Technical Report

Conservation Genetics of Rio Grande Silvery Minnow, Hybognathus Amarus: Genetic Evaluation of Wild and Captive Stocks, 1999 to 2003

URL: https://webapps.usgs.gov/mrgescp/documents/Turneretal_2003_ConservationGeneticsofRGSMHybognathusamarusGeneticEvaluationofWildandCaptiveStocks1999to2003.pdf

Date: 2003/01/01

Author(s): Turner T.F., Osborne M.J., Alo D.

Publication: Annual report submitted to the U.S. Bureau of Reclamation, 80 p.

Abstract:

Part I of this report examines use of ecological and genetic methods to evaluate effects of river fragmentation on the federally endangered Rio Grande silvery minnow. Once widespread and abundant throughout the Rio Grande, this species is now restricted to 5% of its historical range in a river reach fragmented by five dams. Adult census size (N) declined an order of magnitude from 1999 to 2001, and is currently ~ 10⁴ . Microsatellite and mtDNA analyses detected no spatial genetic structure, suggesting that Rio Grande silvery minnow is panmictic over its current range. Analysis of temporal genetic variation indicated substantial shifts in allelic frequencies from 1999 to 2001. Average genetic effective size (Ne ) based on temporal-method estimation was about 70, and the ratio Ne/N = 0.0012 over the study period. Low values of Ne/N probably result from recent changes in demography related to extensive habitat fragmentation. Rio Grande silvery minnow produce passively drifting eggs and larvae subject to downstream transport by river currents, entrainment in diversion canals, and movement to unsuitable nursery habitats, which lead to low retention of yearly reproductive effort and poor recruitment. We propose that the interaction of passive dispersal in early life history stages and extensive habitat fragmentation sharply increases variance in reproductive success in remnant populations of Rio Grande silvery minnow, leading to low Ne compared to historical values. We conclude that river fragmentation has altered demographic and genetic dynamics of Rio Grande silvery minnow populations and reduced effective size to critically low values.

Part II extends results to genetic variation in wild Rio Grande silvery minnow sampled in 2002. A total of 389 fishes were fin-clipped and characterized for genetic 7 diversity at 10 microsatellite loci and one mtDNA locus. No appreciable genetic variation was found among river reaches, which is consistent with 1999-2001 data. Moreover, Ne estimated using temporal method comparisons with previous samples, remains critically low (Ne=55).

Part III considers the genetic effects of hatchery propagation in the Rio Grande silvery minnow. The purpose of this study was to determine whether captive propagation is maintaining the levels and patterns of genetic variation that are present in the wild population. To address this goal, variation in ten variable microsatellite loci and the mitochondrial ND4 gene was assessed in the wild population (2002) and in three hatchery stocks (2000, 2002 and 2003). The results indicate that the offspring of captively-spawned Rio Grande silvery minnow maintain similar levels of heterozygosity to the wild population but have reduced levels of allelic diversity. The converse is true of the captive population derived from wild caught eggs, which shows a marked reduction in heterozygosity whilst maintaining high allelic richness. Inbreeding values (FIS) for the hatchery stocks show an increasing trend, with values twice that of the wild source population in the hatchery 2003 stock. These finding suggest that the goal of preserving the full extent of genetic variation present in the wild population in the hatchery stock is not being realized. The results indicate that the supplementation of the wild population with hatchery stock is likely to cause a further reduction in the effective population size.

Part IV reports on egg collection activities for 2003, and Part V summarizes the major findings of this report and the implications for management and recovery of the Rio Grande silvery minnow.