Bullfrog tadpoles (Rana catesbeiana) and red swamp crayfish (Procambarus clarkii) are widespread introduced taxa that are problematic throughout the western United States. Their impact on native amphibians and crustaceans is well documented, but less is known regarding their influence on native fishes. Predator-prey tank tests showed both species consumed eggs and larvae of the endangered razorback sucker (Xyrauchen texanus) in a laboratory setting. Tadpoles consumed 2.2 razorback sucker eggs/d and 1.4 razorback sucker larvae/d, while crayfish ate 6.0 eggs/d and 3.5 larvae/d. Relatively high densities of bullfrog tadpoles and crayfish in razorback sucker spawning areas suggest that these nonnative taxa might pose a threat to the recruitment success of this and other imperiled native fish.
Razorback sucker (Xyrauchen texanus) is endemic to the Colorado River. Dramatic declines in their number and range caused it to be federally listed as endangered in 1991 (56 FR 54957). The absence of young in the wild has been attributed to predation by nonnative fish (Minckley et al., 1991; Tyus and Sanders, 2000; Minckley et al., 2003). Recruitment levels necessary to sustain populations have only occurred in recent years in isolated ponds where nonnative fishes are absent (Minckley et al., 1991; Minckley et al., 2003; Marsh and Pacey, 2005).
Cibola High Levee Pond (Cibola HLP) is a 2.3-ha human-made oxbow located on the lower Colorado River along the Arizona–California border and represents one isolated location where sustainable recruitment has occurred; however, survival of young razorback suckers is intermittent. Nonnative fish are rare (Mueller et al., 2005), suggesting other predators or factors might be responsible for the periodic absence of young fish. For instance, Horn et al. (1994) illustrated that razorback sucker larvae are highly susceptible to odonate nymphs and suggested they might also be vulnerable to other nontraditional predators. Our discovery of bullfrog (Rana catesbeiana) tadpoles and red swamp crayfish (Procambarus clarkii) among spawners prompted our curiosity whether these introduced taxa could also threaten early life stages of native fishes.
Laboratory Tests
Large numbers of bullfrog tadpoles and sexually ripe razorback suckers were collected at Cibola HLP during routine sampling in 2003. Several hundred eggs discovered in a tub where razorback sucker were held prior to data processing were used in a preliminary experiment, rather than being discarded. One hundred eggs were put into each of 4, 38-L, aerated aquarium tanks in the laboratory. Subsequently, 25 bullfrog tadpoles were added to each of 3 tanks, leaving one tank holding only fish eggs. The tanks were allowed to sit unattended over the weekend and were examined after 72 h. The fish eggs were completely absent in tanks containing tadpoles and were all present in the control tank.
Based on these preliminary results, we designed a more structured and expanded series of tests to examine whether bullfrog tadpole and crayfish would eat razorback sucker eggs, larvae, and fry. Additional bullfrog tadpoles and crayfish were collected from Cibola HLP, and razorback sucker eggs, larvae, and fry were provided by Willow Beach National Fish Hatchery. Experiments were conducted in 38-L, aerated aquariums that were equipped with separation screens that initially isolated predators from prey. Substrate and cover were not provided to allow accurate counts of the number of eggs and larvae remaining after a 24-h exposure experiment.
Tanks contained either 4 tadpoles, 2 cray-fish, or no predators (control) and 20 razor-back sucker eggs or larvae. Controls were used to measure natural mortality of larvae and visibility or deterioration of eggs, because these factors could influence consumption rates. The number of tests and size of test organisms was dictated by their availability. Shortages of larvae made it necessary to use larger (>14 mm) fry for some crayfish experiments. Predators and prey were measured for total length (fish, tadpoles) and cephalothorax length (crayfish) (Table 1). Separating screens were gently removed to start the experiment, and remaining prey were counted at the end of 24 h. These 24-h experiments indicated that tadpoles consumed an average of 2.2 eggs/d (n = 6 tests) and 1.4 larvae/d (n = 7 tests), while crayfish ate an average of 6.0 eggs/d (n = 8 tests) and 3.5 larvae/d (n = 12 tests) (Table 1). Only 3 of 200 control fish died during the experiments (n = 10 tests).
