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 river s and the sea show distinct seasonal pulses: most migration from the s ea into rivers occurs during spring and most migration from rivers int o the sea occurs during summer-mid-winter. Inspection of life stages a t which migrations occur shows that pulses in movements in both upstre am and downstream directions variously involve both larval/juvenile fi sh and adult/prespawning fish. For this reason it is unlikely that the re is a general explanation for what seems, superficially, to be commo n timing of movements by many species. The ability of diadromous fish to move between catchments through the sea gives such species the capa city to recolonise rivers where there has been extirpation as a result of perturbation. This probably explains why fish species on small nea rshore and offshore islands around New Zealand are almost exclusively diadromous. Understanding the timing of these movements is important i f river managers are to minimise the effects of modifying flows in way s that may lead to river mouth closures.