Movement Patterns and Habitat Selection of Lahontan Cutthroat Trout in a Great Basin Stream

doi:10.21203/rs.3.rs-4814789/v1

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Movement Patterns and Habitat Selection of Lahontan Cutthroat Trout in a Great Basin Stream

https://doi.org/10.21203/rs.3.rs-4814789/v1

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Background: Understanding the movement of organisms is critical for species conservation in the context of changing landscapes and climate. As climatic extremes impact the United States Great Basin, quantifying the movements of native fishes like Lahontan cutthroat trout (Oncorhynchus clarkii henshawi) is vital for facilitating their persistence. These climatic extremes are projected to alter flow regimes, specifically, reducing hydrologic connectivity needed to maintain populations. By studying fish movement patterns during streamflow recession and baseflow conditions, we can identify the factors responsible for movement and habitat selection to better manage these factors in a changing world.

Methods: We tagged 57 Lahontan cutthroat trout from early summer to fall in 2021 and 2022 in the Summit Lake watershed (NV, USA). The location of each fish was associated with local hydraulic, physical habitat, invertebrate drift concentration, and water quality data to assess which factors impact habitat selection, abandonment, and overall movement. Multiple linear regression models were used to assess which factors were associated with trout movement, and a two-sample permutation test was used to identify factors associated with habitat selection or abandonment.

Results: Stream-resident trout displayed little movement during streamflow recession and baseflow conditions, with median daily movements of 0.3 m/day and a median home range of 10.2 m; these results suggest even less movement than those reported in previous studies. Abrupt declines in riffle crest thalweg (RCT) depth were the primary factor associated with increases in distance traveled, yet there were only four observed movements below RCT depths of 5 cm and no observations below 4 cm. The only factor that impacted trout habitat selection or

abandonment was fork length and weight, with smaller individuals abandoning habitat more often than larger, dominant individuals.

Conclusions: The findings from this study suggest that trout movement occurs when absolutely necessary, such as escaping drying reaches or being displaced by larger or more aggressive individuals. We suggest that watershed managers implement low-flow hydrologic monitoring to identify vulnerable stream reaches, with an emphasis on preserving streamflow connectivity for stream-rearing salmonids. Additionally, this emphasizes the importance of tracking movements for species of interest as a strategy to identify factors potentially reducing population fitness.

Movement ecology

streamflow recession

Lahontan Cutthroat trout

Great Basin

telemetry

habitat selection

climatic variation

Understanding the movement of organisms is critical for species conservation in the context of changing landscapes. Land use and hydroclimatic changes have impacted landscapes and species distributions throughout the western United States (Cook et al. 2014, Homer et al. 2020, Wilcove et al. 1998, Xue et al. 2017, Hatchett et al. 2016). In the coming decades, landscape-scale impacts and hydroclimatic variation are expected to persist or increase (Zhang et al. 2021, Siirilia-Woodburn et al. 2021). In the Great Basin of the United States, which is characterized by arid hydroclimatic conditions, many of the associated impacts on the native flora and fauna are exacerbated by shifts in temperature and precipitation (Naumburg et al. 2005, Melack et al. 1997, Patten et al. 2008). Species occupying this region's habitats have adapted to harsh conditions, but they are particularly vulnerable to rapid changes in abiotic factors (Hubbs & Miller 1948, Hubbs 1941, Smith 1978, Smith et al. 2002, Comstock & Ehleringer 1992).

The survival and persistence of species in the Great Basin often depend on movement, which can aid in the transition between life history stages (Beever et al. 2003), finding favorable foraging conditions (Collins 2016), avoiding predation (Glaudas et al. 2008), and maintaining homeostasis (Buseck et al. 2005). Climate-driven changes in the quantity and timing of precipitation (Hatchett et al. 2015) are expected to reduce annual baseflow in Great Basin streams (Döll & Schmied 2012), limiting the opportunity for movement of aquatic organisms (Bunn & Arthington 2002, Grantham 2013). By pairing animal movement data with concurrent environmental data, we can create frameworks for understanding the drivers and potential limitations of movement in species of interest (Nathan et al. 2008) to predict how flow alteration impacts animal movement in the Great Basin.

Compared with other fish taxa, stream-resident salmonids have evolved to live in a relatively narrow range of abiotic conditions within stream and river environments. Cool stream temperatures (Dunham et al. 2003), high dissolved oxygen levels (Doudoroff & Shumway 1970), complex habitat (Moore & Gregory 1988), adequate food resources (Wilzbach et al. 1986), and extensive continuous habitat (Hilderbrand & Kershner 2000) are factors needed for trout growth and survival. When reach-level stream conditions deteriorate, habitat connectivity allows salmonids to seek out stream reaches that are more favorable to avoid potentially lethal conditions (Brown & Mackay 1995). In streams of the western United States, trout movement and redistribution among habitats are limited by low flow conditions during the summer months (Goodrich et al. 2018, Rossi et al. 2023). Riffle depths limit passage between habitat units, which can provide access to improved conditions for stream-rearing trout. For example, in years with fast-drying riffles or intermittent conditions, movement is restricted and can lead to poor habitat quality or even complete desiccation (Hwan & Carlson 2016). When riffle depths reach a critically low level, longitudinal stream movements are no longer possible and confine these fish to singular habitat units, and in extreme events of habitat discontinuity, salmonid survival decreases (Obedzinski et al. 2018, May & Lee 2004, Rossi et al. 2023). Woelfle-Eskrine et al. (2017) reported that disconnected habitat units resulted in decreases in dissolved oxygen concentrations and increases in stream temperatures, leading to reduced salmonid survival rates depending upon channel geomorphology. In addition to streamflow-associated water quality conditions, invertebrate availability may influence salmonid habitat selection. Invertebrate drift provides the primary food source for stream-rearing salmonids (Fausch 1984) and is a determining factor for trout habitat selection (Wilzbach et al. 1986). Invertebrate drift density impacts the net rate of energy intake of drift-foraging fishes (Hayes et al. 2007), which affects stream trout survival, growth rates, and habitat carrying capacity (Guensch et al. 2001).

