The term “turbine fish” describes any fish inadvertently drawn through the turbines of a hydroelectric or tidal power facility. This interaction places the global demand for renewable energy in direct conflict with the health of aquatic ecosystems. As water is channeled to generate power, it carries along resident fish, initiating a perilous journey that highlights the need to balance energy production with the protection of fish populations.
The Journey Through a Turbine
A fish’s passage through a dam’s turbine is a violent event exposing it to extreme forces and mechanical dangers. As fish are pulled into a dam’s intake structure, they are funneled towards the powerhouse, where they face hazards that can cause immediate or delayed mortality. The survival rate is highly variable, depending on the fish’s species and size, and the specific type of turbine it encounters.
The most direct threat is a blade strike, where a fish collides with the turbine’s rapidly spinning blades. The speed and size of these blades mean even a glancing blow can cause severe injury or death. The probability of a strike depends on the turbine’s design, including the number of blades and their rotational speed. For example, larger fish like adult eels are more susceptible to being struck than smaller juvenile salmonids.
Fish also experience barotrauma, an injury caused by rapid and extreme pressure changes. As water is pulled through the turbine, the pressure drops dramatically before suddenly increasing upon exit. This shift can cause the swim bladder, an internal gas-filled organ used for buoyancy, to expand and rupture, leading to severe internal injuries. The effect is comparable to a scuba diver ascending too quickly without decompressing.
The intense and chaotic water flow itself poses a danger. Shear stress and turbulence are created as water moves past the edges of turbine blades and other components. These forces can tear tissues, dislodge scales, and cause disorientation. Fish not killed outright may be left stunned and vulnerable to predators after passing through the dam.
Broader Ecological Consequences
The effects of hydroelectric turbines extend beyond individual fish, disrupting entire aquatic ecosystems. Dams act as barriers that alter the natural flow and connectivity of river systems. These changes disrupt the life cycles of many fish species, leading to population-level consequences. The cumulative mortality from turbine passage, combined with other impacts, can cause declines in even once-abundant fish populations.
A primary ecological impact is the disruption of migration for diadromous fish, which travel between freshwater and saltwater to complete their life cycles. Species like salmon, which swim upstream to spawn, find their path blocked by dams. While some structures help fish move upstream, the downstream journey for juveniles is more hazardous as they are pulled through turbines. Similarly, catadromous species like the European eel face high mortality rates when passing through multiple dams to reproduce.
Dams also cause habitat fragmentation, dividing a continuous river into isolated reservoirs and downstream segments. This segmentation can prevent fish from accessing feeding and spawning grounds, shrinking their available habitat. Over time, this isolation can reduce genetic diversity within fish populations, making them more susceptible to disease and environmental changes.
Mitigation and Fish-Friendly Technologies
In response to the ecological damage from turbines, research has focused on developing technologies to help fish pass dams more safely. These mitigation efforts fall into two categories: guiding fish away from danger or designing systems that are less harmful to pass through. The goal is to reduce the injury and mortality rates associated with hydroelectric power generation.
One approach is to physically exclude fish from turbine intakes using screens or racks. These barriers guide fish towards safer alternative routes, known as bypass systems. Fish ladders and elevators are common upstream solutions, allowing adult fish to reach spawning grounds. For downstream-migrating juveniles, bypass channels divert them around the turbines for safe release.
Another innovation is the development of “fish-friendly” turbines, engineered to minimize the physical stresses that cause injury. Designs like the Alden turbine feature fewer, slower-moving blades, which reduces the probability of blade strikes. They also create more gradual pressure changes, lessening the risk of barotrauma. These newer designs can significantly increase survival rates for fish passing through them.
Operational adjustments at dams can also protect fish populations. During peak fish migration, operators can temporarily shut down turbines, allowing fish to pass without risk. Another strategy is to spill more water over the dam, creating a more natural and less hazardous route for downstream passage. Timing these operations to coincide with fish movements can reduce the overall impact on sensitive populations.