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r
<br />MODDE AND IRVING
<br />rtvers, movement is triggered by temperature, and
<br />in others, flow initiated migration (Jonsson 1991).
<br />Undoubtedly, there is interaction between these
<br />stimuli, and fish probably respond to discharge
<br />within some range of temperatures.
<br />Telemetry data indicated that movement of adult
<br />male razorback suckers to spawning sites is influ-
<br />encedmore by chronology of the hydrograph (flow
<br />and temperature) than by uniform stimuli such as
<br />photoperiod. Cuing to natural hydrographic and
<br />thermal regimes in riverine environments may ef-
<br />fectively congregate fish at spawning sites, thus
<br />increasing the efficiency of successful spawning
<br />within small populations. Razorback suckers in
<br />Lake Mohave, Arizona, in the absence of riverine
<br />cues, spawned between November and May
<br />(Minckley 1983). Although there are differences
<br />in the timing of razorback sucker spawning be-
<br />tween the upper and lower Colorado River basins,
<br />the reported water temperatures (10° and 15°C) at
<br />peak spawning in the lower basin (Bozek et al.
<br />1990) are similar to those of the upper basin (Tyus
<br />1987; Tyus and Karp 1990). Thus, discharge cues
<br />aze not needed to successfully reproduce, but in-
<br />creased rates of discharge can initiate movement
<br />and aggregate reproducing adults. The latter would
<br />be important in maximizing reproductive success
<br />in small, genetically isolated populations. The tim-
<br />ing of reproduction is also important if floodplain
<br />inundation optimizes survival of early life stages
<br />of razorback suckers (Tyus and Karp 1990; Modde
<br />1996; Modde et al. 1996).
<br />Tyus and Karp (1990) identified the Yampa Riv-
<br />er and Escalante (Jensen, Utah) spawning areas,
<br />and Tyus (1987) suggested that spawning may also
<br />occur near the mouth of the Duchesne River and
<br />in Island Park. Telemetry from this study sup-
<br />ported the concept that multiple spawning sites
<br />exist, including the Yampa River near its conflu-
<br />ence with the Green Rivet, the Escalante spawning
<br />area, and possibly the lower portion of Island Park.
<br />Although transmitter-implanted male razorback
<br />suckers moved to the vicinity of the Duchesne Riv-
<br />er, movement seemed to occur either before or
<br />following spawning. However, capture data indi-
<br />cated that some razorback suckers were collected
<br />at the mouth of the Duchesne River during the
<br />spawning period. Recapture data during the spring
<br />suggested strong fidelity to the Escalante spawning
<br />area. However, because of the small number of fish
<br />at the Yampa River spawning site, we could not
<br />determine fidelity to this azea by recapture data
<br />alone. Telemetry data indicated that half of the six
<br />males monitored through three spawning years
<br />325
<br />were found in multiple spawning areas. One in-
<br />dividual was initially caught (expressing milt) at
<br />the Yampa River site yet, during the next two
<br />spawning years, was located only at the Escalante
<br />area. In addition to individuals using different
<br />spawning sites among years, fish 181765 and
<br />224B6B visited two sites during the same spawn-
<br />ing period. Presence of adult male fish at multiple
<br />spawning areas suggested a single spawning pop-
<br />ulation in the middle Green River, with most in-
<br />dividuals spawning within a single area and ad-
<br />ditional spawning occurring at other locations.
<br />Acknowledgments
<br />Assistance in field data collection was provided
<br />by T. Hatch, C. Flan, J. Rosendale, R. Small, S.
<br />Persselin, and D. Beers. This study was supported
<br />by the U.S. Fish and Wildlife Service, the National
<br />Park Service, and the Recovery Program for the
<br />Endangered Fishes in the Upper Colorado River
<br />Basin. S. Petersburg, G. Mueller, A. Scholz, and
<br />R. Muth provided helpful comments on earlier
<br />drafts of this manuscript.
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