Prof Townsend, of the University of Otago, was the keynote speaker at the opening of the New Zealand Ecological Society conference, "Biodiversity: 2010 and Beyond", yesterday.
Acknowledging the value and importance of brown trout to the "heart and minds" of New Zealanders, Prof Townsend spoke on the impact of non-native fish such as the trout, asking if it was good or bad for biodiversity.
Brown trout were introduced to New Zealand more than 100 years ago and were widespread throughout the country's rivers, streams and lakes.
For the past 20 years, Prof Townsend and others had been looking at the impact of brown trout on native non-migratory galaxiids in the Taieri and Manuherikia rivers.
They found that although the two rivers were side by side, just how trout affected the galaxiids was different, he said.
In the Taieri River, where there were trout there were no galaxiids, and vice-versa.
"There were a few streams where low densities of both were found but it was mostly one or the other," he said.
What seemed to be an important factor where both were found was the presence of waterfalls - above waterfalls, galaxiids were found; below, trout were found.
"Trout are not very good at climbing waterfalls."
Downstream waterfalls provided a "great upstream refuge" for native fish, he said.
In the Manuherikia, waterfalls were not as common and trout were found above and below them.
The factor that stood out was that trout were found where there were normal river flows, but where there was a high water take for irrigation, galaxiids were found.
In these cases, water temperatures were high and oxygen levels low, which trout did not like.
Galaxiids could burrow in and survive, although it was not ideal for them.
Trout were a "voracious predator", "vacuuming up large numbers" of small galaxiids, he said.
They also displaced galaxiids from their optimal feeding grounds and affected the behaviour of invertebrates such as mayflies, causing them to become nocturnal feeders.
The introduction of non-native species today might be justified if they were of high value and the risk was understood, he said.
However, where there were gaps in knowledge about species' impacts and those could be different in different catchments, a precautionary approach should be taken, as introductions were often irreversible, he said.
Brown trout were in the high-risk category, Prof Townsend said. "There's the probability it wouldn't be introduced now."
