Dynamics of an Increasing Lake Trout Population in Lake Pend Oreille, Idaho

Abstract

AbstractTo quantify movement and abundance of lake trout Salvelinus namaycush in Lake Pend Oreille, Idaho, large trap nets were fished during two periods (30 September 2003–31 March 2004 and 10 September–15 December 2005) and gill nets were fished from 12 February to 6 April 2006. Trap‐net catch rates and recapture rates of previously marked lake trout suggested that sampling locations along the eastern shore were associated with either lake trout foraging or spawning aggregations in both years. Abundance of mostly mature lake trout that were vulnerable to trap‐netting in autumn increased from 6,000 in 2003 to 11,000 in 2005, whereas total abundance of immature and mature lake trout increased from 12,000 in 2003 (based on trap‐net size selectivity) to 36,000 in 2005 (based on gill‐net recaptures). The increase in abundance of all immature and mature lake trout estimated by mark–recapture in 1999, 2003, and 2005 was well described by an exponential growth model (intrinsic rate of population change r = 0.490; annual rate of population change λ = 1.631), which indicated that the population would double every 1.4 years and would reach nearly 400,000 fish by 2010 if carrying capacity was not reached sooner. Based on population densities observed elsewhere within the species' range, an abundance of 400,000 lake trout is possible, but population density at carrying capacity is probably lower in very deep lakes like Lake Pend Oreille, where much of the area is too deep for lake trout. During 2006, suppression of the lake trout population through gillnetting, trap‐netting, and angling was reflected by an exploitation rate of 44% (30–63%) and a total annual mortality rate of 58% (39–82%), well above the sustainable threshold mortality rate of 50%. We recommend the use of population modeling to evaluate the success of ongoing lake trout suppression efforts in Lake Pend Oreille.