Effects of Body Size on Vulnerability to Predation in Juvenile Roundtail Chub Stocked in the Upper Verde River, Arizona

Authors: Laura Tennant, David Ward, Alice Gibb

Year: 2018

Abstract

In the upper Verde River in Arizona, Roundtail Chub, Gila robusta, populations have declined, while non-native fish populations, such as Smallmouth Bass, Micropterus dolomieu, have increased. As one effort toward population recovery, Roundtail Chub are cultured in hatcheries and then released into the upper Verde River. However, predators may target recently-released fish, thereby reducing the effectiveness of stocking programs. We used laboratory and field experiments to examine the influence of non-native Smallmouth Bass on the survival of Roundtail Chub. First, we conducted laboratory trials to determine if Smallmouth Bass gape size influenced consumption of Roundtail Chub of various sizes. Trials were initially conducted with 12 prey fish and four predators; a second set of trials was conducted using two prey fish and one predator. In these trials, Smallmouth Bass consumed chub that were as large as 64% of gape, with a mean of 39.6% gape. In a field experiment where 2,177 pit-tagged Roundtail Chub were released into the Verde River (December 2016), we used electrofishing-surveys 48 hours after release to evaluate predation on stocked fish. In this experiment, we captured and measured the gape of Smallmouth Bass that consumed Roundtail Chub. Prior to stocking, the body depths of Roundtail were measured. Through these two measurements of predator and prey, we were able to quantify the relationship between predator gape and prey body depth when stocked Roundtail Chub were consumed by Smallmouth Bass in the field. From the field experiment, we infer Smallmouth Bass consumed prey that did not exceed 71.0% of gape, with a mean of 52.4% gape. A logistic regression model determines that, when prey body-depth exceeds 75% of predator gape, the probability of survival exceeds 95%. Based on these findings, we suggest that the size of both hatchery-reared fish and existing predators at stocking locations should be considered before native fish are released into the wild to reduce predator-related mortality.