Marine biologists warn that the world tuna supply is plummeting and that a big part of the problem is the growing demand for sushi. That’s quite disappointing news because we enjoy sushi very much.

Fifty years ago, the annual global tuna catch totaled less than 500,000 tons. But that was before a sushi bar occupied the corner of every American suburb. A 2001 measurement put this figure above the 3.7 million ton mark and rising.

Just in case you are enjoying a tuna-on-rye sand as you read this, keep on chewing. Because common tuna varieties like skipjack, sold in cans at the supermarket, fetch low prices and are not in immediate danger.

It’s the enormous bluefin tunas, worth as much as $150,000 per fish on the Tokyo market that are the target of sophisticated global buyers. The bluefin is endangered not only in the Mediterranean, where 20 percent of the world's dwindling supply is normally caught, but also in the Atlantic and Pacific. Uncontrolled fishing, damage from pollution, silt runoffs from over-engineered river systems, and global warming are the main culprits.

As investors, we think about fishing a lot and not just while daydreaming about a weekend by the lake. We fish on the job … for alpha, or excess return. We too face issues of limited, if not diminishing, supply.

No matter how skilled the fisherman, a successful catch depends upon a well-stocked fishing hole. Yesterday’s abundantly supplied locale may disappoint today. A depleted fishing hole sometimes restocks itself naturally. But it often happens that when it’s fished out, it’s done for good.

Many investors are skilled in fishing for alpha at one preferred fishing spot. But knowing

1) when to move on
2) where to move on to, and
3) when & why to return

separates the many who have a few really good days from the few who bring home many fish all year long.