Top Gear: Paired gear comparisons for monitoring Rio Grande silvery minnow

Authors: Ann Widmer, Eric J. Gonzales, Laura Leslie Burckhardt

Year: 2013 (xlv)

Abstract

We conducted paired gear comparisons to validate four sampling gears used to sample the fish community in the Middle Rio Grande: beach seine, backpack electrofisher, mini fyke nets, and bag seine. Beach seines are the primary gear used to monitor trends in abundance in distribution of the endangered Rio Grande silvery minnow, Hybognathus amarus, by the Middle Rio Grande Endangered Species Collaborative Program. This study characterized what additional data could be collected about the fish community and Rio Grande silvery minnow population by supplementing beach seine sampling with a second gear type. Sampling occurred in five 1-km stretches of the main channel during pre-runoff and fall base flow periods in the years 2010-2012. Indices compared among gear types were frequency of species detection, average species richness, species composition of the catch, Rio Grande silvery minnow catch-per-effort (CPE), and Rio Grande silvery minnow size distribution. On average, the backpack electrofisher and beach seine detected a greater number of species and detected individual species more frequently than the other gear types. Supplementing beach seine sampling with an electrofisher and bag seine would yield a higher detection rate for Rio Grande silvery minnow and for larger-bodied species. The beach seine collected fewer Rio Grande silvery minnow per sample and this species comprised a smaller percentage of the total catch compared to the other gear types tested. However, the CPE for Rio Grande silvery minnow was not significantly lower for beach seine than electrofisher when samples were standardized by area so gear supplementation is not recommended for calculation of this metric. Rio Grande silvery minnow CPE paired by site and sampling occasion were not correlated between beach seine and any other gear type, which may have resulted from real differences in the capture efficiency of the gear types or from differences in the fish present in the habitats sampled. No difference was detected in the size distribution of Rio Grande silvery minnow among gear types within a season.