I heard [a story] about attempts to ship fresh North Atlantic cod from Boston to San Francisco during the nineteenth century. At that time the only way to ship the fish to the West Coast was to sail around the South American continent—a trip that took months. As you can imagine, the first attempts to dress the cod in Boston and pack them in ice failed miserably. By the time they reached California, the fish weren't exactly fit for consumption.

Next, the cod were placed in holding tanks full of water, shipped to California alive, and dressed there. The results were less than satisfactory. The fish didn't get much exercise during the trip, and as a result they were pasty and relatively tasteless.

Finally, someone hit upon an interesting idea. "Why don't we put some catfish in with the cod?"

Why? Because catfish are cods' natural enemy. Sure enough, when a few catfish were placed in those tanks with them, the cod were always alert and swimming around. This time, when the fish reached San Francisco, they were in perfect shape.