A Lota lota Consumption: Trophic Dynamics of Nonnative Burbot in a Valuable Sport Fishery

Unintentional and illegal introductions of species disrupt food webs and threaten the success of managed sport fisheries. Although many populations of Burbot Lota lota are declining in the species’ native range, a nonnative population recently expanded into Flaming Gorge Reservoir (FGR), Wyoming–Utah, and threatens to disrupt predator–prey interactions within this popular sport fishery. To determine potential impacts on sport fishes, especially trophy Lake Trout Salvelinus namaycush, we assessed the relative abundance of Burbot and quantified the potential trophic or food web impacts of this population by using diet, stable isotope, and bioenergetic analyses. We did not detect a significant potential for food resource competition between Burbot and Lake Trout (Schoener’s overlap index = 0.13), but overall consumption by Burbot likely affects other sport fishes, as indicated by our analyses of trophic niche space. Diet analyses suggested that crayfish were important diet items across time (89.3% of prey by weight in autumn; 49.4% in winter) and across Burbot size-classes (small: 77.5% of prey by weight; medium: 76.6%; large: 39.7%). However, overall consumption by Burbot increases as water temperatures cool, and fish consumption by Burbot in FGR was observed to increase during winter. Specifically, large Burbot consumed more salmonids, and we estimated (bioenergetically) that up to 70% of growth occurred in late autumn and winter. Further, our population-wide consumption estimates indicated that Burbot could consume up to double the biomass of Rainbow Trout Oncorhynchus mykiss stocked annually (>1.3 × 10 kg; >1 million individuals) into FGR. Overall, we provide some of the first information regarding Burbot trophic interactions outside of the species’ native range; these findings can help to inform the management of sport fisheries if Burbot range expansion occurs elsewhere. Nonnative species continue to be a foremost threat to conservation and management globally, and freshwater ecosystems are among the most highly altered by invasive species (Mooney and Hobbs 2000; Marchetti et al. 2004a). Anthropogenically assisted introductions—intentional and unintentional, legal and illegal—have led to the proliferation of nonnative fishes across the United States (Rahel 2000, 2004). Accordingly, while some fish are experiencing declines in their native range, introductions and subsequent range expansions allow for nonnative populations that are potentially more successful than conspecific populations within the native range (e.g., Brook Trout Salvelinus fontinalis; Peterson et al. 2004; Hudy et al. 2008). Thus, regardless of the introduction pathway, introduced fishes can potentially prove detrimental to the ecological and economic underpinnings of successful fisheries (Dunham et al. 2004; Gozlan et al. 2010). Widespread stocking programs that include nonnative fishes support valuable sport fisheries, and managers are tasked with creating a balance between angler satisfaction and food web dynamics (Eby et al. 2006). Lake Michigan, where Burbot Lota *Corresponding author: [email protected] Received March 18, 2016; accepted August 15, 2016 1386 Transactions of the American Fisheries Society 145:1386–1398, 2016 © American Fisheries Society 2016 ISSN: 0002-8487 print / 1548-8659 online DOI: 10.1080/00028487.2016.1227372 lota are native, provides a classic example of the many complexities associated with fish introductions resulting from attempts to achieve this balance—a struggle that began in the early 20th century and continues today (Stewart et al. 1981; Tsehaye et al. 2014). In Lake Michigan, the balance of an invasive prey base (e.g., Rainbow Smelt Osmerus mordax and Alewife Alosa pseudoharengus) and predators, including native (e.g., Lake Trout Salvelinus namaycush) and nonnative (e.g., Chinook Salmon Oncorhynchus tshawytscha) salmonids, has become destabilized on a number of occasions due to disproportionate stocking densities and nonnative species introductions. Across the western United States, stocked sport fisheries are especially prevalent in numerous man-made reservoirs and are often sustained by the stocking of top predators (Moyle and Marchetti 2006). In accordance with the unnatural characteristics of reservoirs, fish stocking often creates a mixed assemblage of native and nonnative sport fishes, and assemblages consisting almost entirely of nonnative species are relatively common (Martinez et al. 1994; Clarkson et al. 2005). These mixed assemblages typically require the annual stocking of predators, which can decouple the balance of predator consumptive demand and prey availability, as was seen in Lake Michigan (Johnson and Martinez 2000). Reservoirs represent novel