Reproductive Potential of Wild Rio Grande Silvery Minnow (Hybognathus amarus)

The Rio Grande silvery minnow (RGSM; Hybognathys amarus) was listed as federally endangered in 1994 and currently inhabits less than 7% of its historic range. The majority of wild-caught adults are age 1 with a smaller portion represented by 2+.  However, captive propagated stocks have maintained year classes up to 4+ years. Information is needed to determine the fecundity of the various age classes of hatchery-reared and wild-caught RGSM. Our objectives were to determine fecundity of 1, 2, 3 and 4 year old RGSM, compare fecundity among age classes, and compare egg counts to volumetric estimates. Spawning trials were conducted on four age classes (year hatched:age class): 2009:4, 2010:3, 2011:2, 2012:1. We counted all eggs of five individuals (n=5) from each age class through printed photos of eggs. Fecundity (number of eggs) increased among the age classes from an average of 2,113 eggs (± 218.7, standard error) in age 1 (2012) to 10,924 (± 777.7) in age 4 (2009) female RGSM.  Our actual counts were close to volumetric estimates. The average difference between actual counts and volumetric estimates was 4.0% for 2012 age class and 11.0% for 2009 age class. Volumetric estimates were usually slightly less than the actual counts.