"Overfishing" has been blamed for the decimation of wild fish populations worldwide. When we say "over" it should be with respect to some criterion. The popular norm is maximum sustainable yield (MSY), the maximum harvest per period that can be consistently obtained from a fishery. (Economists have an even more conservative norm, but MSY is sufficient for the following.) The typical fishery is harvested at levels way in excess of even just MSY; that means "less" is literally "more", i.e. at the industry level, reduced fishing increases catch.
The reason for this is the well-known "tragedy of the commons": nobody owns the wild fish stock. Nobody can stop another fisher from fishing. Hence nobody has any incentive to keep the fish stock from being overexploited. Keeping the stock higher may increase everybody's catch now and in the future - but if just a few fishers refrain from fishing, they catch less.
Overfishing implies either of two scenarios:
1. Catch can be maintained, but it can be higher.
2. Catch is declining.
Scenario 2 can be further subdivided into two:
2.1. Catch will continue declining at a gradual to moderate pace.
2.2. The decline in catch will accelerate into a full-blown "collapse".
World marine fisheries peaked in the early 90s, and seemed to hold steady since then. So it seems that scenario 1 is happening. Not so, warns many marine biologists. At least three trends are cause for worry:
a. Wherever they are measured, marine fish is much less abundant than they used to be. Large predatory fish populations are down to just one-tenth of pre-industrial fishing levels.
b. Fishing is moving "down the food web". Predator fish are being taken out first, followed by their prey, and their prey, and so on. Biodiversity is rapidly vanishing.
c. Actually fish catch worldwide is declining when we take into account estimated overfishing in China, which accounts for a big share of world marine catch.
At the global level therefore we have observed the start of a gradual decline in catch. I don't know yet whether Scenario 2.1 or 2.2. will materialize. Some biologists are convinced though that the latter is inevitable. They just don't know how soon. Preventing this requires a massive reduction in fishing activity.
If fish stocks could be held privately (just like farmlands), the problem could be solved. Sounds strange? It does, but recall that ownership permits the owner to restrict fishing activity. Then the owner can maintain the stock at commercially appropriate levels, not for environmental reasons, but for the sake of current and future profit. As argued earlier, the way the fisheries are being depleted now does not make commercial sense.
This solution though is moot as it is infeasible. The alternative is state regulation - licensing, catch limits, limits on the fishing equipment used, etc. This too has failed, obviously. Unless alternatives are found soon, we are headed for that bio-simplified fish menu of carp-tilapia. With a few jellyfish on the side.