Freshwater Shashimi: is it Safe?

Ready to cook, freeze or turn into sushi. Photo by author.

Other than slick-talking car salesmen and pretty girls selling raffle tickets, people don’t often “put one over on me” – especially, youngsters. I have nine younger brothers and sisters.

But a ten-year-old nailed me good one day after we’d been fishing. Long story shortened, we each had a freshly caught, freshly cut, small piece of fish in hand and on the count of three, we agreed we’d pop the freshly caught sashimi into our mouth.

One, two, three…I popped, he popped towards his mouth but purposely pitched the raw morsel over his shoulder. Then he laughed at me. I laughed too, but if he’d been one of my five younger brothers, I’d have had the last laugh. Big brothers always get the last laugh.
Good golly! I could have gotten Diphyllobothriasis.

A friend posted on Facebook about having good day of fishing recently. He got the usual, “way to go” comments and one claiming to be jealous because the commentator wanted some fresh, homemade sushi. That prompted a follow-up comment saying “it’s unwise to make sushi with freshwater fish.”

I’d heard that before, but never wondered why, not being a regular consumer of raw fish other on dares from ten-year-olds. So why?

Off the top of my head, I came up with three possible reasons – toxins, germs or parasites. So I dug into the subject a bit more and what I learned revealed science, psuedo-science and a few tales originating from old wives and promulgated on Facebook.

Toxins – I found no evidence of poisonous freshwater fish, especially the species of fish most likely to be consumed raw, such as salmon, trout, walleye or panfish. There is a toxic saltwater fish commonly eaten raw, a dish called Fugu is made from puffer fish and almost all puffers contain a toxin over a thousand times more potent than cyanide.

Almost all fish, fresh or saltwater, contain some measure of chemical contaminants which can be passed on to consumers, whether cooked or uncooked. If these concern you, follow consumption guidelines.

Germs – Bacteria and viruses don’t normally exist inside fish or animal meat, whether it’s a steer from a feedlot, a carp from the Iroquois River or a wicked tuna. Microbes can contaminate the surface of raw meat. The presence or absence of germs on the exterior of the sushi is determined by the handling of the fish and flesh by the fisherman and the sushi-chef. A clean freshwater filet is much better than a not-so-clean saltwater tuna steak.

Parasites – I took a course in college called parasitology. I’ve forgotten far more than I learned, but two things stuck. One is most parasites are host specific. That’s why you can eat those “pepper-flecked” panfish filets without harm. The cysts (pepper flakes) will only pop open and grow into worm if the fish is eaten by a bird.

The other thing is many parasites have a complex life cycle involving more than one host species. The pepper-flecked fish is a fish to bird to snail back to fish cycle.

That’s why I could have contracted Diphyllobothriasis which is the what having a tapeworm (Diphyllobothrium latum) is called. This tapeworm has a complex life style.  Adult D. latums live in the intestines of fish-eating mammals such as mink, otters, humans and others. The eggs produced by the adult tapeworm are passed in feces, get into the water, hatch and are eaten by copepods (a type of zooplankton) which are eaten by minnows or baby fish. The larva grows in the muscle of the parasitized fish and, when eaten raw by a mammal, grows into a tapeworm.

Does this preclude ever eating uncooked other freshwater fish? Not in the least! Heat or cold kills the tapeworm larvae. Cooking fish until it flakes (145 degrees F.) will kill any parasites. So will freezing – most sources suggest a week or longer in a home freezer.

Cold smoking, gravlax, salting or making raw fish into ceviche doesn’t necessarily kill the D. latum larvae which could be present so freeze fish before making these treats, as well. Then thaw and get out the soy sauce or wasabe.

Personally, I’m not all worried about it. Do you think the next ten-year-old will dupe me again?

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Mike Schoonveld

Mike Schoonveld grew up hunting and fishing in rural Northwest Indiana. In 1986 he piggy-backed a career as an outdoor writer onto his already long tenure as a wildlife biologist with the Indiana DNR. Now retired from his DNR position, Schoonveld is a U.S. Coast Guard licensed boat captain, operates Brother Nature Charters on Lake Michigan and spends much of his time trailering his boat to fishing hotspots around Indiana and the Midwest.

Mike can be reached through his website www.brother-nature.com or visit Mike’s Outdoor World Blog at www.bronature.com

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