What Environmental Conditions Reduce Predation Vulnerability for Juvenile Colorado River Native Fishes?

The incompatibility of native Colorado River fishes and nonnative warm-water sport fishes is well documented, with predation by nonnative species causing rapid declines and even extirpation of native species in most locations. In a few rare instances, native fishes can survive and recruit despite the presence of nonnative warm-water predators, indicating that specific environmental conditions may help reduce predation vulnerability. We experimented with turbidity, artificial blue water colorant (artificial turbidity pond treatment), woody debris, rocks, and aquatic vegetation in a laboratory setting to determine whether any of these types of cover could reduce predation vulnerability and confer survival advantages for juvenile Bonytail Gila elegans (mean = 70 mm total length), Roundtail Chub Gila robusta (mean = 35 mm total length), Humpback Chub Gila cypha (mean = 67 mm total length), and Razorback Sucker Xyrauchen texanus (mean = 74 mm total length). We exposed selected species of juvenile native fishes to predation by adult Largemouth Bass Micropterus salmoides, Smallmouth Bass Micropterus dolomieu, Green Sunfish Lepomis cyanellus, Flathead Catfish Pylodictis olivaris, and Black Bullhead Ameiurus melas in overnight trials. Razorback Suckers served as prey in trials conducted with Largemouth Bass and Black Bullhead. Bonytail served as prey in trials conducted with Largemouth Bass and Flathead Catfish. Roundtail Chub served as prey in trials conducted with Smallmouth Bass and Green Sunfish. We matched sizes of predator and prey so that the maximum body depth of the prey never exceeded 40% of the maximum anatomical gape of the predators. Turbidity of 500 nephlometric turbidity units reduced effectiveness of sight-feeding predators such as Largemouth Bass, Smallmouth Bass, and Green Sunfish by up to 50% but also increased predation vulnerability to non-sight-feeding predators (Flathead Catfish and Black Bullhead) by up to 55%. Turbidity was the only treatment that significantly altered predation mortality of native fish. These results may help explain recent patterns of wild juvenile native fish recruitment to adult life stages at the Colorado River inflow in to Lake Mead and at the inflow of the San Juan River into Lake Powell. Both areas possess abundant introduced predatory fishes but are also very turbid.