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3.0 Scientific Basis And Underlying Principles 3-14 April 2004 3.7.2 Larval Entrainment The proportion of drifting razorback sucker larvae entrained at floodplain sites downstream of a spawning bar has not been determined. Given that the razorback sucker is a highly fecund fish species, with average production of about 188,600 eggs per female at 550 mm TL and 1,757 g body weight, the number oflarvae produced by a population of 5,800 adults with a 3:1 male to female effective sex ratio (i.e., 1,740 females) is expected to be about 5.5 million (Floodplain Model, Valdez 2004). Drifting larvae follow a pattern of downstream reduction in numbers of drifting particles described as a negative exponential decay function (Figure 3-7), which assumes ongoing mainstem mortality and periodic entrainment at floodplain sites. Eventually, numbers of drifting larvae become extinguished with distance downstream from a spawning bar. The Floodplain Model predicts that only about 1 % of drifting larvae remain in the main channel 36 miles downstream of a spawning bar at a 90% mile-to-mile survival rate and 10% entrainment at five sites. Hence, downstream floodplain sites closest to a spawning bar are likely to entrain the greatest numbers of drifting larvae and provide earliest refuge for maximum growth and survival of young fish. 6. (l)), coo 5, (l)), coo II If .. 4, (l)), coo .. If ..I "" C 3. (l)), coo .. II .Cl I 2, (l)), coo . I 1 , (l)), coo 0 0 10 15 20 25 3) 35 40 4S 50 55 60 65 10 15 so ~ ~ as 100 Miles Do.. nstream From Sp a.. ning SHe Figure 3-7. Number of larvae surviving to the next river mile as a negative exponential decay function, which assumes ongoing mainstem mortality and periodic entrainment at floodplain sites. Function generated by Floodplain Model (Valdez 2004).