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I I I I I I I I I I I I I I I I I I I INTRODUCTION Information pertaining to post-stocking dispersal and habitat use of endangered razorback sucker (Xyrauchen texanus) is primarily limited to the adult life stage. Radio or sonic transmitters are normally surgically implanted in the fish's abdominal cavity (McAda and Wydoski 1980, Ulmer 1987, Tyus 1987, Tyus and Karp 1989, Marsh and Minckley 1989, Creef and Clarkson 1993, Ryden and Pfeifer 1995, Burdick and Bonar 1997). Due to transmitter size, this approach is not feasible with smaller life stages. Instead of surgery, we externally attached sonic transmitters on juvenile razorback suckers which reduced surgically related stress and mortality. Wild populations consist of old individuals and young fish are rarely encountered (Minckley 1983, Minckley et al. 1991, Gutermuth et al. 1994, Mueller and Marsh 1995, Modde 1996 and 1997). Although juvenile or larval razorback suckers have been stocked in relatively large numbers (Johnson 1985), efforts to reestablish populations have generally failed (Minckley et al. 1991, Mueller 1997). Research attempts to determine habitat use and movements of juvenile suckers has been plagued by small numbers of wild fish, logistical problems associated with sampling remote riverine habitats, and fish disappearance due to dispersal and unknown survivorship. To our knowledge, reports of juvenile habitat use is limited to sparse information collected on pond-reared fish, stocking dispersal studies, and rare observations of wild fish (Brooks 1985, Marsh and Brooks 1989, Minckley et al. 1991, Mueller and Marsh 1993, Modde 1996, Mueller et al. 1998). Similar information is lacking regarding the behavioral stress associated with repatriating endangered fish into historical or new habitats. Fundamental research has focussed primarily on the physiological response of fishes to physical, chemical and handling induced stress. Studies have examined the effect of stress on the body, with mortality the usual endpoint and not fixed on specific, short-term fish behavior. Factors such as anorexia, tendency toward to downstream movement, or abnormal behavior that may increase susceptibility to predation, are difficult, if not impossible to measure (Wedemeyer et al. 1990). Nevertheless, these factors undoubtedly influence the territorial behavior and repatriation of reintroduced fishes. We intended to test if behavioral acclimation would influence short-term dispersal rates of young razorback suckers. If initial stocking behavior can be influenced, it could improve our ability to maintain fish in specific river reaches and improve survival by reducing predation exposure. 1