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Source: Home and Goldman (1994).

Source: Home and Goldman (1994).

TABLE 15.9 Some Common Families of Fish

Family

Examples

Lampreys

Sea lamprey

Anguillids

Freshwater eel

Herrings

Alewife, herring

Salmonidae

Salmon, trout, whitefish

Cyprinids

Minnows, carp

Catostomids

Suckers

Ictalurids

Catfish

Cyprinodontids

Killifish

Gastersteids

Sticklebacks

Percichtyidae

Striped bass

Centrachids

Sunfish, largemouth black bass

Cichlids

Tilapia

Gadidae

Cod

Source: Home and Goldman (1994).

Source: Home and Goldman (1994).

Fish tend to feed selectively in the pelagic, littoral, or benthic zones. Food sources include phytoplankton, zooplankton, detritus, or other fish. Cannibalism is not uncommon, especially by adults on the young. Pelagic fish usually feed at the surface. Shad herring, whitefish, and minnows feed almost entirely on zooplankton. Some of the large predators will also depend on zooplankton in their early life stages. Tilapia feeds on the blue-green alga Microsystis aeruginosa. This makes it a candidate for producing meat on manned space flight, since the algae can also help close the nitrogen cycle on spacecraft.

Most fish live for many years, so the biomass is not subject to seasonal cycles. However, survival of the young produced in a single year, the cohort, depends on predation, the availability of food for the young (usually plankton), and environmental factors. In the presence of intense predation, juvenile fish may restrict their activity to macrophyte beds for protection. Others may venture into deep water to feed on zooplankton in the early hours of the day, retreating to the littoral zone when daylight makes them easy targets for predatory fish. Others move from the depths to the surface and back in a similar way.

Floods and droughts may affect the availability of nesting sites. In tropical rivers, which are much less affected by human activity than other rivers of the world, some fish depend on the floodplains to a great degree. They often breed there and feed heavily during floods.

When there are a large number of species with similar food requirements, species often evolve to avoid direct competition by specializing in a subset of the available food. This is called resource partitioning. In the case of fish, for example, several species that eat zooplankton may restrict their hunt to particular habitats. One species of fish may only eat organisms from the benthic zone, others only from the pelagic surface, and others only from the littoral zone.

Some fish are anadromous, meaning that they live most of their lives in salt water but spawn in fresh water. The best known example is the salmon, which remember the "smell" of the stream that they hatched in after several years in the ocean. They even remember the odor associated with each tributary they passed as a hatchling, and follow them back in reverse order. The six species of Pacific salmon die after spawning; Atlantic salmon and steelhead trout return year after year. The movement of grown fish can also be viewed as a transport of nutrients from the sea to the relatively nutrient-poor headwater streams.

Other fish are catadromous—they live in fresh water but return to the sea to spawn. The eels of the North Atlantic are a remarkable example of catadromous fish. For centuries it was observed that the adults all left their streams in the fall for the ocean, and in the spring a large upstream migration of matchstick-sized elvers appeared. Finally, in the early part of the twentieth century it was realized that all the eels from North America and Europe swam to the Sargasso Sea near Bermuda. There they spawned and the eels died. Over three years the larvae drift with the Gulf Stream and metamorphose into elvers and swim up the estuaries and streams of the North Atlantic. The males stay in the estuaries and the females continue upstream. They remain for 8 to 15 years before returning to their place of hatching. The North American and European species are distinct from each other, although they breed in overlapping areas of the Sargasso Sea and sort themselves out for the reverse migration.

Many human activities have disrupted fish populations around the world. The first apparent effect is usually on populations used for food supply. Direct chemical alteration by pollution can result in eutrophication or toxic exposures. Both can cause fish kills, the former by deoxygenation, the latter by direct toxic effect. Fish can tolerate dissolved oxygen levels down to about 4 to 5 mg/L. If not severe, eutrophication can increase fisheries. Exotic species disrupt the food chain. They may be introduced deliberately or inadvertently. The peacock bass, Cichla ocellaris, was introduced to a lake in Panama as a sport fish. Subsequently, 11 other species suffered serious declines, and gulls and herons lost their main food sources. The loss of several larvae feeders has resulted in an increase in mosquitoes in the area.

Inadvertent introductions may occur as a result of constructed waterways such as canals. The alewife and the sea lamprey were introduced to the Great Lakes in this way after having been limited by Niagara Falls. The sea lamprey, Petromyzon marinus, is a parasitic eel that attaches itself by its mouth to the side of a fish and feeds on the fish's body fluids. The alewife, Alosa pseudoharengus, competes with lake herring for zooplankton. The lampreys killed off the larger fish of the popular salmonid species, forcing fishermen to take smaller sizes. This gradually eliminated the breeding stock, until the fishery suddenly collapsed. Overfishing also caused depletion of some fisheries, such as sturgeon, which were not related to the introduction of exotic species.

On the other hand, construction of dams has created barriers to fish that need to migrate, such as the salmon. Dams may also flood breeding grounds. Flood control eliminates those species that depend on flooding to breed. Development of the river shore eliminates overhanging trees which shade and cool, as well as provide falling insects that some fish depend on, and leaf fall which supplies the detritivores.

Fisheries management is a set of strategies to ameliorate some of these problems. Fish ladders are artificial cascades that can be constructed alongside dams to provide routes for migrating fish. Reservoir releases are controlled with downstream fisheries in mind, controlling flow and temperature. Gravel beds are constructed in streams to furnish fish nesting sites. Competition by overabundant centrarchids in reservoirs can be controlled by lowering the reservoir level at the appropriate time to strand nesting areas. Log jams and fallen trees (snags) serve as sources of invertebrates for migrating fish, as well as resting places in migration. They used to be cleared routinely, but now that their role is understood, they are left undisturbed by fisheries managers. Finally, nutrients are sometimes introduced deliberately to increase primary productivity, ultimately increasing the productivity at higher levels of the food chain as well.

Another important tool of fisheries management is regulation of fishing. A rule of thumb for harvesting fish is that the maximum sustainable yield is obtained if about one-third of the population is taken in each reproductive period. This has been found in experimental systems to result in an equilibrium biomass that is slightly less than 50% of the unexploited fishery. The optimum was found to be independent of the carrying capacity. However, the situation is sometimes complicated by interactions with other species. Harvesting a single species can tip the balance toward a competitor. For example, heavy fishing of the Pacific sardine (Sardinops caerulea) resulted in an increase in the less valuable anchovy (Engraulis mordox).

Natural disturbances to ecosystems tend to show their strongest effects at the lowest trophic levels. The disturbance tends to dampen as it goes up the food chain, since each level often has alternative food supplies. On the other hand, removal of top carnivores tends to have impacts throughout the food chain, even disturbing phytoplankton populations. Fishing usually concentrates on carnivorous species and thus is an example of the latter type of disturbance.

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