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Aquatic Parasites

“We were throwing a cast net on a school of pilchard and within our catch we found a juvenile mackerel. While analyzing the fish a small crustacean-like creature crawled out from its gill plate. I’ve seen parasites within the flesh of fish while filleting and I know most fish are hosts for parasites, but I’ve never seen anything like this. What is it?” – Jeff Wrider

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You’re absolutely right. Most fish serve as hosts for parasites, but parasites are often indicators of a healthy ecosystem even though they can be detrimental to the host species. Aquatic parasites that infect fish come in a wide variety of shapes and sizes, with some preferring to attach to locations on the skin, fins, gills, stomach and oral cavity, with others living within the flesh of the fish.

While the mackerel species in the photo is hard to distinguish, there are certain isopods that only infect Spanish and cero mackerel.

According to Derke Snodgrass, a fishery biologist with NOAA, the parasite in your photo appears to be a common isopod. Isopods are an order of crustaceans, and there are thousands of variations of isopod parasites, some of which even live on land. Isopods that infect marine species frequently occur in a wide variety of species inhabiting a broad range of habitats spanning from shallow intertidal zones to the ocean’s deepest depths.

Fortunately, these parasites have no effect on the edibility of a fish’s flesh, but they can have a significant influence on the host. Sometimes these parasites live in the oral cavity and grow so large they can impede a fish’s ability to feed. They can also damage gill filaments and create points of entry for bacteria and disease. What’s interesting about marine parasites is that they are often so host specific that a number of game fish can be identified by simply observing their parasites. In addition, there have been studies determining primary habitats of fish based on the parasites they harbor. While the mackerel species in the photo is hard to distinguish, there are certain isopods that only infect Spanish and cero mackerel.

While isopods pose little threat to humans, roundworms, tapeworms and flukes are only a few parasites that can be harmful if consumed raw or undercooked. If you’ve ever filleted an amberjack or seatrout, you’ve likely seen worm-like parasites within the flesh. They are often easy to see and you can cut around them, but just to be safe parasitologists highly recommend you thoroughly cook all seafood.

In addition, Food and Drug Administration regulations require all fish (except tuna) to be served raw in the US—whether as sushi, sashimi, ceviche, or tartare—to be frozen first to kill parasites. The ability of freezing fish to kill parasites varies greatly on several factors including temperature, length of freeze, type of parasite and species of fish. FDA recommends freezing at a minimum of -4ºF for seven days or -31ºF for fifteen hours, but unfortunately most home freezers do not reach the extreme temperatures required to kill parasites like those used by commercial operations. Taking all of this into consideration, the risk of health issues arising from eating fish is much more focused on increased levels of bacteria from mishandling and poor preparation practices than it is on parasites. Yum!

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