Seasonal search for secretive black mudfish

This unusual fish species is found in wetland peat habitat and is only found in the Waikato, Auckland and Northland. Adults can measure between 8 cm and 13 cm long.

The species has developed an unusual behaviour to survive the hottest and driest parts of the year by burrowing into the soil beneath tree roots, or into mud and damp leaf litter.

Senior Biodiversity Ranger Nigel Binks says the black mudfish/waikaka is threatened by the loss of its ancestral wetland habitats.

“The draining of wetlands and subsequent lowering of water tables for agriculture have seriously impacted this species, so it’s crucial for us to determine if they’re surviving in their known habitats around Waikato,” he says.

Once burrowed, they undergo a process called aestivation – slowing their metabolism and heart rate to a point where they live out of the water for several months. Staying damp in the mud is essential to the fishes’ survival during the warmer months, and once the water returns to their environment, they become active again.

Twice a year, the team sets 10 traps for the mudfish at each of the surveyed locations around the Waikato. By capturing the mudfish in traps, staff can determine their presence in those locations and long-term monitoring will enable them to estimate the local population size. The 2020 survey has confirmed the presence of historical mudfish populations and these will be a focus for population monitoring in future years.

The trapping work is undertaken in autumn and in spring. Specimens are weighed using battery-powered electronic scales, with weights recorded at least to the nearest 0.1 gm. The individual specimens’ length is also measured as part of the monitoring.

Staff involved in the monitoring wear latex gloves to carefully handle the fish, which have an unusual skin mucus instead of scales, which prevents them from drying out and the spread of infections

“They’re a delicate species, so we place them into a bucket filled with water from their habitat for a few minutes while they’re weighed and measured, before they’re released back into the locations they were found in.”

The species’ spawning period is late winter, and the growth of juveniles is rapid, with individuals reaching up to 10 cm in length within a year. Nigel Binks says the largest specimen found during the recent batch of monitoring clocked in at 11.6 gm and is believed to have been a female carrying eggs.

Nigel Binks says the work is vital to protecting the species, which is listed as At Risk-Declining in the NZ Threat Classification System.

DOC Ranger, Carisse Enderwick was part of the team monitoring Waikato’s black mudfish.

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