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.

Antipredator response of Eurycea nana to a nocturnal and a diurnal predator: avoidance is not affected by circadian cycles of predators

Both predators and prey exhibit cyclic shifts in activity throughout the day, which should cause the threat posed by predators to change in a recurrent pattern. If the threat posed by a predator is dependent on their circadian foraging cycle, prey may respond more or less intensely to predators at different times of day, thereby maximizing the effectiveness and efficiency of avoidance behaviors. We examined whether predator-naïveEurycea nana, a federally threatened neotenic salamander, exhibits a different antipredator response to chemical cues of a diurnal predator, the green sunfish (Lepomis cyanellus), and a nocturnal predator, the red swamp crayfish (Procambarus clarkii). We predicted thatE. nanawould show more intense antipredator responses (reduced activity) to a diurnal predator during the day and to a nocturnal predator at night. We found that, although there was significant antipredator behavior ofE. nanatoward sunfish, there was no detectable response to crayfish and no effect of time of day on responses to either predator, suggesting that eitherE. nanadoes not innately exhibit circadian patterns in avoidance of these species or that those patterns were undetectable in this study. Future studies should examine whether experience with predators may cause these salamanders to be more sensitive to the diel variation in threat, as has been found with some other amphibians and fish. Due to the threatened nature of this species, understanding the factors that influence antipredator behavior are crucial for management.