Detection of Natural Reproduction and Successful Recruitment of Lake Trout in Southern Lake Michigan Using Stable Isotope Analysis

Overfishing and sea lamprey mortality drove Lake Michigan Lake Trout populations to near collapse in the mid-20^th century. In response, a massive stocking program was initiated in 1965 to increase spawner abundance and restore Lake Trout populations. In 2009, nearly 20% of adult Lake Trout captured in fall spawning surveys lacked fin clips that normally indicate a hatchery-reared fish; by 2012 and 2013 nearly 50% of all captured adult Lake Trout were unmarked. Stable isotope signatures were determined using otolith core material (indicative of the location of hatching and early life) and otolith edge material (indicative of locations recently occupied by the fish). Signatures from these two otolith regions enable the detection of likely spawning locations of unmarked adults by comparing their core signatures with core signatures of recently produced hatchery yearlings and with edge signatures from marked, hatchery adults. An analysis of signatures from 206 unmarked adult Lake Trout collected from Julian’s Reef in 2012 and 2013 revealed core stable isotope signatures consistent with the open lake signature found in otolith edge of marked adults, and distinct from core signatures of hatchery-reared juveniles. These results are indicative of successful recruitment of naturally reproduced Lake Trout in southern Lake Michigan.