Understanding migratory life histories is critical for the effective management and conservation of migratory species. However, amphidromous migrations (fish hatch in streams, immediately migrate to the sea for a feeding period and return to fresh water as juveniles) remain understudied owing to the difficulties of tracking tiny larval fish. Despite this, it has widely been assumed that amphidromous fish have open, resilient populations, with marine-rearing larvae dispersing widely during their pelagic phase. In the present study we tested the hypothesis that when an alternative freshwater pelagic habitat is available, non-diadromous recruitment will be the dominant process in sustaining amphidromous fish populations, with implications for their connectivity and resilience. Otolith microchemical analyses of five species (three Galaxias (Galaxiidae), two Gobiomorphus (Eleotridae)) from paired systems on the South Island of New Zealand indicated that when a suitable freshwater pelagic habitat existed downstream, non-diadromous recruitment was the primary population-sustaining process, typically contributing >90% of recruits. In addition, not all species recruited from all lakes, indicating the importance of the largely unstudied role of species-specific amphidromous larval requirements. The results of the present study emphasise the need to better understand the dynamics of individual populations of amphidromous fish, and highlight the importance of understanding species-specific early life history requirements to fully understand their distributions and management needs.

1. Otolith microchemistry was used to identify marine-versus freshwater-derived recruitment of three native freshwater fish species belonging to the southern hemisphere family Galaxiidae, in New Zealand's longest river system, the Waikato River.2. Water chemistry data for trace elements and 87 Sr/ 86 Sr isotope ratios were collected from five lentic and 10 lotic water bodies throughout the lower river floodplain. Potential spawning sites for galaxiids were compared with values obtained by laser ablation inductively coupled mass spectrometry (LA-ICPMS) depth profiling of young-of-the-year otoliths sampled from fish in nine lower river catchment sites.3. Otolith chemical signatures from the larval rearing period indicated that catchmentscale recruitment for two species, Galaxias argenteus (Gmelin, 1789) and Galaxias fasciatus Gray, 1842, was driven predominantly by non-diadromous recruitment from one lake (Lake Waahi). In contrast, diadromous recruitment appeared to be more common for Galaxias maculatus (Jenyns, 1842); however, non-diadromous specimens were also identified for the first time from a New Zealand river. 4. Reversing lake outlet flows linked to river stage appears be important in facilitating the dispersal of rheotactic larvae out of lakes, suggesting that lake outflow management at key times could be used to sustain this ecologically important function.5. This study highlights that some water bodies can supply a disproportionately large number of recruits to support fish populations within the wider riverscape. Identifying these water bodies and managing them to sustain recruitment is key to the conservation of non-diadromous Galaxiidae in this modified lowland environment

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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.

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