Freshwater scientists have rejected claims by a prominent dairy farmer that introduced trout are "freshwater stoats" and could be doing more damage to streams and rivers than dairy farming.
In his final speech, outgoing Federated Farmers dairy chairman Lachlan McKenzie last week told federation members they should distinguish fact from opinion and challenge some of the anti-dairying science.
"We have jaundiced freshwater ecologists blaming land-based industries, when they should be looking at what is eating the basis of the food chain unbalancing our native aquatic biodiversity. It's much easier to blame farmers if you happen to be running a separate agenda."
It was time to test if trout were a benign tourism-friendly icon "or if it is in fact, an aquatic stoat", he said.
Mr McKenzie said he believed the biggest impact of trout was on algal growth. Trout ate the invertebrates, stopping them eating the algae, he said.
"The little critters that graze algae and keep it in check have almost no life expectancy with trout present. Without trout, algal growth is kept in check and our streams and aquatic biodiversity achieves balance. "
Mr McKenzie also said phosphate was a bigger problem than nitrogen in waterways.
"Any call from government, from Dr Mike Joy (a Massey University scientist critical of dairy's environmental impact) and from councils to control nitrogen loss by hook or by nitrogen capping crook, is at best, well less than a 24 percent solution."
Mr McKenzie admitted farming affected water quality but farmers seem to have been attributed with 100 percent of the blame.
A 15-year Niwa report on nuisance periphyton growth found more sites with decreasing cover than increasing cover, which was not expected given the increasing agricultural intensification.
"I interpret Niwa's report as saying they assumed there would be more algae growth but the reality was the opposite."
But freshwater scientists were lining up to debunk Mr McKenzie's claims.
"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," said Otago University zoology professor Colin Townsend.
Insect-eating trout could actually make a small contribution to cleaning up nutrient runoff by increasing the amount of algae but such small changes to nutrient fluxes were swamped by the much larger amounts of nutrients entering from land.
"Soil erosion, and the resulting smothering of the stream bed by fine sediment, can be even more harmful to stream health than nutrient enrichment," he said.
Prof Townsend said farming was important to New Zealand but so was the state of the environment. "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."
Canterbury University's Angus McIntosh said the primary cause of poor waterway health in agricultural areas -- such as Canterbury -- was high sediment and nutrient levels, and that comparing trout with stoats misrepresented the science.
The effects of nutrient enrichment on algal accumulation and nutrient cycling were much more powerful than those of trout.
"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," Prof McIntosh said.
Jenny Webster-Brown, director of the Waterways Centre for Freshwater Management, said trout were the most sensitive species to most contaminants and a useful indicator of water quality.
"Protecting them ensures an additional level of protection for other species from the effects of poor water quality."