8147 - Laserfiche WebLink

Home Browse Search

602 SCOPPETTONE sumed to have developed internally. Substrata were sand and gravel in the main-stem upper Muddy River and primarily silt and sandy silt in tributaries. Substratum was a poor predictor of microhabitat use for adult fish, so it was not con- sidered. Habitat use measurements were made in- termittently from 1986 through 1987 and in the summer and fall of 1992. Moapa dace smaller than 15 mm TL were assumed to be larvae, and fish larger than 40 mm TL were considered adults, approximating Snyder's (1981) determination for speckled dace. Moapa White River springfish smaller than 14 mm TL were considered larvae, and those at least 27 mm, the smallest fish on which I observed spawning colors, were consid- ered adults. Western mosquitofish and shortfin mollies were designated juveniles at sizes less than 20 mm TL and adults at sizes of 20 mm and greater. Depth measurements included depth of the fish from the water surface (focal depth) and depth of stre~m at the' position of the fish (total depth). Water column position (relative depth) was de- termined by dividing focal point depth by total depth, giving a range from 0 at the surface to 1 at the bottom. Velocity measurements were taken at or near the site where the fish was observed hold- ing position in the water column (focal point ve- locity) and at 40% of the depth at the site (mean water column velocity). A Marsh-McBirney mod- el 2101 digital flow meter on a calibrated rod was used to measure depth and velocity. Spatial overlap of species was calculated for adults, juveniles, and larvae; the variables used were mean velocity and position in the water col- umn. Also used was whether the fish occurred in total water depths above or below 10 cm; this distinction helped segregate young fish from adults. These categories were selected because they best defined habitat differences between species and life stages. I followed Moyle and Vondracek (1985) and divided mean velocity into increments of 10 cm/s and relative depth into increments of 0.1. I used Schoener's (1970) model to calculate spatial overlap among upper Muddy River fishes: s= 1 - 0.5(~ IPx.i - Py.il); 1 The use of trade names or commercial product names is solely for the purpose of providing specific informa- tion and does not imply recommendation or endorse- ment by the U.S. Fish and Wildlife Service. Px,i and Py,i are the proportional uses of habitat element i by species x and y. This index ranges from 0 to 1. I followed Brown and Moyle (1991) and considered values less than 0.33 as repre- senting low. overlap and values above 0.67 as in- dicating high overlap. I multiplied calculated val- ues of S by 100 to obtain percent overlaps. For each species and life history stage, I esti- mated niche breadths (B) in terms of mean water velocity, focal point velocity, total water column depth, and relative depth using the following for- mula devised by Levins (1968): B = 1/ ~ (ph Pi is the proportion of physical variable used in each category. Categories were increments of 10 cm/s for mean and focal point velocities, 10 cm for total depth, and 0.1 for relative depth. Introduced fishes may cause a niche shift or re- duction of niche breadth (Brown and Moyle 1991; Moyle et al. 1986). I tested for an induced shift in habitat of Moapa dace and Moapa White River springfish caused by nonnative fish by contrasting habitat use by natives isolated on the Moapa Na- tional Wildlife Refuge with habitat use by natives cohabitating with shortfin mollies and western mosquitofish. Sites selected for contrast were those with discharges similar to those along the refuge stream (0.03--0.09 m3/s). This selection included a 300-m reach immediately downstream of the isolated refuge populations (0.09 m3/s) and a reach of Upper Apcar Stream (0.03 m3/s). A one way analysis of variance was used to test differences expressed by adult Moapa dace and Moapa White River springfish with and without nonnatives. Dietary overlap. - Ten adults each of Moapa dace, Moapa White River springfish, and shortfin molly were captured from each of three Upper Muddy River tributary streams representing three habitat types (glide, pool, and chute); western mosquito fish were taken only in the pool habitat. Fish were captured by seining and with unbaited minnow traps fished no longer than 10 min. All fish from a site were taken at the same time and preserved simultaneously in a 10% formalin so- lution. The anterior third of the gut of Moapa dace, Moapa White River springfish, and shortfin mollies and the entire gut of western mosquitofish was examined. Contents were identified with a dissecting microscope. To evaluate the relative importance of various foods, the Hynes (1950) method of numerically quantifying diet was em- ployed and used in the same model used to de- termine spatial overlap. To add further insight as