Fish in the Forest

The brown mudfish lives in ephemeral pools in West Coast forests, not the kind of place you’d normally expect to find a fish. These pools are very difficult environments to live in, being low in oxygen and high in acidity, conditions known to be deleterious to most fish species. Intriguingly, the brown mudfish can even exist out of water, living in moist forest soils for months at a time when these pools dry out. This indicates that these fish are incredibly tolerant. However, research conducted with the aid of a Brian Mason Trust grant, has shown that this tolerance to environmental conditions may come at a cost.

A large-scale survey of mudfish habitats has shown that they almost never occur in waters where a related fish, the banded kokopu, lives. We established that brown mudfish avoid banded kokopu by inhabiting waters that are too harsh for the banded kokopu to occupy. In this way the mudfish avoids competition and/or predation from banded kokopu. Moving our study into the laboratory we showed that mudfish have a comparatively low resting metabolic rate, meaning they have a reduced need for environmental oxygen relative to the kokopu. This is an adaptation that may allow mudfish to withstand low oxygen waters. However in situations where oxygen is readily available, the higher metabolic rate of the kokopu provides it with greater capacity to exploit the environment, which occurs at the expense of the mudfish.

These data have provided novel insights into the factors that shape the distributions of New Zealand’s native fish fauna, and may eventually be of use in efforts to conserve this important and fascinating group of animals.