1.7.1.1 Downstream passage Injury
Downstream migrating fish are at risk from injury during passage through hydropower infrastructure. Injury can be direct and acute, resulting from
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exposure to rapid and extreme change in pressure or velocity, high sheer stresses arising from greatly differing relative velocities between water bodies, or between structures and water bodies, or mechanical damage (Monten, 1985; Coutant & Whitney, 2000).
Rapid or extreme pressure change
Many teleost fish actively maintain neutral buoyancy with a swim bladder (Bone et al. 1995), which is vulnerable to rupture during rapid decompression (Brown et al. 2012).Fish which regulate the volume of the swim bladder slowly by gaseous exchange through the blood vessels in the wall of the swim bladder (physoclists) are more susceptible to mortality from pressure change than those fish which can quickly alter swim bladder volume through the air canal and mouth (physostomes - for example salmonids) (Larinier & Travade 2002; Brown et al. 2012). In experiments with juvenile Chinook salmon, Brown et al. (2012) found the ratio of acclimation pressure to lowest exposure pressure to be the important factor associated with mortal injury during simulated turbine passage. Turnpenny (2000) also identified haemorrhages to the eyes and other areas of the body, and internal haemorrhages resulting from pressure changes under experimental conditions.
If air is entrained in water which is subsequently pressurised, the dissolved concentration of gases will be above those found at atmospheric pressure. This is termed super-saturation, and can occur where air is entrained into high-head turbine intakes or where water overspills high dams. If the dissolved nitrogen in a fish’s blood equilibriates with nitrogen supersaturated water, this will vaporise when the fish is exposed to lower pressures, a condition equivalent to ‘the bends’ experienced in divers, and which can be fatal. There is a large body of literature evidencing mortality of fish downstream of hydropower schemes from this cause, as reviewed in Cheslak & Carpenter (1990).
Shear stress
Fluid movement parallel to a surface such as a fish’s body results in a stress on that surface which is termed shear stress (Cada et al. 2006). This can occur in hydropower systems where two masses of water moving in different directions
intersect or where moving water slows near a fixed structure. Shear stress experiments by Turnpenny (2000) identified resulting injuries as: removal or rupture of the cornea, torn gill-covers, red-eye and pop-eye. No significant scale loss was found, although mucous stripping was observed. Some delayed mortality was recorded due to these injuries, including those resulting from fungal infections because of loss of surface mucous.
Turbine strike
Turbine strike is recognized as a dominant cause of injury and mortality from passage through conventional turbines. Larinier & Travade (2002) summarize mortality rates for conventional turbine designs as follows: Pelton turbines, 100%; Francis turbines, 5-90%; Kaplan turbines, 5-20%. It is clear that mortality rates are highly variable. For any system they depend both on scheme and operating characteristics (turbine properties, head, mode of operation) and on the characteristics of the fish passing (species, length, mass). More detail on methods for predicting and assessing injury or mortality from mechanical damage is given in Chapter 3.
These injury mechanisms may also cause subtle or latent effects which can affect the fitness of a fish and hence, in the longer term, the population (Budy, 2002). Subtle injury or stress may lead to increased likelihood of disease or predation (Mesa, 1994) after passage, and could act in combination with other process to lead to delayed mortality. For example in a telemetry study on Chinook salmon (Oncorhynchus tshawytscha (Walbaum)) smolts, Ferguson et al., (2006) estimated that delayed mortality contributed 46% to 70% to the total mortality resulting from passage through a hydropower scheme.
Delay
Hydropower impoundments and infrastructure can also halt or delay the downstream movements of fish (e.g. Venditti et al., 2000). As well as the physical barriers introduced by dams, there can also be behavioural obstacles, such as lack directional cues from flow, or hydraulic effects that discourage onward movement. A full consideration of these is given in Chapter 2, especially with reference to Atlantic salmon smolts). Delay may also occur after passage,
potentially due to disorientation, stress, or because of water circulation patterns that hold migrants (Schilt, 2007). Migrations are often seasonal and their timing can be crucial to onward survival (Thorstad, 2012), hence any delay could result ultimately in population decline. Aggregations of migrants due to delay at barriers may also act to increase predation rates.
1.7.1.2 Upstream passage
Large dams can halt upstream fish movement. Historically this has led to the development of fish passage technologies to restore connectivity for upstream migrating fish. Anthropocentric concerns about socio-economically important fisheries have been the main motivation for improving fish passage and protection, and so this effort has been mostly biased towards salmonids for cultural and economic reasons (Clay, 1995). This is changing worldwide (Meyer, 2007) due to conservation concerns manifested in legal requirements (for example the Water Framework Directive and Habitats Directive in Europe, see section 1.4).
Basic research has quantified the movement abilities of some species, with a view to optimising fishway design. Field assessments and experimental evaluations have been performed to assess and optimise the utility of fishway technologies. With these studies has come the recognition of exogenous and endogenous influences on a fish’s ability to traverse an obstacle. Temperature, flow, water quality, species, lifestage, size, physiology and life history are known to affect swimming performance (Wardle, 1975; Beamish, 1978; Blake, 1983; Beach, 1984; Videler, 1993; Pon, 2009). Exogenous and endogenous influences are considered in more detail, particularly for Atlantic salmon, in Chapter 4. Despite substantial research into, and implementation of, technologies to aid fish passage, understanding of the effectiveness of these measures remains poor, and even well targeted species do not always pass as well as expected (Castro-Santos et al., 2009).