Larval drift of amphidromous Gobiomorphus spp. in a New Zealand coastal stream: a critical spatial and temporal window for protection

Amphidromy is a distinct form of diadromy, involving a downstream migration (drift') to a pelagic feeding habitat undertaken by exceptionally small, undeveloped larvae immediately after hatch. Identifying patterns of larval drift may indicate critical periods within which conservation measures can be targeted to protect migrating larvae. This study utilised diel and spatial drift sampling to examine patterns of larval drift in two of New Zealand's endemic eleotrid species, the bluegill bully (Gobiomorphus hubbsi) and the common bully (G. cotidianus) in the Waianakarua River, South Island, New Zealand. A distinct spatial and temporal window was identified within which the vast majority of larvae were migrating to sea, extending only a few hours after sunset and < 10km inland. Similar diel patterns of larval drift have been found in other amphidromous species, suggesting this may be a widespread migratory behaviour. Managing water takes to minimise impacts on migrating larvae within this relatively small spatial and temporal window potentially represents a management strategy that could provide safe passage to the vast majority of larval fish while minimising disruption to human activities.