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Dr Joy said a Wanaka vet told him there used to be three cows in Wanaka and he knew them each by name, now there were upwards of 30,000 cows in the area.
"That is the kind of change we’ve had, that is the reality of it, and a lot of Southland cows are being wintered here and they don’t really count in the statistics properly but their s... sure does."
Dr Joy said the cows were being wintered in Upper Clutha because of the drier climate but the soils around Wanaka were very porous, which allowed pathogens and contaminants to move quickly through the soil and into the waterways. The Massey University academic spoke in Wanaka this week at the invitation of the Lake Wanaka Trust, delivering a public lecture on "The Future of food; our deadly nitrogen and fossil fuel addiction". In an interview, he said artificial nitrogen made from fossil fuels had allowed cow numbers to double in the past 20 years and quadrupled milk production, but the farmers were not making any more money and the nitrogen was ruining rivers and lowland lakes.
"Our rivers can be saved because they flow, so once you stop polluting them they can come right, but our lowland lakes are so full of nutrients and sediments you virtually can’t save them."
He said "there has been such a weakening of the legislation in New Zealand that two of the most nutrient-polluted rivers in the world, like the Yangtze and the Mississippi, would get an A or a B under our new standards."
Dr Joy said New Zealand now had the highest proportion of threatened freshwater native fish species in the world — 74% of its fish were on the threatened species list "so you can’t go much further than that".
The 58-year-old former truck driver, boatie and dairy farmer was 33 years old before he went to university and he has studied the declining health of New Zealand rivers ever since.
He won the Critic and Conscience of Society award for drawing attention to the issue of water quality in New Zealand’s rivers, lakes and drinking water.
In presenting the award, former University of Canterbury deputy vice-chancellor of research and a member of the judging panel Steve Weaver said, "Mike’s work has successfully placed a complex and difficult environmental issue on the public agenda."
"He has single-handedly raised awareness of an issue that is at the heart of our country’s primary economy and environment."
Dr Joy will continue speaking out about water issues in New Zealand for some time.
He said he had been asked to go into politics but "I wouldn’t fit in there because I don’t compromise".
He will speak in Dunedin on Monday, at the University of Otago.