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2.2.2 Life history <br /> Except during periods before and after spawning, adult razorback sucker are thought to be <br /> relatively sedentary and have high fidelity to overwintering sites (Service 2002b). Adults <br /> become sexually mature at approximately 4 years and lengths of 400 mm (16 in.) (Zelasko et al. <br /> 2009), at which time they travel long distances to reach spawning sites (Service 2002b). Mature <br /> adults breed in spring (mostly April—June) on the ascending limb of the hydrograph, <br /> congregating over cobble/gravel bars,backwaters, and impounded tributary mouths near <br /> spawning sites (Service 2002b; Snyder and Muth 2004; Zelasko et al. 2009). Flow and water <br /> temperature cues may play an important role prompting razorback adults to aggregate prior to <br /> spawning (Muth et al. 2000). Tyus and Karp (1990) and Osmundson and Kaeding (1991) <br /> reported off-channel habitats to be much warmer than the mainstem river and that razorback <br /> suckers presumably moved to these areas for feeding, resting, sexual maturation, spawning, and <br /> other activities associated with their reproductive cycle. <br /> Razorback sucker have high reproductive potential,with reported average female fecundity of <br /> approximately 50,000 to 100,000 eggs per fish (Service 2002b). They are broadcast spawners <br /> that scatter adhesive eggs over gravel-cobble substrate (Snyder and Muth 2004). High springs <br /> flows are important to egg survival because they remove fine sediment that can otherwise <br /> suffocate eggs. Hatching is limited at temperatures less than 10°C (50' F) and best around 20°C <br /> (68' F) (Snyder and Muth 2004). Eggs hatch 6 to I I days after being deposited and larval fish <br /> occupy the sediment for another 4 to 10 days before emerging into the water column. Larval fish <br /> occupy shallow, warm, low-velocity habitats in littoral zones,backwaters, and inundated <br /> floodplains and tributary mouths downstream of spawning bars for several weeks before <br /> dispersing to deeper water(Service 2002b; Snyder and Muth 2004). It is believed that low <br /> survival in early life stages, attributed to loss of nursery habitat and predation by non-native <br /> fishes, causes extremely low recruitment in wild populations (Muth et al. 2000). Wydoski and <br /> Wick(1998) identified starvation of larval razorback suckers due to low zooplankton densities in <br /> the main channel and loss of floodplain habitats which provide adequate zooplankton densities <br /> for larval food as one of the most important factors limiting recruitment. <br /> Razorback sucker in the Upper Basin tend to be smaller and grow slower than those in the Lower <br /> Basin, reaching 100 millimeters (4 in.) on average in the first year(Service 2002b). Based on <br /> collections in the middle Green River,typical adult size centers around 510 mm (20 in.) (Modde <br /> et al. 1996). Razorback suckers are long-lived fishes,reaching 40+years via high annual <br /> survival (Service 2002b). Adult survivorship was estimated to be 71 to 73 percent in the Middle <br /> Green River from 1980-1992 (Modde et al. 1996; Bestgen et al. 2002) and 76 percent from 1990 <br /> to 1999 (Bestgen et al. 2002). <br /> Outside of the spawning season, adult razorback suckers occupy a variety of shoreline and main <br /> channel habitats including slow runs, shallow to deep pools,backwaters, eddies, and other <br /> relatively slow velocity areas associated with sand substrates (Tyus and Karp 1989, Osmundson <br /> and Kaeding 1989, Osmundson and Kaeding 1991, Tyus and Karp 1990). Their diet consists <br /> primarily of algae,plant debris, and aquatic insect larvae (Sublette et al. 1990). <br /> 17 <br />