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336 , COPEIA, 1993, NO.2 LITTLE COLORADO RIVER AND CONFLUENCE AREA % I>' t o 2 '--'---' Kilometers Fig. 2. Map depicting the LillIe Colorado River (Navajo Indian Reservation, Coconino County, Ari- zona), at its confluence with the Colorado River (Grand Canyon National Park. Coconino County. Arizona). relates to the unusual anterodorsal (nuchal) hump (Fig. I B); and (2) to perform this evalu- ation using image-analysis technology, which permits rare and/or endangered fish to be cap- tured, filmed, and released unharmed. The hy- pothesis under test in this report is that sexual dimorphism is morphologically pervasive in G. typha, with the distinct nuchal hump of this spe- cies its primary focus. METHODS AND MATERIALS Collection of video i1llages.-Gila typha were col- lected with hoop and trammel nets by Arizona Game and Fish Department (AZGF) during two time periods: in May 1988 at the confluence of the Colorado and Little Colorado rivers (GCNP), and in May ] 989 at the confluence and in the Little Colorado River 11 km upstream from the confluence (Navajo Indian Reservation: see Fig. 2). Adult humpback chub (i.e., those greater than 230 mm TL) were immersed in a 19-1iter bucket of river water treated with MS-222 (San- doz Laboratories). Comatose fish were either Carlin or PIT tagged and sexed using external Fig. 3. Use of camcorder to film Gila cypha along shore of the Little Colorado River (Navajo Indian Reservation, Coconino County, Arizona. morphology of the urogenital papillus (Suttkus and Clemmer, ] 977:4; B. L. Jensen, USFWS, pers. comm.). Chubs were then rewetted and placed onto a styrofoam board (50 x 30 cm; Fig. 3) that had been previously spray painted a neutral color and indented with a shallow de- pression for the body of the fish. A 10-cm rule was placed beside the fish to provide scale. Dor- sal and anal fins were quickly spread and pinned using insect pins, and the chub was filmed for approximately 10 sec (Fig. 3), using a 16mm camcorder (General Electric model 9-9808 SE with a 7-lux low light capability and an auto- matic focus 6: 1 zoom lens). Fish were then re- turned to a ] 9-liter bucket of fresh river water, maintained there until they revived (i.e., were able to right themselves and respond to stimuli), and were then released back into the river. Capture of video images.-Video images were copied from tape and analyzed at Arizona State University (ASU), Tempe. The image analysis system consisted of a Zenith 12 mhz 80286 mi- c'roprocessor with Intel 80287 math coproces- sor, SO-megabyte hard drive, and Mountain Filesafe TD-4000 (40 megabyte) tape backup. A NEC Multi-Sync monitor was used for image