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243 X. texanus (stocked fish; Minckley et al. 1991). Hump enlargement is already in progress in both species at these sizes, and G. cypha of 300 mm and X. texanus of greater than 200 mm would be approaching immunity from even the largest P. lucius. Populations of G. cypha attain sizes of 480 mm in nature and an average adult size of approximately 300 mm TL (Kaeding & Zimmer- man 1983, Karp & Tyus 1990, R. Valdez, unpub- lished). Populations of X. texanus attain an average adult size of about 500 mm TL consisting of adults that range in size from 370 to 740 mm (Minckley 1983, Marsh & Minckley 1989), all of which would be immune from the largest P. Lucius (e.g., 805 mm). Further support for predator-mediated mor- phological change in both of these species is pro- vided by other morphological adaptations linked with predator defense. The lack of, and reduction in size of scales, and the increased relative size of paired fins also are implicated. In G. cypha, scales are absent or greatly reduced in size and number on the dorsal and ventral surfaces. An almost scaleless, and thus, slippery fish can be very diffi- cult for a predator with no jaw teeth to capture and manipulate for effective feeding. Increased maneuverability in more lacustrine-like habitats also would result from laterally flattening bodies, which facilitate complex evasive movements. While these adaptations to predation are consti- tutive in adult G. cypha and X. texanus (i.e., always present), it is possible that these morphologi- cal defense strategies may have been originally predator-induced during pre-adult habitat tran- sitions where they become sympatric with P. Lu- cius. Native Colorado River fish populations have been extirpated from much of their historic habi- tats by reservoir inundation and altered flow re- gimes. Therefore, recovery efforts have been focused on physical habitat restoration. However, non-native predators such as northern pike, Esox lucius, largemouth bass, Micropterus salmoides, smallmouth bass, Micropterus dolomieu, and walleye, Stizostedion vitreum, have been intro- duced with devastating effects (e.g., see Minckley et al. 1991, Tyus 1998, Meretsky et al. 2000). If predation pressure has been the major selective force behind the evolution of enlarged nuchal humps instead of physical habitat constraints as previously reported, this suggests that these species may be especially sensitive to predation by non- native fishes. Hump formation, an interesting adaptation, which presumably resulted in long-life for breeding adults, may no longer provide an effective life strategy. As serious as physical habi- tat loss has been, biological, rather than physical changes may now play the major role in endan- germent of these fishes. In this case, efforts to re- cover these fish will require increased emphasis on the biological attributes of habitat. Acknowledgements This research was conducted as part of the requirements for a Master's thesis in the Depart- ment of Environmental, Population, and Organ- ismic Biology, University of Colorado at Boulder. C. Jordan provided extensive technical support and aided planning during the hydrodynamic portion of this study. We are most grateful for financial contributions, services, and facilities provided by the U.S. Bureau of Reclamation Hydraulics Laboratory, Denver, CO. The U.S.G.S. Biological Resources Division at Fort Collins, Colorado and Dexter National Fish Hatchery at Dexter, New Mexico provided access to fish specimens. We also acknowledge assistance by B. Mefford, T. Rozales, R. Hamman, B. Schmidt, L. Sanabria, M. Rose, and D. Center. This study was conducted under provisions of a Federal Endangered Species permit granted by the U.S. Fish and Wildlife Service (U.S. Federal Fish & Wildlife Permit PRT-704930). References Adler, F.R. & C.D. Harvell. 1990. Inducible defenses, pheno- typic variability and biotic environments. Trends Ecol. Evol. 5: 407-410. Alexander, D.E. 1990. Drag coefficients of swimming animals: effects of using different reference areas. Biol. Bull. 179: 186- 190. Br6nmark, C. & J.G. Miner. 1992. Predator-induced pheno- typical change in body morphology in crucian carp. Science 258:1348-1350. Br6nmark, C. & L.B. Pettersson. 1994. Chemical cues from piscivores induce a change in morphology in crucian carp. OIKOS 70: 396-402.