Wyoming outdoors: Lake trout numbers continue to fall in Yellowstone

2014-03-20T00:00:00Z Wyoming outdoors: Lake trout numbers continue to fall in YellowstoneBy BOB KRUMM For The Gazette The Billings Gazette
March 20, 2014 12:00 am  • 

David Sweet recently emailed me with an update on the ongoing battle to diminish the lake trout population in Yellowstone Lake and increase the numbers of Yellowstone cutthroat trout. In short, things are looking up for the Yellowstone cutthroat as their numbers continue to increase.

According to Sweet, a Cody volunteer who represents the Wyoming Council of Trout Unlimited, distribution netting for cutthroats is the best measure of their population.

“This involves placing nets at the same locations around the system every year on the same dates and letting them soak for the same period of time,” he said. “The number of cutthroats captured gives a relative measure of the increase/decrease in cutthroat numbers. From 2011 to 2012, the number of YCT almost doubled with a significant increase in the number of juvenile cutthroats surviving. In 2013, the number of YCT increased another approximately 30 percent and again there were more juvenile and mid-year class cutthroats. While not a true population estimate, the trend is extremely positive. We were especially happy to see the increased survival of juvenile cutthroats, meaning that a higher number are avoiding lake trout predation.”

Sweet noted that 2013 was a great year for netting lake trout. He said that a total of more than 300,000 lake trout were netted by gill and trap nets. Commercial fishermen the Hickey Brothers with three boats, and the National Park Service with one boat were able to put a dent in the lake trout population. What was encouraging to everyone is that it took more effort to catch the same number of lake trout as they had in 2012.

“The number of net-nights was up about 28 percent to get the same number of fish,” Sweet said. “This increased effort necessary to net the same number of fish is in spite of telemetry aiding the placement of those nets. This is the second year in a row that the catch per unit of effort has gone down (8.23 in 2011, 6.55 in 2012 and 4.7 in 2013). Although not absolutely proof positive, this is a strong indication that the overall lake trout numbers are dropping in the system.”

In 2013 the researchers attempted to obtain a population estimate of the lake trout by using a mark-and-recapture technique. Some of the lake trout that were captured in the trap nets were marked with colored floy tags and released back into the lake. Sweet emphasized that although a final population estimate hasn’t been calculated, it was surprising that 56 percent of the tagged fish were recaptured by season end. That is a heck of a great recapture rate. Sweet also said that some of the lake trout moved considerable distances in a relatively short period of time.

Sweet also reported that the telemetry work was continuing and that 2013 was the second full year. The number of telemetry-tagged lake trout has increased with many of the radio tags having depth and temperature indicators. This telemetry information has enabled the researchers to discern seasonal movement patterns. This information is passed on to the netting crews to help them pinpoint their efforts on a weekly basis. The information has enabled the crews to be more efficient.

Of course, the fall telemetry data also helped the netting crews pinpoint where the lake trout are spawning.

“Fall movement patterns that indicate pre-spawn movements to spawning beds are closer to being predictable,” Sweet said. “These patterns indicated at least three areas of suspected spawning activity: just off West Thumb Geyser Basin; off Solution Creek; and in the Plover Point, Frank Island triangle. Because these were suspected spawning grounds, arrays of receivers along with reference signals were placed in these three areas in early September in an effort to pinpoint any spawning beds in these areas.”

It even looks like the researchers have discovered two or three more spawning sites thanks to the telemetry work.

Another important part of the work on Yellowstone Lake is to understand the spawning site requirements that lake trout have. It is well known that the lake trout spawn at Carrington Island, so researchers placed egg collection baskets in the rocky substrate of the submerged island. They discovered that the lake trout prefer a certain substrate size and that they avoid areas with silt, sand or vegetation in the rocky substrate.

The researchers had hoped to test a prototype electro-shocking device that had been developed by Montana State University’s electrical engineering department. They wanted to test it on the Carrington Island area in October, but the government shutdown negated that so they tested it at Swan Lake. The device is 10 by 14 feet and has an array of electrodes. The device can be lowered into the water and activated with DC voltage.

“The results are amazing,” Sweet said. “Lake trout eggs purposely placed in the rock are destroyed with greater than 98 percent efficiency even if those eggs are buried 8 inches deep in the rock — 100 percent if the eggs are on the rocky surface.”

The group hopes to test the device this spring on lake trout fry after the ice goes out.

Lastly, funding the netting and research work is expensive, but grants have helped to continue the fight. A $1 million annual grant from the Yellowstone Park Foundation will enable the netting efforts to continue at a high intensity.

“Telemetry and ova/fry suppression development is also expensive,” Sweet said. “Prior to 2013, this research effort was funded entirely by the Yellowstone Lake Working Group and the NVO members. Now we have a new partner. The Wyoming Wildlife and Natural Resource Trust Board visited the system in 2013 and awarded a grant to the program for $150,000 for this year. Further, they have now awarded an even bigger grant for the next three years — $621,000. The Wyoming Legislature had to approve this grant and did so in February. This an amazing level of support. When coupled with over $800,000 for the working group partners for years 2014-2016, we now have the resources to complete this applied research effort.”

The tide is turning and, with Herculean effort, the lake trout population in Yellowstone Lake is diminishing and the cutthroat population is increasing. Though the battle is far from won, the disaster of the elimination of the Yellowstone cutthroat fishery has been averted. Let’s hope that with refined techniques and equipment lake trout will be dramatically curtailed.

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