I would not buy fish at a grocery store and eat it raw.... there are so many reasons why you shouldn't and most of them involve bacteria and parasites. where you know the fish is safe to eat raw. Definitely do *not* buy fish from a grocery store and assume it's safe to eat raw, the potential for cross contamination is astounding.The main difference is freshness, or the time it would take to reach your "mouth."All the sushi restaurants I've worked in Los Angeles and San Francisco got their fish directly from the wholesale fish supplier and they all used those fish within days.Because those fish were "sushi" or "sashimi" grade (not an official term, it's just a term used by grocery stores and consumers), they needed to be as fresh as possible and had to meet sushi chef's freshness quality standards.Some of the same fish suppliers deliver those fish to the supermarkets, and they do sit in the fridge until a consumer purchases it and takes it to home to cook.Many factors would make fish a "Sushi" or "Sashimi" grade (in commercial terms), so, unless labeled, grocery stores like Whole Foods can keep the fish a bit longer than sushi restaurants it it were meant for "cooked" consumption.

Fish sold for sushi is handled in a shorter, more direct logistic chain than fish sold for home or restaurant cooking.Fish sold for sushi (i.e.,  fish sold for uncooked consumption) is kept frozen until thawed for preparation of the sushi, unlike fish sold for cooking, which may be shipped on ice, and is thawed for sale.What qualifies fish as sushi-grade?What does the supply chain for sushi grade fish look like? Can you get sushi grade fish at your nearest chain grocery?There's generally five de facto grades for fish at a commercial fish market, usually bought in the "what sells first for how much" order:The best and most expensive stuff the sushi guys buy first.The next best stuff, that restaurants buy for the catch of the day.What the supermarkets buy for the fish freezer.What the canneries buy for canned fish.What the pet food people buy for pet food...and also what the "discount sushi" people buy (i.e. never eat "discount sushi').A lot of places have exclusive deals with fisheries, like CostCo does.

Whole Foods has some direct vendor relationships, as detailed here:SeafoodIt's difficult to understand the "real" or intrinsic difference between excellent quality fish and not-excellent quality fresh fish unless you've worked with it for a period of time.  Once you know it though, great fish sticks out like a sore thumb.Depending on the species, great, fresh fish has a unique translucence and texture.  It's more "clear" than stuff of lesser quality and the texture is more like jello jigglers.  It has a bounce.  Conversely, less than pristine fish is cloudier and has more give.  Great quality fish is great not only because it tastes and eats better, but it's about 10x easier to work with and cut.  Lesser quality fish are more delicate and are more easily damaged.Now that we've covered the flesh itself, there's one common misconception to take care of.  Most people think that sushi is all about the "freshest fish," and have the idea that the less time that the fish spends between the ocean and your mouth, the better. 

The reality is that getting peak flavor from fish all depends on the fish.  Most sushi fish benefit from some sort of aging, curing, pickling or any combination of the three.  Just like with land animals, you develop flavor with time.takeout sushi downtown calgaryHere's a picture of 10 day aged fish from Zaiyu Hasegawa of Den in Tokyo:And just like with land animals, there is a difference between aging and just plain rotting.  sushi maken onlineThe former is the result of deliberate handling and the latter, one of neglect.sushi the global catch onlineSome fish, endangered Blue Fin Tuna is a good example, are flash frozen on the boats, auctioned frozen, and then further kept in deep freezers for months until ultimately thawed out by the restaurant to be cut and served.  sushi online leuven

Most of the most expensive tuna that you'll have at the nicest sushi restaurants in the US will have gone through this process.  Never frozen Japanese Bluefin tuna is highly sought after, but it's a minority of the trade and almost never leaves Japan.Going back to fish before any processing, aging, or storing, great quality fish is a matter of good fishing practices, selection and proper handling.  how to get sushi grade salmonCulturally, the Japanese are of a completely different mindset compared to the western world in each of those aspects.  corso di sushi onlineThrough experience, they readily identify and classify the quality of the fish and sell it accordingly.  mac sushi kiss ukThere are US importers of fish that have gone through the Japanese system, and so it is possible to get high-quality fish if you're willing to pay for it and seek it out. 

Getting fish through the US fishing system, my experience is that "fish is fish" so it's more luck-of-the-draw and your own personal selection required to get a great quality fish.  I've used the best local fish purveyors that I could find, and even then, it's not unusual to get just one excellent quality fish out of a batch of  60 fish.So, given how difficult it is for me, as someone who actually cares and knows the difference to get excellent quality fish out of the local wholesale system, you can imagine the quality of fish that end up at your local fish counter.  At my favorite local supermarket, they do from time to time luck out and get a few high-quality pieces of fish.  I always buy it as soon as I see it.  This happens roughly once every 6 weeks.  That's not to say that it's impossible to get great fish at a whole foods or other fishmonger, just that it takes quite a bit of effort.So even at the retail level, just start being observant, look at the fish on display, check to see how "alive" it looks.