Cutthroat trout (Oncorhynchus clarkii) rely heavily upon movement and streamflow connectivity at each stage of their life histories (Pringle 2003, Gowan et al. 1994, Rieman & Dunham 2000). In the face of climate change, reductions in habitat connectivity and quality pose a direct threat to cutthroat trout persistence. Lahontan cutthroat trout (Oncorhynchus clarkii henshawi), hereafter referred to as LCT, is a federally threatened subspecies of cutthroat trout which persists in disconnected watersheds of the basin due to hydroclimatic changes and reductions in range since the late 1800s. These reductions can be attributed to anthropogenic activities such as commercial fishing and harvest, habitat degradation, water diversions and overallocation of water resources, competition with introduced species such as brook trout (Salvelinus fontinalis), and hybridization with rainbow trout (Oncorhynchus mykiss) (Coffin & Cowan 1995, Dunham et al. 1997, Dunham et al. 1999, Null et al. 2017). LCT exhibit one of three life history strategies that require movement within and across aquatic ecosystems: stream-resident (individuals who live their entire lives in streams), lacustrine (individuals who live their whole lives in lakes), or adfluvial (individuals who spawn in streams and rear in lakes) (Gerstung 1988). Life history strategies of stream-resident LCT in populations with interconnected lake and stream populations may be key to overall population persistence but are understudied since few remaining populations display both life histories within the Great Basin (Campbell et al. 2018, USFWS 2009).

The Summit Lake watershed (Figure 1) is an endorheic basin within the Great Basin of northern Nevada (USA) that hosts the state's last, self-sustaining adfluvial LCT population, supporting both stream-resident and adfluvial LCT populations. The indigenous people (Agai Panina Ticutta or “Lake Trout Eaters” in the Paiute Language) of Summit Lake have relied upon and managed this resource for millennia, and it has been central to their culture. Since the 1960s, the Summit Lake Paiute Tribe’s Natural Resources Department has enumerated the adfluvial spawning population into the stream to better understand population dynamics within the watershed. Recently, there has been heightened concern by the Summit Lake Paiute Tribe that changes in climate and hydrology, particularly during low flow periods, may influence LCT movement within the primary inflowing stream, Mahogany Creek, and the persistence of cutthroat trout populations. Understanding which physical habitat characteristics impact the movement of stream-resident LCT during the low flow hydrological bottleneck period is critical for guiding management and stream restoration decisions.

Here, we examined the movement patterns of stream-resident LCT through weekly radio telemetry tracking during the early summer streamflow recession to baseflow period in the fall. We quantified the physical habitat, water quality, in-stream hydraulic properties, and invertebrate food conditions contributing to the movement of trout within the stream. We address the following questions regarding stream-resident LCT movement patterns: 1) How much do stream-resident LCT move during declining flow and low flow periods? 2) What factors impact stream-resident LCT movement? and 3) What factors (i.e., physical habitat, water quality, invertebrate drift characteristics, in-stream hydraulics, fish size) affect stream-resident LCT habitat selection?

Study site

The Mahogany Creek watershed is a 34.5 km² tributary and the primary input to Summit Lake (41.54423° N, 119.03127° W), a terminal lake in Humboldt County, Nevada, USA. It is located on the Summit Lake Paiute Reservation and Bureau of Land Management Black Rock Desert Wilderness (Figure 1). Two tributaries enter Mahogany Creek: Summer Camp Creek and Pole Creek. Because Pole Creek dries in the summer, it was excluded from this study. The remaining watershed provides 19.7 km of stream habitat and is characterized by silt, sand, gravel, and cobble substrates with stream gradients ranging from <2% to 6%. Mahogany Creek is fed primarily by snowmelt and reaches its maximum and minimum flows in May (mean discharge = 3.0 m³/s) and September (mean discharge = 0.5 m³/s), respectively (site ID: 10353750, USGS 2016). The riparian vegetation is dominated by willow (Salix spp.) and invasive reed canary grass (Phalaris arundinacea) in the downstream reaches of the watershed, and dense aspen (Populus tremuloides) stands with intermittent willows in the upstream reaches. Although the downstream reaches have been degraded by historic livestock grazing and invasive plants, much of the upper watershed remains relatively undisturbed.

The fish community of Mahogany Creek consists of three species: Lahontan cutthroat trout, Lahontan redside (Richardsonius egregius), and speckled dace (Rhinichthys osculus). The redsides and dace are confined to the lower watershed and have never been observed in the upper watershed (Summit Lake Paiute Tribe, pers. comm.). The watershed supports adfluvial and stream-resident populations of LCT. Campbell et al. (2018) reported the highest density of stream-resident LCT beyond river kilometer 4 from the confluence with the lake and fewer adfluvial LCT. Campbell et al. (2018) reported that the proportions of stream-resident LCT in the upstream reaches of the watershed were as high as 55% compared to 38% in the lower watershed. Additionally, a higher rate of outmigration from adfluvial juvenile LCT was observed in the lower watershed during this study. As a result, we restricted our sampling locations to the upper Mahogany Creek watershed to avoid potential tag loss to outmigrating adfluvial LCT. By restricting our sampling sites to above river kilometer four (the delineation for the upper watershed for this study), we were able to better focus our results and final data interpretations on stream-resident LCT movement.

Stream Habitat Survey

Throughout the watershed, twenty stream reaches (~100 m) were surveyed once each to measure and categorize channel geomorphology, vegetation, in-stream cover, substrate composition, streamflow, and water quality (USFS 2016). Within each survey reach, a new habitat unit was recorded at the delineation of a new stream channel unit (i.e., pool, riffle, run, etc.), where habitat data was collected. Additional data was recorded at locations of fish found outside of the initial habitat survey reaches. The habitat survey data at each habitat unit was simplified to encompass the following factors relating to LCT habitat: (1) large woody debris count, (2) shade availability, (3) undercut bank habitat, and (4) dominant substrate composition (see Table 1 with terms, variables, definitions, and units).

Fish Sampling, Tagging, & Tracking

LCT were captured and tagged in June 2021 and from May through August 2022. All individuals were captured and sampled under the Institutional Animal Care and Use Committee Protocol ID 21-02-1134. Fish were collected in both sample years using a combination of hook-and-line sampling and electrofishing with Smith-Root LR-20B backpack electrofishers. The captured LCT were anesthetized with CO₂-saturated water, and 0.3 g Lotek NTF 2–1 tags were inserted through the pelvic girdle with the tag antenna perforating the ventral wall (Thorstad et al. 2013). Tags were activated at 8 AM and operated on a 12-hour duty cycle, allowing us to track fish between 8 AM and 8 PM. All fish were tagged with passive integrated transponder (PIT) tags as well to ensure that any fish migrating out of the Mahogany Creek watershed would be recorded by PIT antennas in the lower watershed. To ensure that the tag weight did not exceed 2% of LCT body weight (Table 1) and thereby impact swimming ability, only fish with body weights exceeding 15 grams were tagged (Winter 1996). Fish were then held in flow-through buckets in the river until their swimming ability was regained.

