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.