BY RYAN WEHSE, GREENBAY FWCO
Since the 1960’s, Lake trout have been stocked in Lake Michigan to restore a self-sustaining population that were extirpated in the 1950’s due to overfishing and sea lamprey predation.
The first signs of successful reproduction were reported from the western shores of Lake Michigan in 2013 whereby 10-60% of lake trout recoveries from the 2007-2010 year-classes did not have fin-clips. These were fish that hatched in the lake (wild) as all stocked lake trout are marked by removing one or more fins (fin-clips) although roughly 3% of stocked fish are inadvertently released into the lake without fin-clips. Therefore we infer natural reproduction when unclipped fish comprise more than 3% of the lake trout recovered.
Tracking the relative strength of wild year-classes is a major focus of the Green Bay Fish and Wildlife Conservation Office’s Native Species team. The Great Lakes Mass Marking Program sampling of the recreational fishery provides a robust source of data for tracking trends in natural reproduction. In 2015, nearly 5,800 lake trout caught by the recreational fishery were sampled and 14.5% of these were unclipped. Throughout this past winter, native species technicians Ryan Wehse, Zach Kleemann and Parker Kilsdonk aged otoliths from these unclipped fish to access which year-classes were contributing to the recreational fishery catch.
Ages are derived from fish otoliths, the fish’s inner ear bones that contain growth rings, or annuli, much like annular rings found on a tree. Otolith preparation involves encasing the otolith in an epoxy, cutting a 20 micron thin-section from the core of the otolith, and counting the annuli under a microscope. Most of the unclipped lake trout were sampled from southern Lake Michigan ports in Indiana, Illinois and southern Lake Michigan and these fish ranged between four and 21 with most between 5 and 9 years of age. Our aging results corroborate that large numbers of wild fish between 5 and 8 years of age, from the 2007-2010 year classes where natural reproduction was confirmed, were caught in the recreational fishery. We also aged considerable number of 9 year old fish which suggests there may have been successful natural reproduction in the 2006 year-class but we cannot confirm recoveries from this year class exceeded the 3% rate of marking error as there are no comparable age data for fin-clipped (stocked) fish caught in the recreational fishery.
In the coming years the 2011 and later lake trout year-classes will be vulnerable to the recreational fishery. Hopefully numbers of wild lake trout will continue to grow and give hope that a naturally sustaining population is soon to come.