Fish movement was assessed with a Lotek SRX1200 telemetry receiver and a three-element Yagi handheld wildlife antenna. Preliminary testing confirmed that this equipment was capable of identifying tagged individuals at distances of up to 800 meters. Individuals were tracked until the tag batteries expired or until the location of the tagged individuals became inaccessible due to snowfall. In 2021 and 2022, the final tracking events occurred on September 18th and November 11th, respectively. The locations of the tagged LCT were recorded using a Garmin eTrex 10 GPS unit with 3-meter accuracy. Since the average habitat unit length in the upper watershed was 11.6 m, fish that had moved more than 3 m or moved into a new habitat unit were assigned new GPS coordinates to account for movement within individual habitat units. This allowed us to reduce false movement values from GPS inaccuracies while still recording short movements within larger habitat units. Location coordinates were converted to river meter values (Table 1) using QGIS software (QGIS Development Team 2024). The distance traveled was normalized for days between sampling periods, as it varied from 6 to 28 days during this study. Tagged individuals not displaying movement were verified as alive through bankside observation and/or underwater GoPro video placed in the stream. No in-stream mortalities were observed. However, during the first year of the study, 16% of the tags transmitted a radio signal for less time than anticipated.

In-stream hydraulic measurements, water quality, and precipitation

Seven pool habitat units were selected throughout the watershed in areas adjacent to the highest densities of tagged LCT and were used in both the 2021 and 2022 sampling periods. These sites were representative of the average geomorphologic conditions present within their respective stream reaches and were composed of step-pool and riffle-pool habitats. Within each site, high-frequency samples (15-minute intervals) of dissolved oxygen (DO) and water temperature (Table 1) were collected via PME miniDOT Clear loggers. The loggers were cleaned weekly to avoid biofouling. At the end of each year, the loggers were removed and calibrated using cycles of oxygen saturation through water aeration and zero-point calibration through light deprivation and dissolved yeast.

During each sampling event, the pool maximum depth and riffle crest thalweg (RCT) depth (Table 1) were recorded to monitor habitat depth. Since there are no active discharge measurements recorded in the watershed, changes in stage were used to document changes in streamflow and pool hydraulics. Onset HOBO MX2001 water level loggers were installed at the four sites farthest upstream; however, they were only installed at the start of the 2022 sampling period. Measurements were collected every 15 minutes throughout the 2022 sampling period. Stage logger measurements were verified and calibrated using HOBOware software paired with physical stage measurements. These measurements were used to determine the streamflow recession and baseflow periods. To determine this for the 2021 sampling year, stage measurements were obtained from a HOBO RX3000 monitoring station in the lower watershed (river km 0.8). Precipitation data was collected from a SNOTEL site (Site ID: 1194) at the headwaters of the Snow Creek watershed located approximately 2 km south of the most upstream hydrologic site, which has been operational since 2012.

Invertebrate collection

Macroinvertebrate drift samples were collected in July, August, and September 2021 and August, September, and November 2022 at each site to assess LCT foraging potential. Samples were collected from riffles immediately upstream of pools with a 250 µm Nitex drift net and preserved in 90% ethanol. Stream velocity measurements were taken immediately upstream of the net opening with a SonTek FlowTracker 2. The samples were sorted and identified as aquatic or terrestrial (Table 1) and dried in foil packets for 24 hours at 70 °C. Samples were weighed, and the drift concentrations were calculated using the following equation:

Drift concentration = invertebrate dry weight / (drift net area * time of net inundation * streamflow velocity entering net)

Data analysis

The LCT locations determined using telemetry were matched with habitat, in-stream hydraulic, water quality, and invertebrate drift data from the nearest habitat unit using simple distance formula matching. Resulting fish locations were classified as “site abandoned” or “site occupied” if their observed location had a different associated habitat unit (nearest habitat unit) from the previous sampling event or they remained in the same habitat unit, respectively. Analysis of habitat characteristics impacting LCT habitat selection was performed by separating the dataset into “site abandoned” and “site occupied” and performing a two-sample permutation test on all predictor variables for each group. To investigate the associations between LCT movement and streamflow variation, multiple linear regression models were developed to test for correlations between in-stream hydraulic measurements, water quality, and changes in drifting macroinvertebrate concentrations and LCT movement within the watershed.

Trout Movement

For the 2021 sampling period, 29 LCT were tagged (fork length mean±se=167±7 mm) and tracked from June 28^th through September 18^th. In 2022, 27 LCT were tagged (fork length mean±se=153±5 mm) and tracked from June 1^st through November 16^th. In 2021 and 2022, the lowest stage measurements occurred on August 17^th and September 9^th, respectively; these dates were used to identify absolute baseflow conditions—the lowest annual stream stage measurement. Across both sampling years, the mean±se daily distance traveled for all LCT movement observations during streamflow recession to absolute baseflow conditions was 4.8±1.2 m/day (median= 0.3 m/day, range=0–95.5 m/day, n=163; Figure 2a). Across both sampling years, the mean±se home range (the difference between most upstream and downstream recorded locations) for each individual during streamflow recession and absolute baseflow conditions was 61.0±26.0 m (median=10.2 m, range=0–938.3 m, n=56; Figure 2b). From August 12^th to September 5^th, 2021, and September 7^th to October 3^rd, 2022, the lowest range streamflow conditions were observed in the watershed, and this range was used to assess baseflow movements. During this period, movements were even further reduced with a mean±se daily distance traveled for all LCT movement of 2.3±1.3m/day and a mean±se home range of 37.5±13.1 m (median=0 m/day, range=0–58.7 m/day, n=46 & median=2.6 m, range=0–293.3 m, n=46, respectively).

