It is nearly impossible to exterminate zebra mussels once they’ve entered a body of water, yet biologists were able to do that in a Texas lake three years ago, so is there hope for Manitoba’s Clear Lake?
Only if you act fast and catch them early, says Monica McGarrity, a senior scientist for aquatic invasive species with the Texas Parks and Wildlife Department.
“Once they’ve spread beyond a localized introduction to other areas of the lake, it’s really, virtually impossible,” she told CBC Information Radio host Marcy Markusa.
In September 2014, employees with the City of Waco, Texas, found zebra mussels at a single boat ramp on Lake Waco.
Further surveys found approximately 75 more adult mussels as well as a few zebra mussel larvae, but all were in a localized area around the ramp and adjacent marina, according to a 2021 news release from the Texas Parks and Wildlife Department, when it was declared the invasive mollusks had been abolished.
“An infested barge came into a marina and we believe it dislodged some zebra mussels right around a boat ramp. So we did extensive shoreline surveys, diving surveys, and found that they were localized just around this one boat ramp in an area of less than one acre,” McGarrity said.
“That turns out to be basically .0001% of the total lake. So this is a very small portion of this lake.”
Chemical control options, including potash or a low dose copper application, were examined but determined to be questionable with mixed results in other jurisdictions, McGarrity said.
Instead, benthic mats — thick plastic sheeting to smother the mussels — were used after showing a 98 per cent success rate against Asian clams in Lake Tahoe.
In October 2014, nearly an acre of the sheeting was rolled out over the shoreline and lake bottom, where it stayed until March 2015.
“In order for the benthic barrier method to work, you really have to catch the muscles very, very early in the introduction. You need to know that they’re localized and they haven’t spread throughout the lake, and you need to try to get that barrier down before they start to reproduce,” McGarrity said.
When the tarps were removed there was a black residue on the underside, indicative of anoxia, McGarrity said, indicating the mussels had been deprived of oxygen.
“We did find two live zebra mussels which were removed from the water body. So we know that this method did not kill 100 per cent of the mussels, but we believe that if there were any remaining, there were too few or too far apart to be able to establish a population.”
No larvae or DNA was detected for five years afterwards and the lake was declared mussel-free.
Found in Clear Lake
Parks Canada officials found live zebra mussels in Clear Lake at Boat Cove in Riding Mountain National Park last November.
Since then, it has conducted sets of water sample tests, including one set between Jan. 8 and Feb, 2, and another between Feb. 2 and Feb. 20.
Though both sets of tests came up negative for zebra mussel environmental DNA, Parks Canada’s website says the invasive species could still be present in the lake and it is considering banning watercraft this summer.
Mireille Kroeker, a Parks Canada spokesperson, says there is no perfect solution when it comes to eliminating zebra mussels from a large body of water and Parks Canada is therefore considering multiple eradication methods in Clear Lake, including benthic mats.
“Parks Canada’s first priority is to the ecological integrity of Clear Lake and surrounding ecosystems, [and] any eradication effort considered would be carefully evaluated and only used if feasible,” she said in a Tuesday statement to CBC.
Parks Canada also continues research to figure out the zebra mussel situation in Clear Lake, which will be used to ensure that it can make timely decisions about mitigation, Kroeker said.
“Discussions on temporary boating management measures during the monitoring and potential eradication period (if needed and feasible) are underway.”
Premier against boat ban
Scott Higgins, a research scientist with the Experimental Lakes Area, told CBC News earlier this month that zebra mussels don’t reproduce until water temperatures exceed about 10 C to 12 C, which is why they could still be present despite the negative water sample tests.
Once that temperature is reached and they begin reproducing, the veligers — microscopic larva — will be free floating and spread to new areas, McGarrity said.
“A watercraft ban could certainly be highly effective for preventing the spread of zebra mussels to new water bodies, because transport via watercraft is really the main way that they’re transferred,” she said.
Premier Wab Kinew has urged Parks Canada to not ban the boats in the lake, which is a highly popular resort area in Riding Mountain National Park.
His government has promised an additional $500,000 to increase watercraft inspection and decontamination stations.
Inspection and decontamination stations won’t eliminate existing zebra mussels but could help prevent more from being introduced, McGarrity said.
“But that comes with a few drawbacks. It can be very expensive, it requires a lot of staffing, it causes delays for boaters leaving the lake, which they’re not going to be too happy about, and it also requires authority to require inspection and decontamination of all those watercraft,” she said.
But that still leaves the zebra mussels that possibly reside in a lake, and they will soon begin multiplying as it warms.
“You need to try to get that barrier down before they start to reproduce,” McGarrity said.