Developing performance standards in fish passage: Integrating ecology, engineering and socio-economics

The construction of dams and weirs has fragmented many rivers and streams globally, and this is a major threat to biodiversity. The most common method used to reduce these impacts is to construct fishways. Published examples show that while fishways can be effective, their performance can also be highly variable. Given this variability, it is critical that transparent targets are set and criteria are developed to assess fishway performance. Often though, this is not done, leading to uncertainty about what constitutes acceptable fishway performance. We present a conceptual framework that illustrates how fishway performance standards sit within a wider process involving objective setting, fishway design, and assessment, and then outline the key principles in the development of fishway performance standards. We highlight the importance of setting clear ecological objectives based on a ‘guiding image’ (the desired characteristics of the fish assemblage above and below a barrier), and fish passage objectives (i.e. required proportion of the fish assemblages, either individual species or life stages, that needs to be passed without delay and over an expected range of flows). We describe the biological and hydraulic characteristics that need to be considered in performance standards, and highlight the relevance of these characteristics to fish attraction, passage and exit. We use four case studies from diverse riverine systems to provide examples of how performance standards have been set and progress towards their assessment. We conclude by highlighting the potential benefits and risk of using performance standards and identifying areas of uncertainty for future research. Keywords: ecological objective, fishway, fish migration, fishway efficiency, performance metric, rehabilitation.