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Predator/Prey Trials Razorback Suckers Versus Flathead Catfish The predator/prey trials were designed to test whether exercise and predator exposure reduced short-term predation of razorback suckers compared to control fish. The predator/prey trials for razorback sucker and flathead catfish were conducted in a large 7-m-diameter fiberglass tank to allow fish adequate room to maneuver (fig. 4). The tank was partially buried into the ground and was shaded by a sun screen. Catfish shelters provided the predators a dark cavity in which to hide. Six shelters were fabricated by cutting plastic garbage cans (120 L) in half and attaching steel rebar along the cut edges to weigh the shelters down. Shelters were always used by the flathead catfish and were easily removed to capture the surviving prey. Water depth was maintained at 1 m, and water temperatures were recorded. Figure 4. Photograph showing the 7-m-diameter tank used to conduct predator/prey experiments. Flathead catfish were captured from the lower Colorado River. Five were used in the 2006 trials and six to seven were used in the 2007 experiments. Their length averaged 680-mm TL (range: 572- to 838-mm TL) and their weight ranged between 2.2 to 14.8 kg (Appendix A). Catfish were nocturnal, hiding in the shelters during daylight hours and roaming freely at night. 2006 Trials Failure to precondition razorback suckers to predation left two test groups in 2006: razorback suckers that had been exercised and those that had not. Trials began with the selection of 10 fish from either group; they were measured and marked with a small pectoral-fin clip. Then ten similarly sized (:t5-mm TL) fish were selected from the other test group and marked on the opposite pectoral fin (Appendix B). All 20 fish were introduced into the predation tank at the same time. 2007 Trials These experiments were repeated in 2007; however, the comparison involved two treatment groups and a control; one new group had been exposed to predation. These trials started with 12 razorback suckers being selected from the exercise group; they were measured, marked. and placed in (\ tank with a flathead catfish to experience their first predation event. Then 12 similarly sized (IS-111m TL) fish were selected from the exercise and control groups. They were also marked and placed in a holding tank (Appendix C), so that all three groups experienced similar handling stress. The following 6