During this study, there were 12 fish with two or fewer detections, with two detections being the minimum number of location positions used to calculate movement data. Fish displaying abnormal behavior due to injury or mortality from the capture and tagging process would have been excluded from our analysis. However, four fish from this study that had stopped emitting a signal prior to their anticipated end of operation date were observed by tribal staff at a later date. Additionally, two fish were recorded with underwater video with a visible antenna protruding from their body cavity with no signal emitted during a period where a radio signal was expected to be actively transmitted. These observations suggest a batch of faulty tags which was later corroborated with the tag manufacturing company. With these field observations as well as no outlier data originating from these 12 individuals, we elected not to exclude their data from the analysis. Among these 12 individuals, there were four LCT with single detections that did not yield any movement data and thus were excluded from our analysis. The eight remaining tags from this pool with only two detections were included in our analysis. To reduce the effects of the actions of individual fish on our model, their unique tag identification was included as a covariate.

The global model predicts that a higher rate of movement is correlated with a decline in RCT depth (ΔRCT) and stream temperature (WaterTemp) across both years, with the sample year, site, and individual LCT included as covariates (r²=0.32, p<0.001; Table 2, Figure 3). The top-performing models predict log transformed movement values demonstrating that LCT movement increases exponentially as the RCT depth decreases (Figure 4a). In addition to changes in RCT depth, we observed declines in significant movement (>1 m/day) below RCT depths of 5 cm, and there were no observed movements below 4 cm (Figure 4b). Notably, despite stream temperature being a significant variable in the top-performing model, the change in RCT depth was the primary predictor variable of significance, as it appeared in all the top-performing models with a greater coefficient magnitude. Additionally, these two metrics were found to be inversely proportional to one another using a Pearson correlation test (r(190)=-0.38, p<0.001).

While our anticipated operation time for our radio tags was approximately four months, two of these fish were later recaptured and identified by their PIT tag ID through tribal electrofishing efforts. One radio tagged individual from the 2022 sampling period was recaptured in August 2023 during tribal electrofishing sampling, where the fish remained in the exact same pool it was originally captured in and remained in throughout the entire 2022 sampling period. Another tag from the 2021 sampling period was recaptured at the beginning of the 2022 sampling period and was also found in the exact same pool where it was captured the year prior. These LCT were 160 mm and 46.0 g and 199 mm and 87.6 g, respectively.

The cumulative annual precipitation collected from the SNOTEL site measured 455 and 582 mm during the 2021 and 2022 water years, respectively. Since 2012, the watershed has received a mean cumulative precipitation of 522 mm (range: 384–721 mm), placing the 2021 and 2022 values in the 36th and 81st percentiles of cumulative precipitation years, respectively. As a result, in 2021, the mean±se minimum RCT value across all sites was 3.97±0.74 cm (n=7), and in 2022, the mean±se minimum RCT value across all sites was 4.97±1.13 cm (n=7). In 2021, the mean±se daily distance traveled for all LCT movement observations during streamflow recession to absolute baseflow conditions was 7.2±2.0 m/day (median= 0.5 m/day, range=0–95.5 m/day, n=92). While 2022 data showed that the mean±se daily distance traveled for all LCT movement observations during streamflow recession to absolute baseflow conditions was 1.6±0.7 m/day (median= 0 m/day, range=0–42.9 m/day, n=71). These two years of data collection suggest that years with less streamflow result in greater movement among the stream-resident LCT population within the creek. However, given the variation in sampling periods and only two years of available data, we chose not to assess the relationship between precipitation and LCT movement.

Habitat selection

Across both sampling years, fish size was the strongest predictor of whether a chosen habitat unit remained occupied or was abandoned by an individual (Table 3). Fish that retained their previously occupied habitat had a mean±se fork length and weight of 163±2mm (range=116–241mm, n=229) and 53.9±2.2g (range=16–153g, n=229), respectively. Those who abandoned their previously occupied habitat had a mean fork length and weight of 152±3 mm (range=127–197, n=46) and 39.8±2.5 g (range=23–81 g, n=46), respectively.

Hydraulic conditions were a poor predictor of LCT habitat selection across all observable metrics (Table 3). Although changes in RCT depths were among the main predictors of distance traveled in LCT throughout the watershed, this did not impact their habitat selection. The mean value±se of longitudinal stream position within the watershed was 10020±330 m (n=46) for LCT that abandoned their habitat and 9460±160 m (n=229) for those that abandoned their habitat. Although longitudinal stream position was one of our strongest predictors across all hydraulic metrics, the relationship was not statistically significant (p = 0.15; Table 3).

Neither dissolved oxygen nor stream temperature content was a significant predictor of habitat retention or abandonment. Dissolved oxygen concentrations across all sites never crossed minimum thresholds that would create lethal stream conditions for cutthroat trout (Doudoroff & Shumway 1970) (Figure 5a). Similarly, stream temperatures across all sites never exceeded any accepted threshold (range: 0.02–17.6 °C) that would impact bioenergetics or create lethal stream conditions for LCT (Dickerson & Vinyard 1999) (Figure 5b). While the literature body on dissolved oxygen concentration thresholds for LCT specifically is limited, Null et al. (2017) identified 5–6 mg/L as a management threshold to maintain ideal conditions for LCT which is comparable to values reported for other stream-rearing salmonids (Davis 1975). This threshold was not reached or crossed at any of our sites throughout this study.

None of the physical habitat variables collected were significant predictors of stream-resident LCT habitat selection. The percentage of shaded habitat was the strongest predictor of LCT habitat selection among all the measured physical habitat variables (p=0.09, Table 3). However, the differences between the mean values for sites abandoned and occupied by LCT were minimal. The mean percentage of habitat shaded in occupied habitats was 81%, and the mean value for abandoned habitats was 85%. Across all habitat survey sites within the range where tagged fish were found, the mean ± se shade percentage was 82 ± 0.8%, the mean ± se large woody debris count was 1.1 ± 0.1, and the mean ± se undercut bank length was 3.9 ± 0.2 m (n = 275). Among the 489 sites surveyed in this subset of habitat data, sand was the dominant substrate at 408 of the sites within the survey reaches. Cobble and gravel were the dominant substrates at 46 and 28 sites, respectively. The homogeneous nature of the habitat used by LCT in this study did not allow us to confirm or reject our hypothesis that LCT would be less likely to abandon dense riparian habitat.

