Changes to fish communities
Some of the effects on river ecology described above have direct effects on fishes, as mentioned, but - because of their importance to human welfare - their impacts deserve special attention.

Changes to the Aquatic Communities
Substantial changes occur to biota below a dam. The most direct of these is the blocking of the pathway by which invertebrates and fish travel upstream and downstream. This may result in the complete elimination of species dependent of migrations upstream for breeding, or at least a drastic change in their abundance. It can also mean the genetic segregation of different blocks of the same species and the failure to recolonise a reach of river that is overfished. These effects are not the only ones and fundamental changes occur to plants and animals both upstream and downstream of dams.

Upstream Effects
Effects upstream of the dam are felt in the channel of the river and in the new waterbody that is formed.

Effects in River Channels. The bar to migration is felt upstream, because riverine invertebrate and vertebrate species that would have come from downstream to upstream habitats are no longer able to pass the dam. Localised populations of migratory species may be retained if the tributary rivers above the reservoir are of significant size and are not disturbed. In these cases, the retention of species that are able to continue to live in the river and upstream tributaries of the reservoir depends on the preservation of a suitable upstream environment for breeding and feeding by the organisms affected. Building further reservoirs upstream destroy any longitudinal migration, and the animal populations that previously depended on the migratory pathway will be at risk.

The slower water in reservoirs means that invertebrates, and fish eggs and larvae that are normally transported by the current in rivers in the drift, settle to the bottom and are killed. Therefore reservoirs and dams pose a barrier to downstream migration of many invertebrates and young fish.

The Reservoir. Reservoirs can be divided into two effective zones, each of which has a characteristic fauna. Upstream, in the deposition area, there is a riverine zone where many of the original riverine invertebrate and fish species persist. Excessive silting in this zone may affect the bottom living invertebrates that rely on clean, sediment-free conditions in the main river. In the body of the reservoir there is a lacustrine (= 'lake-like') zone where true lacustrine phyto- and zooplankton develop. Floating vegetation such as the water fern (and the water hyacinth) may form extensive mats covering large areas of the reservoir. Lacustrine insects also colonize the reservoir and organisms such as lake flies (chironomids and chaoborids) can cause a potential nuisance.

Furthermore many disease vectors, particularly the snail hosts of bilharzias and the mosquitoes that carry malaria, thrive in the new environment. Fish faunas become modified to favor lake dwelling species, some of which may be introduced by humans. The open waters of this area are particularly likely to become colonised with small pelagic fish species. The most lacustrine area of all is that closest to the dam where the waters are deepest.

The following changes to the fish fauna can be anticipated after the filling of a reservoir:

• The disappearance of most migratory species from the body of the dam. Some of these may persist in the upper riverine parts of the reservoir;

• The rise in abundance of species that spawn in lacustrine situations;

• The rise in abundance of introduced species such as tilapias that are better adapted to lacustrine conditions;

• The rise in abundance of small pelagic species that are normally rare in the river;

• The dramatic rise in fish catch and later stabilization at a reasonable level of yield.

Whether the catch from the reservoir willcompensate for the loss of fisheries downstream of the reservoir depends on local geographical circumstances.

Reservoirs may support valuable fisheries based on the fish populations that develop in them. In many reservoirs, the fisheries provide more value than the primary purpose for which the dam was built. Reservoirs also provide a valued recreation resource with possibilities for sport fishing, bathing and boating. For this reason, reservoirs should be carefully managed to give the maximum benefit to human populations. The fish populations in particular should be carefully conserved by proper management, avoidance of excessive fishing and by stocking to give the maximum yield for food and recreational fisheries.


Downstream Effects
Other effects of dams on fishes are felt downstream, as highlighted below.

Short Distance Effects. The water discharged from dams is usually poor in nutrients, and the carbon flows of a natural river channel are interrupted (see Section 7, the Serial Discontinuity Concept). Much of the invertebrate life depends on these food sources and tends to be diminished. Migrating fish that would normally have migrated upriver past the dam site tend to accumulate in the tailwater areas below dams. Here they are especially vulnerable to discharges of deoxygenated and cold water from the spillways. The accumulations of fish are also vulnerable to predators including other fish, birds and human fishers.

Long Distance Effects. The major downstream effect is the failure of normal floods for considerable distances below the dam. This means that the floodplains that were essential to many of the fish species are now detached from the river, and seasonal waterbodies and marsh environments dry out. As a result, the vegetation structure of the natural floodplain and its former waterbodies changes from water-tolerant species to more arid varieties. Similarly, many of the invertebrates that are characteristic of seasonally flooded environments may disappear. Species of fish that are adapted to floodplain habitats are eliminated or decline in abundance, and are replaced by other species. New species colonizing the habitat are often less desirable for food and commerce. In particular, the altered conditions favour introduced species, which may increase in abundance and eventually dominate the fishery. Foodplains are the preferred environments of many species of waterbirds and populations of these may also decline.

In summary, the following changes can be anticipated downstream after the creation of a major dam:

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