Unique Allacustrine Migration Patterns of a Bull Trout Population in the Pend Oreille River Drainage, Idaho

Abstract

We captured and radio-tagged six adult bull trout Salvelinus confluentus in a spawning tributary of the East River basin, Idaho. These fish were tracked for a year to determine the type of migration they endured to reach their overwintering and spawning locations. Our tracking efforts revealed that the fish made complex postspawning migrations downstream and then upstream either towards or into Lake Pend Oreille. To reach the lake, bull trout migrated at least 12 km out of the East River basin into the Priest River, traveled 34 km down the Priest River into the Pend Oreille River, and then turned upstream and migrated 36 km to Lake Pend Oreille. Three of the six bull trout returned to the East River basin during the subsequent spring. These movement patterns are uniquely complex and extensive for outlet-spawning or allacustrine bull trout. This work illustrates the type of allacustrine migrations bull trout can have and suggests the need for new approaches for accomplishing bull trout population expansion into historically occupied habitats. Eliminating barriers downstream of lakes could potentially contribute to and increase bull trout populations considerably.