Terrestrial and aquatic invertebrate concentrations had no significant effect on LCT habitat selection (Table 3), which deviated from our hypothesis that LCT would select habitat based on changes in invertebrate concentrations. Sites that were abandoned by LCT and sites that were occupied by LCT had mean±se aquatic invertebrate densities of 6.4x10^-3±1.1x10^-3 mg/m³ and 6.7x10^-3±5.5x10^-4 mg/m³, respectively. Fish that abandoned their habitat and fish that remained in their habitat had mean±se terrestrial invertebrate densities of 3.6x10^-3 mg/m³ and 3.2x10^-3 mg/m³, respectively. Invertebrate drift samples were collected at irregular intervals due to unforeseen field conditions (smoke conditions, frozen stream sections, etc.); however, aquatic invertebrate densities were highly variable for both years (mean±se = 1.2x10^-2± 6.1x10^-3mg/m³, n = 28). There was less variability observed among the terrestrial samples (mean±se = 5.7 x10^-3 ± 1.6x10^-3mg/m³, n = 28).

Freshwater fishery managers are trying to understand the factors that influence the movement and habitat selection of focal species such as endemic trout and answer questions such as, how humans have altered the ability of animals to redistribute between suitable habitats?, and how can they restore the connectivity and movement which allows the persistence of species? Recent advances in telemetry technologies have enabled researchers to track individual fish at high resolution over time and space. Although movement studies have been conducted on stream-resident LCT in larger tributaries (Alexiades et al. 2012), significant uncertainty remains regarding the drivers of LCT movement and habitat selection, particularly in small streams. In Mahogany Creek, stream-resident LCT appear to move even less than previously observed during streamflow recession and summer baseflow periods compared to prior stream-resident LCT movement research from the Truckee River, Nevada, USA (Alexiades et al. 2012). This may be attributed to differences in stream size and available habitat (Taylor & Cooke 2012; Boavida et al. 2017). The movements we observed, however, were strongly associated with declining riffle crest thalweg depths. This metric is used to identify hydraulic conditions that influence fish passage between pools (Rossi et al. 2023, Kastl 2023) and habitat quality within passage thresholds for migration between contiguous habitat pools for salmonids (Rossi et al. 2021a & 2021b). Although the riffle crest depth may not be the causal factor of movement, it likely serves as an indicator of the changing ecological and physical habitat conditions that lotic organisms respond to (Rossi et al. 2021a). Additionally, increasing stream temperatures had a weaker but still significant influence on stream-resident LCT movement. The primary driver for whether LCT would abandon or retain previously occupied habitat was the fork length and weight of the individual. Larger individuals retained their previously occupied habitat more frequently than smaller individuals. This may be due to dominance hierarchies, a social structure in fishes primarily influenced by body size that dictates access to preferable resources (Drews 1993, Newman 1956), which can influence habitat selection and habitat displacement of LCT. This behavior has previously been observed in cutthroat trout in small streams (Heggenes et al. 1991).

A majority of LCT (82%) demonstrate no movement outside of the ~100 m reach they were initially captured in during the summer streamflow recession to absolute baseflow conditions—a period considered to provide the most significant bottleneck for growth (Harvey et al. 2006, Rossi et al. 2022) and survival (Grantham et al. 2012, Obedzinski et al. 2018) in stream-rearing salmonids. LCT experienced reduced movement during the baseflow period, but they were not restricted within singular habitat units. Thirty-nine percent of individuals observed during baseflow conditions abandoned the habitat unit they were found in at the beginning of this period. This can likely be attributed to deeper associated RCT depths with LCT that did move during this period.

Changes in streamflow are documented as an environmental cue for cutthroat trout movement to explore adjacent habitats (Gowan 2007), a behavioral adaptation to seek out more bioenergetically favorable habitats (Gowan & Fausch 2002). Habitat conditions that are bioenergetically unfavorable for salmonids can result in salmonid emigration from previously occupied habitats (Fausch et al. 1997). When streamflow declines beyond a certain threshold, however, fish movement over shallow water habitat (i.e., pool-riffle-pool movement) declines due to increased risk of stranding (Nagrodski et al. 2012) and predation (Power 1984, Lonzarich & Quinn 1995). This suggests three conclusions regarding the stream-resident LCT population in this watershed. First, in situ habitat within this watershed during the water years in our study provides favorable conditions for stream-resident LCT, resulting in substantially less habitat emigration (Penaluna et al. 2021) than that reported in comparable cutthroat trout movement literature (Young 1996, Starcevich 2005, Hilderbrand & Kershner 2000). Second, stream-resident LCT are extremely well adapted to the conditions in the upper watershed. The apparent lack of movement on a large scale leads us to believe that most of the LCT population found conditions optimal for growth and survival within small home ranges, suggesting high levels of local adaptation (Carim et al. 2017, Heggenes et al. 2007). Lastly, this suggests that differences between habitat units and reaches within the watershed demonstrated such little variance in physical habitat that LCT did not attempt to emigrate to adjacent habitats to seek improved conditions. Alternatively, if they relocated to a nearby habitat unit and moved back to their previously recorded location between sampling intervals and were not detected, this would further emphasize the homogeneity of the habitat quality concerning bioenergetic potential.

Previous studies have identified diel movement in stream trout (Hilderbrand & Kershner 2000). Although not incorporated into our experimental design, individuals in more regularly foot-trafficked areas were monitored daily throughout the week. There were no recorded observations of diel movement among tagged individuals. Our tags were transmitting signals on a 12-hour duty cycle. Therefore, obtaining location data for LCT at night was impossible; movements during this period may have occurred; however, this was outside the scope of our study. Nocturnal movement has been observed in cutthroat trout with differing foraging territories between day and night (Hilderbrand & Kershner 2000). Nakano et al. (1999) reported notable spikes in drifting and benthic invertebrate concentrations in nocturnal drift samples; however, this food source may be less accessible due to low-light conditions (Fraser & Metcalf 1997). We suggest further research into the impacts of nocturnal movement and foraging behaviors of cutthroat trout, as these potential foraging opportunities have implications for species fitness (Railsback et al. 2005).

