Are trout to blame for declining water quality?

Scientists are questioning claims by a prominent dairy industry representative, suggesting trout are a 'disastrous species' -- no better than 'freshwater stoats' -- and that farmers have been unfairly blamed for their impacts on declining water quality.

These allegations were made in a speech last week by outgoing Federated Farmers chair Dairy chairperson Lachlan McKenzie, urging members to use science and their own judgement to distinguish fact from opinion.

An excerpt:

"We can easily and graphically see the effect of stoats, possums and rats on our terrestrial environment... but what if we have an equally disastrous species in our waterways devouring native invertebrates, increasing algal growth and forcing our native fish to the brink?

"It's time to test if trout is the benign tourism-friendly icon it is held up as. ...

"Farming affects water quality, yes, but we seem to have been attributed with 100 percent of the blame."

The following experts in freshwater, aquatic ecosystems and water quality contacted the Science Media Centre with their reactions to statements made in the speech.

Feel free to use these quotes in your stories. Comments are excerpted below -- to read them in full, see our website.

To follow up with these or other local experts, contact the Science Media Centre.

Professor Colin Townsend, Dept of Zoology, University of Otago comments:

"In his speech, Lachlan McKenzie advises us not to accept someone's opinion as gospel but to interrogate it and check it out. 'If robust it will stand scrutiny', he says, 'if not, there is cause for concern'.

"As a researcher with 40 years experience and someone intimately involved in both trout research and the effects of agriculture on stream ecology I wish to comment on a few relevant points.

"Our research has shown that by reducing grazing by stream insects, trout can lead to a modest increase in algae on the streambed. These extra algae have the effect of sucking up some nitrogen from stream water and so the trout actually make a small contribution to cleaning up the mess caused by nutrient runoff from farms. In any case, the small changes to nutrient fluxes in streams associated with trout are swamped by the much larger amounts of nutrients entering as diffuse pollution from the land.

"It's worth noting too that our research in Otago shows that soil erosion, and the resulting smothering of the streambed by fine sediment can be even more harmful to stream health than nutrient enrichment. Unless Lachlan McKenzie has witnessed trout emerging from streams and churning up the land with their big fat hooves, he will find it difficult to shift responsibility from cows to trout.

"Farming is important to New Zealand but so is the state of our environment. Thankfully, many farmers are already doing their best to be good stewards. What is needed now is more discussion, education and collaboration between all sectors with an interest in land and water management, not an untutored and distorted analysis of the evidence."

Professor Angus McIntosh, Chair in Freshwater Ecology, University of Canterbury comments:

"I have studied the influence of trout on native biodiversity for most of my career, and as a researcher, I'm also involved in developing ways to mitigate the effects of land-use intensification on waterways.

"Yes, it is true that trout have negatively influenced native biodiversity and they do alter nutrient cycling. However, to compare their effects to stoats, and to imply they are somehow worse than, or equivalent to, the effects of land use intensification on water ways is a misrepresentation of the science.

"Firstly, they are clearly not 'eating the basis of the food chain' since we have highly productive trout fisheries in clean water streams. Native fish populations have been affected, but the resilience displayed by stream invertebrates in supporting predation by both native and introduced fishes is remarkable. Secondly, the effects of nutrient enrichment on algal accumulation and nutrient cycling are much more powerful than those of trout.

"Experiments conducted in New Zealand and elsewhere clearly establish that elevated nutrient concentrations quickly overwhelm any effect of trout on algae, which is actually small by comparison. Trout have not been responsible for what could be described as 'algal blooms' in New Zealand or elsewhere. Moreover, the effects of nutrient enrichment on stream invertebrate communities are also likely to be much stronger than those of trout. ...

"The health of water ways in places like lowland Canterbury is very poor at present. The primary causes in agricultural areas are high sediment levels, low flow and high nutrient levels. In urban areas, storm water contaminated with heavy metals and sediment (especially since the earthquakes) are to blame. Discussion regarding urban and rural waterways should be revolving around plans for rehabilitation and management, and needs to based firmly on the best science possible."

Prof Jenny Webster-Brown, Director -- Waterways Centre for Freshwater Management comments: "Trout may be an introduced species, but they are also the most sensitive species to most contaminants, and the first to show the effects of water quality decline. Because they are widespread globally, there is also a great deal of toxicity data (data that tell us when a particular species will be affected by increasing contaminant concentrations) available for trout; certainly when compared to the data available for species which are endemic to New Zealand.

"Trout are therefore a very useful indicator of water quality, and protecting them ensures an additional level of protection for other species from the effects of poor water quality."

ends

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