Observations on the spawning habitat of giant kokopu

New Zealand’s native freshwater fish fauna is dominated by the galaxiid genera, of which there are currently thirty recognised taxa. Of these, all but two (Galaxias maculatus and G. brevipinnis) are endemic to New Zealand and 96% are classified as either At Risk or Threatened. 

The five most widespread and well recognised of the galaxiid species (G. maculatus, G. fasciatus, G. argenteus, G. brevipinnis, and G. postvectis) in New Zealand are collectively known as whitebait and the juvenile life stage makes up an important recreational and culturally significant fishery. Perceived long-term declines in the fishery have led to demands for improved conservation of these iconic species. Various factors have been suggested for the decline including habitat loss and alteration, barriers to migration, introduced fish and over-harvesting. However, the biology and ecology of these species is still relatively poorly understood making attempts to conserve and restore their populations challenging in an increasingly modified landscape subject to multiple stressors.

Understanding of the spawning biology of these five species is particularly poor and, with the exception of G. maculatus (inanga), is based on only occasional observations. However spawning habitats of the giant kokopu (G. argenteus) have been recently discovered. In common with the other kokopu species (banded kokopu (G. fasciatus) and shortjaw kokopu (G. postvectis)), it has been found that giant kokopu lay their eggs amongst bankside vegetation during high flows. Egg development then occurs out of the water. A subsequent high flow event is then required to re-immerse the eggs, prompt hatching and allow downstream transport of larvae to the sea.

Observations to date indicate that giant kokopu spawning in Bankwood Stream, Hamilton, occurs over the period from April to June, that multiple spawning events occur during the spawning season, and that the same spawning sites are used between years. Eggs have been found deposited in wandering willie (Tradescantia fluminensis) and Yorkshire Fog grass (Holcus lanatus). This relatively unique spawning strategy makes this life stage highly susceptible to changes in flows and the management of river banks.