Rainbow and hybrid trout production dynamics and spread in the Flathead River, Montana

Abstract

Invasive species are one of the foremost drivers of global biodiversity loss, and managing their spread is critical to preserving the integrity of the affected ecosystems. Traditionally, management of invasive species often relied on spatial patterns of presence that may poorly reflect true production sources, as individuals may disperse from where they are produced. Therefore, identifying production sources is critical for understanding invasion dynamics and targeting effective control. The Flathead River, Montana is a large and connected river system that is a stronghold for native trout species in the continental US, including robust populations of westslope cutthroat trout. Historical stocking of non-native rainbow trout (Oncorhynchus mykiss) has led to an established and spreading invasive fish population that, through hybridization, is threatening the genomic integrity of native westslope cutthroat trout (Oncorhynchus clarkii lewisi) populations across the region. Otoliths (ear stones) are a calcium carbonate structure which accumulate tree-like growth rings and elemental signatures of surrounding waters and therefore can be used to age fish and identify their natal origins. Natal origins of 181 fish were assigned by visually identifying natal regions of the otolith and then using microchemical data from that region in an assignment algorithm to assign most likely origin locations along the Flathead River network. In addition, we created a novel genetic modeling tool which greatly improved precision of natal origin assignments. Our analysis provides evidence of strong hybrid source populations in creeks draining Glacier National Park and continued high production of rainbow trout the lower river near the initial site of introduction and where most of our sampling took place. Rates of spread differ significantly between rainbow and hybrid trout with hybrid trout spreading faster and more per year than rainbow trout. This suggests that hybrid trout are contributing more to the spread of invasive populations than rainbow trout themselves. Invasion risk increases with extent of hybridization and should be accounted for when developing future management plans. Our study provides a baseline for future work tracking and understanding invasive species production in complex, interconnected river systems.