A Study of the Spawning Ecology and Early Life History Survival of Bonneville Cutthroat Trout

We completed a large-scale field experiment in four tributaries of the Logan River, Utah, where the largest metapopulation of imperiled Bonneville cutthroat trout Oncorhynchus clarkii utah persists. We documented the spatial and temporal distributions of spawners, quantified substrate use versus substrate availability, and evaluated differences in hatch and emergence fry success between and among sites in relation to habitat characteristics. We observed considerable variability in the timing, magnitude, and duration of spawning among study areas (streams), in part as a function of a variable, multipeaked hydrograph. Nevertheless, across study areas, >70% of redds were constructed on the final descending limb of the hydrograph. Despite large differences in the amount of spawning substrate available, Bonneville cutthroat trout utilized a narrow range of substrate and sizes (3–80 mm) similar to that utilized by other subspecies of cutthroat trout, albeit biased towards larger sizes. Water temperatures generally remained below the recommended range (6–17◦C) for spawning; however, the viability of this metapopulation of cutthroat trout suggests that the recommended temperature range for spawning is overestimated for this subspecies and (or) does not account for local thermal adaptation. Hatch varied from 43% to 77% and emergence survival from 39% to 65% among streams, and within-stream variability was substantial; both survival rates declined significantly as a function of increased fine sediment concentrations. Egg development rates were nearly 50% greater in a highelevation tributary where redd counts were also lowest. In high, mountain systems with short growing seasons, this incubation delay likely presents a significant growth disadvantage for age-0 trout. Our research enhances our understanding of Bonneville cutthroat trout spawning ecology and early survival and provides critical information for aiding in the development of benchmarks for their recovery. Effective conservation efforts should be directed towards minimizing anthropogenic activities that result in excess sedimentation in their critical spawning tributaries. In the last century, cutthroat trout Oncorhynchus clarkii have experienced large, range-wide reductions in distribution and abundance, due to the combined effects of habitat loss and fragmentation, competition and hybridization with nonnative species, disease, and overharvesting (Behnke 1992; Duncan and Lockwood 2001; Fausch 2008) and now, the additional effects of climate change (Williams et al. 2009). Today, the range of *Corresponding author: [email protected] Received April 15, 2011; accepted February 7, 2012 this species is extremely fragmented, with subspecies limited primarily to high elevation lakes and rivers (Behnke 2002). Consequently, of the 14 recognized subspecies of cutthroat trout, 2 are extinct, 3 are listed as threatened under the U.S. Endangered Species Act, and the 9 remaining subspecies are generally imperiled (Williams et al. 2009). Cutthroat trout prefer habitat with clear, cold water, sufficient stream flows, adequate 436 D ow nl oa de d by [ N at io na l F or es t S er vi ce L ib ra ry ] at 1 2: 36 0 7 Ju ne 2 01 2 EARLY LIFE HISTORY SURVIVAL OF CUTTHROAT TROUT 437 streamside vegetation, and habitat complexity and heterogeneity. Their criteria for spawning are thought to be quite specific and require a narrow range of substrate and hydrologic conditions (Hickman and Raleigh 1982; Bjornn and Reiser 1991; Behnke 1992). Because of such stringent habitat requirements, cutthroat trout are particularly sensitive to human disturbances (e.g., livestock grazing, irrigation diversions, road construction); such sensitivity is probably most pronounced in the important spawning and highly variable early life history stages (Duff 1988; Behnke 1992; Kershner 1995). As spring spawners, typical in high mountain streams, the spawning and early life history stages of cutthroat trout often correspond with the snowmelt and spring spates and are thus extremely difficult to study. While considerable information exists describing the spawning ecology and early life history of other salmonids (e.g., salmon species; Beauchamp et al. 1994; Isaak et al. 2007), significant gaps remain in our understanding of these life stages for Bonneville cutthroat trout Oncorhynchus clarkii utah, the focus of this study (see also Hilderbrand 2003). As with other springtime spawners, cutthroat trout spawning is thought to be initiated in response to seasonal changes, when environmental conditions reflect the transition from winter to spring with increasing water temperatures, increasing day length and receding flows from spring runoff (Behnke 1992). The environmental conditions that follow spring runoff (e.g., stable flows and warm water temperatures) are representative of high mountain rivers and provide ideal conditions for embryo