The two risks we need to address to serve fish raw at home are bacteria and parasites.
When the fish dies, bacteria start decomposing its flesh and multiplying very quickly. Bacteria growth can be minimized by bleeding and gutting the fish immediately after death, and storing it on ice. As the temperature drops, bacteria growth drops off exponentially. Exposing the fish flesh to air introduces more bacteria, so you want to fillet the fish as close to serving raw as possible.
To ensure that these conditions are met, buy your fish from a high end fishmonger who has a good turn over, and from a fishmonger who is comfortable answering your questions. Ask the fishmonger when the fish was filleted. If the answer is “today”, you are in good hands. But please don’t panic if the fish came to the store a few days ago. Whole fish can last very well on ice. If you want to eat the fish raw, I suggest you do so within 24 hours after purchase.
Keep the fish between ice-packs as soon as it’s in your possession (even in the car and in your fridge).
Now let’s talk about parasites. Although the kingdom of marine parasites is extremely diverse, the only 3 that you need to worry about are are cod worm, anisakis, and tapeworm. Those can live in mammals, which is you. The other ones can’t, so they are harmless. Fish that are prone to these species of parasites should not be consumed raw (at least not without freezing). Of particular concern are freshwater fish and pacific fish, like wild salmon. The incidence of parasites in some of those fish is as high as 90%.
Now that I’ve ruined fish for you, let me assure you that not all is lost. Among wild denizens of the seas, tuna, scallops, and oysters are free from cod worm, anisakis, and tapeworm.
Farm-raised fish that are aqua-cultured in huge pools (vs the ocean) are completely parasite free. This includes arctic char, and Mediterranean bass (branzino).
Farm raised salmon is raised in cages in the ocean. Whether these fish pose a parasite risk or not depends on how the farm is set up. Since it’s not easy to find out, it’s best to freeze farm-raised salmon before eating it raw.
Wrap it tightly in plastic, place in the freezer bag and freeze. 7 days in the home freezer is sufficient to kill parasites. To defrost, move it to the fridge 24 hours before serving.
I can’t get a straight answer from any source about parasites in hamachi. But since the only hamachi available to me in Boston is imported from Japan in the frozen state, I don’t need to worry about parasites.
Does freezing solve all the problems?
I know what you are thinking — can’t we just freeze all fish? In sushi restaurants, most fish is frozen, but they have a freezing technology that doesn’t allow water to crystallize. This prevents fish from turning mushy. Freezing halibut, fluke and most other white fish in a home freezer completely destroys their texture. But salmon freezes quite well in a home freezer because it’s fatty.
Oh, and by the way, the freezer section in your supermarket is not a good option for raw consumption. Although the parasites in those fish are long dead, the bacteria are not. Freezing pauses bacteria growth, but doesn’t kill bacteria. Most of those fish have been through a lot of transportation. In some cases, they were frozen and defrosted numerous times. Even if they were safe, their flesh is often mushy and the flavor is dull.
If you like your fish cured in acid or salt, like ceviche or gravlax, keep in mind that unlike heat, acid and salt do not kill parasites. So the same precautions that apply to raw fish, apply to cured fish.