Lake Trout Fins Indicate Restoration Success:
Natural Reproduction of Lake Trout in Lake Michigan
Dale Hanson holds a “wild” lake trout captured during the bloater egg-take surveys, from a gill-tug of the northern Door Peninsula, Michigan. (U.S. Fish and Wildlife Service photo by Todd Kinn)
By Dale Hanson
Green Bay FWCO
Something really cool is happening on Lake Michigan. Anglers and biologists are capturing lake trout that have a complete set of fins. This first evidence suggesting successful natural reproduction of lake trout was obtained from large numbers of small lake trout recovered during Green Bay Fish and Wildlife Conservation Office's bloater egg collection surveys.
Lake trout disappeared from Lake Michigan in the 1950s due to overfishing and sea lamprey predation. In the 1960s a reintroduction program began in Lake Michigan with widespread stocking programs to restore self-sustaining populations. Over the last 50 years, between 2 and 3 million lake trout yearlings were stocked annually into Lake Michigan waters. These fish were marked, by clipping one or more fins, so they could later be identified as having originated from hatchery stocking. In the last two years a notable percentage of lake trout caught in the lake have a full complement of fins. This is a clear sign that these fish originated from natural reproduction in the lake.
Between 2010 and 2013, the Green Bay FWCO worked with commercial gillnetters and bottom-trawlers, in offshore waters of western Lake Michigan, to capture and spawn bloater, a deep-water species of cisco. The primary objective of these surveys was to obtain fertilized eggs to support a bloater reintroduction effort in Lake Ontario. However, lake trout were commonly caught as by-catch. Unclipped, wild lake trout accounted for 20 percent of all lake trout caught from surveys in the Southern Refuge. In waters off the northern Door Peninsula, between 10 and 27 percent of the 2007 – 2009 year-classes were wild. Preliminary data from 2013 indicates 22 percent of the lake trout by-catch from the northern Door Peninsula was wild, and 21 percent of lake trout by-catch was wild from bottom trawling near Manitowoc, Wisconsin.
These findings of wild recruitment were just published in the latest issue of North American Journal of Fisheries Management. More evidence is mounting that recent natural reproduction may be widespread in Lake Michigan: multi-agency fall lake trout spawn surveys reported 9 to 50 percent of the fish recovered were unclipped and these included surveys in Grand Traverse Bay and Illinois waters.
Why, after more than 50 years of stocking, are we only now seeing widespread reports of lake trout natural reproduction? There is no simple answer. Several impediments to lake trout natural reproduction have been suggested, including contaminants, stocking of lake trout in poor habitats, insufficient numbers of spawning lake trout, predation of newly hatched lake trout by alewife, and a deficiency of thiamine in lake trout eggs caused by a diet rich in alewives. Any one of these factors may have played a role in preventing natural reproduction of lake trout over the years.
Most contaminants have dropped precipitously in Lake Michigan since the 1960s. Lake trout stocking practices have changed and most are now stocked in offshore areas with suitable spawning habitat. Lake trout harvest is more effectively managed these days and reducing the abundance of sea lamprey remains a priority of the Great Lakes Fishery Commission and U.S Fish and Wildlife Service. Finally, alewife populations in Lake Michigan are near their lowest levels since the 1950s, reducing the potential for fry predation and increasing the likelihood that lake trout now consume a more diverse diet that has led to increased thiamine concentrations in lake trout eggs.
Restoration of self-sustaining lake trout populations is not complete. Lake trout densities are still far below target levels and natural reproduction is only just starting to reach detectable levels. Still, after 50 years of stocking it is great to see a lake trout with all of its fins.