incubation, fry emergence, and juvenile rearing (Behnke 1992; Kershner 1995). While information describing the spawning ecology and early life history of Bonneville cutthroat trout is limited, a considerable body of literature provides insight into this critical stage for other species of cutthroat trout. Such studies include the description of physical characteristics of redds (e.g., Thurow and King 1994; Schmetterling 2000), and the female age at maturity, fecundity (e.g., Downs et al. 1997). The relationship of habitat availability, habitat type, and substrate characteristics (e.g., percent fine sediment) to spawning distribution, along with redd composition and redd densities have also been characterized (e.g., Magee et al. 1996; Joyce and Hubert 2004). However, to our knowledge, there has been a paucity of research focused on the spawning ecology of Bonneville cutthroat trout, specifically, the quantification of the spawning distribution, spawning duration and timing, fecundity, egg incubation period, emergence time, and egg-to-fry survival. While restoring these imperiled cutthroat trout populations and protecting and preserving remaining healthy populations remains a top priority and concern (Budy et al. 2007; Williams et al. 2009), these data gaps challenge our ability to identify links between land management and cutthroat trout viability and thus limit the effective prioritization of conservation and recovery actions. In addition to being difficult to quantify and sensitive, for most salmonids, these early life stages typically exhibit high rates of natural mortality for both incubating embryos and emergent fry (e.g., Knight et al. 1999; Kershner 1995). Furthermore, a wide suite of abiotic variables (e.g., water temperature, dissolved oxygen, water velocity, gravel size, percent fine sediment) can be influential in determining early survival (Hickman and Raleigh 1982; Bjornn and Reiser 1991; Kondolf 2000). In addition, disturbances to habitat via land-use activities can alter these key physical factors. Hickman and Raleigh (1982) suggest that suitable spawning criteria for cutthroat trout in general include (1) water temperatures of 6–17◦C, with optimal embryo incubation at 10◦C, (2) mean water column velocities suitable for embryo incubation of 0.11–0.90 m/s, with optimal velocities of 0.30–0.60 m/s, and (3) a range of substrate sizes for embryo incubation of 3–80 mm and optimal at 15–60 mm. The critical value for dissolved oxygen is not known at this time for cutthroat trout embryos but is assumed to be similar to that of adults; optimal dissolved oxygen levels for adult cutthroat trout are >7 mg/L at ≤15◦C and ≥9 mg/L at >15◦C (Hickman and Raleigh 1982). Overall, the abundance of spawning gravel is perhaps one of the most critical and limiting factors for both successful redd construction and embryo incubation (Kondolf et al. 1991). Despite these general criteria, we know extremely little about how spawning criteria differ among the different subspecies of cutthroat trout, subspecies that are adapted to very different environments. Land-use activities, such as livestock grazing, can have direct and indirect detrimental impacts on spawning because redds are extremely vulnerable to trampling by livestock and to fine sediment accumulation via bankside disturbances from grazing livestock (Gregory and Gamett 2009). Anthropogenic activities such as road construction and irrigation diversions also have the potential to negatively affect spawning either by fine sediment increases or redd dewatering (Hickman and Duff 1978; Kershner 1995). The effects of fine sediment accumulation can be significant because sediment caps can form over redds and smother or suffocate incubating embryos and prevent fry emergence (Tappel and Bjornn 1983; Lisle 1989). Given the challenges of quantifying early life history survival in the wild, laboratory-based studies have evaluated the relationship between some of these key abiotic variables and cutthroat trout survival at the early life stage in controlled laboratory settings. Young (1991), for example, used Colorado River cutthroat trout O. clarkii pleuriticus eggs in a laboratory setting to assess the degree to which different proportions of sediment (≥13.8 mm) impacted early survival and concluded that egg-to-fry survival declined in respect to the percent fine sediment. Similarly, while studying the effects of water temperature reduction on survival of cutthroat trout embryos fertilized at 7◦C, Hubert and Gern (1995) found survival to the hatching stage to be lower for those embryos that were at an earlier development stage when water temperatures were reduced to 3◦C. While these studies have advanced our knowledge of the early life stages of cutthroat trout in general, they have not identified mechanistic linkages between habitat quality and quantity, and survival as they occur in nature. D ow nl oa de d by [ N at io na l F or es t S er vi ce L ib ra ry ] at 1 2: 36 0 7 Ju ne 2 01 2