Re-introduction of giant kokopu to Nukumea Stream, New Zealand

Summary

The giant kokopu (Galaxias argenteus), New Zealand's largest galaxiid and listed Vulnerable amid ongoing habitat loss, had apparently disappeared from Auckland's ecologically intact Nukumea Stream, last recorded there in 2002, likely due to a perched culvert blocking juvenile migration. As partial mitigation for a new road through the catchment, researchers from New Zealand's National Institute of Water and Atmospheric Research removed the culvert in mid-2009 and, after securing permits and consulting local Maori groups, ran a pilot reintroduction using 30 juvenile fish reared to taggable size and surgically implanted with PIT tags before release in December 2009. Fixed PIT antennae showed the fish dispersing rapidly from the release reach within three months, likely accelerated by unusually low summer flows that eliminated much of the preferred deep-pool habitat, forcing monitoring to shift to trapping surveys across a wider reach. Over the following two years, three of the 30 tagged fish were recaptured in good condition with normal growth rates, confirming successful adaptation, though no tagged fish were recaptured from 2012 onward aside from three suspected release-group fish identified by healed tagging scars, and a separately discovered large adult indicated a small remnant wild population also persisted. Difficulties included securing multi-agency approval, sourcing enough fish large enough for tagging, and tracking a small, widely dispersing cohort. The project rates itself partially successful, crediting culvert removal and confirmed short-term establishment, while noting that securing support for a larger follow-up release, needed for a self-sustaining population, has so far proven unsuccessful.