Body condition in an endangered minnow is variable but not strongly related to environmental conditions

Abstract

Seasonal patterns in body condition may help identify sensitive time periods or environmental stressors in fishes. Our objectives were to document spatiotemporal patterns of body condition of wild and hatchery-released Rio Grande silvery minnows (Hybognathus amarus), an endangered species endemic to the Rio Grande basin of North America. We collected fish weekly or bi-weekly from February to September, 2017 through 2019, in the Rio Grande, New Mexico, USA. Our time-series spanned an extreme drought year in 2018, allowing us to observe changes in body condition in relation to markedly different flow conditions. We collected body measurements from 2520 RGSM during the 3 years of our study. Spatiotemporal patterns in body condition were evident among and within years and sampling reaches, but within-sample variation in body condition was high. Annual patterns did not appear to be related to annual flow regime. Recaptured hatchery-reared fish were in slightly lower body condition, but followed the same seasonal patterns as did wild fish. Contrary to our expectations, body condition in both females and males was higher during the extreme drought year of 2018. Body condition may not be useful for identifying environmental stressors in small-bodied, short-lived fishes because individual variability is greater than spatial or temporal variation.