Bull Trout Redds Research Articles

Fine-Scale Characteristics of Fluvial Bull Trout Redds and Adjacent Sites in Rapid River, Idaho, 1993–2007

From 1993 to 2007, we used single pass, September surveys to locate and measure fluvial bull trout (Salvelinus confluentus) redds in Rapid River, Idaho. Here we describe substrate sizes, redd dimensions, and water depths, velocities, and temperatures within and adjacent to 337 redds. Most (79%) spawning sites had fewer than 20% surface fines (< 2 mm) and mean, annual water depths and velocities ranged from 14.2–23.0 cm and 11.6–30.5 cm s-1, respectively. Mean, annual completed redd total lengths and surface areas averaged from 1.03–1.47 m and 0.37–1.07 m-2, respectively. Pea gravel (2 to < 8 mm) and gravel (8–64 mm) were dominant substrates (> 60%) in redds. Bull trout altered channel water depths and velocities during redd construction; pits averaged 5 cm deeper, leading tailspill edges 1.2 cm shallower, and tailspill crests 6.2 cm shallower than adjacent, undisturbed sites. Conversely, pit velocities averaged 2.1 cm s-1 slower, tailspill edge velocities 2.3 cm s-1 faster, and tailspill crest velocities 10.1 cm s-1 faster than adjacent sites. Mean, annual pit water temperatures ranged from 4.5 to 7.7 °C. Water depths and water velocities over undisturbed sites adjacent to bull trout redds were significantly correlated with water depths and water velocities inside completed redds. Improving our understanding of fluvial bull trout redds will increase the accuracy of redd counts, especially in streams with sympatric, fall spawning salmonids. Data describing fine-scale characteristics of redds and adjacent sites will assist efforts to conserve and restore critical bull trout spawning habitats.

Recruitment of Young Bull Trout into the Metolius River and Lake Billy Chinook, Oregon

AbstractThe abundance of Bull Trout Salvelinus confluentus in the Metolius River and Lake Billy Chinook, a hydroelectric reservoir, increased dramatically from 1998 to 2004, following implementation of restrictive fishery regulations. However, both adult Bull Trout and kokanee Oncorhynchus nerka (prey) populations have declined in more recent years. We investigated the relationships between Bull Trout spawner densities, emerging fry densities, and numbers of juveniles migrating into the reservoir to determine the potential role of juvenile production in this decline. We hypothesized that age‐1 and older juvenile production is now limited by natal habitat capacity and that excess fry production is lost to density‐dependent mortality. Bull trout redd densities in surveyed spawning reaches ranged from 14 to 39 redds/km during recent years. The estimated abundance of emerging Bull Trout fry in monitored spawning reaches has varied from about 1.0–2.5 million annually since 2005, while mean estimated densities of newly emerged fry varied from over 35/m2 in Roaring Creek during 2005 to less than 1/m2 in Jefferson Creek during 2009. Fry capture numbers from high versus low escapement broods at the Metolius River downstream trap varied by a factor of four, but capture of age‐1 and age‐2 juveniles did not differ substantially. These findings indicate natal Bull Trout habitats are seeded at spawner densities of 14 redds/km and higher. Fry in excess of natal habitat capacity are probably lost to density‐dependent mortality factors. We estimate natural recruitment of age‐1 and older Bull Trout into Lake Billy Chinook of at least 9 individuals/ha annually since 2001. Recent population bottlenecks limiting adult Bull Trout abundance occurred after juvenile recruits entered the lake and are probably related to prey availability.Received February 23, 2015; accepted July 8, 2015

Multiscale hydrogeomorphic influences on bull trout (Salvelinus confluentus) spawning habitat

