Sample collection
The samples used in the current study were a subset of those described in Roth et al. (2018) which assessed the effects of air exposure and angling on short- and long-term survival as well as progeny production of adult Yellowstone Cutthroat Trout. Results identified that air exposure had no statistically significant effect on the proportion of fish that successfully spawned and that neither angling nor air exposure significantly affected progeny production (Roth et al. 2018). Given the lack of observed impacts of air exposure and angling on reproductive contributions and reproductive success, we concluded that data on mating systems for Yellowstone Cutthroat Trout derived from the samples of Roth et al. (2018) would be reflective of patterns in naturally occurring populations.
A detailed description of all sampling methods can be found in Roth et al. (2018). Briefly, sampling was conducted on Burns Creek, Idaho (Figure 1 ), a tributary to the South Fork Snake River, from May to October 2016. A large portion of Yellowstone Cutthroat Trout in this system display a fluvial life history strategy with adults moving from the mainstem of the South Fork Snake River into tributary systems to spawn (Thurow et al. 1988). Spawning typically begins in late May and continues into July. Fry in Burns Creek emerge between mid-July and September and out-migrate to the mainstem river as age-0 fish (Moore and Schill 1984; Thurow et al. 1988).
Adult Yellowstone Cutthroat Trout were sampled from May through July 2016 at an adult weir operated on Burns Creek, 0.9 km upstream of the creek mouth. Fish migrating upstream enter a fish trap which is located at the top of a fish ladder which is utilized to reach upstream habitats from the weir. The trap is operated annually by the IDFG with the purpose of removing straying Rainbow Trout and hybrids. The total length was recorded for each adult. A genetic fin clip was taken from each migrating adult and stored on Whatman 3MM chromatography paper (Thermo Fisher Scientific, Inc., Pittsburgh, Pennsylvania). The phenotypic sex of each adult was identified in the field and confirmed using a genetic assay with accuracy (e.g., 99%; Schill et al. 2016).
Out-migrating age-0 Yellowstone Cutthroat Trout were collected later the same year using a combination of trapping and electrofishing methods. Fry were collected using a modified picket weir approximately 25 m downstream of the adult weir (described above) as well as with one Kray-Meekin trap placed in the thalweg downstream of the weir. Trapping occurred continuously between July and October in 2016. Additionally, single-pass backpack electrofishing was used to supplement fry collection and was performed over two 2 d periods in September and October. Backpack electrofishing was performed from the IDFG picket weir upstream for 4 km. A subset of fry collected via traps and electrofishing were randomly sampled for genetic tissue. We assumed that the cumulative fry sampling was an accurate representation of total production/outmigration and made every effort to minimize bias that may have arisen due to sampling and subsampling.