Monitoring Demographic and Genetic Responses of a Threatened Inland Trout to Habitat Reconnection

Abstract

Freshwater fishes living in streams and rivers can be affected strongly by isolation, which causes a disproportionate degree of fragmentation in such dendritic systems. Isolation disrupts important ecological and migratory processes as well as the ability to access refuge habitats during disturbances. The restoration of habitat connectivity, then, should be a productive strategy for improving the resiliency of freshwater fish populations, but the local and broader ecological benefits of barrier removal are still poorly understood. We report on a long-term, spatially intensive effort to monitor the responses of inland trout to stream habitat reconnection at a watershed scale, using both demographic and genetic techniques. Individual-based genetic assignment uncovered clear evidence of movement into the primary tributary of interest, which had been blocked by an assumedly complete barrier, but the source population generating this movement varied over time. A linear mixed-effect model suggested trout densities in this target stream increased threefold and the population sustained more large migratory-sized individuals after habitat reconnection. Densities overall did not respond to fire or variable spring flows which occurred during the course of the study, but a negative parameter estimate for “year” suggested a possible decline in densities over the study period (although 95% CIs barely overlapped with 0). Population genetic metrics showed no change in population differentiation or metrics of genetic diversity in most cases, except for a significant decline in allelic richness in the target population. The effective biological reconnection documented in our study should improve population resiliency in the future, but we discuss the benefits of spatially intensive monitoring at a watershed scale and combined inference from both demographic and genetic metrics to uncover unexpectedly complex fish responses to habitat reconnection.

Received September 18, 2015; accepted December 10, 2015 Published online April 27, 2016

Acknowledgments

This study was thoughtfully initiated in 2001 by Amy Harig (Trout Unlimited) and Carol Evans (BLM, Elko District Office). We thank Warren Colyer, Dana DeGraaf, Aaron Cushing, Jim Hearsey, John Rodstrum, Diana Gould, Michael Patton, Richard Walker, and John Zablocki for assisting with field sampling, and appreciate permission to sample on private property owned by the Maggie Creek Ranch, TS Ranch, and 26 Ranch and Newmont Mining Corporation. Veronica Kirchoff and Kristina Kruse completed laboratory analyses; Figure 1 was created by Matt Mayfield, and Sean McFall helped with R scripts. This manuscript benefited greatly from comments by two anonymous reviewers and the Associate Editor. This work was funded by Trout Unlimited via support from the National Fish and Wildlife Foundation, with additional support for H.N and D.D. provided by a BLM–Trout Unlimited Cooperative Agreement and National Aeronautics and Space Administration grant NNX14AC91G.