Water infrastructure and the migrations of amphidromous species: impacts and research requirements

Abstract Amphidromous species undertake a number of migrations throughout their life-history, migrating to sea immediately after hatching in freshwater, migrating back to freshwater after a pelagic larval period, and potentially undertaking adult spawning migrations. Amphidromous species are therefore likely to be highly susceptible to water infrastructure, having different migratory directions, objectives, and requirements at different life stages. We review the current state of knowledge on the migrations of amphidromous species, identifying the requirements for successful migrations, and potential threats from water infrastructure associated with anthropogenic activities. Newly hatched larvae migrating downstream are susceptible to numerous hazards associated with water infrastructure, including larval retention and starvation in freshwater, entrainment and impingement at water intakes, and barotrauma and physical damage associated with weirs and turbines. Distinct patterns of larval drift (spatial and temporal) may provide opportunities to alleviate mortality during larval emigration. While instream barriers inhibit the upstream migrations of amphidromous post-larvae and juveniles, climbing abilities are common, allowing for creative solutions facilitating upstream migration. Downstream spawning migrations are common in numerous amphidromous taxa, and are often associated with natural changes in flow regime, highlighting the need for bidirectional passage, and the potential for artificial flow alteration to negatively affect reproduction. Much research on the passage of amphidromous taxa has focussed on upstream migrating juveniles, while downstream migrating adults and larvae, which may be far more susceptible to water infrastructure, have largely been ignored. This life-stage bias represents a key research gap that must be addressed to safeguard amphidromous species in future.