I remember watching a movie once where a rich criminal was bragging to his guests that the lobster they were eating had been flown in special from Maine that morning. It was fresh out of the sea, and he wanted them to know it and be impressed. When you think about it, that's the message we always see in the various forms of media. Fresh seafood means it's the best seafood. Or at least that's what you are led to believe.
I'll agree with our movie criminal friend on one point - fresh is the best when it comes to shellfish. Oysters and lobster are prime examples of seafood that you want right out of the ocean if you can. When it comes to sushi though, it's not always the case. You may sit down at your favorite sushi bar and ask for their freshest stuff. However, if the chef is doing things right, some of your fish wasn't caught that morning.
Sometimes fish for sushi is better when it's been aged. Maguro (Tuna,) Hirame (Halibut,) and Kampachi (Amberjack) are all prime examples of fish that should be aged. When a fish is caught it's generally under a tremendous amount of stress. It's muscles and tissues all tighten up. Think about what happens inside your own body when you are in trouble or scared. It's the same thing. Aging the fish will loosen things back up again.
Aging fish properly is a labor of love. The fish must first be properly gutted and thoroughly cleaned. It's then wrapped in a moisture-absorbing cloth. The fish is then kept cold as the cloth draws the moisture out of the fish. The cloth needs to be changed daily, and the whole process takes four or five days.
The result is sublime. By taking the time to reduce the moisture in the fish you draw away impurities and concentrate its flavor. This fortifies the umami taste that sushi lovers crave. The aging also allows the fish's natural enzymes to break down the muscle and soften the texture.
Many of us have a favorite sushi bar. We generally find it by trying a bunch of places where the sushi is ok, but then we come across that one place where the fish tastes like butter. Once we find that, nothing else will do. "Butter" happens when you properly age fish, and it's the only way I do things. Spend time in one of my classes and you'll experience the difference when things are done right!