Kokanee population of Lake Granby takes a dive
March 19, 2013
The Kokanee population in Lake Granby is “crashing” this year, according to Jon Ewert, aquatic biologist of Colorado Parks and Wildlife in Hot Sulphur Springs.
Kokanee salmon were first introduced to Lake Granby in 1951, and the reservoir became the state’s principle source of kokanee salmon eggs until the early 1990s when egg production was reduced to nearly zilch.
Kokanee spawn-take totals have been declining since at least the 1980s, and this year is an especially bad notch in the cycle due to high mysis shrimp numbers affecting the food web since 2007, according to Ewert.
When lake Granby levels are high, he said, the mysis proliferate, which reduces zooplankton called Dalphia, making it difficult for kokanee to survive.
Parks and Wildlife predicts the 2013 spawn-take at Lake Granby will be 40,000 to 70,000 eggs, a far cry from more than 2 million taken just a few years ago.
Although the upcoming season is predicted to be a low water year, meaning a building year for the kokanee, a low water year can be disadvantageous to lake trout, Ewert said.
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“When this has happened in the past, we have seen body condition of large lake trout decline and growth rates of large fish slow or stop,” Ewert said. “We expect to see that happen again over the next few years.”
For this reason, Colorado Parks and Wildlife plans to increase the number of stocked fish in Lake Granby to 75,300 catchable rainbow trout this year, and is still lobbying for 300,000 fingerling rainbows, which as of last week were not yet approved, according to Ewert.
“It’s coincidental there are shortages in rainbows to stock right when kokanee stocks in Granby are about to crash,” he said.
Ewert said lake trout predation on kokanee is just one of several factors in the decline of the species.
“There is a fundamental change in the Granby food chain that is more than just predator-prey,” Ewert said to a roomful of expert anglers last Wednesday during this year’s “State of the Fish” meeting in Granby. He said Parks and Wildlife will be comparing fisheries data of the Three Lakes with data from the U.S. Geological Survey to learn more about whether nutrient imbalances ultimately are causing declines in fish populations.
The lake is “struggling” in terms of large-fish populations, Grand Lake anglers told Ewert at the State of the Fish meeting.
To protect the fishery, Grand Lake anglers pressed their case for a change in the Parks and Wildlife’s wordy Grand Lake-specific fishing regulation, preferring instead that all lake trout over 20 inches be released immediately upon catch, meanwhile keeping the four-fish possession limit for fish under 20 inches.
But Ewert resisted a change this year, saying more fish data need to be collected from Grand Lake to make that determination. This summer, Colorado Parks and Wildlife will be using an advanced Canadian method of surveying the lake, which is expected to give a more accurate number of lake trout. The steep sides and deepness of the lake have made it difficult to sample up to now, Ewert said.
Colorado Parks and Wildlife plans to stock 8,000 catchable rainbow trout in Grand Lake, and another 50,000 fingerling rainbows, with the option to stock more fish from an allocation of 15,000 catchable 10-inch rainbows designated for Shadow Mountain Reservoir.
The biomass of brown trout has been on a decline since 2007 in the Colorado River, meaning an erosion of the adult population of brown trout, according to Ewert’s presentation during the State of the Fish meeting. In the Parshall to Sunset reach, the river is “dangerously close to slipping below the gold medal standard,” Ewert said.
But according to Colorado Parks and Wildlife officials at the State of the River, an agreement forged in the Colorado River Cooperative Agreement allows the release of 5,412 acre-feet of water in late July and August starting in 2014, which should help to reduce temperature exceedances on that reach that create stressful conditions for trout.
On the positive side of the Colorado, the rainbow population is on a rebound. Parks and Wildlife had been stocking 5-inch whirling disease-resistant rainbows with little recruitment, Ewert said, then switched to about 100,000 1.5 to 2-inch fingerlings stocked in fry habitat earlier in summer each year. What has resulted is an increase in the population.
“If things keep going in this direction, we’re not that far from the rainbow trout and brown trout populations flipping and the river being more rainbows than browns,” Ewert said.
Parks and Wildlife plans to stock 400,000 2-inch rainbows in the Colorado this year.
Thanks to the 2003 Fraser River Enhancement Project behind Safeway that succeeded in improving river habitat, Ewert has seen great strides in the health of the fish population there, he said.
“You may catch any species at any given time on that stretch of river,” Ewert said.
And the strategy of stocking small whirling disease-resistant rainbows in the Fraser is a strategy that is working well, he said.
So well, Ewert has discovered that at Confluence Park in Winter Park, where Vasquez Creek meets the Fraser River, the species of rainbows is displacing the brook trout population, a species also non-native to the river. It wasn’t Ewert’s intention, he said, and he plans to back off on the number of rainbows stocked in that portion of the river.
Down river at Kaibab Park in Granby, near where the Fraser feeds into the Colorado River at Windy Gap, the fish population may be telling of the big flushing flows of 2011.
2012 fish surveys showed the majority of brown trout in that area to be smaller fish, perhaps carried down and deposited there during the previous big water year.
“I can’t stand in front of the world and say the Fraser River is crashing,” Ewert told anglers at the meeting. “That’s not what I see.”
When people ask him how the Fraser River is doing, he said it is always a difficult question for him to answer.
“Every mile is different from every other mile,” he said. And on the nuanced river, “There is a lot of diversity from the headwaters to Windy Gap.”
Parks and Wildlife plans to stock 50,000 2-inch rainbows in the Fraser this year.