Fish out of water: black mudfish

Department of Conservation —  30/09/2020

By Carisse Enderwick, Community Ranger

Waikaka: The cunning ones. Galaxiidae: The fish that wear the stars.

It’s early morning and a small team of rangers is heading out to three Waikato peat lakes to check on the minnow traps set the day before.

They’re hoping to find waikaka/black mudfish, Neochanna diversus, a native fish with a clever little trick up its proverbial sleeve. Adapting to the ebb and flow of a wetland habitat, this 9 – 15 centimetre scale-free fish has found a way to do what most fish cannot: survive out of water.

During the summer months when a wetland naturally dries out, black mudfish burrow under tree roots, into mud or damp leaf litter, and undergo a process called aestivation. By slowing down their heart rate and metabolism, these fish can live out of water for several months. When the water returns, they wriggle back into life almost immediately. Their skin, covered in a tear-like coating, absorbs oxygen, helps keep them moist out of water and protects against infection. It’s a set of survival skills that has served them well in a naturally changeable habitat.

But there are things even the cunning ones cannot outsmart, things that despite their adaptations, pose serious threats to their ongoing survival. When you’re a swamp-dwelling mudfish, you will likely make your home in a low-nutrient, acidic peat lake or bog with clear water and overhanging vegetation. Perhaps even in a farm drain on peat soil with good cover. But these habitats are being modified and habitat loss is a serious threat. Clearance of vegetation means wetlands are drier for longer spells over summer, and there is less plant cover for the fish to hide in. They are becoming, quite literally, fish out of water pushed beyond the limits of their incredible biology.

And then there are the pest fish. Gambusia, a small aggressive fish with a taste for fish-eyes, fins, and small fry, are the biggest predator to mudfish. Catfish and koi are also a problem, their foraging habits disturbing the lakebed and stirring up silt. This creates murky water and limits sunlight, which affects plant quality and ultimately water quality.

We are trying to understand just how much of an effect these threats are having on our Waikato mudfish populations. It’s been a while since the last survey was done – records go back to the 1990s – so there is work to be done. Establishing the presence and absence of fish is the work that, for the next two years, will establish a baseline from which to measure threats and interventions.

On this particular morning there is a feeling of excitement as our rangers wade through damp, dense grasses toward the lake edge. Lifting out a minnow trap feels like Christmas morning, and today there are smiles and cheers. Twenty-five fish are caught over four sites: Whangamarino Wetland, two Horsham Downs lakes and Lake Rotopiko. They are caught, measured, weighed and then released back into the water. The catch of the day is an 11.6g mudfish, a giant compared to the usual 2.88g fish recorded in this survey. She is likely carrying eggs.

It’s been a good day and while there is a sense of achievement, the mahi is far from over. Habitats are changing, landscapes are shifting.  It’s a time of observing and gathering data so that we can take what we learn and maybe – just maybe – rewrite the stars for our waikaka, the cunning ones and our Galaxiidae, the fish who wear the stars.

One response to Fish out of water: black mudfish


    I enjoy and learn a lot from these reports.