INTRODUCTION
Freshwater fishes display a diversity of mating systems and reproductive
behaviors (DeWoody and Avise 2001), and knowledge of reproductive
ecology and drivers of reproductive success are relevant to both
conservation and management efforts. For example, effective population
size (N e) measures the rate of genetic drift
within a population (Charlesworth 2009), and reproductive
characteristics such as family size, variance in reproductive success,
and adult sex ratios directly influence N e. In
particular, populations that display large variances in reproductive
success, unequal family sizes, and heavily skewed sex ratios will have
lower values of N e, experience greater losses in
genetic diversity via genetic drift, and be at an elevated risk of
extirpation (Frankham 2005). In addition to potentially impacting
persistence, mating systems and knowledge thereof may be directly
applicable to hatchery supplementation efforts whereby managers may wish
to emulate patterns observed in the wild.
Pacific salmonids (Oncorhynchus spp.) are of profound
recreational, ecological, and economic importance throughout their range
(Quinn 2018). Rates of imperilment among salmon are high (Gustafson et
al. 2007), and efforts to assess the viability of distinct population
segments include quantifying levels of genetic diversity (McElhaney et
al. 2000). Given the direct link between reproductive characteristics
and levels of genetic diversity, knowledge of mating systems is directly
relevant to conservation planning and management. The mating systems and
predictors of reproductive success of many salmonid species have been
described in detail (Brook Trout, Salvelinus fontinalis : Kanno et
al. 2011; Steelhead, Oncorhynchus mykiss : Seamons et al. 2004a;
McMillan et al. 2007; Atlantic salmon, Salmo salar : Garant et al.
2001); however, there is no such data for species such as Yellowstone
Cutthroat Trout (Oncorhynchus clarkii bouvieri ).
Yellowstone Cutthroat Trout are an iconic species of the western United
States, and are one of three subspecies of Cutthroat Trout native to
Idaho. Yellowstone Cutthroat Trout are thought to occupy less than half
of their historical habitats rangewide (May et al. 2003; Meyer et al.
2006), and are considered to be a high-priority species with a
standalone management and conservation plan (IDFG 2007). Primary factors
associated with declines include hybridization with non-native Rainbow
Trout (O. mykiss ), competition with invasive species (e.g., Brook
Trout), overharvest, water diversion, and habitat alterations (Varley
and Gresswell 1988; Behnke 1992; Kruse et al. 2000). Despite a range
contraction relative to historical records, the abundance and size
structure of Yellowstone Cutthroat Trout in Idaho have remained
relatively stable over the last 20 years (Meyer et al. 2003; Meyer et
al. 2014). Although Yellowstone Cutthroat Trout have been extensively
studied, we found no published literature describing their mating
systems. Recent efforts that have sought to understand how angling
impacts the reproductive success of Yellowstone Cutthroat Trout (Roth et
al. 2018), and the associated data sets have afforded the opportunity to
use extensive field sampling of parents and progeny to describe mating
systems and patterns of reproductive success.
This study used an existing data set to describe the genetic mating
system of Yellowstone Cutthroat Trout. Specifically, we were interested
in identifying the number of mates and number of offspring produced by
males and females, testing for evidence of inbreeding avoidance, and
exploring the effect that arrival timing at spawning grounds and total
length had on offspring production. To accomplish these goals, we made
use of data collected by Roth et al. (2018) in which prespawn adults
were sampled at a weir as they entered a spawning tributary as well as
their outmigrating juvenile offspring as they left the system. All
sampled adults and juveniles were genotyped using a panel of single
nucleotide polymorphisms (SNPs), and a combination of parentage analysis
and sibship estimation were used to document the mating system (e.g.,
monogamy, polygamy, etc.) and estimate the reproductive success of
sampled parents. Additionally, we used generalized linear models to
estimate the relative predictive importance of arrival date and total
length on offspring production. We also tested for the presence of
inbreeding avoidance among adults via comparisons of simulated and
observed levels of genetic relatedness for parents that did and did not
contribute to reproduction. Lastly, we estimated metrics of the
effective number of breeders (N b) and effective
population size (N e) via sibship analysis applied
to collections of juveniles and parents respectively. Combined, we
present the first detailed description of the genetic mating systems of
Yellowstone Cutthroat Trout and drivers of reproductive success.