Influence of flow on the migration and capture of juvenile galaxiids in a large river system

Abstract

The upstream migrations of juvenile galaxiids, or ‘whitebait’, form the basis of an important New Zealand fishery. This study used mark and recapture experiments with juvenile galaxiids in the Mokau River to examine the effect of flow on migration and capture rates. Recapture rates of marked fish ranged between 2.7% and 26.9%, with the proportion of fish recaptured positively correlated with increasing river flow. Increasing river flow negatively affected the upstream migration rates of fish. Migration rates through the tidal zone could be rapid, with fish travelling upstream at speeds up to 1.4 km h−1 within 24 h of release. However, not all marked fish exhibited rapid upstream movement. Fish captured on the second day following release had travelled at slower speeds up to 0.7 km h−1, suggesting that these fish did not initiate their migration on the tidal cycle subsequent to their liberation. A small proportion of fish were also found to travel up to 15 km downstream following release at the upper end of the tidal zone. Results indicated that some fish migrated on both diurnal and nocturnal tidal cycles. Nocturnal migrations by juvenile galaxiids could reduce capture rates and increase recruitment, especially in smaller rivers and streams where fish can pass the tidal fishing area on a single overnight tidal cycle. These results have management implications for river systems where juvenile galaxiids are exposed to multiple days fishing pressure, as reducing fishable zones could be one means of reducing pressure on unstable or declining galaxiid populations.