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238 We measured mouth gapes of P. lucius at the Dexter National Fish Hatchery, Dexter, New Mexico. We took measurements from 103 live specimens. We took total and standard lengths using a fish board. We determined weights with a spring scale. We measured mandibular arch width and maximum mouth gape (both laterally and dorsoventrally) using digital calipers. In addition, we took a measurement of the maximum pharyn- geal opening with tong spreaders placed inside the pharynx. We used an inside dimension spring caliper to measure the pharyngeal opening on fish too small for the spreaders. Measurements of the mouth and pharyngeal cavity revealed that the dentary and maxillary bones of the P. lucius lim- ited maximum sizes of prey that could be swal- lowed. Therefore, we calculated gape as the maximum distance between the maxillary and the dentary bones dorsoventrally in the mouth. We evaluated the relationship between mouth size (gape) of the predator and maximum dorsoventral body depth of the hump of the prey with regres- sion analysis. Results Drag assessment Drag coefficients using our apparatus and tech- niques were similar to that reported for a sphere (CD = 0.45 vs. 0.47 reported by Vogel 1994) and 0. mykiss (CD = 0.025 vs. 0.022 reported by Webb 1975, Table 1). Also, drag coefficients demon- strated the importance of streamlining. The sphere (no streamlining) had the highest drag coefficient, and among the four Colorado River fishes that we tested, the catostomid and cyprinid without humps (i.e., C. latipinnis and G. robusta) had the lowest drag coefficient (Table 1). A comparison of CD of the two humped forms from which humps were removed and sympatric forms were identical for Gila and almost identical for the catostimids (X. texanus = 0.24 and C. latipinnis = 0.25). When looking at the fish casts with humps vs. humps removed, there was a reduction in the drag force and therefore a reduction in the drag coefficient after the removal of the nuchal humps (Table 1; Figure 3; p < 0.05). Thus, the extra mass of the nuchal hump resulted in appreciable drag. An increase in area or wetted surface of the fish casts always resulted in increased drag, and drag force was greatest on fish with the largest frontal areas proportional to their length (Table 1). Drag coefficients obtained were consistent with pub- lished values (Table 1; Webb 1975, Vogel 1994), and diminished as fish became more streamlined (Table 1). There was a significant reduction (p < 0.05) in the drag force upon removal of the nuchal hump from the X. texanus and G. cypha casts (Figure 3, Table 1), as reflected by a 27% reduction in CD for X. texanus and a 14% reduc- tion for G. cypha (Table 1). However, drag forces seemed to be Reynolds number dependant because of the characteristic length, which takes into ac- count the different shapes of the fish and were not Table 1. Drag coefficients obtained from published sources and experimental results at water velocities of 0.3 m s-1 and their respective type of reference area. Objects Drag coefficients Reference area Theoretical sphere 0.47' Frontal area Experimental sphere 0.45 Frontal area 0. mykiss 0.0226 Wetted area 0. mykiss 0.025 Wetted area X. texanus 0.033 Wetted area X. texamm (hump removed) 0.024 Wetted area C. latipimeis 0.025 Wetted area G. cypha 0.029 Wetted area G. cypha (hump removed) 0.025 Wetted area G. robusta 0.025 Wetted area 'Vogel (1994). bWebb (1975).