Abstract
Dams are important structures for generating electricity and irrigation, but they often severely modify river hydrology and geomorphology and thus impact freshwater communities. These effects are broadly predictable and therefore, at a catchment scale, biological communities should respond in some predictable manner. We interrogated the New Zealand Freshwater Fish Database to determine whether dams have predictable catchment‐scale effects on fish assemblages. Seven selection criteria were applied to over 2000 dams, resulting in 30 dams being considered suitable for analysis. Comparisons were made above and below dams and with data from undammed sites which showed that community composition was significantly different above and below dams. Sites above dams had lower species richness, a lower percentage of diadromous species and a higher percentage of exotic species compared to below dams. None of these differences were present at undammed sites. In summary, dams restrict the movement of diadromous fish generally resulting in their reduction or loss from above‐dam habitats and also create artificial lentic ecosystems that exotic species can successively exploit. These changes to fish communities were found across New Zealand and should be viewed as a general consequence of any new dam construction unless fish management and conservation measures are undertaken.