Influence of spatial positioning within stream networks on fish assemblage structure in the Kansas River basin, USA Darren J. Thornbrugh and Keith B. Gido Abstract: We found that riverine confluences had localized effects (within 20 km) on stream fish assemblages of the Kan- sas River basin. The majority of variation in fish assemblages occurred from east to west and along a stream size gradient. After controlling for the influences of longitude and stream size, distance of sample sites from streams ‡ 5th order ac- counted for a small proportion of taxonomic variability. However, species richness was significantly higher and assem- blage structure was different in tributary stream segments directly connected to larger-ordered streams, suggesting that the effects of spatial position within this stream network were greatest in close proximity to tributary–mainstem confluences. Fish collections from three intensively sampled tributaries also indicated an abrupt change in species assemblages between mainstem river sites and tributary sample sites above confluences, followed by a gradual taxonomic change with increasing distance up to 20 km from the confluence. Changes in fish assemblages were associated with the reduced abundance of adult stream species near the confluence with the mainstem, rather than the occurrences of riverine species in the tribu- tary. Re ´sume ´: Les confluences des cours d’eau ont des effets bien localise ´s (sur une distance de moins de 20 km) sur les peu- plements de poissons d’eau courante dans le bassin versant de la rivie `re Kansas. La plus grande partie de la variation dans les peuplements de poissons se produit d’est en ouest et suivant un gradient de taille des cours d’eau. Une fois qu’on a tenu compte des influences de la longitude et de la taille des cours d’eau, la distance entre le point d’e ´chantillonnage et les cours d’eau d’ordre ‡ 5 explique un petit pourcentage de la variabilite ´ taxonomique. Cependant, la richesse spe ´cifique est significativement plus e ´leve ´e et la structure du peuplement diffe ´rente dans les sections de tributaires qui sont relie ´es di- rectement a ` des cours d’eau d’ordre supe ´rieur, ce qui indique que les effets de la position spatiale dans le re ´seau hydrogra- phique sont maximaux pre `s des confluences des tributaires et du cours principal. Des re ´coltes de poissons dans trois tributaires fortement e ´chantillonne ´s montrent aussi un changement abrupt dans les peuplements de poissons entre les sites du cours principal de la rivie `re et les sites d’e ´chantillonnage des tributaires en amont de la confluence, suivi d’un change- ment taxonomique graduel en fonction de la distance pouvant se manifester jusqu’a ` 20 km de la confluence. Les change- ments dans les peuplements de poissons s’expliquent par une abondance re ´duite d’espe `ces adultes de ruisseau pre `s de la confluence avec le cours principal pluto ˆt que par la pre ´sence d’espe `ces de rivie `re dans le tributaire. [Traduit par la Re ´daction] Introduction Stream fish assemblages are influenced by the availability of aquatic habitats across a variety of spatial scales and the ability of individuals to move among those habitats (Schlosser 1982; Angermeier and Winston 1998; Grossman et al. 1998). Accordingly, measures of local fish assemblage structure are often predicted by the species composition of the surrounding drainage basin, as well as the position of that locality within the drainage (Osborne and Wiley 1992; Matthews and Robison 1998; Schaefer and Kerfoot 2004). Such spatial dependencies of fish populations and commun- ities among stream habitats (e.g., streams of different size or basins with different geology) have been conceptualized by several authors. Recently, Fausch et al. (2002) developed the riverscapes perspective, which posits that dendritic stream networks cannot be viewed in the context of disjunct parts but should be considered in their entire interconnected network architecture. Campbell-Grant et al. (2007) further emphasized the importance of the structure of dendritic net- works in shaping biotic assemblages and highlighted the im- portance of movement among branches and the heterogeneity of habitats associated with a branch–node structure. These studies are important because they concep- tualize how regional factors operating at spatial scales of tens to hundreds of kilometres can influence local assem- blage structure, which is typically measured at spatial scales of <0.5 km. Despite this increasing recognition that land- scape and stream network architecture influence fish as- semblages, empirical data on the relative importance of Received 24 December 2008. Accepted 23 September 2009. Published on the NRC Research Press Web site at cjfas.nrc.ca on 11 December 2009. J20956 D.J. Thornbrugh 1 and K.B. Gido. Kansas State University, Division of Biology, Manhattan, KS 66506, USA. 1 Corresponding author (e-mail: dthor@msu.edu). 143 Can. J. Fish. Aquat. Sci. 67: 143–156 (2010) doi:10.1139/F09-169 Published by NRC Research Press