New visitors not welcome at caves

Researchers have been getting their feet wet in the hunt for fishy invaders under the twinkle of Waitomo Caves’ glowworms.

The renowned tourist attraction hosts thousands of visitors per year but not all newcomers are welcome.

National Institute for Water and Atmospheric Research (Niwa) principal scientist for natural hazards and hydrodynamics Dr Graeme Smart and hydrometric technician Carl Fischer were alerted to koi carp lurking in the cave system last year.

The pest fish were released into New Zealand waters about 60 years ago and pose a serious threat to native ecosystems.

Smart said the invaders’ appetite was destroying indigenous species and freshwater habitats at an alarming rate.

‘‘People were worried because they may be competing for food sources in the caves, which could severely impact the glowworms and other native animals.’’

The regional council’s proposal of building a fish barrier downstream of the caves to stop carp migrating up into them could flood the caves.

Discover Waitomo Group tasked Niwa with measuring water levels through the cave system and identifying fish species that make Waitomo caves home, particularly the invasive carp.

This would help them decide whether a weir could worsen any flooding.

However, a hitch was that modern surveying equipment will not work in a cave. ‘‘It is GPS operated, so functions perfectly well outdoors but is completely useless down there. So, we went back to the good old-fashioned way of doing things. Which meant getting our feet wet,’’ Smart said.

One of the methods they used was a level traverse which involves linking water levels measured in the cave to water levels at the cave inlet and resurgence, or exit, by a path of intermediate level stations.

The Smart-Fischer team is now in the process of evaluating their findings and planning further studies. It is likely that the cave system could need computer modelling to get a true idea of the impacts of inserting a downstream fish barrier.

Pest fish koi carp pose a serious threat to native ecosystems.