In southwestern North America, consumption of native fish larvae by nonnative predators has imperiled native populations. Field-acquired dietary analyses have provided little evidence of this cause-effect relationship. In this study, small, nonnative green sunfish Lepomis cyanellus, bluegills L. macrochirus, red shiners Cyprinella lutrensis, fathead minnow Pimephales promelas, and yellow bullheads Ameiurus natalis were each fed a single larva of the native razorback sucker Xyrauchen texanus. Gut content analysis revealed that prey detection generally became increasingly difficult over a short postconsumption time period under laboratory conditions. For green sunfish, bluegills, and yellow bullheads, significant relationships between prey detection and time were revealed; the probability of prey identification was initially 50% or greater for about 30 min postconsumption, whereas few prey (3%) were identifiable at 60 min postconsumption. For red shiners and fathead minnow (pooled for analysis), no relationship was evident; these two species completely masticated their prey, thus hindering identification. Green sunfish and bluegills swallowed prey whole, and yellow bullheads damaged larvae during consumption. Many larvae were discovered in the foregut, and 25% were regurgitated during predator fixation. Use of gut content analysis as evidence of predation on native fish larvae by small, nonnative fish is problematic and unreliable due to rapid mechanical and chemical digestion of fragile larval tissues.

Acoustic telemetry and scuba revealed immediate and high post-stocking mortality of razorback sucker Xyrauchen texanus in Lake Mohave, U.S.A., a Colorado River impoundment. At the conclusion of this 6 month study, only three of 19 (16%) study fish remained active. Concurrently, 20 X. texanus implanted with acoustic transmitters and held in a hatchery raceway remained healthy throughout the experiment and no transmitters were shed. Poststocking mortality was probably due to predation by striped bass Morone saxatilis, a piscivorous non-native fish.

Attempts to maintain refuge populations of Devils Hole pupfishCyprinodon diabolis in artificial tanks have achieved limited success. Previous studies have documented changes in morphological, behavioral, and genetic characteristics of refuge populations, which suggest that environmental conditions (and thus selective pressures) differ from those found in Devils Hole (DH). Physical, chemical, and biological characteristics were compared among the three Devils Hole pupfish refuges (Hoover Dam, School Springs, and Point of Rocks) and between each refuge and DH. In contrast to the thermally constant environment in DH (∼33 • C), mean refuge temperatures were cooler and fluctuated on a weekly and seasonal basis. On two occasions, extreme temperature fluctuations lasting several weeks (due to water supply malfunctions) were recorded at the Hoover Dam (6 • C decrease) and School Springs (22 • C decrease) refuges. The physical design of the refuges precludes surface runoff from entering them; thick layers of anoxic sediment accumulated, particularly in School Springs (mean depth = 19.1 cm; range = 1-30 cm) and Point of Rocks (mean depth = 8.9 cm; range = 5-39 cm), thus burying the rocky substrate designed to replicate the upper shelf in DH. All three refuges had higher mean oxygen concentrations and lower coefficients of variation (CVs; range = 12.2-16.9%) than the DH upper shelf (CV = 52.5%), which receives limited direct sunlight. Attached algal standing crops differed several-fold among refuges (range = 31.0-231.7 g of ash-free dry mass/m 2 ) but were not significantly different between seasons. In contrast, benthic biomass values reported from DH were smaller and varied seasonally. Aquatic invertebrate taxa that were abundant in DH were rare or absent in the refuges. These results demonstrate how artificial refuge environments deviate from natural conditions in DH and further illustrate the challenges faced when attempting to establish and maintain long-term refuge populations as a conservation strategy to preserve threatened and endangered fishes.

Mastication, digestion, and rapid evacuation rates make visual identification of larval fish remains in the digestive tracts of predatory fishes problematic. Recent advances in molecular technology, however, have increased the likelihood of identifying remnants of partially digested larval prey, thereby enabling assessment of predator impacts on local populations. Conducting controlled laboratory experiments, we evaluated the utility of quantitative polymerase chain reaction (qPCR) for identification of Razorback Sucker Xyrauchen texanus larvae in the digestive tracts of Green Sunfish Lepomis cyanellus and Western Mosquitofish Gambusia affinis. Primers and a probe were developed to amplify a fragment from Razorback Sucker mtDNA. Tests using a suite of potential predators and prey indicated that these primers and probes amplified only mtDNA from Razorback Sucker and Flannelmouth Sucker Catostomus latipinnis, an allopatric and allochronic species in the lower Colorado River. Amplification with primers for Razorback Sucker-specific DNA fragments identified Razorback Sucker DNA in the digestive tracts of 87.5% of Green Sunfish and Western Mosquitofish at 2 h postfeeding. After 12 h, DNA fragments were identified in 75.0% of Green Sunfish but in only 28.6% of Western Mosquitofish. No Razorback Sucker DNA was found in 24-h postfeeding samples, though it was detected in 12.5% of both Western Mosquitofish and Green Sunfish at 48 h postingestion. The sensitivity of qPCR provides a useful tool for extending the time period in which Razorback Sucker larvae can be identified beyond that of traditional visual analyses of stomach contents.

Global wildlife trade exacerbates the spread of nonindigenous species. Pathogens also move with hosts through trade and often are released into naïve populations with unpredictable outcomes. Amphibians are moved commercially for pets, food, bait, and biomedicine, and are an excellent model for studying how wildlife trade relates to pathogen pollution. Ranaviruses are amphibian pathogens associated with annual population die-offs; multiple strains of tiger salamander ranaviruses move through the bait trade in the western United States. Ranaviruses infect amphibians, reptiles, and fish and are of additional concern because they can switch hosts. Tiger salamanders are used as live bait for freshwater fishing and are a potential source for ranaviruses switching hosts from amphibians to fish. We experimentally injected largemouth bass with a bait trade tiger salamander ranavirus. Largemouth bass became infected but exhibited no signs of disease or mortality. Amphibian bait ranaviruses have the potential to switch hosts to infect fish, but fish may act as dead-end hosts or nonsymptomatic carriers, potentially spreading infection as a result of trade.