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L Z ? j J GV ?1 Y?1/n /p?1 [I?fi " $J / ''+v/J • ?? C c% t_ °`Y't . _f??? _ . ?y? 1 61- Ors _0 WE Aspects of Razorback Sucker (Xyrauchen texanus, Abbott) 67 Life History Which Help Fxnlnin Their Decil- William E. Loudermilk L D to e?v?t r ? California Department of Fish and Game 407 W. Line Street Bishop, California 93514 ABSTRACT Efforts to federally list and develop a recovery plan for razorback sucker (Xyrauehen texanus) in 1978-1980 failed despite their critical status. Now, their abundance is further reduced, threats have increased, and they are unsuccessfully attempting reproduction only in the Yampa River (northwest Colorado), the Colorado River (near Clifton and Grand Junction, Colorado), in Lake Mohave (Arizona/Nevada) and in Senator Wash Reservoir (California). Federal/State agreements have been en- acted to begin reintroduction efforts as a method of recovery in some areas although a substantial portion of their life history is unknown. A review of historic river conditions, and the relationship to the currently known life history of razorback sucker is summarizer) to indicate possible t, r_ ..,_e popui aa. , _ L mCcsia.il.??. [^.?. l::l X11 t. a.ii_n uel.l llle• Incorporation of critical early life history requisites into a recovery program is the most likely way to reestablish natural recruitment and recover the species. INTRODUCTION Razorback sucker (X. texanus) were proposed for a federal "threatened" status in 1978, but listing efforts were dropped in 1980. They are now in serious trouble both in the upper and lower Colorado River Basin. They have been listed as endangered in California since 1974. The only remaining groups known to be attempting natural reproduction are in the Yampa River (northwest Colorado), the upper Colorado River (near Clifton and Grand Junction, Colorado), Lake Mohave (Arizona/Nevada), and Sena- tor Wash Reservoir (California). Two dams proposed on the Yampa River above spawn- ing areas will likely modify the physical habitat conditions along that drainage and both physical and biological changes occurring or planned at Lake Mohave will further jeopardize the existence of the species. All other areas of their historic range contain only remnant individuals or groups making the remaining areas where natural reproduction is now attempted even more valuable to preserve. I'll briefly describe the river conditions that historic populations were exposed to, summarize known and some suspected life history aspects, and present some possible reasons for their de- cline. Defining the critical reason(s) is the first step to recovery. I believe predation is an important factor now that broodstock abundance is low and spawning is restricted to isolated locations, but there are suggestions that other mortality, associated with egg and larval development, is of equal or greater importance in razorback sucker (X. texanus) population declines. PHYSICAL CONDITIONS Near the end of the Pliocene or early Pleistocene epoch the area from Parker, Arizona south (River Mile 176) was a brackish arm of the Gulf of California. The desert mountain ranges appeared only as islands. Fossils in strata from this epoch indicate both freshwater and marine species were present. The water receded 1-3 million years ago, the Colorado River entered the area and much of the original substrate in the first channel was washed out. After that, the mountains rose :5 '?L ,c. 081