But the waters close to Japan have not held many fish for decades. So to feed the population, fishing boats got bigger and traveled farther. 

The farther the fishermen went, the longer it took to bring in the fish. If the return trip took more than a few days, the fish were not fresh. The Japanese did not like the taste.

To solve this problem, fishing companies installed freezers on their boats. They would catch the fish and freeze them at sea. Freezers allowed the boats to go farther and stay longer. However, the Japanese could taste the difference between fresh and frozen. This wasn't a good thing. Soon the frozen fish brought a lower rice.

So fishing companies installed fish tanks. They would catch the fish and stuff them in the tanks, fin to fin. After a little thrashing around, the fish stopped moving. They were tired and dull, but alive.

Unfortunately, the Japanese could still taste the difference. Because the fish did not move for days, they lost their fresh-fish taste.

So how did Japanese fishing companies solve this problem?

They added a small shark to each tank. The shark eats a few fish, but the rest of them are continuously challenged. They stay in a very lively state--and arrive tasting fresh.