The genetic legacy of more than a century of stocking trout: a case study in Rocky Mountain National Park, Colorado, USA

Abstract

Human introductions can obscure the diversity and distribution of native biota; hybridization with and replacement by introduced congeners is a primary conservation threat, particularly in salmonids. Cutthroat trout (Oncorhynchus clarkii) are an important component of biodiversity in the American West, and all recognized subspecies are targets for state and federal conservation efforts. Rocky Mountain National Park (RMNP) in northern Colorado is a microcosm of trout introductions that happened worldwide. We used a combination of extensive stocking records and molecular genetic data to ask whether native trout populations persist despite stocking and whether patterns in the distribution of cutthroat trout clades could be explained by source and intensity of stocking. Nearly 15 million cutthroat trout were stocked into RMNP from a mosaic of sources in the 20th century. A single lineage of cutthroat trout was historically native to each side of the Continental Divide in RMNP, but we detected at least five divergent clades of cutthroat trout in 34 localities on both sides of the Divide. The distribution of lineages was predicted by stocking pressure and source but not by which lineage was historically native. The future of mixed and non-native cutthroat trout populations in RMNP poses a substantial conservation challenge.

Limited hatchery introgression into wild brook trout (Salvelinus fontinalis) populations despite reoccurring stocking

“…Other studies of domestic introgression focus on populations that were last stocked over 20 years ago, when broodstocks had been held in captivity for fewer generations. Although significant genetic change can occur over short time spans (Christie et al., ), minimal genetic differences between historic hatchery and wild stocks are not expected to significantly influence patterns of genetic diversity in present‐day wild populations (Ford, ; Naish et al., ; but see Stowell et al., for a counterexample in cutthroat trout ( Oncorhynchus clarkii ) populations).…”

“…Supplementation of wild populations with captive‐raised individuals is an increasingly important management strategy for species of social, commercial, and recreational values (Araki, Cooper, & Blouin, ; Naish et al., ; Stowell, Kennedy, Beals, Metcalf, & Martin, ). With continued population declines in many fish species due to habitat loss, climate change, and historic overharvest, stocking programs have expanded to meet growing recreational demands and conservation goals (Araki & Schmid, ).…”

Thermal regimes of Rocky Mountain lakes warm with climate change

“…High-elevation lakes, like those in the SRM, provide critical fish habitat, and changes to thermal characteristics are likely to alter fish life histories. Although it is unknown how many of these lakes were naturally fishless, owing to extensive stocking of native and non-native fishes initiated around the turn of the 20 th century [ 68 , 69 ], it is likely that some of the lakes in our analysis were fishless. Of the 27 lakes in our analysis, 23 currently have fish populations.…”

Connections and containers: Using genetic data to understand how watershed evolution and human activities influence cutthroat trout biogeography

“…Coastal, westslope, Lahontan, and Rio Grande cutthroat trout, all have mtDNA haplotypes that appear to diagnose specific geographic areas and can therefore be used to define subspecific boundaries [ 41 , 93 ]. In other subspecies, intermixed mtDNA haplotypes may be the result of natural admixture from historical events or from more recent translocations [ 33 , 46 , 94 ]. While it would be logical to conclude that haplotypes from geographically distant locations are from non-native introductions, if admixed populations are in adjacent watersheds then one must be cautious when inferring whether the population is introduced or not [ 95 ].…”