Population Dynamics of Bull Trout in Lake Pend Oreille, Idaho

Abstract

Introductions of invasive species, habitat loss, and stream fragmentation have caused bull trout Salvelinus confluentus to significantly decline throughout their native range. Consequently, bull trout were listed under the U.S. Endangered Species Act as a threatened species in June, 1998. Bull trout have existed in Lake Pend Oreille and its surrounding tributaries since the last ice age, and the lake was once a world-renowned bull trout fishery. However, non-native species introductions, stream habitat loss and fragmentation in the Lake Pend Oreille system have resulted in a decline of bull trout. To effectively manage bull trout in Lake Pend Oreille, the population dynamics of the species must be understood. Bull trout were captured, marked, released, and recaptured in gill and trap nets during the current lake trout suppression effort. Abundance was estimated using Chapman’s modification of the Petersen estimator. Bull trout killed during sampling were used to estimate age structure, growth, and maturity. Dead bull trout were dissected to retrieve otoliths for age estimation and to estimate maturity based on development of gonads. Bull trout numbers did not increase as expected in Lake Pend Oreille, but instead abundance remained stable. I found that bull trout residing in Lake Pend Oreille between 2006 and 2008 were between 4 and 14 years of age. I found that growth slowed following maturation of Lake Pend Oreille bull trout and that growth was greatest between age 1 and 2. Males began to mature at a shorter total length than females and most bull trout were mature when they reached 450 mm. My findings improved understanding of bull trout population dynamics in Lake Pend Oreille and suggest that current management techniques are improving the status of bull trout in Lake Pend Oreille.