Dwarf inanga (Galaxias gracilis)

The dwarf inanga looks like an inanga and is probably closely related to it. However, it is found in only 13 lakes near Dargaville in the North Island, and does not co-exist with other galaxiid species. As its name implies, it is the smallest member of the Galaxiidae family in New Zealand. Specimens of over 80 mm in length are rare, and mature adults may be only 40 mm.

Dwarf inanga populations have declined over the past 30 years, and it is now considered to be a threatened species. The introduction of rainbow trout into the lakes it inhabits was initially blamed for this decline, as trout are known to eat inanga. However, joint NIWA/Department of Conservation management trials showed that removing trout from Lake Waikare, one of the 13 lakes it inhabits, had no affect on the abundance of dwarf inanga. Biologist began to look for another culprit, and they did not have to look very far!

The prolific and aggressive gambusia is known to inhabit the edges of Lake Waikare, habitat that adult dwarf inanga use for feeding and breeding. After the trout were removed in this lake, gambusia numbers increased by up to 300%. In the shallows of the lake, gambusia were seen attacking healthy dwarf inanga and causing severe damage to the tails and heads of the majority of the fish. Fish that survived the initial attack would have later died from starvation because they were unable to swim, see, or feed properly. Ironically, the trout that had been eliminated helped previously to ensure the survival of the dwarf inanga, at least in this lake, because they would have preyed on the gambusia and held their numbers in check.

These studies show that the management of populations of native fish is not as straightforward as you might imagine, and that results from one area might not universally apply. In another lake, dwarf inanga became extinct even though trout were present along with the gambusia. However, this study and others indicate that gambusia pose a serious threat to some native fish species. We urgently need to find methods that eradicate or at least control gambusia numbers. And on no account should further liberations of gambusia occur.

[This page last modified September 2005]