3.3.4. Environmental impacts
Tidal plants sited at the mouths of estuaries pose many of the same environmental threats as large dams. By altering the flow of saltwater into and out of estuaries, tidal plants could impact the hydrology and salinity of these sensitive environments. Estuaries serve as a nursery for many marine organisms as well as a unique and irreplaceable habitat for estuarine organisms, and alteration of this habitat by the construction of large tidal plants should be avoided. During the construction phase for the tidal plant at La Rance, the estuary was entirely closed off from the ocean for 2–3 yr, and there was a long period before the estuary reached a new ecological equilibrium. Changes caused by the barrage include a reduction in intertidal area, slower currents, reduced range of salinities, and changed bottom water characteristics, all of which led to changes in the marine community there [34]. In the future, any new tidal barrages should be constructed taking care not to close off the estuary from the ocean during construction, and these plants should not be built until detailed environmental assessments demonstrate a minimal impact on the marine ecosystem.
Tidal fences and tidal turbines are likely to be more environmentally benign [29]. Tidal fences may have some negative environmental impacts, as they block off channels making it difficult for fish and wildlife to migrate through those channels. However, Blue Energy claims that the slow-moving turbines allow both fish and water to flow right through the structures, and have no effect on silt transport. A 20 kW prototype built in 1983 by Nova Energy, Blue Energy's predecessor, in the St. Lawrence Seaway found zero recorded fish kill [31]. In longer-term situations, some fish kill would be inevitable, but fences could be engineered so that the spaces between the caisson wall and the rotor foil were large enough for fish to pass through, and the turbines could be geared down to low velocities (25–50 rpm), keeping fish kill to a minimum [32]. Marine mammals would be protected by a fence that would keep larger animals away from the structure and a sonar sensor auto-breaking system that shuts the system down when marine mammals are detected [31]. The tidal fences would not alter the timing or amplitude of the tides.
Tidal turbines could be the most environmentally friendly tidal power option. They do not block channels or estuarine mouths, interrupt fish migration or alter hydrology [29]. Tidal turbines and tidal fences both may offer considerable generating capacity without a major impact on the ocean, while tidal barrages are probably too damaging to the marine ecosystem. Research in tidal energy should focus on turbines, fences and similar technologies. These projects should be sited and built so that major migration channels are left open. Turbines should turn slowly enough that fish mortality is minimized and nutrient and sediment transport is largely unaffected. Tidal fences should be built across narrow channels, but not blocking an entire bay or corridor.