The Japanese have always loved fresh fish. But the waters close to Japan have not held many fish for decades. So the fishermen had to go further out to sea to catch their supply of fish, the fishing boats got bigger and bigger.
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 and they lost their fresh 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 and they did not like frozen fish. Also, the frozen fish brought a lower price. 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.
How did the Japanese fish company solve this problem? To keep the fish tasting fresh, the Japanese fishing companies added a small shark to the tanks that the fish were kept in. The shark of course eats a few fish, but most of the fish arrive in a very lively state due to the fact that the fish are challenged.
Keep that shark inside you alive!