In marine fisheries there are the added difficulties of never knowing enough about stocks, and of the participants being largely occupied with the mechanics of getting a catch and staying alive while doing so. The net result is that fisheries closures almost invariably come too late, usually after stocks have been severely depleted or permanently devastated. In the past three years, overfishing in the Strait of Georgia has necessitated indefinite closures for commercial harvesting of ling cod and abalone. And while fishermen will resist the designation of harvest-free zones, most will agree that it makes sense to set some areas aside to preserve marine life, if only to provide a little crop insurance. It might make a difference in whether or not there will be anything left for the next generation of fishermen.
Fisheries selectively harvest the largest individuals of desired species.
By targeting those animals that are reproducing the population, fisheries walk a narrow line between this year s harvest and next year s stock recruitment. The larvae produced by these fish are often widely dispersed, allowing MPAs to effectively seed surrounding areas with new recruits. By targeting desirable top predators such as rockfish, fisheries dramatically reduce the diversity of coastal waters. Removing predators offsets the intricate balance of natural systems, allowing particular prey species to become very abundant and out-compete other species. Studies done in South Africa have shown that the density of fish species is five times greater inside MPAs than in surrounding waters where rockfish are harvested.