Postrelease growth and maturity of hatchery-reared Rio Grande Silvery Minnow

ABSTRACT Objective Captive propagation and supplementation are used for conservation of endangered fishes. Breeding programs have been used to prevent extinction of the endangered Rio Grande Silvery Minnow Hybognathus amarus. However, the postrelease effectiveness of supplementation programs is not often evaluated. Our objective was to compare the growth rate and size at maturity between wild and hatchery-reared individuals of this short-lived species. Comparing growth and maturity patterns of postrelease hatchery individuals to those of wild individuals will provide insight into hatchery optimization and improve conservation hatchery strategies. Methods We used multiple long-term data sets of hatchery-reared and wild Rio Grande Silvery Minnow length measurements to estimate growth rates and size at maturity. We used age data obtained from otoliths of wild individuals and compared them to known-age hatchery individuals released in the middle Rio Grande, New Mexico. First, we estimated von Bertalanffy growth parameters based on monthly intervals. Next, we used logistic regression on data from Rio Grande Silvery Minnow collected in 2018–2019 to estimate and compare size at maturity for hatchery and wild individuals. Results Hatchery-reared individuals were smaller than their wild counterparts at release and remained so until an age of about 1.5 years. Hatchery fish grew slower (growth coefficient k = 0.08) than their wild counterparts (k = 0.10) but had a larger estimated average maximum length (86.6 mm versus 78.6 mm standard length). We estimated that over 50% of wild females were sexually mature by April 14 during our study, compared to April 19 for hatchery females. Conclusions Since hatchery Rio Grande Silvery Minnow are smaller upon release and mature at a smaller length, they have lower fecundity compared to wild individuals. Fish culture methods that raise Rio Grande Silvery Minnow to a larger size may be more effective by improving survival to spawning and increasing fecundity relative to wild individuals. Our results may guide the design of conservation hatchery programs for other small-bodied, short-lived fishes.