Why do some environmental groups oppose tidal energy?

Because it could harm wildlife. 

Environmental groups oppose tidal energy because of the following potential negative consequences:

By changing the flow of tides with dams, there can be negative impact on the aquatic shoreline of the ecosystem;building of barrages may affect the sedimentation and turbidity of water within water basinsthere are animals what depend on the flow of tides for existence feeding and movement. The techology which captures the energy produced by the ties and converts them to electricity may affect the natural flow fo the tides thus impactin negatively on the aquatic and non-aquatic habitat.

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