Spatially Continuous Analysis of Fish–Habitat Relationships

American Fisheries Society Symposium 48:473–492, 2006 © 2006 by the American Fisheries Society Landscape Influences on Longitudinal Patterns of River Fishes: Spatially Continuous Analysis of Fish–Habitat Relationships Christian E. Torgersen*,1 and Colden V. Baxter2 Oregon Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife Oregon State University, Corvallis, Oregon 97331, USA Hiram W. Li Oregon Cooperative Fish and Wildlife Research Unit (U.S. Geological Survey) Department of Fisheries and Wildlife, 104 Nash Hall Oregon State University, Corvallis, Oregon 97331, USA Bruce A. McIntosh Oregon Department of Fish and Wildlife, Corvallis Research Laboratory 28655 Highway 34, Corvallis, Oregon 97333, USA Abstract.—Longitudinal analysis of the distribution and abundance of river fishes provides a context-specific characterization of species responses to riverscape heterogeneity. We exam- ined spatially continuous longitudinal profiles (35–70 km) of fish distribution and aquatic habitat (channel gradient, depth, temperature, and water velocity) for three northeastern Ore- gon rivers. We evaluated spatial patterns of river fishes and habitat using multivariate analysis to compare gradients in fish assemblage structure among rivers and at multiple spatial scales. Spatial structuring of fish assemblages exhibited a generalized pattern of cold- and coolwater fish assemblage zones but was variable within thermal zones, particularly in the warmest river. Landscape context (geographic setting and thermal condition) influenced the observed rela- tionship between species distribution and channel gradient. To evaluate the effect of spatial extent and geographical context on observed assemblage patterns and fish–habitat relationships, we performed multiple ordinations on subsets of our data from varying lengths of each river and compared gradients in assemblage structure within and among rivers. The relative associations of water temperature increased and channel morphology decreased as the spatial scale of analysis increased. The crossover point where both variables explained equal amounts of variation was useful for identifying transitions between cool- and coldwater fish assemblages. Spatially continuous analysis of river fishes and their habitats revealed unexpected eco- logical patterns and provided a unique perspective on fish distribution that emphasized the importance of habitat heterogeneity and spatial variability in fish–habitat relationships. INTRODUCTION *Corresponding author: [email protected] 1 Present address: USGS-FRESC Cascadia Field Station, Studies of river fish assemblages often focus on 344 Bloedel Hall, University of Washington, Seattle, describing and understanding patterns in species Washington 98195-2100, USA. composition that occur along the length of river 2 Present address: Department of Biological Sciences, systems. In general, fish assemblage structure is Idaho State University, Pocatello, Idaho 83209, USA. thought to change predictably from headwaters 473 22torgersen.p65 473 7/28/2006, 10:00 AM 474 Torgersen et al. to downstream reaches, with biotic zones (e.g., of spatial variability in river fish–habitat relation- cold- and warmwater assemblages) occurring in ships remain unanswered: How finely tuned are which species are added or replaced in response longitudinal patterns in fish assemblages to key to continuous gradients in temperature, chan- habitat factors such as thermal heterogeneity, nel morphology, and water velocity (Huet 1959; channel morphology, and velocity? Can the ef- Sheldon 1968; Horwitz 1978; Hughes and fects of temperature on fish assemblages be iso- Gammon 1987; Li et al. 1987; Rahel and Hubert lated from the effects of other factors? How are 1991; Paller 1994; Belliard et al. 1997). Biotic assemblage patterns at one scale mediated by zonation and species addition are dominant, context at larger spatial scales? Does perception coarse-scale patterns that have been described of habitat relationships change with the spatial by numerous studies during the last century extent of a study? We propose that these ques- (Matthews 1998). Beyond these patterns, how- tions can be addressed only by adapting and ever, there is little understanding of spatial het- changing the manner in which fish assemblage erogeneity in fish distribution and habitat and habitat data are collected and analyzed. relationships within biotic zones or the effects Here we illustrate a new approach to collect- of spatial scale and context on observed fish as- ing and analyzing fish assemblage and habitat semblage patterns in rivers (Collares-Pereira et data that provides a more spatially continuous al. 1995; Duncan and Kubecka 1996; Poizat and view of fishes and the riverine landscapes, or