Our study focused primarily on the streamflow recession and baseflow periods; however, the duration of tag operation and incidental LCT recaptures allowed us to also examine LCT movement patterns during streamflow increases and across sampling periods. Only 6.7% of individuals’ movements exceeded 3.8 m/day during the increasing streamflow period, the mean movement value across both sampling years, compared with 15.7% for all recorded movements during the streamflow recession period. These findings are consistent with those of previous cutthroat trout movement studies that reported a significant decline in movement during fall and winter periods (Hilderbrand & Kershner 2000, Brown & Mackay 1995). Additionally, the two individuals recaptured across sampling years using PIT tag identification demonstrate the long-term use of singular habitat reaches and even habitat units. This corroborates findings by Campbell et al. (2018) from Mahogany Creek, our study system, that stream-resident LCT tend to remain within the same habitat reaches.

Two trout demonstrated the significance of movement as a response to declining streamflows despite the overall lack of movement during our sampling period. The stream reach containing these LCT during the 2021 sampling season (Figure 1 Site 4) experienced intermittent streamflow and, ultimately, completely desiccated. This stream reach was impacted by wildfire 20 years prior and, as a result, has incised and has increased sediment supply. Both trout exited this reach before intermittency and found refuge downstream within one week of each other as the streamflow and minimum passage thresholds (RCT depths) abruptly declined. The last recorded RCT depth while these two fish were within this reach was 7.7 cm, which was within seven days of their emigration. This value was not near the minimum RCT depth recorded across all survey reaches; however, it declined the fastest, with complete stream reach desiccation occurring just over one month after their emigration. While this reach was not sampled to quantify the survival of non-tagged individuals, these conditions would have resulted in mortality for nearly all present aquatic biota (Larimore et al. 1959). Had this reduction in streamflow occurred more quickly in the case of more extreme drought conditions, fish could have been trapped in this drying reach with no possibility of escaping through downstream movement.

Across all variables analyzed as potential drivers of LCT movement, change in RCT depth was the strongest predictor across all models (Table 2). Our study did not examine RCT depths at exact fish location sites, but rather at nearby sites with similar channel geometry and bed roughness; however, RCT depths within stream reaches of similar channel geometry and roughness tend to have similar relationships with streamflow (Rossi et al. 2021a). We observed an associated minimum RCT depth of 4 cm across all fish emigration observations (n=46), suggesting a minimum passage threshold for movement between habitat units. Kastl (2023) and Rossi (2023) produced similar findings for stream-rearing juvenile salmonids in coastal streams and reported that movement between habitat units abruptly declined below RCT depths of 3.8 and 4.0 cm, respectively (Kastl 2023, Rossi unpublished). Although stream desiccation was an isolated phenomenon in this study, the response of LCT to changes in minimum passage thresholds highlights the importance of baseflow-oriented management strategies. Water years with decreased baseflow can be expected to increase as climate projections predict increases in precipitation variability in the region. Management strategies for at-risk watersheds should be developed to reduce the risk of salmonid mortality. In the long term, stream reaches experiencing connectivity issues should be monitored to better understand hydraulic and geomorphologic processes and impairments. Rossi et al. (2023) demonstrated that stream reaches with seasonal intermittency can benefit from streamflow restoration efforts. If stream reach assessments indicate that there is significant degradation, habitat restoration efforts should be prioritized to improve streamflow connectivity for stream-resident trout during streamflow recession.

While a majority of movement observations were limited to less than 10 meters per day, we observed larger movements that hinted at transitions between trout life history events. Many of these movements (>10 m/day) occurred at the beginning of our sampling efforts each year, with the bulk of these observations occurring from late June to early July 2021 (Figure 4c). This finding coincides with declining streamflow as we have documented; however, this trend of larger early season movements can also likely be attributed to seasonal life history transitions. Cutthroat trout occupy a much larger home range leading up to and following the spawning period in late spring and early summer. Young (1996) reported an identical trend in Colorado River cutthroat trout (Oncorhynchus clarkii pleuriticus), with weekly movements greater than 200 meters per week in early June declining to less than 20 meters per week in August. This trend was exemplified by one LCT in particular in upper Summer Camp Creek, which was tracked each week from June through September 2021. This fish’s mean daily movement narrowed from 48 m/day to nearly zero during baseflow as it was seeking out suitable habitat for the low flow period. This demonstrates the necessity for deep minimum passage thresholds in the late spring and early summer for fish to ‘test the waters’ of local stream reaches following their spawning period. With changes in flow regimes stemming from increasing precipitation variability and reduced snowpack in cutthroat trout habitats, this has the potential to reduce LCT fitness as it could impact their ability to find suitable summer habitat (Williams et al. 2009).

The only observed factor that impacted LCTs’ decision to continue occupying or abandon a given habitat unit was their fork length and weight due to the homogeneous and relatively undisturbed nature of the habitat in the Mahogany Creek watershed. However, our observations regarding the impacts of fork length and weight contradict our hypothesis. Larger individuals were less likely to abandon their habitat than smaller LCT. This suggests that dominance hierarchies among stream-resident LCTs impact their decision or ability to retain or emigrate from specific habitat units. Nakano (1995) reported that dominant stream salmonids had priority to optimal foraging territories within habitat units. Habitat units can support multiple large individuals in watersheds with greater habitat volume; however, in small streams such as Mahogany Creek (mean±se habitat unit area = 14.7±0.8^m2, n = 489), the ability for multiple stream-resident LCT to share the same habitat unit may be limited (Horan et al. 2000). We hypothesize that the relatively limited habitat availability within individual habitat units paired with dominance hierarchy effects on habitat selection led to smaller, less dominant LCT to be displaced from their previously occupied habitat units. Habitats where fish were displaced were not sampled to identify the presence of potentially more dominant fish. In one instance, however, a 173 mm LCT (fork length: 69th percentile) was displaced from its habitat by an oversummering adfluvial LCT that was approximately 600 mm long and was visible from the stream bank. While competition from oversummering adfluvial LCT is considered uncommon in this watershed, this observation demonstrates how dominance hierarchies drive habitat selection and abandonment in stream-resident LCT.

Movement can be a useful strategy for improving fitness; however, it comes with associated costs and risks. For stream-rearing trout, especially those inhabiting a small, desert stream that hosts an extremely localized food web, movement comes with the risk of emigrating from cover and exposing themselves to predators. This exposure, as we observed in this study, is exacerbated by reduced minimum passage thresholds during low flow periods leading to a tipping point at which movement becomes untenable out of fear of predation or stranding (Brown et al. 1999, Nagrodski et al. 2012). During this period, the benefit of remaining in place outweighs the value of seeking out alternative habitats, especially in streams with high-quality, homogeneous habitat and no distinct areas of greater invertebrate densities.

