Lake Trout Population Dynamics in the Northern Refuge of Lake Michigan: Implications for Future Rehabilitation

Abstract

The Northern Refuge was established in 1985 as part of the lake trout Salvelinus namaycush rehabilitation effort for Lake Michigan. To evaluate progress toward lake trout rehabilitation in the Northern Refuge, we conducted annual (1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) gill-net surveys in the fall to assess the adult population and beam trawl surveys in the spring to assess naturally reproduced age-0 lake trout. Our criteria for evaluating progress included the density of ''wild'' age-0 fish within the Northern Refuge, the proportion of wild fish within the adult population, density of spawners, adult survival, growth, and wounding rate by sea lampreys Petromyzon marinus. No wild age-0 lake trout were caught in the Northern Refuge during 1991-2008. Overall, wild lake trout did not recruit to the adult population to any detectable degree. The mean density of spawning lake trout decreased from 45 fish 305 m of gill net À1 d À1 during 1991-1999 to only 4 fish 305 m À1 d À1 during 2000-2008. Although the sea lamprey wounding rate more than doubled between these two time periods, catch curve analysis revealed that mortality of adult lake trout actually decreased between the two periods. Therefore, the 90% decrease in abundance of spawning lake trout between the two periods could not be attributed to increased sea lamprey predation but instead was probably due in part to the reduced lake trout stocking rate during 1995-2005. The paucity of natural reproduction in the Northern Refuge during 1991-2008 most likely resulted from alewife Alosa pseudoharengus interference with lake trout reproduction and from the relatively low lake trout spawner density during 2000-2008. Our results suggest that the annual stocking rate of lake trout yearlings should be increased to at least 250,000 fish/reef to achieve greater densities of spawners.

Diet of lake trout and burbot in Northern Lake Michigan during spring: Evidence of ecological interaction

“…Wild recruitment to the adult lake trout population in the Northern Refuge has been negligible for nearly a quarter of a century, despite stock sizes in the mid-1990s that may have been sufficient to generate successful natural reproduction (Madenjian and Desorcie, 1999). In the 2000s, however, lake trout spawner abundance in the Northern Refuge has remained low and, not surprisingly, natural reproduction remains undocumented (Madenjian and Desorcie, 2010). To explain this lack of wild recruitment, researchers have proposed several factors, including thiamine deficiency in eggs and fry (owing to high thiaminase in lake trout diets, Honeyfield et al, 2005), predation on lake trout eggs and fry by interstitial predators (Claramunt et al, 2005), and predation of lake trout fry by alewife (Krueger et al, 1995).…”

Factors Affecting Postrelease Survival of Coded‐Wire‐Tagged Lake Trout in Lake Michigan at Four Historical Spawning Locations

“…Notably, our data are from spring surveys and focus on fish stocked in the Northern Refuge but recovered anywhere in Lake Michigan. The age structure of Lake Trout recovered strictly within the Northern Refuge during fall surveys from 2000 to 2008 included substantially more fish from older ages (Madenjian and Desorcie ). Nevertheless, the age structure observed in the Northern Refuge is consistent with populations experiencing higher mortality from exploitation and/or Sea Lamprey‐induced predation.…”

“…These CPUE values are surrogates for postrelease survival, are similar to how relative return rates have been calculated in the past (e.g., Bronte et al. ; Madenjian and Desorcie ), and may be treated as a multi‐age index similar in design to studies that focus on one age as an index of relative survival (e.g., Hansen et al. ; Wilberg et al.…”

“…Consideration of only A1, A2, and A3 wound types follows conventional methods for examining Sea Lamprey wounding (e.g., Sitar et al. ; Rutter and Bence ; Madenjian and Desorcie ). We constrained our analysis to Lake Trout that were released and captured in the same location to limit, although not completely eliminate, the potential for bias from Lake Trout that moved and may have been wounded elsewhere.…”

“…Third, there are several factors, such as site‐specific mortality at the time of stocking and predation on newly stocked Lake Trout (Jacobs et al. ; Madenjian and Desorcie ), that could affect Lake Trout survival but either have not been evaluated or have only been evaluated at select sites. Finally, performance of hatchery fish stocked in the past has implications for current patterns in wild reproduction and harvest; thus, a contemporary analysis would be useful to help inform future stocking strategy modifications.…”