The kōura were thought to have vanished from the river until monitoring for eDNA material such as skin, hair, scales, mucus and faeces was carried out for fish and other species earlier this year.
Opuha Water’s latest quarterly water quality report confirmed the existence of the threatened species.
Opuha freshwater specialist Dr Jared Panther said in the report the finding of kōura eDNA was significant as local reports had suggested it was no longer present in the Kakahu.
"But it was found upstream and downstream of the Opuha Water discharge point into the Kakahu River, including upstream and downstream of the gorge. Kōura are more active at night and usually seek shelter from predators during the day, which is one reason why they are not easily observed in waterways during daylight hours."
The eDNA monitoring traced freshwater mussels, longfin eel, shortfin eel, upland bully, common bully, Canterbury galaxias, torrent fish and brown trout upstream and downstream of the discharge point.
Water released at Lake Opuha and passing through the Kakahu Irrigation Scheme goes into the Kakahu River.
In the past local people have been concerned about the water quality of the river near Winchester.
Environment Canterbury (ECan) has just put out a commissioned review of the ecological health of the Opuha and Kakahu Rivers.
This assessed the Kakahu River near the Kakahu Irrigation Scheme discharge as moderate and declining to poor further downstream.
Opuha Water’s own water quality report scored it average or poor using the eDNA method, with a macroinvertebrate community index (MCI) giving good and fair ratings.
Another macroinvertebrate community index scored the river higher upstream of the discharge point compared with downstream, with waterway ratings of good and fair.
Longfin eel, shortfin eel, upland bully, common bully and trout were caught in the river and identified upstream and downstream of the discharge point.
In the report SLR Consulting said the collective biomonitoring results from the survey and previous surveys did not indicate any consistent patterns or significant adverse effects of the consented discharge on the freshwater communities of the Kakahu River.
Dr Panther said the main ecological parts of the survey investigated whether the impact of the discharge could be found on macroinvertebrates such as kōura, fish and periphyton material growing on rock or the river bottom, which provided a habitat and food for macroinvertebrates.
"The levels of periphyton cover in the Kakahu River in March remained below guidelines set by the Ministry for the Environment and ECan for thick mats and long filamentous algae. There was an increase in periphyton cover at the sampling site immediately downstream of the discharge point, relative to other sites, which could be due to a localised effect of the discharge but could also be due to other factors such as river works undertaken at the start of the irrigation season. However, there were no patterns evident in periphyton cover at the other downstream sites, which indicates there are no wider impacts of the discharge on the periphyton community."
MCI scores found little difference between the site immediately upstream of the Opuha discharge and all downstream sites in the Kakahu River.
"Comparing the 2024 survey with the previous two surveys, there are no consistent patterns in macroinvertebrate community diversity, abundance or quality that would indicate any significant adverse effects of the discharge. Attributing a discharge effect is difficult, as different habitats in the Kakahu River will naturally give rise to different macroinvertebrate scores. High abundance and diversity of fish at sites downstream of the discharge indicate that the discharge is not adversely affecting the fish community."
Ecan will have scientists going through the main findings of its review on the health of the Opuha and Kakahu Rivers at a community meeting in Pleasant Point on June 13.
tim.cronshaw@alliedpress.co.nz
