Around the world, fisheries employ approximately 40 million people who regularly harvest some 1,500 species. The total value of fisheries and aquaculture around the world is estimated at $217.5 billion. Target species include molluscs, such as scallops and clams; crustaceans, such as lobster, crab, and shrimp; freshwater fish, such as catfish and trout; and marine fish, such as tuna and anchovy. Out of the 90 million metric tons (99 million tons; 1 metric ton equals 1,000 kilograms, or 2,204 pounds) of fish captured around the world in 2011, about 90% comes from marine fisheries; the remaining 10% comes from inland rivers and lakes, according to the United Nations Food and Agriculture Organization (FAO).

An additional 60 million metric tons come from aquaculture, the controlled growing of aquatic organisms, including fish, shellfish, algae, or plants, as a crop, mainly for food. From 1950 to 2011, global fish consumption increased from less than 20 million metric tons per year to about 150 million metric tons. In addition to the fish specifically targeted by fishers, other marine species have also been impacted through bycatch, the killing and discarding of noncommercial fish, birds, dolphins, sea turtles, and other wildlife that occurs as a result of contact with fishing gear. Because bycatch can affect so many nontarget species, it can have widespread negative effects on the marine food web.

The earliest fishers practiced subsistence fishing, catching enough for themselves and their families, including a small amount to be bartered or sold. Some of the simplest techniques involve catching fish by hand in shallow waters, spearing or trapping them, or simply scooping them out of the water with a net or other vessel. Today, subsistence, or noncommercial fishing, is still practiced in many indigenous and rural communities around the world. For instance, the state of Alaska allows residents to catch a certain number of salmon each year using a handheld dipnet or a snag hook—a hook that allows fishers to yank fish out of the water without the use of bait. Alaskan residents with the proper permits are allowed to subsist on a wide range of other species, including halibut, crab, and clam. In general, subsistence fisheries tend to be small and have a limited impact on fish populations.

In contrast to subsistence fishing, commercial fishing involves catching fish for profit, and it represents the vast majority of the fish captured around the world. More than 90% of commercial fishing around the world is done by small-scale fishers, who use minimal gear, such as handlines or hand nets, and may fish from small boats such as motorized skiffs or nonmotorized canoes. They typically stay close to the coastline and fish for only a few hours or days at a time.

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Industrial fishermen use more expensive and technologically advanced gear to catch fish and may often travel for weeks at a time, processing and refrigerating or freezing their catch on board. Bottom trawlers drag weighted nets along the ocean floor in order to catch groundfish, including cod and flounder, along with scallops, shrimp, and crab. Longline fishing involves laying out a very long line with hundreds or thousands of baited hooks, used to catch tuna (near the surface) or groundfish (e.g., halibut, cod). Gillnetting involves placing a net with large mesh in the water column to selectively catch fish, including salmon, in the Pacific Northwest. The size of fish caught depends on the net mesh size. When a fish cannot pass all the way through the gillnet, it is ensnared by its gill covers when trying to retreat. Other methods, such as baited pot-traps, are used to catch lobster and crab.

Finally, sport or recreational fishing may include fly-fishing on a stream in Montana or hiring a tourist charter boat to catch trophy-sized fish, including sharks, swordfish, and tuna. Some sport-fishers eat their catch, whereas others engage in catch-and-release fishing, in which they release the fish back into the water where they caught it. However, care must be taken to ensure that the captured fish set free will survive the encounter.

The major question for fisheries regulators, such as Alaska Fish & Game or the National Marine Fisheries Service in the United States, is determining the level of sustainable harvest of a particular stock. A stock is loosely defined as a discrete subpopulation of a species, which is reproductively isolated from other stocks. Some wide-ranging species like southern bluefin tuna consist of a single stock, whereas dozens of salmon stocks are known, depending on the specific river to which they return for breeding. Fisheries managers perform a stock assessment to estimate the size of the fish stock, the rate at which it is growing, and the rate of sustainable harvest. A key piece of data for making stock assessments is the catch-per-unit effort, which is a measure of how many fish can be caught using a specific piece of gear—a net or a line—for a certain period of time. In “mark and recapture” studies, fisheries biologists may also release tagged fish and then try to catch them again to get a better idea of the size of a population or the boundaries of a particular stock.

Although fisheries statistics can be complex, the basic principles are simple. When fishing pressures are high, fish populations decline. When fishing pressures are reduced, fish populations grow. However, scientists have noted that when fish exist at high densities, their level of reproductive success is lower. Consequently, harvesting fish to prevent them from reaching high densities can make the fishery more productive. However, if fish populations are reduced to extremely low levels, the fish have trouble finding suitable mates, and fishery productivity may thus decrease.

The goal of fisheries managers has long been to manage fish populations so that harvest rates are at or near a theoretical level called maximum sustainable yield (MSY), which is the maximum harvest of a renewable natural resource that does not reduce future yields. If we assume S-shaped, or logistic, population growth (see Figure 3.11, page 69), the maximum sustainable yield is expected when the population size is approximately one-half of the carrying capacity (Figure 8.1). At this size, the rate of population growth is highest, and recovery from harvest should be most rapid. In well-managed fisheries, ongoing scientific study provides information to help managers and fishers avoid depleting stocks below these levels by reducing or sometimes closing a fishery to continued harvest. When a fish stock drops below the population size that provides the MSY, it is considered overfished.