Detection of Natural Reproduction and Successful Recruitment of Lake Trout in Southern Lake Michigan

Overfishing and sea lamprey mortality drove Lake Michigan Lake Trout populations to near collapse in the mid-20th century. A massive stocking program began in 1965 with the goal of increasing spawner abundance and restoring Lake Trout populations. Beginning in 2009, nearly 20% of adult Lake Trout captured in fall spawning surveys lacked fin clips that normally indicate a hatchery-reared fish, and in 2012 and 2013 nearly 50% of all captured adults were unmarked. Stable isotope signatures were determined using otolith core material, which are indicative of the location of hatching and early life, and otolith edge material, which is indicative of locations recently occupied by the fish. Signatures from these two regions of the otolith enable the determination of the origin of unmarked fish by comparing otolith cores of unmarked adults with otolith cores of recently produced hatchery yearlings and otolith edge material of adults captured in L. Michigan and L. Huron. Otolith core signatures for unmarked adult Lake Trout collected from Julian's Reef in 2012 and 2013 were consistent with the open lake signatures found in otolith edgematerial of L. Michigan and L. Huron fish, and distinct from the hatchery signatures found in the core of hatchery-reared juveniles, suggesting natural recruitment to the adult life stage.