Giant Kokopu

The Rees-Webbe property at Mangakura is a licenced freshwater fish farm, fully self-contained with its own water supplies and facilities, for breeding kokopu species for release into the wild in restored waterway habitats to combat the decline of the species. Releases of endangered endemic species of  giant kokopu have occurred and more are planned for 2021. With sufficient notice from landowners with suitable streams, giant kokopu juveniles can be supplied for next season. The 8-year conservation project is the passion of Jerry Rees-Webbe and his aquaculture consultant Lyn Hamilton-Hunter.

Giant kōkopu (Galaxias argenteus) is a threatened species of ray-finned fish in the genus Galaxias, found only in New Zealand. Galaxias are typically found at temperate latitudes across the Southern Hemisphere and are frequently referred to as galaxiids. Galaxiids are the dominant group of native freshwater fish in New Zealand. Galaxiids are scaleless and somewhat tubular in body form, ranging from very slender to quite bulky. They are somewhat torpedo-shaped, with the dorsal and anal fins positioned close to the tail. With more than 40 species, they are generally small and the Giant kokopu is the largest of the genus. Some galaxiids include a marine stage in their lifecycles where larvae are washed out to sea to develop and return to rivers as juveniles. In the returning river runs they are called whitebait and caught as a fish delicacy. The main whitebait species are inanga, giant kōkopu, banded kōkopu, short-jawed kōkopu and koaro. A major threat to wholly freshwater galaxiids is exotic salmonid species like trout and many galaxiids species here and in Australia are restricted to trout-free habitats. Diadromous galaxiids (which use marine waters in their life cycle) in New Zealand can co-exist with trout and salmon.

Adult giant kōkopu are found in fresh water, mostly near the coast. Spawning has never actually been observed in the wild, but it occurs during autumn/winter (first discovered in 2013). What is fairly certain is that the eggs hatch and the small larvae head to the ocean. After spending 4–6 months at sea the larvae head back to the harbours, entering fresh water streams in the spring. On their return they are called whitebait and as juveniles they swim upstream to grow into adults.

Giant kōkopu are classed by the Department of Conservation as “at risk, declining”. They are slow growing and can live for 20-30 years. Not the most adept of climbers, giant kōkopu are generally found at low altitude. They inhabit wetlands, lakes and forest streams, and rely on good bush surrounds. Giant kōkopu are skulking predators, lurking under cover and making dashes to nab their prey, which include aquatic and terrestrial insects, spiders and occasionally other fish. They are endemic to New Zealand and are found also on the margins of the two main islands, at the Chathams and on Stewart Island. Canterbury and Hawke’s Bay do not have populations.

Giant kōkopu prefer to live at temperatures between 11°C and 15°C but are commonly captured in waters with higher temperatures. These fish have a preference for slightly acidic waters with an approximate pH of 6. This explains why this species can be found in brown peat-stained waters.

Giant kōkopu are being bred in captivity by Jerry and Gayle Rees-Webbe at Mangakura, on the Kaipara Coast Highway (SH16), assisted by Lyn Hamilton-Hunter, the hands-on aquaculture consultant. They are building on the previous work of Charles Mitchell (from where Jerry purchased his original broodstock), Matt Wiley of Otago University and Quentin O’Brien of the formerly named Mahurangi Technical Institute. Hamilton-Hunter was employed and trained there. No one has yet been able to get giant kōkopu to breed naturally in captivity. Females are fecund and the stripping, spawning and fertilising can be mastered by a trained operator. Females produce as many as twice the number of eggs of other common galaxiid species. Hamilton-Hunter and Rees-Webbe are aiming for high hatch rates and good larval survival rates (60%) and are developing intellectual property in improving the outcomes. 

Young adult fish are fed on a generic pelleted fish food with natural supplements to acclimatise them for release – such as meal worms, earthworms, insects and crushed dried oysters.

The Mangakura licenced fish farm carries 400 female broodstock giant kōkopu (plus other species) and about 2000 replacement fish of different ages. With improved fertilisation and hatch rates being achieved this year, there is the capability for thousands of whitebait to be reared for release. The objective is not to produce commercial quantities of whitebait but to breed for populating and re-populating streams with this endangered species. At 2 to 3 years old they are large enough to survive predation, but the preferred option is release in larger numbers when fish are aged 9 to 12 months with an expectation that some natural losses will occur. Two years ago there was a release of the Mangakura fish into Nukumea stream, Orewa, arranged by NIWA and Whitebait Connection. An application has been made to DOC for another release into a stream off the Mahurangi river. A third release is planned for a stream flowing into the Kaipara harbour. 

Showdown Productions Ltd.   Rural Delivery Series 16 2021