Fish have been in the news, recently, as major world fisheries have been overexploited and are now very unproductive. In the North Atlantic Region, in particular, many fishermen have been thrown out of work in the 1990's because there are just not enough fish to catch.
Of course, this is an evironmental problem and a biological problem, but first and foremost, it is an economic problem, a problem of resource allocation. Here is what I mean.
When a fisherman catches a fish and sells it, that fisherman gets a private benefit -- the revenue from selling the fish. But there is a cost -- because there are fewer fish to reproduce, there will be less fish caught (ceteris paribus) in the next and future years. This cost is spread out over all the fishermen and consumers of fish, and thus is "external" to the individual fisherman's decision how many fish to take. The individual fisherman does not take into account the influence of his fishing on the fish that will be available to other fishermen in the future -- it he isn't going to catch them, why should he care? The result is that he does not limit his catch in such a way as to conserve the fish population to reproduce, and this is why fisheries are overexploited.
In effect, when a fisherman cuts back on his catch to leave fish in the water so that the fish can reproduce, the fisherman is investing. He is creating a capital resource: breeding fish. But since he doesn't get (most of) the benefits of this investment, the fishing industry as a whole doesn't invest enough in breeding fish. The world as a whole has allocated too little to investment in breeding fish, and too much to other purposes. This is a problem of misallocation of resources.
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