River research is a net gain for students and urban fishery

An abundance of brown trout in a river flowing through the Otago University Dunedin campus has surprised a group of Zoology Department third-year students.

Niamh Tasker didn’t have high expectations of the Waters of Leith, due to a large portion of it running through highly urbanised areas.

“I expected low numbers of fish because of this, however, I was surprised to see an encouraging number of brown trout in the waterway, as well as some other animals,” Ms Tasker said.

Above: Owen Dabkowski (left) helps Otago Fish & Game ecologist Jayde Couper electric-fish in Lindsay Creek at Bethunes Gully. Photos: Bruce Quirey, Otago Fish & Game

She is studying towards a Batchelor of Science, majoring in Marine Science and minoring in Agricultural Innovation

However, the Leith and its tributary Lindsay Creek were clearly less healthy the further they moved into the city, the students found.

Marine Science and Ecology student Jemma Bezuidenhout said it was great to have access to waterways so close to where the students lived and worked.

“Within the upper reaches the streams are clean and hold many interesting species,” Ms Bezuidenhout said.

“But is clear that the further these waterways move into the city and the more redirected and manipulated they are, the less healthy they seem.”

Otago University has been researching the fish population in the Leith and its tributary Lindsay Creek for seven years.

Field work with Otago Fish & Game at various sites in the catchment was taking place last week and this week. (March 8 and 15)

Owen Dabkowski, another Marine Science and Ecology student, was helping to gather data at Bethunes Gully.

One aspect of analysis would look at where the brown trout population came from – the ocean or freshwater.

“Understanding the freshwater ecology and the species that interacted there was important for the management of the trout fishery and the health of the stream,” he said.

Ms Tasker said by gaining abundance, age, and size data of brown trout, they could begin to assess the health of the ecosystem and waterway as a whole.

“We can compare this data with previous years to see how current management systems are working and whether anything needs to be reassessed.

“This allows us to provide data to organisations such as Fish & Game as well as Government groups to help drive positive management in these areas.”

Professor Gerry Closs, from the Zoology Department, said overall, the numbers of juvenile trout were very variable from year to year.

“Autumn floods are important in allowing upstream migration to spawning sites,” Prof Closs said.

“However, big floods in winter and spring can knock juvenile trout numbers down.

“If the floods hit soon after the juveniles emerge from spawning gravels in late September to October, then most are lost from the stream.”

However, the trout population was resilient as there are always adults and juveniles further upstream, contributing to populations downstream, Prof Closs said.

Otago Fish & Game ecologist Jayde Couper said the organisation appreciated the opportunity to work with the university, both the expert freshwater ecologist on the staff and the students.

“This work has shown that a high proportion of the trout in the Waters of Leith system may be sourced from fish that have spent time in the tidal reaches and harbour, meaning enhancement of the lower reaches of the river may improve the overall fishery, Mr Couper said.