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.