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The efficiencies of fish passes specifically designed and constructed to facilitate the movement of a limited number of species and lifestages past structural barriers are likely to decline as site-specific conditions shift with a changing climate. There is a need to develop realistic fish passage criteria based on understanding swimming capability and behaviour of multiple species in relation to temperature and flow. The influence of temperature and discharge on behaviour and ability of groups of migrating adult river lamprey (Lampetra fluviatilis), a threatened species, to pass a small overshot or undershot weir at night was investigated using a large experimental flume. Lamprey approached the weirs less, and more often maintained station by using the oral disk to attach to structure, under high flows. Oral disk attachment was more commonly observed during tests employing the undershot weir. Upstream movement tended to be in close proximity to the channel walls and floor where, compared to the mid-channel, velocities were generally lower and velocity vectors more likely to be in a direction other than the bulk flow. Upstream movement was positively related to temperature, and was higher for the overshot weir. Weir passage rate was higher for undershot than overshot weirs, and negatively related to the maximum velocity at the weir. Passage rate was low when maximum velocities at the weir exceeded 1.5 m s À1 , although some fish passed at c. m s À1. Passage efficiency, the number of weir passes as a percentage of the number of approaches, was also higher for the undershot weir, but was not affected by discharge because lamprey approached less frequently at high flows. This study provides fish passage criteria under realistic conditions for an infrequently studied anguilliform species of conservation concern and provides a methodological perspective by which to improve fishway suitability for a wider range of species subject to changing climate.

This study demonstrated that juvenile (glass) eels used a specific substrate (eel tiles) to circumvent a model Crump weir under an experimental setting. Upstream passage efficiency was 0 and 67% for the unmodified (no studded eel tiles on the downstream face; control) and modified (with studded eel tiles on the downstream face; treatment) set‐ups, respectively, and was greater for a small (59%) compared to large (41%) stud configuration. Eels were active and motivated to ascend the weir during both control and treatment set‐ups. Approach and attempt rates were elevated during the first few minutes of the treatment compared to control trials. Eels were edge‐oriented under both set‐ups and ascended the weir through the tiles during single burst swimming events (reaching estimated speeds of 68.5 cm·s−1). Eel tiles may provide a cost‐effective solution for mitigating impacts of anthropogenic barriers to juvenile eel migration. Further research is required to determine passage efficiencies under higher flows, for a greater size range of eel, and for other migratory anguilliform fish (e.g. lamprey, Lampretra spp. and Petromyzon marinus L.). The performance of eel tiles should be validated through robust field studies.

Worldwide, fishways are increasingly criticized for failing to meet conservation goals. We argue that this is largely due to the dominance of diadromous species of the Northern Hemisphere (e.g., Salmonidae) in the research that underpins the concepts and methods of fishway science and management. With highly diverse life histories, swimming abilities and spatial ecologies, most freshwater fish species do not conform to the stereotype imposed by this framework. This is leading to a global proliferation of fishways that are often unsuitable for native species. The vast majority of fish populations do not undertake extensive migrations between clearly separated critical habitats, yet the movement of individuals and the genetic information they carry is critically important for population viability. We briefly review some of the latest advances in spatial ecological modelling for dendritic networks to better define what it means to achieve effective fish passage at a barrier. Through a combination of critical habitat assessment and the modelling of metapopulations, climate change‐driven habitat shifts, and adaptive gene flow, we recommend a conceptual and methodological framework for fishway target‐setting and monitoring suitable for a wide range of species. In the process, we raise a number of issues that should contribute to the ongoing debate about fish passage research and the design and monitoring of fishways.