Transactions of the American Fisheries Society © 2019 American Fisheries Society ISSN: 0002-8487 print / 1548-8659 online DOI: 10.1002/tafs.10203 ARTICLE Effects of Multiple Nonnative Fish on an Imperiled Cyprinid, Hornyhead Chub Brian T. Hickerson* Wyoming Cooperative Fish and Wildlife Research Unit, Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming 82071, USA Bryan M. Maitland Program in Ecology and Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming 82071, USA Annika W. Walters U.S. Geological Survey, Wyoming Cooperative Fish and Wildlife Research Unit, University of Wyoming, Laramie, Wyoming 82071, USA Abstract Nonnative fish can have substantial negative effects on the abundance and distribution of native fishes through pre- dation and competition. Nonnative predators are of particular interest because they represent novel threats to native prey species that are not adapted to their presence. Prey species with limited distributions or population sizes may be particularly vulnerable to the effects of nonnative predators. In the Laramie River, four nonnative predators—Brown Trout Salmo trutta, Rainbow Trout Oncorhynchus mykiss, Brook Trout Salvelinus fontinalis, and Smallmouth Bass Micropterus dolomieu are present along with a state-imperiled population of Hornyhead Chub Nocomis biguttatus. The abundance of Hornyhead Chub has declined with increasing abundance of nonnative predators, with the probabil- ity of occurrence of Hornyhead Chub dropping drastically when Smallmouth Bass were present. All four nonnative species preyed on native cyprinids, but Smallmouth Bass relied most heavily on fish as a prey item. Isotopic niche overlap occurred between Hornyhead Chub and all of the nonnative predator species. Our results demonstrate that nonnative predators have the potential to negatively affect the abundance and distribution of Hornyhead Chub through the mechanisms of predation and competition, and predator identity is important in determining the extent of effects. Smallmouth Bass are a greater concern than nonnative salmonids because of their more piscivorous behavior, and their recent upstream expansion may be limiting the downstream distribution of Hornyhead Chub in the Laramie River. Interactions between predator and prey species play an interactions for space or resources (Mills et al. 2004). important role in structuring aquatic communities Assessing the relative importance of competitive versus (Hoeinghaus and Pelicice 2010; Cucherousset and Olden predatory interactions in driving changes in the abundance 2011). Piscivorous fishes can cause declines in the abun- of prey species is difficult (Crowder 1980; Ross 2013), but dance and distribution of prey species through direct understanding the importance of these drivers is critical to predation (Garman and Nielsen 1982; Kershner and the effective management of fisheries resources, particu- Lodge 1995; Jackson et al. 2001) or through competitive larly in contemporary ecosystems with nonnative species. *Corresponding author: [email protected] Received June 27, 2019; accepted September 4, 2019 1 2 HICKERSON ET AL. Nonnative predators represent a novel threat to native Trout, Brown Trout, and Brook Trout occurring in the prey populations that are not adapted to their presence late 1930s and early 1940s (S. Gale, Wyoming Game and (Hughes and Herlihy 2012). The number of predator spe- Fish Department, personal communication). Smallmouth cies that are present in a local community and their Bass were first introduced into Grayrocks Reservoir, respective identities are likely important factors that deter- which impounds the Laramie River, in 1981, and they mine the extent of predator effects on prey species. Fur- have rapidly expanded their upstream distribution during ther, the identity of nonnative predators will be the last decade (Quist et al. 2005; Hickerson 2018). Small- particularly important when they vary in their feeding mouth Bass were not detected upstream of Grayrocks strategy or habitat use (MacRae and Jackson 2001; Reservoir during sampling in 2004 or 2005 (Bear and Bar- Hughes and Herlihy 2012). rineau 2007), but in 2008 and 2009 Smallmouth Bass were Understanding the competitive and predatory effects of detected 19 and 26 km upstream, respectively (Moan et al. nonnative predators is especially important for native spe- 2010; Bestgen 2013). By 2015, Smallmouth Bass had cies that are of conservation concern. These species are moved approximately 47 km upstream (Peterson 2017). inherently less resilient to new threats because they typi- Currently, the recruitment of Smallmouth Bass occurs cally have limited distributions and small population sizes throughout the lower Laramie River and individuals have (Labbe and Fausch 2000). For