Field Monitoring
Spawning activities of razorback sucker and bonytail (Gila elegans), another native fish found in the Cibola HLP, were recorded using underwater video equipment. During this monitoring, bullfrog tadpoles and crayfish were commonly observed feeding among spawning fish. These filming sessions were expanded to gain a better understanding of the relative abundance of bullfrog tadpoles and crayfish among spawners.
Two 12-volt (VDC), black-and-white, underwater video cameras were mounted on small submersible tripods and aimed at the bottom. These cameras were linked to surface monitors and VHS recorders. Four areas were filmed: a razorback sucker spawning area, a bonytail spawning area, and 2 areas that were randomly chosen that were not being used by spawners. Recordings were reviewed using a VHS film editor and stopped at precise 5-minute intervals to count tadpole and crayfish observed in that single frame. Density estimates were calculated from the number of organisms observed divided by a size estimate of the viewing area (the viewing area varied due to camera angle). Average densities were multiplied by the total area of the pond to develop a simple estimate of population size.
Twelve video sessions (2 h each) taken during daylight hours from 18 February to 17 April 2003 were analyzed for the presence of bullfrog tadpoles and crayfish. Bullfrog tadpole densities (n = 286 frames) averaged 2.1 tadpoles/m2, with densities increasing (0.9 to 3.7 tadpoles/m2) during the course of the study. Adult crayfish densities averaged <0.1 crayfish/m2 (0.0 to 11.1 crayfish/m2) during the same period. The tadpole and crayfish community was estimated at approximately >48,000 tadpoles and >2,000 crayfish.
Given that bullfrog tadpoles and crayfish consumed razorback sucker eggs and larvae under laboratory conditions, their abundance and presence among spawners at Cibola HLP suggests they might pose a threat to native fish eggs and larvae if their densities are relatively high. The intermittent recruitment of razor-back sucker at Cibola HLP might be attributable to bullfrog tadpole and crayfish predation, because nonnative fish predators were rare (<0.1% of the 3,760 fish sampled) (Mueller et al., 2005).
Introduced bullfrogs and crayfish are widespread and abundant not only in the wild, but also in many culturing facilities (Bills and Marking, 1988; Kane et al., 1992). Bullfrog tadpole predation of eggs and larvae of native anurans and salamanders is well documented (Ehlrich, 1979; Kiesecker and Blaustein, 1997; Murray et al., 2004), but their threat to native fish is less recognized (Kane et al., 1992). Boyd (1975) suspected tadpole predation on fish, but Nguenga et al. (1997, 2000) were the first to document and measure toad (Bufo regularis) tadpole consumption (17 fish/d) of African catfish (Heterobranchus longifilis). They found fish larvae were most vulnerable prior to developing fins (<6 d).
Crayfish predation on eggs of recreational and native fishes is well documented (Horns and Magnuson, 1981; Dorn and Mittelbach, 2004). Evidence of crayfish feeding on live fish larvae is less common. In laboratory settings, crayfish fed on young lake trout (Salveninus namaycush; Savino and Miller, 1991) and juvenile Gila chub (Gila intermedia), suckers (Catostomus), and speckled dace (Rhinichthys osculus) (Carpenter, 2000). Gut content analyses provided evidence of P. clarkii consuming Gambusia in a freshwater marsh (Gutiérrez-Yurrita et al., 1998). Introduced crayfish negatively impacted several benthic fish communities in British rivers via competition and predation (Guan and Wiles, 1997).
Predator removal programs aimed at restoring razorback sucker recruitment within the Colorado River basin have typically focused on the removal of large, nonnative fish predators (Mueller, 2005). When large predators that depress nontraditional predators (i.e., predaceous insects, crustaceans, and amphibians) are removed, the latter typically increase in abundance (Horn et al., 1994; Mueller and Burke, 2005). These cause-and-effect reactions deserve closer scrutiny in predator control programs, because of the potential negative effects the nontraditional predators might pose to the early life stages of fish.
Literature Cited
Table 1
Size of prey and predators (diameter of fish egg, total length of fish larva and tadpole, and cephalothorax length of crayfish) and predation rates (number of individuals consumed/day) of bullfrog (Rana catesbeiana) tadpoles and red swamp crayfish (Procambarus clarkii) preying on razorback sucker (Xyrauchen texanus) eggs and larvae in laboratory tank experiments.