habitats for both native species (e.g., creating standing water in naturally flowing river systems) and nonnative species alike, creating sport fisheries that can be relatively harmless to native fishes and highly popular among anglers targeting stocked nonnative fishes (Marchetti et al. 2004b; Johnson et al. 2009; but see Rubidge and Taylor 2005). However, when unauthorized introductions occur, managers are confronted with uncertainty about food web dynamics and overall ecosystem stability in these highly managed fisheries (Leung et al. 2002; Maguire 2004). In the face of such uncertainty, management can be difficult and further complicated by the need to balance ecological, social, and economic concerns (Quist and Hubert 2004). In the upper Colorado River basin, many reservoirs fragment the region but also create valuable sport fisheries as a result (Dibble et al. 2015). The prized sport fishes in these reservoirs are usually nonnative salmonids. However, illegal introductions of other fishes have led to many populations of coolwater and warmwater fishes across the basin, which pose threats to actively managed species (Johnson et al. 2009). Flaming Gorge Reservoir (FGR), Wyoming–Utah, is one of the largest reservoirs and supports among the most popular fisheries in the upper Colorado River basin but is threatened by the relatively recent introduction and expansion of Burbot. Within FGR, angling opportunities for trophy Lake Trout are abundant, and Lake Trout are primarily supported by a kokaneeOncorhynchus nerka population that is popular among anglers. Rainbow Trout Oncorhynchus mykiss and Smallmouth Bass Micropterus dolomieu provide further angling opportunities. Although these sport fishes are also nonnative, they are actively and intentionally managed within FGR. In the early 2000s, Burbot began to expand downstream in the Green River drainage of the upper Colorado River basin from Big Sandy Reservoir, Wyoming, where Burbot were established from illegal introductions in the 1990s (Garduino et al. 2011). By 2006, Burbot were established in FGR. Burbot have been demonstrated as voracious predators in their native range and therefore could pose a threat to highly valued fisheries within FGR via direct and indirect competition (Rudstam et al. 1995; Jacobs et al. 2010). Unlike the Lake Trout and kokanee populations that FGR is noted for, Burbot can inhabit both lotic and lentic waters. As such, Burbot pose a threat of further expansion throughout the upper Colorado River basin, and managers fear that the species’ expansion will be detrimental to the sport fishes of FGR. As a Holarctic, circumpolar species, the Burbot is relatively well studied in its native ranges throughout Asia, Europe, and North America; however, many native populations have experienced dramatic declines (Stapanian et al. 2010), and very little is known about Burbot in their nonnative range (but see Klein et al. 2015). Niche partitioning between Burbot and fishes in their native range has been explored (e.g., Guzzo et al. 2016), but we require an understanding of trophic dynamics that have contributed to the success of Burbot in their nonnative range, especially as nearby native populations are in decline (e.g., Krueger and Hubert 1997). We investigated the population status of Burbot in FGR and assessed the potential direct and indirect impacts of Burbot on valued sport fish, with a primary focus on trophy Lake Trout. We collected diet and stable isotope samples and extended our findings by using a bioenergetics model to quantify predation potential and competition. Collectively, we used these data to show how a newly established population of Burbot across a range of population densities could affect highly managed sport fisheries, especially in water bodies where annual stocking promotes angler success. Our findings demonstrate that efficient predators such as Burbot can consume many stocked sport fishes (e.g., Rainbow Trout) as well as important prey resources (e.g., crayfish) that are used by these sport fishes. The present study provides a baseline for better understanding and managing fisheries that might be affected if Burbot expansion continues. METHODS Study site.—Flaming Gorge Reservoir, which was created by impoundment of the Green River in 1962, lies on the border of northeastern Utah and southwestern Wyoming (Figure 1). At capacity, FGR is approximately 145 km long, encompasses 17,000 ha of water, and has a mean depth of 34 m. Previous studies (e.g., Yule and Luecke 1993) separated the reservoir into three distinct regions based on physical and chemical characteristics: (1) the Canyon region—a narrow and deep (maximum depth = 134 m), stratified, nearly oligotrophic region extending approximately 38 km north of the dam; (2) the Open TROPHIC DYNAMICS OF NONNATIVE BURBOT 1387