Restoring giant kokopu (Galaxias argenteus) populations in Hamilton's urban streams

Abstract

In this study, options for restoring fish populations in Hamilton City (37.47'S, 175.19'E) were explored. Habitat and fish populations in Hamilton urban streams were manipulated using a two-fold experimental design. Firstly, habitat was enhanced in ten urban streams with three continuous treatments in a 60-m reach at each site (20 m with 10 ponga logs, 20 m with 20 hollow clay pipes, and 20 m with no added structure). Secondly, juvenile farm-reared giant kokopu (Galaxias argenteus), were stocked into five of the enhanced stream sections. Giant kokopu are threatened and occur naturally in Hamilton urban streams in sparse populations. The abundance of wild fish was monitored before and after enhancement and fish release from November 2006 to November 2007. Stocked fish were monitored for eight months, from April to November 2007. Over this time electric fishing was conducted three times, trap nets (Gee minnow and fyke nets) were set monthly and spotlighting was conducted monthly at three release sites where water clarity allowed.

Anticipated outcomes of this research were; to determine whether giant kokopu abundance in Hamilton urban streams is limited by recruitment or by habitat, and to assist with the development of methods to restore fish populations in Hamilton City urban streams.

Logs used as enhancement structures in Hamilton urban streams provided more stable habitat for fish and created more suitable microhabitat than pipe structures. Pipes moved considerably during high flows, and their instability made them less effective at providing habitat. Within the study sites there appeared to be complex interactions with turbidity, stream width and depth, which complicated the effect of the habitat structures. The limited replication and variability among sites contributed to statistically insignificant results using analysis of variance.

Retention and recapture rates of stocked juvenile giant kokopu were greatest at Site M11, where the stream was narrow, shallow, clear and had lower numbers and biomass of shortfin eels, compared to other survey sites. Marked and released giant kokopu were retained in the release reaches at four of the five sites, for a minimum of four months, and exhibited substantial growth. Daily growth of juvenile giant kokopu ranged from 0.19 to 0.33 mm day-1 and from 0.03 to 0.11 g day-1, exhibiting substantial growth over winter. Giant kokopu appeared to have a slight bias to the log section of enhanced habitat, but habitat selection appeared to be overwhelmingly controlled by initial habitat selection.

The stocking of farm-reared fish into urban streams was largely successful, but the success of the habitat enhancement was variable and further work is required to determine better techniques for habitat enhancement in these urban environments. It is concluded that releasing farm-reared giant kokopu can be used to restore populations especially where recruitment limitations control fish abundance and diversity.