8022 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I .1 I I I I I Marsh and Langhorst 1988; Papoulias and Minckley 1990, 1992; Modde 1997), Papoulias and Minckley (1990) suggested that many wild-caught razorback sucker larvae from Lake Mohave are in the "critical period" (sensu Hjort 1914) of transition from dependence on endogenous to exogenous foods (within about the first 3 weeks after hatching), They concluded that mortality related to low abundance of appropriate foods may contribute to year-class failure of razorback suckers in Lake Mohave, Papoulias and Minckley (1992) found that survival of larval razorback suckers reared at 12-170C in earthen ponds fertilized at three levels (low, medium, and high) was independent of invertebrate densities (survival ranged from 67.4% in the low-fertilization treatment to 89.8% in the medium-fertilization treatment), but total larval growth was significantly greater at the two higher invertebrate densities. Based on their observations of mean TL at each fertilization level by 7-d intervals, growth rates oflarvae during the first 56 dafter swimup averaged about 0.17, 0.16, and 0.13 mm TL/d for the high, medium, and low treatments, respectively. By comparison, our estimated mean growth for otolith-aged larval razorback suckers less than 35 d old (posthatching) ranged from 0.27 mm TL/d (1994, lower Green River) to 0.35 mm TL/d (1996, middle Green River). Our estimates of growth on wild-caught fish are likely biased high because many slow-growing individuals had probably already been "removed" from the populations by natural selection (e.g., Miller et al. 1988; Rice et al. 1993; Bestgen et al. 1997). Nonetheless, based on observations of larval diet, composition and abundance of invertebrates in quiet-water soft-sediment riverine habitats, and growth and survival of captive larvae, it appears that food abundance in existing razorback sucker nursery habitats on the Green River is adequate to meet the minimum nutritional requirements for larval survival. However, the growth potential of razorback sucker larvae is greater than we observed. Mean growth rates of larval razorback suckers reared in the laboratory for 28 d after the start of exogenous feeding and fed nauplii of Artemia sp. ad libitum twice daily were 0.39, 0.58, 0.65, or 0.72 mm TL/d at constant water temperatures of 16.5, 19.5,22.5, or 25.50C, respectively (K. R. Bestgen, Larval Fish Laboratory, personal communication). Relatively minor differences in growth rates can be biologically significant if size-dependent processes, such as predation by small, gape-limited predators, ;ll"e important regulators oflarval survival. For example, Bestgen et al. (1997) demonstrated through experiments and recruitment-model simulations that the predatory effects 21