Given the demonstrated lack of movement, the results from this study support the restricted-movement paradigm (Gerking 1959; Rodriguez 2002). Gowan et al. (1994) outlined issues leading to sampling bias that would artificially inflate the amount of restricted movement observations; however, in this study, we were able to address these issues. We captured high-frequency, high-resolution data and leveraged radio tagging methods paired with passive tagging ‘gateways’ to confirm a lack of emigration from our study reaches. While our study supports the body of literature on cutthroat trout movement patterns, it is important to recognize the literature that has produced alternate findings (Table 4). Alexiades et al. (2012), Brown and Mackay (1995), and Schrank and Rahel (2004) reported that cutthroat trout had maximum home ranges of 3.8 km, 7.6 km, and 82 km, respectively, in their study systems. These studies took place in watersheds with drainage areas ranging from 6300 km² to 17800 km², in contrast to the smaller, 34.5 km² drainage area of the Mahogany Creek watershed. These findings suggest that cutthroat trout movement may scale with basin size. Since cutthroat trout habitats across the western United States vary greatly, understanding the movement requirements of individual populations is imperative for guiding management actions given changing streamflow regimes.

Monitoring the movement of stream-resident trout can be used to identify intrinsic and extrinsic factors that impact movement and habitat selection. Habitat selection and emigration were exclusively affected by LCT fork length and weight, emphasizing the impact of dominance hierarchies on habitat selection. Fish movement during streamflow recession and baseflow conditions was much lower than anticipated. Nevertheless, the most significant movement was observed as more abrupt declines in the minimum passage threshold (RCT depths) occurred. Even after streamflow increased later into the sampling period, LCT movement was largely limited to within individual habitat units and reaches; additionally, two individuals were recaptured a year after tagging that had remained within the same habitat units. This suggests that LCT movement may be a behavioral adaptation exhibited only when necessary (i.e., stream intermittency, increased risk of predation, displacement by dominant individuals) to avoid fitness impacts. We found that baseflow conditions created scenarios that force stream trout to move in areas with severely reduced streamflow. These results highlight the importance of baseflow-oriented management strategies to improve streamflow connectivity for native trout, especially for small, high-elevation streams during streamflow recession. These streams are expected to experience disproportionately high impacts on natural flow regimes and thermal habitat because of future climatic variation and decreased snowpack (Isaak et al. 2010, Wenger et al. 2011). We recommend that watershed managers carry out hydrologic monitoring with an emphasis on RCT depths to identify stream reaches at risk for intermittency. By identifying these reaches, managers can focus restoration efforts toward improving connectivity, which would benefit stream-resident salmonids.

LCT

Lahontan cutthroat trout

RCT

riffle crest thalweg

Ethics approval and consent to participate

The animals in this study were handled and underwent surgical procedures following the guidelines of the Institutional Animal Care and Use Committee (IACUC) outlined by the University of Nevada, Reno and following all state and federal regulations.

Consent for publication

Not applicable.

Availability of data and materials

The datasets generated and/or analyzed during the current study are not publicly available due to data sharing policies set forth by the Summit Lake Paiute Tribal Council but are available from the corresponding author upon reasonable request.

Competing interests

The authors declare that they have no competing interests.

Funding

The following study was supported by contracts from the National Fish and Wildlife Foundation (Contract number: R20AV00010) via the Summit Lake Paiute tribe to S. Chandra, A. Harpold, E. Hanan, and J. Greenberg.

Authors' contributions

KF conducted fieldwork, performed analyses, and was the primary contributor to the writing of the manuscript. SK served as a PI on this project, conducted fieldwork, provided support for the experimental design and statistical analyses, and provided manuscript revisions. GR provided support for the experimental design and manuscript revisions. ZB assisted with the statistical design and manuscript revisions. JS, AC, and ZH assisted with manuscript revisions. SC served as a PI and provided support for the experimental design and manuscript revisions.

Acknowledgements

We thank the Summit Lake Paiute Tribe for allowing us to carry out this project on their land. Members of the University of Nevada, Reno’s Aquatic Ecosystem Analysis Laboratory provided technical support in experimental design and analysis. Field technicians who assisted with this project included Victoria Dugan, Sky Russell, Craig Sande, Craig Wesner, Gary Chang, and Jeff Thompson. To Sky, thank you for your extremely hard work; this paper would have not been possible without your assistance.

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Table 1

Glossary of predictor variables collected throughout the study for habitat, biotic, hydraulic, and water quality metrics. (Placement: between line 226 & 227)
Terms	Definition	Units
Fork Length	Measurement from fish snout to caudal fin fork	mm
Weight	Fish weight	g
River Meters	Longitudinal distance of river from mouth to location of interest	m
Large Woody Debris	Pieces of wood within a habitat unit	count
Shade	Amount of shade cast on habitat unit during survey	%
Undercut Bank	Length of undercut bank in habitat unit	m
Riffle Crest Thalweg Depth	Depth taken where the deepest longitudinal part of the stream channel meets at the riffle crest (Rossi et al. 2021a)	cm
Max Pool Depth	Depth taken at deepest part of hydrologic site pool	cm
Water Temperature	Water temperature measured at hydrologic site by PME MiniDOt sensor	°C
Dissolved Oxygen	Dissolved oxygen measured at hydrologic site by PME MiniDOt sensor	mg/L
Aquatic Invert Concentration	Aquatic macroinvertebrate mass captured in drift net at pool head normalized for duration and net area	mg/m³
Terrestrial Invert Concentration	Terrestrial macroinvertebrate mass captured in drift net at pool head normalized for duration and net area	mg/m³
Substrate	Dominant substrate observed within a habitat unit (Sand, Gravel, Cobble, Boulder, or Bedrock)	categorical
Channel Type	Mid-channel, Scour Pool, Rapid, Riffle, Run, or Cascade	categorical