Pont 1996; Bult et al. 1998; Fausch et al. 2002). “riverscapes,” they inhabit (Fausch et al. 2002). Consequently, there are likely many more pat- Our objectives were to (1) collect spatially con- terns and spatial relationships that have yet to tinuous data on fish assemblage structure and be described, and these may be essential to un- habitat along the length of three rivers with con- derstanding river fish assemblages. trasting physical environments, (2) characterize Discovery of new patterns and spatial rela- and compare longitudinal patterns and habitat tionships may be constrained by repeated use of relationships (water depth, velocity, channel gra- traditional study approaches. Relatively short dient, and water temperature) among and within sampling reaches (<500 m) spaced at wide in- these riverscapes, and (3) evaluate the effect of tervals (>10 km) along the longitudinal profile spatial extent and geographical context of sur- or throughout a channel network may provide vey data on observed fish–habitat relationships. the information necessary to detect coarse gradients in fish assemblage structure associated METHODS with factors such as temperature and stream or- der (Vannote et al. 1980). However, such site- Study Area based studies lack the spatial resolution necessary for detecting patterns in fish–habitat relation- We studied fish assemblages in three small riv- ships across a range of spatial scales. Conse- ers in the Blue Mountains of northeastern Ore- quently, the perception that river fish gon: the Middle Fork John Day (MFJD; upper assemblages change gradually with respect to 49 km), the North Fork John Day (NFJD; upper longitudinal habitat gradients may be driven 70 km), and the Wenaha River (WEN; lower 35 largely by the resolution and extent of data col- km; Figure 1). Study section elevations ranged lection and analysis (Naiman et al. 1988; Wiens from 500 m in the lower WEN to 1,700 m in the 1989; Poole 2002). As a consequence of the dis- upper NFJD and shared a similar geology of continuous and spatially limited manner in Columbia River basalt at lower elevations and which river fishes are traditionally sampled, fun- folded metamorphosed rocks partially overlain damental questions about the nature and extent by volcanic tuff in headwater reaches (Orr et al. 22torgersen.p65 474 7/28/2006, 10:00 AM Landscape Influences on Longitudinal Patterns of River Fishes 475 Figure 1. Study area and river sections surveyed for fish assemblages in northeastern Oregon. Study rivers included (A) the Middle Fork John Day (MFJD), (B) the North Fork John Day (NFJD), and (C) the Wenaha River (WEN). Black dots indicate the spatial extent and continuity of underwater visual surveys. 22torgersen.p65 475 7/28/2006, 10:00 AM 476 Torgersen et al. 1992). Although the NFJD study section had the ranches. Land-use impacts are minimal in the largest drainage area and the highest elevations, relatively pristine WEN compared to the NFJD the WEN received more annual precipitation and and the MFJD, which have experienced exten- had higher summer base flow (Table 1). Longi- sive mining, grazing, and logging during the last tudinal gradients in elevation and annual pre- century. cipitation were steepest in the WEN, followed by the NFJD and the MFJD. Maximum summer Fish Assemblages water temperature patterns reflected differences in streamflow among basins and represented a Native fish species common in the study rivers range of cool and cold thermal environments included four salmonids, three catostomids, four (Table 1). cyprinids, and two cottids. Two nonnative fishes Seasonal weather patterns throughout the (brook trout Salvelinus fontinalis and small- study area are typical of high desert climates with mouth bass Micropterus dolomieu) were ex- hot, dry summers and cold, relatively wet win- tremely rare and therefore not included in our ters (–15–38°C; Loy et al. 2001). The Blue Moun- analysis. We selected a subset of species for as- tains ecoregion is characterized by contrasts in semblage analysis based on their relative abun- temperature, precipitation, and vegetation cor- dance and ease of identification underwater responding with steep elevation gradients (Figure 2). We noted sculpins Cottus spp., (Clarke and Bryce 1997). Canyons and alluvial longnose dace Rhinichthys cataractae, and moun- valleys in the Wenaha and John Day River ba- tain sucker Catostomus platyrhynchus during sins are vegetated with mixed conifer forest (pon- surveys but did not include them in analysis be- derosa pine Pinus ponderosa, grand fir Abies cause they were difficult to detect and identify grandis, Douglas-fir Pseudotsuga menziesii, west- underwater, as determined by comparisons of ern larch Larix occidentalis, and lodgepole pine

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