When the bacteria gets down to where oxygen is depleted, this is where bacteria methylate the mercury. This, she said, is another reason to worry about cyanobacteria and try to find ways to limit those harmful blooms. Bacteria that converts mercury to methyl mercury can live anywhere with dissolved oxygen, meaning anywhere from the thermal cline down to the bottom of the lake.
This research, McMahon said, is a new and emerging field. Her research is ongoing, and she hopes to better understand how this conversion is taking place and what can be done to limit the inorganic form of mercury's conversion or the organic form. Her entire presentation can be found on the Clean Lakes Alliance YouTube channel or by searching YouTube for "Clean Lakes 101."
Beckie Gaskill may be reached via email at [email protected].
Comments:
You must login to comment.