Seasonal pulses in migrations of New Zealand diadromous fish and the potential impacts of river mouth closure

Movements of 16 diadromous New Zealand freshwater fishes between rivers and the sea show distinct seasonal pulses: most migration from the sea into rivers occurs during spring and most migration from rivers into the sea occurs during summer‐mid‐winter. Inspection of life stages at which migrations occur shows that pulses in movements in both upstream and downstream directions variously involve both larval/juvenile fish and adult/prespawning fish. For this reason it is unlikely that there is a general explanation for what seems, superficially, to be common timing of movements by many species. The ability of diadromous fish to move between catchments through the sea gives such species the capacity to recolonise rivers where there has been extirpation as a result of perturbation. This probably explains why fish species on small nearshore and offshore islands around New Zealand are almost exclusively diadromous. Understanding the timing of these movements is important if river managers are to minimise the effects of modifying flows in ways that may lead to river mouth closures.