Many diadromous fish populations have declined over the years due to reduction in habitat quality and quantity, and from overharvesting. In fact, several species are listed as threatened or endangered on the federal Endangered Species List. These species are likely to become extinct if we do not protect their populations and habitats. The combination of overharvest, habitat loss, and decreased water quality makes it difficult for diadromous fish to survive.
Because fish use a variety of habitats throughout their life, the loss of only one of those types of habitat can seriously threaten a specie's chance for survival. As a result, it is important to look at all habitat needs throughout the entire lifecycle of a species.
In addition, we can not overlook the fact that the overall amount of adequate habitat is decreasing, and that the quality of the remaining habitat is declining. Some of the most significant contributors to habitat decline include the following:
Barriers to Migration: Some barriers are natural (such as waterfalls) and some are man-made (such as dams). Dams have been built for a variety of purposes including water supply, navigation, flood control, and hydroelectric power. While some dams have fish passage facilities, there is often significant mortality for the fish that attempt passage.
Other man-made barriers include culverts that are too steep, too high, or just too difficult for fish to negotiate successfully. These barriers not only prevent the movement of diadromous species upstream and downstream, but also cause some migratory populations to become landlocked.
The U.S. Army Corps of Engineers and the Federal Emergency Management Agency (FEMA) maintains a database of dams for the United States. This database includes structures greater than 25 feet tall or store at least 50 acre-feet of water. There are over 76,000 such structures in the United States!
Flow Alteration: As a result of dams, dikes, and water diversions, stream flows in many cases have become very altered, resulting in lower stream flows and less variation in the magnitude of flows. Reduced stream flows, primarily in drier seasons, can be problems because the stream can become too shallow and too warm for fish to survive. Reduced variation in flows is a concern because it hinders natural stream and floodplain processing of pollutants.
Conversion of Habitat: As discussed above, fish need adequate spawning, feeding, and rearing areas to survive. Due to human activities such as development, agriculture, and dredging, fish habitat is often damaged or converted into something unsuitable for fish use. An example is the filling of wetlands for commercial or residential development. Wetlands are used by fish for spawning and rearing, and are critical pieces of habitat. When wetlands are filled in, the result is a decrease in quantity and quality of habitat.
Overharvest: Many species of diadromous fish face not only habitat loss, but also the threat of over-exploitation by commercial and recreational fishing interests. The Atlantic salmon was fished to near-extinction by the early 1900s. The Pacific salmon have faced overharvest as well as habitat loss. Many other diadromous fish species have faced overharvest, including striped bass, sturgeon, American shad, and herring.
Water Quality: Many land use activities leave residual pollutants such as metals, oils, chemicals, and fertilizers on the ground. Rainfall eventually washes these pollutants into streams, often resulting in significant harm to aquatic species when ingested either directly from the water, or from eating plants and animals that contain these pollutants. Other water quality parameters include dissolved oxygen, sedimentation, turbidity, temperature, pH, and nutrients. Each of these parameters can be affected by human activities.
