Fish migration is a natural process in which the school of fish moves from one place to another on the body of water on a larger scale and duration apart from normal activities. This procedure can occur daily or annually. Some kinds of fishes depart from freshwater to marine water while others may migrate from marine water bodies to freshwater bodies. This natural procedure may occur due to different reasons such as for better feeding, reproduction, protection from predators and dangers or even for a better living.
However, the main challenges faced by these kinds of fish are challenges associated with hydropower systems which are established to produce electricity using the kinetic energy of water flow. Hydropower systems include hydroelectric power plants, hydroelectric dams, run-river systems and wave energy converters. As they are associated with water systems they can harm migratory fish species and the ecosystems that they are related to, which results in a decline in fish population, disrupting fish life cycles, ecological imbalance and loss of biodiversity. In this blog, we are going to explore some of the main challenges faced by migratory fish species due to implanting hydropower plants and the actions that can be taken to avoid those consequences.
Barrier Creation and Fish Passage Design
The structure of dams acts as a barrier to the migratory fish species; especially the ones that are involved in upstream migration. The height of the dams and weirs which are used in streaming rivers to get elevated, obstruct the upstream migration causing difficulty to migrate between feeding and breeding zones resulting in disruptions to their life cycles and ability to reproduce. Furthermore, as they get stuck at the bottom of the hydropower systems, limitations for the fulfilment of necessities due to increasement of the fish population in that particular area occur. These areas may be successful hunting places for particular predators which is a threat to the immigrant fishes. Changes that occur around the dams and wieners, such as the quality of water, biodiversity, and dissolved oxygen concentrations can damage or even kill the fish. On the other hand, downstream migratory fishes face similar consequences.
Due to the barriers of hydropower plants to fish species, the accelerated of extinction rate increases and the confinement of certain species to a very restricted part occurs. Although certain species have special abilities to face these obstructs, as a collection it’s a serious threat to most of the migratory species. To overcome this situation researchers have suggested some operations.
To avoid the consequences due to barriers of dams and wieners, they can be built with a design specific for fish passage, which allows them to migrate upstream with minor impacts. Some already implanted types of fish ladders are pool-type fish passages,denil fish passes, nature-like bypass channels, and rock lamp fishways. Except for fish passages, we can entrap them in a specific location and release them in upstream areas known as fish lifts which are effective for smaller-scale hydropower plants. Another method is to attract them to a downstream far away location and allow natural upstream migration. It is important to consider making ways for downstream migration as well, which allows returning of upstream migrated fish species back to the ocean and downstream migratory species to travel
Turbine-Related Mortality
When talking about the sustainability of hydropower, turbine-related fish mortality act as a vital point which turns the whole concept upside down. Researchers have found out that everyone of every five fish gets damaged, suffers fatal injuries or even gets killed whenever they pass through a hydropower turbine. These potential risks are associated with abrasion, scrapes, freefall from passing over a spillway and mechanical strikes from turbine blades of the hydropower system. Along with the physical injuries water pressure may vary due to changes in pressures, shear stress, turbulence and cavitation which will affect the life of the fish. The probability of damaging the fish depends on the size, life stages, species, and other biological characteristics such as natural abilities along with the type of turbine associated may vary.
Most of the downstream migratory fishes face these consequences affecting their lifecycle and their entire existence resulting in increasing the rate of extinction of fishes belonging to these types. Furthermore, as the downstream migrant fishes are not very well observed, historically far fewer considerations and a minimal amount of research were done on these species. Therefore strategies and structures which are specific for accommodating downstream passage were not frequently introduced.
However, there are some techniques which have been introduced to reduce fish mortality. Introducing ‘fish friendly’ turbines is one of the methods where the turbines rotate slowly which include water wheels and VLH turbines. These are comparatively less harmful than most conventional turbines. Other than that fish protective turbines with appropriate technical operations and configurations that can prevent from entry of fish into turbines can be implanted. Another method is the installation of bypass systems which prevent the pathways to turbines.
Altered River Flow
The altered river flows refer to any changes that can occur to the natural flow of water on rivers, streams or the water level of a certain water body. The volume of water, amounts of sediments, type of soil, nature of the rock structure and the slope of the terrain and the effect of the river bank determine the flow velocity, density, volume and temperature of the certain water body which impacts the flow rate and the water level. This can be altered by external introductions to water bodies such as hydro power plants including dams.
Water Levels change due to mechanisms resulting from the circulation of water at the foot level of the dams which allows water behind the dams to move towards intakes and get pushed by the turbines against blades allowing them to turn. Furthermore, the velocity of the water flow can be changed due to the flow of water at a high speed from the top of the dam and at the location of the turbines. Due to the sudden change of water levels when dams are closed and opened with relative velocity the soil and sediments of rocks can get washed away changing the volume of water and also changing the bottom structure of the stream. This results in increasing bank erosion by affecting the stability of river banks, which causes flooding affecting the vegetation, habitats, flora and fauna.
Due to the alteration of water movement, a reduction of fish movements occurs as these altered waterways and flows make it impossible attempt for fishes to complete their journey. As the swimming ability against different water flows varies with the species, some can survive under the conditions of altered waterways. However, in most cases, many migrant species avoid these types of waterways. Furthermore, as the reproduction process of certain species is tied to the level of water, it gets affected too. Some conditions like less variable flows may result in an increase in temperature especially for the non-shady area of the stream. This force damages the ecosystem and habitats making it unsuitable for fish to live in migratory species to complete their life cycle.
To avoid these consequences refilling of water bodies can be done, naturally through rainwater which fills the river or river basins that can seep down to these channels. Artificially basins can be transplanted upstream to the channel which acts the same as the natural water basins. Other than that, restoration of aquatic habitats and aquatic ecosystems can be done which include reconstruction of past physical conditions, chemical adjustments on water and soil, and reintroduction of native flora and fauna which were absent can be done. Also, steps must be taken to avoid bank erosion around the river banks such as increasing vegetation in the particular area can be done.
Conclusion
Though hydropower systems act an important role as a renewable source of energy, they are a kind of threat to fish species, especially the migrants. These threats include barrier creation, turbine-related mortality and altered river flow. Solutions can be introduced to reduce their impacts such as fish passages and bypasses, fish-friendly turbines, and artificial basins. However, their successful implementation requires careful consideration of various factors including the specific characteristics of the river and the species involved. It’s crucial to continue research and innovation in this area to sustain our rivers’ biodiversity while meeting our energy needs.
