Genetic Monitoring of the RGSM

Abstract

Negative genetic impacts to a population can occur over relatively short time frames for fishes characterized by a short lifespan, when abundance fluctuates substantially from year to year and when populations are subject to augmentation with captive reared fish. For these reasons, genetic monitoring (i.e., temporal evaluation of genetic diversity) was implemented in 1999 for Rio Grande silvery minnow and has continued annually. As such, the monitoring program has bracketed multiple collapses of the middle Rio Grande population and includes years where captive-born repatriates comprised the majority of spawners in the wild. Genetic analyses of samples representing the 2021 population revealed that genetic diversity has been maintained across the time-series; a result we attribute to augmentation of the wild population. Although augmentation buffers the population against loss of diversity there may be unintended consequences, including increased opportunity for breeding between related individuals in some years and reflected by increases in measures of inbreeding estimated from SNPs. Genetic effective population size estimates based on changes in allele frequencies from one year to the next revealed a substantial decline in 2021 with effective size estimates among the lowest values (<100) since genetic monitoring began. Likewise, linkage disequilibrium effective size (which refers to the effective size of spawning population in 2020) decreased from values in the previous year. These results are consistent with substantially reduced density of Rio Grande silvery minnow in 2020. Both estimators of genetic effective size are negatively associated with the fraction of augmented fish in the population; such that genetic effective size is smaller when the breeding population is dominated by captive reared fish. Preliminary results for 2022, are indicative of a slight increase in both measures of effective size for the middle Rio Grande population.