Trout numbers in the South Island are under threat from the invasive freshwater algae didymo, a new study has found.
University of Canterbury researcher Professor Jon Harding found trout, previously in 20 South Island rivers, are now absent in 60 percent of the rivers with a high didymo biomass.
"The results of our study are of particular concern. We have assumed for some time that didymo will have an impact on fish, but these results show both native fish and introduced sports fish are all being affected by didymo," Prof Harding says.
Researchers also found that fish were being starved of small insects such as mayflies, stoneflies and caddisflies in rivers with the commonly known 'rock snot'. The hardest hit is brown trout where its body mass can be decreased by 70 percent.
"Thus rivers with didymo are liable to have much smaller trout and other fish, which could have major impacts on New Zealand's sports fishery industry," Prof Harding says.
Didymo has spread through 150 waterways throughout the South Island. There has been no recording of didymo in the North Island.