We investigated multiscale hydrogeomorphic influences on the distribution and abundance of bull trout (Salvelinus confluentus) spawning in snowmelt-dominated streams of the upper Flathead River basin, northwestern Montana. Within our study reaches, bull trout tended to spawn in the finest available gravel substrates. Analysis of the mobility of these substrates, based on one-dimensional hydraulic modeling and calculation of dimensionless shear stresses, indicated that bed materials in spawning reaches would be mobilized at moderate (i.e., 2-year recurrence interval) high-flow conditions, although the asynchronous timing of the fall–winter egg incubation period and typical late spring – early summer snowmelt high flows in our study area may limit susceptibility to redd scour under current hydrologic regimes. Redd occurrence also tended to be associated with concave-up bedforms (pool tailouts) with downwelling intragravel flows. Streambed temperatures tracked stream water diurnal temperature cycles to a depth of at least 25 cm, averaging 6.1–8.1 °C in different study reaches during the spawning period. Ground water provided thermal moderation of stream water for several high-density spawning reaches. Bull trout redds were more frequent in unconfined alluvial valley reaches (8.5 versus 5.0 redds·km−1 in confined valley reaches), which were strongly influenced by hyporheic and groundwater – stream water exchange. A considerable proportion of redds were patchily distributed in confined valley reaches, however, emphasizing the influence of local physical conditions in supporting bull trout spawning habitat. Moreover, narrowing or “bounding” of these alluvial valley segments did not appear to be important. Our results suggest that geomorphic, thermal, and hydrological factors influence bull trout spawning occurrence at multiple spatial scales.

Impact of Reservoir Elevation during the Spawning Season on the Distribution of Bull Trout Redds

AbstractAdfluvial Bull Trout Salvelinus confluentus reside in Chester Morse Lake (CML), a major reservoir in the City of Seattle water supply system on the central west slope of the Cascade mountain range, Washington. Annual surveys over a 12‐year period (2000–2011) were used to investigate the spatial distribution of adfluvial Bull Trout redds in river systems of CML relative to reservoir elevation. Because reservoir elevations were lower in the spawning season than during the incubation period, some redds placed in river reaches near the reservoir were vulnerable to cumulative effects from impending inundation as reservoir levels increased and as fine sediments settled across gravel substrates. Higher average elevation of CML during the peak period of spawning caused significantly lower percentages of Bull Trout redds in lowermost river reaches, where fluctuating water levels could adversely affect incubating eggs (R 2 = 0.73). Annually, the proportion of redds distributed within habitat reaches where inundation, as indicated by mean CML elevation during the incubation period, could affect them, varied from 1% to 18%. However, a greater proportion of annual Bull Trout redds (10–68%) fell within the maximum reservoir elevation during the incubation period. These results suggest that maintaining moderate reservoir elevations during the spawning season and holding the reservoir near this elevation through the incubation period can lower the effect on incubating Bull Trout redds. The implications for water level management to benefit adfluvial Bull Trout derived from this data set could also be applicable in similar systems, especially where knowledge of temporal and spatial distribution of salmonid spawning is available.Received November 13, 2012; accepted June 17, 2013

An Evaluation of Redd Counts as a Measure of Bull Trout Population Size and Trend

AbstractThe use of redd counts to monitor abundance and trend of bull trout Salvelinus confluentus has been clouded by uncertainties concerning measurement error, life history variation, and correspondence of redd counts to adult population size. We compared census redd counts with population estimates of mature females for a migratory fluvial population of bull trout (primarily ≥ 300 mm fork length) and for a population of small (&lt;200 mm), likely resident, bull trout. We also compared the measurement error of the experienced surveyors who conducted the redd counts to that of a group of inexperienced surveyors. Although the regression of redd counts on adult females for the migratory population was statistically significant, a large proportion of the variation in the relationship was unexplained (r 2 = 0.47). Despite that variation, redd counts accurately reflected a greater than 50% decline in the population over 10 years; however, 5‐year trends in redd counts could be misleading. Power analysis parameterized by using the variation in the number of females per redd and measurement error of experienced surveyors indicated that minimum declines of 44–56% or increases of 78–118% over 10–15 years would be necessary for detection using traditional statistical criteria. Geometric mean abundance of migratory adults derived from redd counts and adults‐per‐redd values from the present study and published averages were similar to measured adult numbers in most cases. For both migratory and resident populations, redd counts by experienced surveyors were substantially more accurate and precise than those by inexperienced surveyors. Counts of migratory bull trout redds were more accurate and precise than counts of resident bull trout redds, which were significantly smaller and consistently underestimated. Thus, bull trout redd counts can be used to estimate abundance levels and to detect substantial longer‐term changes in abundance, particularly for migratory populations. However, the reliability of the counts depends on the skill of the surveyors.Received April 4, 2011; accepted August 8, 2011

