Budy, P., P.D. Thompson, M.D. McKell, G.P. Thiede, T. Walsworth, and M.M. Conner. 2020. A multi-faceted reconstruction of the metapopulation structure and life history expressions of a remnant metapopulation of Bonneville Cutthroat Trout (Oncorhynchus clarkii utah): implications for maintaining intermittent connectivity. Transactions of the American Fisheries Society 49: 443-461.
Abstract
Fishes that evolutionarily demonstrated a fluvial life history expression and migrated to spawning and rearing habitat using lotic corridors are increasingly impacted by fragmentation. The overall goal of this study was to identify the contemporary importance of mainstem connectivity and tributaries to maintaining life history expression, population structure, and viability of a large metapopulation of Bonneville Cutthroat Trout (BCT; Oncorhynchus clarkii utah) persisting in the Weber River, a highly fragmented Utah river. We used a multifaceted approach including active sampling, mark-recapture, passive PIT-tag detection, otolith microchemistry, and genetics. We collected BCT in all tributaries and the mainstem, encountering age-0 fish in three tributaries, indicating successful reproduction. In tributaries, the size structure was bimodal and consisted of smaller fish deemed to be resident and larger fish deemed to be fluvial, whereas all sizes and ages (≥ age-1) were present in the mainstem. We identified up to eight age classes; tributaries were dominated by ages 2 and 8, and the mainstem was dominated by ages 2, 5, 6, and 7. Tributary BCT had lower growth rates than BCT in the mainstem. We observed a surprising degree of fluvial life history expression, and fish also demonstrated very complex movement patterns across their life span. Average apparent survival (33%) was within the range estimated in similar studies for BCT, and resight rate was best explained by angler management regulations. The fact that BCT in the Weber River and tributaries still reproduce successfully in most years and are still able to grow into large, fluvial fish, suggests connectivity must be occasionally available despite considerable fragmentation. Therefore, this metapopulation may need little further human intervention if barriers to fish passage could be removed and thus connectivity could be improved and represents a high priority metapopulation for conservation, thus also highlighting the utility of our approach.