What impacts are dams having on New Zealand freshwater fish communities and are some of our mitigation strategies effective?

Dams play an important role in generating electricity and providing a reliable supply of water for irrigation but their construction also has significant environmental consequences.

In addition to severe changes to river hydrology and geomorphology, dams often create a physical barrier for the upstream and downstream movement of aquatic species, particularly fish. To lessen the impact of such dams on freshwater fish, structures such as fish ramps and passes are often installed to allow fish passage. To investigate whether dams have any broadly predictable effects on fish communities at the catchment scale, we interrogated the NZ Freshwater Fish Database.

In conjunction with this desktop study, we also conducted experiments on the effect of ramp angle and substrate type on juvenile eel climbing success. The desktop study showed that fish communities above dams had lower species richness, a lower percentage of diadromous species (i.e., species that migrate between fresh water and the sea) and a higher percentage of exotic species compared to fish communities below dams. Eel climbing experiments showed how differences in ramp angle and substrate type can have a major influence on the percentage of fish that might successfully negotiate a fish ramp.

In summary, by impeding diadromous fish migrations and creating artificial lake ecosystems that exotic species can successively exploit, dams have altered fish communities to the general detriment of native fish species. These changes to fish communities were found across New Zealand and should be viewed as a general consequence of any new dam construction although appropriate mitigation measures should lessen the impact of a dam.