Millions of dollars pumped into cleaning Lake Taupo of excess nitrogen levels may be damaging the lake's valuable trout-fishing industry by stripping the lake of valuable food sources for trout.
Anglers from Advocates of the Tongariro River say the acclaimed Lake Taupo fishery is in "crisis", with catch numbers down and smaller fish being caught than before.
The fishery - estimated to contribute $70 million a year to the local economy - was far from being the best trout fishery in the world, the group says.
Licence sales have dropped as much as 20 per cent in five years as anglers bypass Taupo and head to Rotorua lakes for bigger limit bags.
Conservation Department Turangi Taupo area manager Dave Lumley said Lake Taupo may be too clean, with the lack of nutrients in the water contributing to the trout not getting enough to eat.
Shallower Rotorua lakes contained more nutrients from nitrogen runoff and wastewater, and bigger fish, he said.
The Conservation Department, together with Taupo District Council and Advocates for the Tongariro River, is about to launch a three-year study programme to find out why the Taupo fishery has declined.
The $108,000 project, undertaken by a PhD student, will examine the interaction between the lake's phytoplankton and zooplankton and smelt, which are a main food source for trout.
The study would help identify the "missing link" in the lake's ecosystem, Mr Lumley said.
"We don't know enough about how the lake functions or the link between phytoplankton and zooplankton, and how they contribute as a food source for smelt."
It was clear bigger fish were being caught in Rotorua lakes, but it would be too easy to label less nutrients, or cleaner water, as the main factor behind the decline, he said.
The decline was reversible, and the fishery was showing improvement after lean years from 2004-06 when floods and weather patterns disrupted spawning runs, he said.
Advocates for the Tongariro River member Eric Wilson said lower numbers of smelt might be a factor behind smaller trout in the lake.
"The study will give a better understanding of the food chain in the lake and how much food source is available," he said.
Skinny trout were being caught, which suggested food was low.
During Taupo's fishing heyday, trout "gorged" on a ready supply of native species for food, he said. Those species had almost gone, and been replaced by smelt.
"Ten years ago I could fish for an hour and catch 3-4 fish. Now I fish for an hour and get one fish."
Local and central government are investing $81 million to remove 20 per cent of nitrogen from leaching into Lake Taupo by 2020.
Farmers now need resource consent to farm, and more than 100 dairy and drystock farms bordering the lake have had nitrogen levels benchmarked to reduce runoff.
Since 2007 the amount of nitrogen from farms entering the lake has been reduced by 140 tonnes a year.
FISHING FINE, SAYS ANGLER
During the past 12 years the fishing on Lake Taupo and surrounding rivers had been consistent. "I've never had any trouble catching trout. We had a blip from 2004-08 caused by floods that swept away a lot of young trout, and weather factors didn't ‘stir' the lake up and mix the nutrients from the bottom."
Later on there were too many trout for the amount of food available and many anglers were catching poor conditioned trout. Since maximum fish sizes were reduced from 45cm to 40cm there was less pressure on the food supply, meaning better trout.
"The quality of the fish is back to where it was six years ago. You can have a dirty lake or a clean one, the key is to balance trout numbers with food supply but it is difficult if you don't know how much food there is in the lake."
- © Fairfax NZ News
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