By Kevin Bunch, IJC
A study of lake trout stocked into Lake Michigan has found a wild population rising in the southern basin of the lake, but struggling in the north where sea lamprey predation and fishing pressure prevents most fish from living long enough to spawn.
The native top predator in four of the five Great Lakes, lake trout are important ecologically and as a game and sport fish. The lake trout – also known as siscowet, lake char, or mackinaw – inhabits cold, pristine, oxygen-rich waters and mature slowly. That slow growth rate led to a population crash in the mid-20th century, when overfishing and invasive sea lamprey predation ravaged the species. A change in the food web due to other invasive species also has impacted common food sources for lake trout. Fishing limits and sea lamprey control programs have helped reduce pressure on the species, however, and restoration efforts are paying off.
US Fish and Wildlife Service fish biologist Matt Kornis said lake trout have been stocked into Lake Michigan for decades in a bid to restore the species. Some of these fish were tagged with coded-wire tags at the hatchery beginning in the mid-1980s. Those tagged fish were stocked in four important spawning areas – one in the southern refuge, a cluster of reefs in the dead center of the lake; one in a northern refuge, a cluster of reefs in the northeast part of the lake; one at Julian’s Reef in offshore Illinois waters; and a nearshore shoal in Wisconsin waters called Clay Banks. The lake trout came from genetic remnant stocks from lakes Michigan and Superior alongside lake trout from New York’s Seneca Lake.
“Not only is the restoration timeframe (from the 1960s to now) long, but the spatial scale is very large,” said US Fish and Wildlife Service Senior Biologist Chuck Bronte. “We’re talking about one of the largest lakes in the world (fifth by area). That’s a big scale for trying to restore a keystone predator.”
In Lake Michigan, Kornis said the US Fish and Wildlife Service (USFWS), along with state and tribal partners, have been cooperatively sampling lake trout using gillnets every year since 1998 to analyze the recovery of tagged fish and get a better idea of survival rates and where they were found in relation to where they were stocked. They’ve found that the survival of stocked lake trout and positive growth in their population were heavily dependent on where the fish were stocked.
“The fish that were stocked in the northern refuge … had a substantially lower survival rate that we attribute to sea lamprey predation and fishing harvest,” Kornis said. “The downside is that there is poor survival in northern Lake Michigan, but the upside is we observed high survival of fish stocked in the southern basin, where we also saw more recent increases in wild recruitment (where fish spawn naturally).”
The problem in the northern refuge with sea lamprey stems in part from a failed dam on the Manistique River which allows sea lamprey access to a large, ideal system to spawn in. The Great Lakes Fishery Commission has been controlling sea lamprey populations in the Great Lakes by using lampricide in spawning habitats and physical barriers, but the Manistique system is not easy to treat with lampricide due to its size, making the dam a vital barrier to keeping lamprey from getting into the river system in the first place. Bronte said that dam will be replaced and upgraded within the next few years, shutting out sea lamprey from that spawning habitat that replenishes their numbers, in turn dramatically reducing their numbers in the area and helping lake trout in the northern refuge recover.
The trout harvesting is done primarily by Native American tribes exercising Great Lakes treaty fishing rights guaranteed under the 1836 Treaty of Washington, Bronte said, which are negotiated jointly by the tribes, the state of Michigan and the US Department of Interior as a consent decree. The current agreement was approved in 2000 and has seen minor revisions as circumstances change in the lakes; it runs until 2020.
Kornis said only a handful of older, mature lake trout were caught in the northern refuge, which means the fish don’t have a large enough parent population size to properly breed. A fecund population, he said, needs a high abundance of older fish from multiple age classes, something that’s been seen in southern sites over the past 10 years but not yet seen in the northern refuge.
“You can stock fish, but if they don’t survive to maturity that’s a problem,” Bronte said. “If you want lake trout restoration (to work) you’ve got to let them live longer and get to higher densities.” Lake trout take six to 10 years to become sexually mature.
All lake trout stocked everywhere in Lake Michigan – and not just those four reefs – started being tagged in 2010, but since lake trout take around five years to reach harvestable size and thus enter the fishery, those fish have yet to be included in the surveys, Kornis said.
Lake trout may be benefitting from Lake Michigan’s reduced alewife population too, as that invasive fish will prey on lake trout fry. Adult lake trout predation on alewives also can lead to deficiency of thiamine, a critical vitamin. Thiamine deficiency reduces the survival of the eggs and larvae of affected parents, Kornis said, an affliction known as “early mortality syndrome.”
Success stories in restoring lake trout to other Great Lakes, thanks to Canadian and US cooperation and planning, provide a sense of optimism for Lake Michigan. Binational programs to limit the harvest, control sea lamprey and stock the fish have been successful in Lakes Superior and Huron, according to Jolanta Kowalski, senior media relations officer at Ontario’s Ministry of Natural Resources and Forestry. Stocking was particularly important given how the fish was nearly wiped out in Lake Huron and had lost much of its adult population in Lake Superior when stocking began in the 1950s.
The lake trout population in Lake Huron is recovering well, Bronte said, with roughly half or more of the fish in the lake being entirely wild. While part of that is related to the alewife population collapsing, sea lamprey control efforts and a consent decree limiting the amount of lake trout that could be harvested also played a role in allowing the parental stocks to recover there. Bronte believes that same situation (low sea lamprey and fishing mortality) may be playing out in the southern end of Lake Michigan to some degree, but the recovery is still in the early stages and requires a low mortality rate to be successful.
Kowalski said Lake Huron still seems to have higher sea lamprey marking rates in the North Channel of Lake Huron than officials would like, suggesting there are still tributaries where the invasive species is reproducing with limited controls. Ontario still stocks lake trout in the Georgian Bay and the North Channel of Lake Huron, though the species has recovered enough in the lake’s main basin that it has ceased there.
Lake Superior’s lake trout population is fully restored and large scale binational stocking ended in the mid-1990s, Bronte added, providing hope that rehabilitation efforts can achieve similar success elsewhere.
Kevin Bunch is a writer-communications specialist at the IJC’s US Section office in Washington, D.C.