Table 2

Top model selection of mean daily LCT movement (Movement) using change in riffle crest thalweg depth (ΔRCT), water temperature (WaterTemp), change in water temperature (ΔWaterTemp), change in dissolved oxygen (ΔDO), and site, year, and individual fish ID (Tag ID) included as covariates during the LCT telemetry study from 2021–2022. Model selection was based on the Akaike information criterion corrected for small sample sizes (AICc). All LCT movement values were logarithmically scaled for analysis; values were transformed to avoid log-zero calculations using an increase of 0.0001. (Placement: between line 347 & 348)
Model	AICc	AIC Weight	Model Likelihood	K	P-Value	Adjusted r²	Parameter Estimates
Log(Movement) ~ ΔRCT + WaterTemp + Site + Year + Tag ID	966.61	0.74	1.0	11	1.79e−13	0.32	ΔRCT:−6.1 WaterTemp:−0.2
Log(Movement) ~ ΔRCT + Site + Year + Tag ID	968.73	0.25	0.34	10	3.48e−13	0.31	ΔRCT:−4.9
Log(Movement) ~ ΔRCT + ΔWaterTemp + Year + Tag ID	976.85	0.01	0.01	6	2.79e−12	0.26	ΔRCT:−2.9 ΔWaterTemp: 1.9
Log(Movement) ~ ΔRCT + Year + Tag ID	977.65	0.00	0.01	5	2.14e−12	0.25	ΔRCT:−4.2
Log(Movement) ~ ΔRCT + ΔWaterTemp + ΔDO + year + Tag ID	978.58	0.00	0.00	7	1.04e−11	0.26	ΔRCT:−2.9 ΔWaterTemp: 3.1 ΔDO: 0.2
Log(Movement) ~ ΔRCT + Tag ID	984.77	0.00	0.00	4	3.02e−11	0.22	ΔRCT:−5.7

Table 3

Mean predictor variable values for fish that abandoned their previously selected habitat (n = 46) and fish that remained in their previously occupied habitat (n = 229) ± 1 standard deviation. Significance was calculated using a permutation test over 20,000 iterations. Significant predictor variables (p < 0.05) are bold. (Placement: between line 418 & 419)
Predictor Variables	Site Abandoned Mean Values	Site Occupied Mean Values	P-value
Fork length (mm)	152 ± 23	163 ± 34	0.03
Weight (g)	39.8 ± 17.1	53.9 ± 34.0	0.01
River meters (m)	10020 ± 2219.5	9460.1 ± 2430.3	0.15
Large woody debris count	1 ± 1.8	1.1 ± 1.3	0.73
Shade (%)	85 ± 11.8	81.3 ± 14.1	0.09
Undercut bank (m)	3.9 ± 5	3.9 ± 3.9	0.99
RCT depth (cm)	6.8 ± 1.2	7 ± 1.8	0.37
Max pool depth (cm)	20 ± 3.9	19.9 ± 3.9	0.8
RCT change (cm)	−0.6 ± 1.4	−0.4 ± 1.5	0.3
Max depth change (cm)	−0.5 ± 3.3	−0.4 ± 3.5	0.9
Water temperature (C)	11.5 ± 2.9	11.9 ± 3	0.46
Dissolved oxygen (mg/L)	8 ± 0.9	8 ± 0.7	0.67
Aquatic invert concentration (mg/m³)	0.0064 ± 0.0075	0.0068 ± 0.0079	0.82
Terrestrial invert concentration (mg/m³)	0.0036 ± 0.0049	0.0032 ± 0.0046	0.43
Aquatic invert concentration change (mg/m³)	0.00083 ± 0.0058	0.00014 ± 0.0029	0.3
Terrestrial invert concentration change (mg/m³)	0.00032 ± 0.0022	−0.00048 ± 0.0031	0.17
Sand	0.9 ± 0.3	0.9 ± 0.4	0.51
Gravel	0.1 ± 0.2	0.1 ± 0.3	0.44
Cobble	0 ± 0.2	0 ± 0.2	1
Mid-Channel	0.2 ± 0.4	0.2 ± 0.4	0.85
Scour Pool	0.3 ± 0.4	0.2 ± 0.4	0.57
Rapid	0.1 ± 0.3	0.1 ± 0.3	0.65
Riffle	0.3 ± 0.5	0.3 ± 0.4	0.37
Run	0 ± 0.2	0.1 ± 0.3	0.28
Cascade	0 ± 0	0 ± 0.1	1
Plunge Pool	0 ± 0.1	0 ± 0.2	0.7

Table 4

Literature review of cutthroat trout (Oncorhynchus clarkii) movement and their primary drivers. (Placement: between line 600 & 601)
Taxa	Location	Time of Year	Movement Summary	Factors Impacting Movement	Citation
O.clarkii henshawii	Truckee River, NV, USA	Annual	853 m mean movement; 24% <100 m	Stream reach, season (varied by reach)	Alexiades et al. 2012
O. clarkii	Ram River, Alberta, Canada	Aug - Nov & Oct - Dec	Range: 0−7.6 km; most didn’t move	Decline in streamflow & ice presence	Brown & Mackay 1995
O. clarkii lewisi	John Day River, OR, USA	Annual	Home ranges 104 m & 112 m (summer)	Fork length (positive)	Starcevich 2005
O. clarkii	Musqueam Cutthroat Creek, Vancouver, Canada	Jan - Aug	18% moved > 50 m; 48% moved < 3 m	Fork length (positive)	Heggenes et al. 1991
O.clarkii pleuriticus	North Fork Little Snake River, WY, USA	May - Aug	Median home range: 332 m; 0−2.4 km	Julian day (negative)	Young 1996
O. clarkii lewisi	Blackfoot River basin, MT, USA	Annual	< 200 m in tributaries, post-spawn < 100 m	Season	Schmetterling 2001
O. clarkii utah	Beaver Creek, UT, USA	Annual	Summer: median 55 m; Fall & Winter: median 0 m;	Season, no environmental variables assessed	Hilderbrand & Kershner 2000
O. clarkii utah	Bear River, WY, USA	July - Aug	Summer < 500 m; post-spawn: 0.5–82 km	Fork length (positive)	Schrank & Rahel 2004
O. clarkii clarkii	Lookout Creek, OR, USA	Annual, movement assessed in summer	Old forest: 26% >2 m; Young forest: 76% >2 m	LWD presence & pool habitat availability	VerWey 2018

No competing interests reported.

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Movement Patterns and Habitat Selection of Lahontan Cutthroat Trout in a Great Basin Stream

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