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2 EcologV of Bonytail and Razorback Sucker Chapter 1. The Fish Overview Bony tail (Gila elegans) and razorback sucker (Xyrauchen texanus) are part of the Big River Fish Community described by Minckley (1973) and are endemic to the Colorado River in the southwestern United States of America (fig. 1). They evolved in one of the most physically diverse and environmen- tally harsh river basins in the world. Catastrophic events such as floods and droughts periodically devastated this community (Stanford and Ward, 1986; Douglas and others, 2003). For example, historic flows at Yuma, Arizona, dwindled to as little as 1.4 m3fs during droughts and swelled as much as 11,200 m3fs during floods (Stanford and Ward, 1986). During peak flooding, the river could be represented as a wall of water 10 m deep and 1 km wide. Water temperatures were also extreme, ranging from OQC to >35QC (Mueller and Marsh, 2002). Both species were historically widespread and abundant in main- stem habitats throughout the Colorado River system (Jordan and Evermann, 1896). Their survival depended upon their abil- ity to live and reproduce in an unpredictably harsh environ- ment. Harsh climatic and riverine conditions are reflected in the reproductive strategies for long-lived species. For example, razorback sucker females have the ability to produce several million eggs during their 50-year lifespan. During adverse conditions such as droughts, recruitment was low, possibly non-existent. Recruitment was not necessarily a yearly occurrence. Their fitness (reproductive success) depended on survival and reproduction of just two young. Needless to say, survival of each young presumably was one chance in several million! The ability of both species to adapt to the fluctuating conditions is recognized collectively in the scientific literature but is seldom acknowledged in scientific circles. This is due in part to a lack of historical data and the fragmented nature of to day's populations caused by the physical and biological alterations to the basin during the past century. Floods and droughts are now regulated through large upstream reservoirs, and floodplain and oxbow habitats, which were once abun- dant, have been drained and buried under desert farmland and altered by urban development. Recovery of these fish has been further complicated by conflicting uses of the resource, includ- ing political agendas, institutional boundaries, and different philosophical approaches (Marsh, 2004a; Clarkson and others, 2005). These fish were highly adaptive to changing environmen- tal conditions, but their young were extremely vulnerable to introduced predators. Historically, the Colorado pikeminnow (Ptychocheilus lucius) and large Gila spp. were the major piscivores in the Colorado River system. Unfortunately, they have been replaced by several dozen nonnative predators that are aggressive and more numerous than their native predeces- sors. These fishes benefited from the creation of large storage reservoirs and regulation of stream flows that moderated historic drought and flood conditions in the basin (Mueller and Marsh, 2002). The vast majority of young bony tail and razorback suckers disappeared nearly half a century ago and with them the ability of the Colorado River system to sustain native populations (USFWS, 1984; USFWS, 1998a). Popula- tions declined as surviving fish died of old age and were not replaced by young adults (Minckley and others, 1991). As a result, both species are now listed as endangered under provi- sions of the Endangered Species Act of 1973. Human intervention has done little to reverse the loss of native communities. Forty years of research, with nearly 20 years in recovery efforts and 10 years of active nonnative pred- ator control programs, have failed to recover, or even slow, the decline of these species (Minckley and Deacon, 1991; Mueller and Marsh, 2002). Predator removal programs have been inef- fective and costly, with targeted species rapidly recolonizing treatment areas (Mueller, 2005). In many areas, the spread of northern pike (Esox lucius), smallmouth bass (Micropterus dolomieui), and other introduced species has only increased (Tyus and Saunders III, 2000; Mueller and Brooks, 2004; USFWS, 2004). Effective control of undesirable nonnative fishes in a large riverine environment is highly unlikely since it requires substantial manpower on a continuous basis and results in only partial removal of such fishes. Fish that remain after control efforts exhibit faster growth and higher fecundity that allows undesirable fishes to be highly resilient to control efforts (Wydoski and Wiley, 1999). Effectiveness of nonnative fish control is compromised by conflicting issues involving recreational fisheries, salvage requirements, trespass issues, and angler sentiments (Marsh, 2004a; Clarkson and others, 2005). The two most important factors in the extinction of 40 native North American fishes (27 species and 13 subspecies) during the past century were habitat alteration and nonnative fish introductions (Miller and others, 1989). These are the same two factors impacting the bony tail and razorback sucker. Today, wild bony tail are most likely extirpated from the Colorado River mainstem and only a few hundred wild razor- back suckers remain. Some of these long-lived fish survive and spawn but natural recruitment does not occur because of predation by nonnative fishes. The presence of these two endangered species is being maintained through propagation and stocking of large fish. Current Status of Bonytail and Razorback Sucker Status of the Bonytail Bony tail were federally listed as endangered under the Endangered Species Act of 1973 on 23 April 1980. Wild fish are believed extirpated from the Upper and possibly the entire Colorado River Basin (45 FR 27710). Critical habitat was designated on 4 September 1994 (59 FR 13374). The last