example, nonnative Brook been captured more than 71 river km upstream of Gray- Trout Salvelinus fontinalis displace native Cutthroat Trout rocks Reservoir (Hickerson 2018). The increased upstream Oncorhynchus clarkii populations from important habitats movement coincided with a rapidly expanding Small- through competitive exclusion (Shepard et al. 2002; mouth Bass population in Grayrocks Reservoir after 2009, McGrath and Lewis 2007). Elsewhere, the introduction of which is attributed to filling Grayrocks Reservoir after a nonnative Northern Pike Esox lucius has led to the decline number of drought years and higher flows—which also of geographically isolated and regionally imperiled cypri- likely facilitated the upstream dispersal of Smallmouth nids in Montana and darters in Colorado due to predation Bass (Wyoming Game and Fish Department 2011). (Labbe and Fausch 2000; Stringer 2018). Similarly, studies Our objectives were to (1) quantify the relationships have shown that nonnative Brown Trout Salmo trutta and between the abundance and distributions of Hornyhead Rainbow Trout Oncorhynchus mykiss negatively affect Chub and nonnative predatory species and (2) evaluate populations of endangered Humpback Chub Gila cypha, the potential for competition, predation, or both between primarily through predation (Yard et al. 2011). Therefore, the nonnative predator species and Hornyhead Chub. We species with geographically restricted distributions may be examined the four nonnative predator species individually at an increased extirpation risk from nonnative predators and combined to determine the importance of predator due to a lack of population redundancy across the land- identity. scape. Hornyhead Chub Nocomis biguttatus, with its highly restricted range, is a species that is greatly in need of con- METHODS servation efforts in Wyoming (Bestgen 2013). Western Study area.— The Laramie River flows northeast from populations of Hornyhead Chub have historically the Medicine Bow Mountains in Colorado through south- occurred in the Platte River tributaries in Wyoming, Col- east Wyoming, and it is impounded by two major reser- orado, western Nebraska, and western Kansas. Western voirs before reaching its confluence with the North Platte populations are currently restricted to the Laramie and River near Fort Laramie, Wyoming (Figure 1A). The North Laramie rivers in Wyoming (Propst and Carlson lower Laramie River, downstream of the Wheatland Tun- 1986; Baxter and Stone 1995; Bestgen 2013). The Laramie nel Diversion, has one of the most intact and diverse River also has naturally reproducing populations of four native fish communities in the North Platte River water- nonnative predatory species: Brown Trout, Rainbow shed of Wyoming, where sensitive species like Common Trout, Brook Trout, and Smallmouth Bass Micropterus Shiner Luxilus cornutus and Stonecat Noturus flavus are dolomieu. These four nonnatives can all negatively affect relatively abundant (Quist et al. 2005). The flows in this the distribution and abundance of the native fish popula- reach are regulated year-round by a series of reservoirs tions (Garman and Nielsen 1982; Dunham et al. 2002; and diversion dams. Historically, Hornyhead Chub were Fritts and Pearsons 2006; Johnson et al. 2008; Yard et al. found in the Laramie River from the Wheatland Tunnel 2011). The sympatric occurrence of these four species pro- Diversion downstream approximately 126 km to the con- vides a rare opportunity to study the effects of multiple fluence with the North Platte River near Fort Laramie, nonnative predators and predator identity on a native fish Wyoming (Baxter and Simon 1970; Baxter and Stone species that is of significant conservation concern. 1995; Quist et al. 2005; Figure 1A). We sampled the cur- Nonnative salmonids have inhabited in the lower Lara- rent occupied range, a 42-km reach of the Laramie River mie River for decades, with the first stockings of Rainbow from the Wheatland Tunnel Diversion to just downstream NONNATIVE FISH AND DECLINE OF IMPERILED CYPRINID 3 to 1,417 meters (Bestgen 2013). The mean stream tempera- ture in August within the study reach ranges from 17.6°C near the Wheatland Tunnel diversion to 19.8°C down- stream near Palmer Canyon Road, which is well within the reported thermal range for Hornyhead Chub (spawn- ing: 16–26°C; Vives 1990; thermal maximum: 35.6°C; Smale and Rabeni 1995) from other parts of their range. The Laramie River sampling reach was stratified into three subreaches to account for variations in stream habi- tat and fish community: (1) upper reach (11 km), (2) mid- dle reach (16 km), and (3) lower reach (15 km; Figure 1A). The upper reach is characterized by a moderate gradient; it has few nonnative predatory fish, and Smallmouth