Overexploitation

Overfishing
People harvest a wide variety of marine life, mostly for food but also for curios, aquariums, medicine and raw materials. Theoretically, any species, no matter how long-lived can be harvested sustainably if the level of removal is less than the ability of the species to replace those individuals. Before large scale fisheries, coastal communities would harvest small numbers of animals without noticeably damaging the ecosystem. Now our ability to take vast quantities of fish very quickly has put many fisheries in jeopardy of collapse from over-exploitation. Atlantic cod off the northeastern US was historically so abundant that when the pilgrims arrived they noted the fish were so thick you could walk on them. Today the populations of cod are so low that the fishery is in danger of closing. The most obvious result of overfishing is a severely depleted population which eventually becomes uneconomical to fish and the population is ‘commercially extinct’. At this point, there are still fish left but there may be so few that the population cannot recover. This has happened with spawning aggregations of Nassau grouper throughout the Caribbean. These fish gather in large numbers in the same location every year, which makes then an easy target for fishers. The spawning aggregations have been so depleted (with very few exceptions) that the fishery collapsed and populations of Nassau grouper have not recovered. Fishing often targets the largest individuals, but since these produce the largest numbers of eggs, their removal compromises the ability of the population to withstand fishing pressure, and to recover from over-exploitation. Fishing usually targets a particular subset of the fish community, often the predators, which are larger and higher value. With sustained overfishing, changes in community composition may occur as the large species are depleted, followed by the smaller lower-value species. This process changes food webs, reduces diversity and changes predator-prey relationships within an ecosystem, sometimes irreversibly.