Following complaints from Norsewood angler Gavin Mulinder about the state of a section of the Manawatu River, a Horizons Regional Council team has been in the water trying to identify the problem.

Mulinder had told the Dannevirke News it had been like fishing in an "open sewer" on two occasions.

Mulinder had gutted a fish to discover it was smelly and green inside, and was worried someone would eat fish from the area and become ill.

Horizons freshwater scientist Michael Patterson has told the Dannevirke News Horizons science staff went to sites on the river on last Monday, December 11, at Mangatera at Timber Bay to undertake invertebrate and periphyton monitoring.

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"This is at the confluence of the Mangatera Stream [into which Dannevirke sewage treatment plant discharges] and the Manawatū River," he explained.

"This is also approximately 8km upstream of the Manawatū River at the end of Kakahukura Rd which we understand to be approximately the location the fisherman was referring to."

Both the Tamaki River and the Kumeti (Mangapuaka) stream flow into the Manawatū between Timber Bay at the road end and the Oruakeretaki joins it a short distance downstream.

"The Tamaki, Kumeti and Oruakeretaki were all visited on Monday for periphyton monitoring, as was the Manawatū at Weber Rd [upstream of the Mangatera confluence] and Manawatū at Hopelands [approximately 20km further downstream]," Patterson said.

"I am confident that we have a reasonable coverage of the river in this reach and surrounding major tributaries."

Samplers have noted the presence of cyanobacteria at some of these sites and also filamentous green algae.

"This is consistent with a period of extended low flows and high water temperatures at this point," Patterson said. "Generally speaking there is a lot of periphyton around the region at the moment because of these weather conditions."

Mulinder believed the stench was coming from human waste in the river, but Patterson said the sampling team did not note anything unusual in the Manawatū or any of the aforementioned tributaries, nor anything that would be linked to a wastewater treatment plant discharge outside of normal periphyton growth.

"Due to the extended period of low flows, it is very likely that periphyton has been exposed and is slowly degrading which can result in a smell or unpleasant appearance," he said.

"Likewise, cyanobacteria can result in a musty smell at times that can be quite noticeable and it is likely these factors explain what the complainant experienced."