Displacement of migratory Bull Trout (Salvelinus confluentus) spawning by a newly constructed beaver (Castor canadensis) dam in a west-central Alberta stream

Abstract

Despite broad overlap of the ranges of Bull Trout (Salvelinus confluentus) and North American Beaver (Castor canadensis),relatively little is published in the peer-reviewed literature on their interaction. Beavers are ecosystem engineers with the potential to transform lotic environments, while Bull Trout are of conservation concern and subject to various protective measures throughout their range. The impact of beaver activity on trout populations is context dependent, and our appreciation of the scope of this impact depends on documentation. We report on the disruption of Bull Trout spawning migration by a newly constructed beaver dam in Fall Creek, a west-central Alberta stream in the North Saskatchewan River drainage system. Large Bull Trout have been documented travelling up to 74 km to spawn in Fall Creek. Since 2007, we have used autumn redd counts to monitor the abundance of spawning Bull Trout in the stream. In 2022, we documented displacement of Bull Trout spawning in Fall Creek by a newly constructed beaver dam. Over 60% of Bull Trout redds were observed in a reach downstream of the dam rarely used by spawning fish. We also observed seven post-spawn Bull Trout, representing over 10% of the spawning run, apparently stranded in a shallow impoundment upstream of the dam. It is unclear whether this displacement will have a long-term impact on the Fall Creek Bull Trout population. However, the seasonal impact on spawning adults is unprecedented in the 15 years we have monitored the population