Ecology of Young Stream Resident Warner Sucker (Catostomus Warnerensis) in Warner Basin, Oregon

Article excerpt

ABSTRACT.-

We visually censused Warner sucker cohorts in a low gradient stream reach and determined microhabitat use from random availability data. For above-water visual censuses (AVC), we counted aggregations while slowly wading through Honey Creek three times both years. Underwater visual censuses (UVC) were done in 1993 by snorkeling. Mean focal point velocity (FPV) during AVC significantly differed by year but not by census date, and mean depth did not differ for either. In 1993 we found suckers used microhabitats with FPV between >3 to 6 cm/s (χ^sub 1^^sup 2^ = 3.93, P < 0.05) and depth between >20 to 40 cm (χ^sub 1^^sup 2^ = 4.5, P < 0.05). Suckers avoided areas where FPV exceeded 15 cm/s (χ^sub 1^^sup 2^ = 18.7, P < 0.001) and depth between >60 to 80 cm (χ^sub 1^^sup 2^ = 8.1, P < 0.005). Aggregate abundance was significantly related to both distance to (r^sub 64^ = -0.45, P = 0.0002) and percentage (r^sub 64^ = 0.44, P = 0.0003) submerged vegetation. We found 97% of suckers (N/m^sup 3^) in riffle/run habitat during UVC. When mean flow was used as a continuous variable, we found suckers occupied habitats >3 cm^sup 3^/s and avoided habitats <2 cm^sup 3^/s in a disproportionate manner relative to availability. These results suggest young suckers select vegetated areas with moderate flow and relatively shallow depths during first few months of life. This information improves our understanding of ecological habits of early life stages of a rare western catostomid during stream residence, which could be useful for conservation management in low gradient stream reaches of Warner Basin.

INTRODUCTION

Warner sucker is endemic to Warner Basin and listed as threatened (U.S. Fish and Wildlife Service, 1985). General description of Warner Basin, reproductive habits of Warner sucker, stream and lake distributions, and conservation threats are described in Kennedy and Vinyard (1997). With the exception of drift ecology (Kennedy and Vinyard, 1997), litde is known about ecological habits of young Warner sucker. Recognition of the ecological requirements of these life stages is important because mortality in early life can affect year-class strength and total population size (Fuiman, 2002). We focused our efforts on a low gradient stream reach because it is important habitat for reproductive ecology of this migratory species (Kennedy and Vinyard, 1997). Documenting information pertinent to catostomid ecology in lotie ecosystems is valuable in general because they function as indicators of biotic integrity (Hughes and Gammon, 1987).

STUDY AREA

Warner Basin drains 6858 km^sup 2^ of high desert in south-central Oregon, extreme northwest Nevada and northeast California, with spring runoff from three drainages (Twelvemile/Twentymile, Deep, Honey creeks) filling a chain of shallow lakes remnant of pluvial Lake Warner. Seven weir structures in lower Honey Creek from near the confluence with Hart Lake to ~3.2 km upstream are used in summer to irrigate ditches. Our study site was a 220 m section bounded by the uppermost diversion weir and a shallow riffle 2 cm deep (Fig. 1; Kennedy and Vinyard, 1997). Respectively, mean (±SD) width and depth were 5.7 ± 2.3 m (n = 123) and 26 ± 17.5 cm (n = 126) in 1992, 7.9 ± 3.2 m (n = 94) and 50 ± 33 cm (n = 94) in 1993. Public access to all other low gradient stream sections near lake confluences was restricted. The study site represents ~4.5% of stream habitat occupied by Warner sucker and is listed as critical habitat (USFWS, 1985).

METHODS

Above-water visual censuses (AVC).-We conducted above-water censuses three times in 1992 (30 May, 6 June, 2 July) and in 1993 (30 June, 10 July, 29 July). For each survey, an observer slowly waded through the study site in an upstream direction for ~3 h. The same observer, positioned <50 cm downstream of each aggregate, estimated abundances followed by microhabitat measurements (see below). …