The evolutionary potential of a species is determined by its genetic diversity. Thus, management plans should integrate genetic concerns into active conservation efforts. The copper redhorse (Moxostoma hubbsi) is an endangered species, with an endemic distribution limited to the Richelieu River and a short section of the St Lawrence River in Quebec, Canada. The population, gradually fragmented since 1849, is characterized by a decline in population size and a lack of recruitment. A total of 269 samples were collected between 1984 and 2004 and genotyped using 22 microsatellite loci, which indicated that these fish comprise a single population, with a global FST value of only 0.0038. Despite a small census size (∼500), a high degree of genetic diversity was observed compared to common values for freshwater fishes (average number of 12.5 alleles/locus and average HO of 0.77 ± 0.08). No difference was observed between expected and observed pairwise values of relatedness (rxy: −0.00013 ± 0.11737), suggesting an outbred population. Long-term Ne was estimated at 4476 whereas contemporary Ne values ranged from 107 to 568, suggesting a pronounced yet gradual demographic decline of the population, as no bottleneck could be detected for the recent past. By means of simulations, we estimated Ne would need to remain at more than ∼400 to retain 90% of the genetic diversity over 100 years. Overall, these observations corroborate other recent empirical studies confirming that long generation times may act as a buffering effect contributing to a reduction in the pace of genetic diversity erosion in threatened species.