Population structure, individual movement, and growth rate of shortjaw kokopu (Galaxias postvectis) in two North Island, New Zealand streams

Abstract

The shortjaw kokopu (Galaxias postvectis), although widely distributed throughout New Zealand, is only known from a few sites and generally with extremely low population numbers. Two sites in Taranaki (Katikara Stream and Stony River tributary) were found to contain relatively high densities of shortjaw kokopu. These sites were monitored at 6‐monthly intervals over 3½ years to gather site fidelity and growth rate data. Half‐way through the sampling period the riparian vegetation alongside the Katikara Stream site was destroyed. Although this was unfortunate, it allowed examination of the impact of a disturbance event of this nature on shortjaw kokopu growth and behaviour. The density of shortjaw kokopu in Katikara Stream was about twice that in the Stony River tributary, whose population was characterised by fewer but larger individuals. Growth rate was significantly greater for smaller fish, and varied between the two sites. Fish in the Katikara Stream were generally site loyal with only a small percentage of mobile fish; however, greater movement from sites was noted immediately after the disturbance event.