Cattle Trampling of Simulated Bull Trout Redds

AbstractListing of the bull trout Salvelinus confluentus under the Endangered Species Act and concerns over livestock stepping on bull trout redds have led many U.S. Department of Agriculture Forest Service managers to remove livestock from bull trout spawning areas once spawning begins. While this policy has extensive ramifications for livestock producers who graze their cattle on national forests, a lack of data precludes evaluation of the benefit of livestock removal to bull trout populations. To fill this information gap, we assessed the probability that cattle would step on simulated bull trout redds in grazing allotments within the Little Lost River drainage in Idaho. Bull trout redds were simulated using clusters of clay shooting targets. Simulated redds were placed in streams within three grazing pastures where bull trout have historically spawned or currently spawn. During the 14–21‐d grazing period, 15–83% of the simulated redds were affected by trampling. When the control period was standardized to the same time period as the treatment, cattle were found to be responsible for affecting 12–78% of simulated redds and breaking 6–49% of the clay targets. Impacts were higher in pastures where cattle stocking intensity was higher, but impacts were also determined by site conditions adjacent to the simulated redds.

Observer Error Structure in Bull Trout Redd Counts in Montana Streams: Implications for Inference on True Redd Numbers

AbstractDespite the widespread use of redd counts to monitor trends in salmonid populations, few studies have evaluated the uncertainties in observed counts. We assessed the variability in redd counts for migratory bull trout Salvelinus confluentus among experienced observers in Lion and Goat creeks, which are tributaries to the Swan River, Montana. We documented substantially lower observer variability in bull trout redd counts than did previous studies. Observer counts ranged from 78% to 107% of our best estimates of true redd numbers in Lion Creek and from 90% to 130% of our best estimates in Goat Creek. Observers made both errors of omission and errors of false identification, and we modeled this combination by use of a binomial probability of detection and a Poisson count distribution of false identifications. Redd detection probabilities were high (mean = 83%) and exhibited no significant variation among observers (SD = 8%). We applied this error structure to annual redd counts in the Swan River basin (1982–2004) to correct for observer error and thus derived more accurate estimates of redd numbers and associated confidence intervals. Our results indicate that bias in redd counts can be reduced if experienced observers are used to conduct annual redd counts. Future studies should assess both sources of observer error to increase the validity of using redd counts for inferring true redd numbers in different basins. This information will help fisheries biologists to more precisely monitor population trends, identify recovery and extinction thresholds for conservation and recovery programs, ascertain and predict how management actions influence distribution and abundance, and examine effects of recovery and restoration activities.

SAMPLING VARIABILITY AND ESTIMATES OF DENSITY DEPENDENCE: A COMPOSITE-LIKELIHOOD APPROACH

It is well known that sampling variability, if not properly taken into account, affects various ecologically important analyses. Statistical inference for stochastic population dynamics models is difficult when, in addition to the process error, there is also sampling error. The standard maximum-likelihood approach suffers from large computational burden. In this paper, I discuss an application of the composite-likelihood method for estimation of the parameters of the Gompertz model in the presence of sampling variability. The main advantage of the method of composite likelihood is that it reduces the computational burden substantially with little loss of statistical efficiency. Missing observations are a common problem with many ecological time series. The method of composite likelihood can accommodate missing observations in a straightforward fashion. Environmental conditions also affect the parameters of stochastic population dynamics models. This method is shown to handle such nonstationary population dynamics processes as well. Many ecological time series are short, and statistical inferences based on such short time series tend to be less precise. However, spatial replications of short time series provide an opportunity to increase the effective sample size. Application of likelihood-based methods for spatial time-series data for population dynamics models is computationally prohibitive. The method of composite likelihood is shown to have significantly less computational burden, making it possible to analyze large spatial time-series data. After discussing the methodology in general terms, I illustrate its use by analyzing a time series of counts of American Redstart (Setophaga ruticilla) from the Breeding Bird Survey data, San Joaquin kit fox (Vulpes macrotis mutica) population abundance data, and spatial time series of Bull trout (Salvelinus confluentus) redds count data.

