Study finds low levels of microcystin in fish

Posted 7/3/19

PORT MAYACA – An Ocean Research and Conservation Association (ORCA) study on microcystin levels in fish in the St. Lucie Canal found very low levels of microcystin in most fish, well within the …

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Study finds low levels of microcystin in fish


PORT MAYACA – An Ocean Research and Conservation Association (ORCA) study on microcystin levels in fish in the St. Lucie Canal found very low levels of microcystin in most fish, well within the levels deemed safe for human consumption by the World Health Organization. The only fish found to be high in microcystin were armored catfish, sometimes called “algae eaters.”

“Exposure to Toxic Algal Blooms: The vulnerability of Martin County’s subsistence fishing communities” was recently published by OCRA.

Special to the Lake Okeechobee News
The Florida Department of Health advises pregnant or nursing women and women who may become pregnant to limit their consumption of some fish due to concerns about mercury in the fish. For more information, the 2019 guide to eating fish caught in Florida is available online at

“The World Health Organization (WHO) has set a provisional total daily intake (TDI) for chronic microcystin exposure (40 nanograms per kilogram of body weight) and set acceptable drinking water levels (less than 1 microgram per liter). Several studies have demonstrated the accumulation of microcystin and other cyanotoxins in species of plants, bivalves, crustaceans, and fish destined for human consumption. Given these findings, a case has been made for including fish consumption in estimates of microcystin exposure,” the report states.

Martin County hosts a large subsistence fishing community, anglers who fish primarily to feed their families, the authors explain. ORCA completed this study to estimate the exposure of a subsistence fishing community in Martin County to microcystins.

The study was conducted from August 2018 through May 2019. The researchers collected fish at the areas they found people fishing, primarily on the St. Lucie Canal near the Port Mayaca Lock and adjacent waterways.

“Based on interview results identifying fish species eaten, ORCA researchers collected the same species of fish — caught at the same locations — for microcystin analysis. Fish were collected by angling, and through donations of fish from the fishers. Samples of skin, fillet, and liver were collected from each fish,” the report explains.

A survey of 50 people who fish the St. Lucie Canal and adjacent waterways found crappie (speckled perch) is the most frequently eaten fish, followed in frequency by bass, catfish and bluegill. All of the people surveyed reported eating the filet, while 60% eat skins. “None of the fishers reported eating organs, but 6% did eat other parts of fish including eggs, head (no eyes), and tail meat. The majority of fishers fry their fish (96%). Fifty-four percent bake their fish, and 22% used additional methods of preparation including broil, boil, steam and grill,” the report states.

The study found that most of the microcystin in all fish was found in the liver. None of those surveyed indicated they would eat the fish liver. In addition to the fish most commonly eaten, the researchers also tested mullet, gar, tilapia and armored catfish. None of those surveyed said they eat armored catfish.

Of the fish tested, only the armored catfish had high levels of microcystin in the fillets.

“Based on the WHO’s tolerable daily intake for microcystin exposure of 40 nanograms per kilograms of body weight, a 155 pound (70 kg) person should consume less than 2800 ng of microcystin per day. Taken together, the data collected in this study do not indicate exposure levels approaching this tolerable daily intake among subsistence fishers in Martin County. The one exception is armored catfish. This species has consistently been found to have very high levels of microcystin,” the report states.

Based on that formula, a 40-pound child should keep consumption below 720 ng of microcystin per day.

The average serving of fish consumed by the study participants was well below the level considered safe by WHO.

For example, the study found a 6-ounce crappie fillet contained about 204 ng of microcystin; bass, 102 ng; channel catfish, 119 ngs; bluegill, 85 ngs; mullet 170 ng; and tialpia, 578 ng. By comparison, a 6-ounce fillet from an armored catfish contained 2,924 ngs of microcystin.

The authors noted that while the study found the microcystin levels for most fish were low compared to the EPA maximum consumption limits, there are other factors that consumers should consider, such as mercury. It is especially important to keep all of the food safety factors in mind when feeding pregnant women and children.

The study authors recommend reducing the blue-green algae concentrations in the waterways by reducing the nutrient load into the waterways.

“Ultimately, the upstream solution of reducing the nutrient load to our local waters to reduce the frequency, size and duration of algae blooms is urgently needed to prevent the public risks associated with the blooms. In the meantime understanding and addressing human health concerns associated with the transfer of cyanotoxins is warranted,” the study states.

The report also includes microcystin levels in the water in the St. Lucie Canal at the Port Mayaca Locks. Microcystin was detected most months in low concentrations. In August, September, tests found microcystin slightly above 2 micrograms per liter. In November, it was less than one microgram per liter. No microcystin was detected in the December test. In January, very low levels were detected (less than 0.5 micrograms). In February, no toxins were detected. October was the only month with comparably high concentrations of microcystin in the water, with almost 10 micrograms per liter. While the World Health Organization recommended level for human recreational contact is 10 micrograms per liter, earlier this year, the United States Environmental Protection Agency set the recommended limit to no more than 8 micrograms per liter.

Florida Department of Environmental Protection (FDEP) samples taken over the past year found Lake Okeechobee samples tested lower in microcystin than did samples tested in the St. Lucie Canal. A study published by Florida Atlantic University in 2018 found the toxin levels were higher in the St. Lucie Canal closer to the east coast. The study also found the nitrogen levels in basin runoff were higher as the water neared the coast.

Armored catfish are invasive
Armored catfish, sometimes called “algae eaters,” are considered a harmful invasive species in Florida waterways. The fish were imported from South America for use in aquariums. It is believed some people released these nonnative fish into Florida waterways when the fish grew too big for their aquariums. The armored catfish quickly reproduced to become a major hazard to Florida waterways, where they burrow into the banks of canals which degrades the stability of the banks. They have also been known to harass manatees by latching onto the manatees to suck algae off their skin. The distressed manatees are exhausted by the efforts to rid themselves of the catfish. There have even been reports of manatees going into colder water to escape the armored catfish; water temperatures below 68 degrees can cause hypothermia in manatees.