If you’ve ever eaten sushi, you’ve probably scrolled through a list of different raw fish on the menu, in addition to all the spider rolls and dragon rolls. Salmon, tuna, and shrimp are some of the most common, but what about other, commonly eaten fish, such as catfish or trout? Why don’t we see these in any Western sushi restaurants? 

The big difference is in where the fish were caught. The science behind freshwater or ocean fish consumption is fairly simple: freshwater fish, at least in the United States, tend to have more parasites. 

Scientists are calling for caution when selecting raw fish for consumption as our demand for it expands across the globe. In a review published earlier this year, researchers updated the current understanding of food safety hazards of raw fish. 

Microscopic creatures such as Diphyllobothriidae (a type of tapeworm) can cause horrible illness when consumed, leading to diarrhea and intestinal issues in humans. One of the best ways to get rid of these parasites is by cooking the fish, whether by frying or baking at a certain temperature. If you’ve ever caught food poisoning from seafood, you’ll know it is perhaps the most painful version of a stomach bug. There are exceptions to this need for ocean caught fish rule for raw consumption in other countries. Namely, that is the eel, which is smoked and then prepared with a sugar-based sauce. Other problematic fish, such as mackerel, are pickled.

How fish is prepared is another reason why sushi is often ocean fish. Most ocean caught fish are frozen immediately after catching and butchering. This kills most of their potential parasites. Once the fish is frozen, it can be safely transported to a wide variety of destinations, including places far from local coasts, and inspected for parasites.