Geomorphology, hyporheic exchange, and selection of spawning habitat by bull trout (Salvelinus confluentus)

The distribution and abundance of bull trout (Salvelinus confluentus) spawning were affected by geomorphology and hyporheic groundwater - stream water exchange across multiple spatial scales in streams of the Swan River basin, northwestern Montana. Among spawning tributary streams, the abundance of bull trout redds increased with increased area of alluvial valley segments that were longitudinally confined by geomorphic knickpoints. Among all valley segment types, bull trout redds were primarily found in these bounded alluvial valley segments, which possessed complex patterns of hyporheic exchange and extensive upwelling zones. Bull trout used stream reaches for spawning that were strongly influenced by upwelling. However, within these selected reaches, bull trout redds were primarily located in transitional bedforms that possessed strong localized downwelling and high intragravel flow rates. The changing relationship of spawning habitat selection, in which bull trout selected upwelling zones at one spatial scale and downwelling zones at another spatial scale, emphasizes the importance of considering multiple spatial scales within a hierarchical geomorphic context when considering the ecology of this species or plans for bull trout conservation and restoration.

Geomorphology, Logging Roads, and the Distribution of Bull Trout Spawning in a Forested River Basin: Implications for Management and Conservation

The Swan Basin in Montana is considered a stronghold of regional significance for the bull trout Salvelinus confluentus, a native char whose populations are fragmented and declining throughout its range. We used correlation analysis to examine spatial and temporal variation of bull trout redd count data (1982–1995) relative to geomorphic and land-use factors among nine principal spawning tributaries of the Swan River. Bull trout redd numbers were positively correlated with the extent of alluvial valley segments bounded by knickpoints and negatively correlated with the density of logging roads in spawning tributary catchments. The density of logging roads in spawning tributary catchments was not significantly correlated with geomorphic factors. Temporal trends among the principal spawning streams were variable. In four of the nine principal spawning streams, redd numbers increased significantly during the survey period, and in the remaining streams, redd numbers showed no significant change. Changes in redd numbers with time were negatively correlated with catchment road density and positively correlated with the extent of bounded alluvial valley segments. The significance of bounded alluvial valley segments to bull trout spawning habitat may be related to groundwater–surface water exchange occurring within these segments. Our results emphasize the importance of valley geomorphology to bull trout, and our results suggest that prior land use may have adversely affected bull trout populations in the Swan Basin. Protection of critical spawning tributary catchments from additional road building and associated land-use disturbance will likely be necessary for the maintenance of viable bull trout populations in the Swan Basin. Geomorphic context and land-use status of spawning tributaries are important considerations for future monitoring and management of this species.

A Power Analysis on the Monitoring of Bull Trout Stocks Using Redd Counts

Abstract The bull trout Salvelinus confluentus is listed as a federally threatened species in the Columbia and Klamath river drainages. A priori establishment of levels of decline or increase in bull trout redd numbers that will be considered biologically significant and levels of statistical significance that will be used to identify changes in redd numbers is essential to the success of future recovery plans. A prospective statistical power analysis indicates that with standard significance levels and two-tailed testing procedures the yearly variation in redd numbers typifying many stocks of bull trout in Montana limits the power of detecting less than 50% of changes in population size per generation to less than 0.8 during the first 15 years of a monitoring program. The limitations of monitoring bull trout stocks with redd counts, coupled with the critical nature of identifying future population changes, justifies the need to (1) identify and reduce the level of measurement error involved in redd count...

Spatial and Temporal Variability in Bull Trout Redd Counts

Abstract We analyzed redd counts of bull trout Salvelinus confluentus in northern Idaho and northwestern Montana and found evidence of stronger correlation in the number and year-to-year change in number of redds between streams that were closer together than between streams that were far apart. The pattern was weak, however, indicating that spatial heterogeneity in habitat, in population demographics, or in life history at a local scale is important to stability of regional populations. The weak correlations also indicate that monitoring only a few index populations may not clearly represent the dynamics of larger regional populations. If synchrony is weak and not masked by sampling error, conservation management should favor the maintenance of high-quality habitats and strong local populations in proximity to each other to facilitate dispersal and demographic support. Common declining trends among all streams within a single lake basin show that even well-dispersed regional populations face important ri...