Tradeoffs of using barriers to upstream movement to conserve native fishes at risk from nonnative fish invasions

Collaborators: Dr. Steven Kalinowski (MSU), Chris Downs (GNP), Joel Tohtz (Montana Fish, Wildlife & Parks)

Invasive species are one of the greatest threats to global biodiversity. Nonnative fish introductions have been a significant factor in the biotic homogenization of freshwater ecosystems worldwide. In the western United States, the distribution and genetic diversity of many native salmonids have declined due to competition and hybridization with introduced species. Consequently, many remaining trout populations are restricted to small, fragmented headwater habitats in protected areas, where their long-term sustainability is uncertain due to the risk of invasion by nonnative salmonids from downstream sources. These often irreversible changes have contributed to the extinction of many rare and endangered fishes. Consequently, fisheries managers frequently consider using barriers to prevent upstream movement of nonnative fishes as a conservation strategy, yet isolating populations may increase their risk of extinction. Therefore, this trade-off between invasion and isolation presents a serious conservation dilemma faced by many resource managers interested in protecting critical populations and species adaptations across the landscape. Such data for endangered and rare species in nature, however, are limited in most situations and the genetic, ecological and evolutionary consequences of these conservation measures are often poorly understood.

Glacier National Park (GNP) is considered a regional stronghold for the threatened bull trout (Salvelinus confluentus) and westslope cutthroat trout (Oncorhynchus clarkii lewisi), a species of special concern. However, populations of both species have dramatically declined over the past 30 years due to introductions and invasion of nonnative salmonids into historical cutthroat and bull trout habitats. Nonnative lake trout have displaced and replaced native bull trout as the dominant predator in most of the large lakes along the west slopes of the Continental Divide in GNP, as they have in several other systems throughout the western United States (more info). Currently, there are 17 lake dwelling populations on the west side of GNP, ten of which are already compromised to the point of extinction and two (Akokala and Cerulean) are at high-risk of invasion. In addition to threats to bull trout posed by lake trout invasion, these aquatic systems may be suitable for future invasion by brook trout, which can hybridize with bull trout and can outcompete cutthroat trout at young ages.

Non-hybridized westslope cutthroat trout populations in these same systems are also threatened by introgressive hybridization with introduced rainbow trout (O. mykiss). Recent studies have shown that hybridization between these taxa is spreading upstream in the upper Flathead River system following continent-island and stepping-stone models of invasion and may not be limited by biotic or abiotic factors. Rainbow trout produce fertile offspring when crossed with cutthroat trout (O. clarkii), and introgression often continues until a hybrid swarm is formed and the native cutthroat genomes are lost. A major consequence of such interspecific hybridization may be outbreeding depression due to the break-up of co-adapted gene complexes and disruption of local adaptations. Therefore, there is an urgent need to protect remaining populations to prevent further population losses in GNP and the upper Flathead River system.

Akokala Lake hosts one of the last remaining intact assemblages of lacustrine-adfluvial bull and cutthroat trout in GNP. However, these ecologically and genetically unique populations are at risk of invasion by nonnative salmonids, including invasive lake, rainbow and brook trout. Therefore, GNP resource managers are considering isolating these native populations in the lake by installing a barrier to upstream movement. However, isolation of these populations may disrupt migratory life history types within the drainage, restrict these populations to habitats unsuitable for long-term persistence, and prevent future recolonization of these habitats following catastrophic events (e.g., fire, floods, climate change). At NOROCK, a new project aims to provide ecological and genetic information on native salmonid populations in the Akokala Creek system which will provide a unique opportunity to assess the trade-off between invasion and isolation in a protected area.

For more information contact:
Clint Muhlfeld
406-888-7926
cmuhlfeld@usgs.gov