Obstructions to Migrations: Dams
Humans need fifty liters of water per person per day on average (World Commission on Dams, 2000). Less than .007% of the water on earth is liquid fresh water that is regularly cycled and renewed (Human Appropriation).
The world population is increasing at an unprecedented rate and urbanization is occurring on a similarly impressive scale. These increases will result in a larger demand for limited water resources; due to uneven water distribution, it is expected that one-third of water-stressed countries will experience severe water shortages in the next century (WCD, 2000). Currently, most water is used for agricultural purposes, especially in developing countries. Currently, dams are a tool for obtaining the water we so desperately need.
In the 1970s, there was a major boom in dam construction, especially in China, the United States, Japan, Spain, and India (WCD, 2000). Currently, of the thousands of large dams two thirds are in developing countries (WCD, 2000). These dams fulfill a variety of functions including but not limited to water storage, hydroelectric power generation, and flood control. The figure at the right shows the distribution of dams by functions. One third of countries rely on hydropower for over half their energy needs (WCD, 2000). Overall, it is easy to see the incredible importance and economic impact of dams. However, there are many environmental and social problems associated with dams. Dams have a significant impact on the marine fisheries, either directly by destroying spawning habitat or blocking migration or indirectly by increasing pressures on marine fisheries.
The problems associated with large dams can be broken down several categories:
- Changes to the chemical and physical properties of a river
- Biotic changes to the ecosystem resulting from the aforementioned riverine changes
- Human impact due to change in either the river or ecosystem
According to the World Commission on Dams, 46% of the 106 primary watersheds on earth are affected by dams. These effects can include temperature changes (water held in a reservoir warms, while water which is released over the dam's head is cooled) and dissolved oxygen level changes (the warmer water in a dam's reservoir will have lower dissolved oxygen levels resulting from higher water temperatures and slower water velocity, while water below the dam may become super-saturated with oxygen and poison fish). These changes often favor invasive species, which can then outcompete the native biota. Dams also change the natural flow regimes, which are important triggers for biological cycles. Flow levels can enhance or suppress reproductive success for many species, as well serving to redistribute substrates (material comprising the river bottom) and bed-loads (large particles carried along the bottom) (Young, 1997). Furthermore, starvation of sediments because of retention by dams can alter the substrate composition downstream with huge effects on fish; studies on the Colorado River indicated that natural reproduction of fish species was suppressed because sandbar formation had ceased due to a lack of sediments (Young, 1997). The WCD reports that in many cases wetlands dry out and recharge of groundwater is diminished. Besides "trapping" water behind them, dams also act as particle traps, holding back nutrients and sediment. The downstream ecosystems that rely on these nutrients can suffer severely; the crash of Kokanee salmon was attributed to the drastic decrease in nutrient loading caused by the construction of two dams (Wuest). The changes in sediment transport can heavily influence the channel, floodplain, and delta morphology. In coastal areas, the erosion caused by waves is no longer counteracted by deposition of sediment; the WCD reports that the coastline of Togo and Benin has decreased by 10-15 meters per year after the Akosombo Dam on the Volta River was completed. There are indications that erosion may also result in a lack of floodplain fertility.
One of the largest problems dams cause for fish is obstruction of their migrations; dams provide large, physical barriers to passage up and down rivers. Diadromous fish, which live in salt water and spawn in fresh water or vice versa, are in many cases entirely unable to reach their spawning grounds. Salmon and shad have died out in areas due to dam construction (WCD, 2000); in the United States, shad populations rebounded only after extensive stocking and fish passage efforts (Richardson); in the Caspian Sea, sturgeon must be stocked because dams entirely obstruct their reproduction (WCD, 2000). Dams can also obstruct the movements of aquatic insects and larval clams (glochidia); reductions in these populations, which serve as food for organisms higher on the food chain, can have indirect effects on fish populations. Dams have been reported as the largest cause of freshwater species extinction (WCD, 2000). Loss of freshwater species as a food source (6% of fish caught are from fresh water) may result in more pressure being placed on marine species, so it is important to regard the loss of those species as important to the fate of marine fisheries (WCD, 2000). It is estimated that 20% of freshwater fish have become extinct, endangered, or threatened in recent years.
However, it is not just by obstructing fish passage that dams affect marine fisheries. Dams have been shown to decrease catches of fish in upstream portions of rivers such as the Senegal and Niger Rivers, Nile Delta, and Zambezi River which again may put more stress on marine fisheries (WCD, 2000). Downstream, changes in flows of fresh water and in nutrient levels can influence the estuarine habitats where many marine fish come to spawn. Lowered nutrient levels can result in lowered overall productivity from a diminished primary food source (i.e. less primary production), as occurred with the Aswan High Dam in Egypt (WCD, 2000). Furthermore, increases in salinity from lessened freshwater flows can allow marine predators to invade, lowering recruitment rates (WCD, 2000). The overall effects of these changes can be significant; in the Zambezi Delta, dam-related changes cause an estimated $10 million annual loss to the shrimp fishery (WCD, 2000).
Other problems associated with dams that are not related to fisheries at large but are large-scale impacts of dams, include displacement of native people (40-80 million) and a diminished ability of native people to collect the river's resources (WCD, 2000). Dam reservoirs also emit greenhouses gases, at times at levels larger than the area in a pre-dammed state, which can be a factor when dealing with climate change issues and legislation (WCD, 2000). It is also notable that in solving these issues, international politics may come heavily into play, as 261 watershed cross political boundaries and water security issues have been heated in the past (WCD, 2000).