Big question: Are fish safe to eat?
Toledo, Ohio — Microcystis is the dominant noxious cyanobacteria, widely called “blue-green algae,” present each year in Western Lake Erie and other lakes throughout Ohio.
Depending upon environmental conditions, such as water temperature and nutrient levels, some microcystis blooms produce various forms of microcystin, a neurotoxin having various levels of toxicity.
Exposure in the field comes from direct contact, inhalation of mist, or accidentally swallowing raw water during recreational water activities. It is capable of causing digestive problems, liver tumors, tremors, and, in high enough doses, even death to animals and people.
In light of the widely publicized algae blooms starting up again in Lake Erie, fishermen now want to know if they may become ill from eating fish. Unfortunately, there are few studies that investigate the risk of ingesting the toxin through the consumption of fish in North America.
The World Health Organization levels for daily exposure for microcystin from fish consumption are based upon amount consumed per body weight. In many of its worldwide study sites, people also drink raw water from where the fish are sampled, combining methods of ingestion.
Thus, it is difficult to provide a threshold of fish which should not be exceeded without taking into consideration the species of fish being eaten, determining which forms of the toxin are present, the levels of each toxin in the fish about to be eaten, the portion size, and the weight of each consumer contemplating having a fish meal.
Most researchers believe that fish ingest microcystis mainly through their food, not through their gills. In laboratory trials, microcystin concentrations peaked 12 hours after eating microcystin-infused food, but were mostly absent 24 hours later. Their livers contained 10-100 times more of the toxin than their muscle tissue.
In another test, pumpkinseed sunfish were fed doses of microcystin-infused food and then toxins in their liver and muscle tissue were measured. These results showed that their livers and muscle tissue both showed rapid initial uptake of the toxin.
But, after about six days of eating the toxin-infused food, the fish were able to digest and eliminate toxins through their gall bladder bile and other bodily waste. So, it appears that sunfish become less contaminated over time even when microcystin levels remain high – good news for inland lake fishermen.
More than one researcher mentioned their suspicion that microcystin can likely bio-accumulate through the food chain, so there was the suspicion that walleyes and other fish-eating fish may reach higher readings.
In a small sample size study of fish from the Maumee Bay collected in 2006 and 2007, there were microcystin levels in walleye, white bass, and smallmouth bass at or near WHO thresholds.
But, in 2011 Ohio Division of Wildlife tissue tests, there were no detectable levels of microcystin in walleye samples, according to Lake Erie Fisheries Research Administrator Jeff Tyson. He adds that new samples have been collected this year and microcystin testing will likely be a permanent contaminant tested for the annual Lake Erie fish consumption advisory.
Fishermen should remain cautious about eating fish caught in and near noxious algae blooms. The researchers were not willing to conclude that there was no risk of microcystin toxicity for consumers of fish.
But, some factors working in favor of Lake Erie fish still being low risk include:
1) Algae blooms are typically seasonal and spatial, with algae present only for a defined period of time in some locations, and some areas remain devoid of algae altogether.
2) Not all microcystis varieties are toxic, so the presence of a visible algae bloom doesn’t necessarily translate into risk of exposure.
3) Of the many zooplankton varieties, only the water flea bosmina consumes substantial amounts of microcystis. Fish that are eating copepods and daphnia water flea species avoid high exposure to toxins.
4) Bosmina have peak abundance in June, which is normally before widespread microcystis blooms, then again in September and October.
5) When toxic algae blooms are peaking in late summer, perch are eating fewer zooplankton and more baitfish and invertebrates (animals without backbones) such as mussels, aquatic worms, and insect larvae.
6) Zebra and quagga mussels select algae other than microcystis, and thus do not provide a pathway to yellow perch or other fish species feeding on them.
7) Fish come and go through the algae blooms, so if fish are taken from clean water, it is likely that the fish have purged the toxins from their systems by the day after any exposure to toxic algae.
8) Blue-green algae tend to be scattered throughout the water column on windy days, but concentrated near the surface on calm days – well away from the bottom of the lake where perch and other sport fish spend the majority of their time. Even walleyes caught at the surface are not there all day.
9) Fish livers, with the exception of cod, are generally not consumed by humans but have more than 10-100 times as much algal toxin in them than is found in the fillets.
10) Microcystin levels are likely low enough to not cause consumption advisories any more restrictive than the ones already in place for other contaminants, such as PCBs and mercury.
Strategies for avoiding algal toxins in fish
1) Anglers should avoid consuming fish that are caught in newly appearing, visible, dense blue-green algae blooms.
2) Use the available satellite images to stay outside of the bloom as the season progresses.
3) Give fish a day or two to purge any built up toxins after an algae bloom disappears before targeting them again for the table.
4) Rinse/freeze fillets in clean water – not lake water.
5) In the case of perch, keep and eat larger ones, which are more likely to be feeding on invertebrates and small fish instead of zooplankton.
6) Space your fish meals throughout the year, regardless of species or time of year caught, so that peak levels of toxin exposure are not exceeded over a short period of time.
7) A larger amount of fish in a smaller person is more dangerous than a smaller amount in a larger person, so match portion size to body weight.
8) During a bloom, consume more yellow perch and sunfish (rock bass, bluegill, pumpkinseed) instead of walleye, bass, steelhead, or white bass.
9) Additional contaminants can be reduced further by removing the skin. Do not feed fish guts to pets, especially the livers, which test the highest for toxic algae.
10) Practice catch and release on larger game fish, such as walleyes, smallmouth bass, and white bass.
Even though the levels of microcystin in the fillets rarely exceeded recommended WHO levels in fish tested so far, given the growth of the algae bloom problem, some fish are more likely to be at or above WHO guidelines during the onset and peak of the algae blooms.
Anglers would be wise to minimize their risk by following the strategies listed above and err on the side of caution with respect to the quantities consumed by children until further information is made available after more testing.
By being careful, the benefits of eating fish will still far outweigh the risks.