Selection of Measures of Substrate Composition to Estimate Survival to Emergence of Salmonids and to Detect Changes in Stream Substrates

-Biologists have anempted to link intragravel survival of juvenile saimonids to changes in stream substrate quality caused by land management, but the failure to standardize measures of substrate composition has hindered this effort. We compared 15 such measures in iaboratory tests that evaluated survival to emergence of Colorado River cutthroat trout Oncorhvnchrcs cfarki plmriticus in substrates of different composition. We also evaluated the sensitivity of three measures of subsmite composition to the modificauon of stream substrates by spawning brook trout Salvelinus fontinafis and to the deposition of sediment in former redds of Colorado River cutthroat trout. Different estimates of the geometric mean panicle size accounted for the greatest proportion of the variation in survival to emergence in laboratory tests. but the percentage of substrate less than 0.85 mm in diameter was the most sensitive measure of known changes in s u b u composition in the held. We concluded that a single rneasurc of substrate composition may be inadequate to both asscs~ the potenuai survival to emergence in a substrate and detect changes in substrate! composition c a d by land use. It has been demonstrated that fine sediment can reduce survival to emergence (STE) of juvenile salmonids (Tappel and Bjornn 1983) and that certain land management practices can increase the proportion of fine sediment in spawning gravels in streams (Plats et al. 1989). Managers have attempted to link the effects of land management on STE by assessing changes in substrate composition (Stoweil et al. 1983); however. the inconsistent definition of substrate composition. in addition to other problems (Chapman 1988: Young et al. I990), has obscured this linkage. Two approaches have been widely used to describe substrate composition. In the b t , the proportion of substrate particles less than a given size is quantified by weight or volume. Reference particle diameters have included 6.4 mm (Stowell et al. 1983), 4.0 mm (MacCrimmon and Gots 1986), 3.33 mm (Koski 1975; Ringler and Hall 1988), 3.0rnm(HallandLantz 1969;Phillipseta.l. 1975), 2.0 mm (Hausle and Coble 1976; Witzel and * Present addrtss: U.S. Forest Service. Rocky Mountain Forest and Range Experiment Station, 222 South 22nd. Laramie. Wyoming 82070, USA. * The Unit is jointly supported by the University of Wyoming, the Wyoming Game and Fish Department, and the U.S. Fish and Wildlife SeMct. MacCrimmon 1983a), 1 .O mm (Crisp and Carling 1989), 0.84 mm (Reiser and White 1988), 0.83 mm (McNeil andAhneU 1964), and0.75 mm (01sson and Persson 1988). In addition, Tappel and Bjornn (1983) used two sizes of sediment (9.5 mm and 0.85 mm) to describe substrate composition (also see Reiser and White 1988). In the second approach, aspects of the central tendency of the entire particle distribution are described. Such measures inciude the geometric mean particle size (Platts et ai. 1979), W e index (Lotspeich and Everest 1981), modified fredie index (Beschta 1982), arithmetic mean particle size (Crisp and Cariing 1989), median particle size (Witzel and MacCrimmon 1983b), sorting coefficient (Sowden 1983), and skewness (Crisp and Carling 1989). Graphical and mathematical techniques have been used to calculate most of these measures (Shirap and Seim 1979); however, these techniques produce Merent estimates for a particular substrate, especially if the distribution of particle sizes in a sample is not lognormal (see Folk and Ward 1957). I W e know of no studies of the relation between the several measures of substrate composition and STE. Often, a single measure of substrate composition is arbitrarily selected and rekited to STE (usuaily as the percentage of fines less than a given