Investigating the Maturation Cycle of Invasive Lake Trout Salvelinus namaycush in Yellowstone Lake

Since the introduction of Lake Trout to Yellowstone Lake in 1994, the National Park Service has been actively removing these invasive predators via gillnetting.  More recently, emphasis has shifted toward alternative suppression techniques.  Following 13 years of removal efforts, we investigated the reproductive development in Lake Trout at various sizes to enhance removal efforts.  To determine stage of maturity and potential for consecutive-year or alternate-year spawning by Lake trout, we evaluated gonadosomatic indices (GSI) and egg development via histological analysis.  Preliminary results indicate that most males matured between 450 and 500 mm TL, but mature females were all greater than 500 mm TL.  For females greater than 550 mm TL, GSI gradually increased throughout the year, with a sharp increase in early October.  The GSI for males increased sharply in early summer, decreased slightly through late summer, and then increased again in early October.  Histological gonadal staging is being used to determine sexual maturity, provide insight into reproductive allocation and condition, and to determine a gonadosomatic cut-off score for assigning future samples into gross reproductive categories.  In addition, data will be useful for assessing population response to changes in lake trout abundance resulting from suppression activities.