Inland Fisheries

Harmful Effects

Harmful effects of dams and other structures on the fish populations can be categorized into (1) Obstructional (2) Ecological

1. Obstructional effects of dams on fisheries

If the riverine fishery is sustained by stocks of migrating fishes that become blocked by a dam, the riverine fishery can be severely impacted. If the migrating fishes are anadromous or catadromous species, linked to ocean fisheries, or those of inland seas or large lakes, the negative impacts to these stocks and their associated fisheries can be catastrophic. Major concern throughout Asia is that movements of migratory fishes along river courses will be blocked by dams. Additionally, dewatering of stream channels immediately downstream from dams can be a serious problem. Dams also can block the flow of nutrients from ocean environments upstream into riverine environments by preventing anadromous fishes that die after spawning (eg, Pacific Salmon) from depositing these nutrients via carcass decay in upstream reaches.

2. Ecological effects of dams on fisheries

• Ecological changes affect both migratory and non-migratory species of fishes
• The most obvious effects from placing dams on rivers result from formation of new lentic or semi-lentic environments upstream from the dam, and tailwater environments downstream from the dam. Both environments can be conducive to the establishment and maintenance of fish stocks appropriate for exploitation by fisheries.
• Cumulative effects of dams in catchment basins and tributary streams can significantly block nutrient flow throughout the ecosystem, affecting fisheries production in downstream reservoirs, river channels and estuary and marine environments.
• Development of reservoir plankton reflects nutrients captured by the reservoir. This plankton generally relates directly to fisheries production of the respective reservoir. However, when several dams are constructed on upstream tributaries of a river ecosystem, the cumulative effects of these dams can be that of blocking the flow of nutrients originating from the catchment basin from the lower reaches of the ecosystem, thereby negatively affecting fisheries production in downstream portions of the ecosystem (including estuary and marine environments).
• Some species shift to new spawning and migration range
• Anadromous fishes tend to settle down
• Local stocks of fish form
• Intra-specific biological differentiation of fish occurs
• Egg-laying substrata change
• Spawning grounds inundate
• Water levels fluctuate
• Physico-chemical conditions of spawning areas in the upper reaches, marshlands, constituting the spawning and feeding grounds of some important food fishes may disappear.
• Turbidity and silting pattern change. It may result in the failure of spawning or ineffective spawning of many important fishes
• Considerable reduction of water flows in the residual rivers result in the formation of shallow areas, which impede or obstruct fish movements.
• When dams, weirs etc., are constructed in estuaries due to reduction in discharge of water, changes occur in temperature and salinity regimes of brackishwaters and of current velocities and directions at the mouth of rivers. This may affect the migration of fishes particularly the anadromous varieties resulting in successful migration or total failure of runs.