Redd Counts Research Articles

Bias and variation in salmonid redd counting using remotely piloted vehicles

AbstractRedd surveys estimate spawning population size for many salmonid species. Studies of field‐based redd counting methods highlight observer bias caused by redd density, observer experience, and environmental factors. Researchers have begun using remotely piloted vehicles (RPVs, drones) to count redds; yet, no studies have quantified bias and variability in counts. This study aimed to quantify the influence of redd density, observer experience, and environmental factors (namely, water clarity) on redd counting bias and variability when using RPVs. We found that technological and procedural improvements from our previous study increased precision and reduced variability among observers (coefficient of variation, сυ = 11% compared to сυ = 42%). Redd density was the leading covariate causing differences between RPV and both “best counts” (p < 0.05) and field counts (p < 0.05). We found a reduction in variability with experience level (no experience сυ = 78%; semi‐experienced сυ = 33%; experienced сυ = 20%), with no directional bias in counting. Our paper is the first to quantify observer bias in RPV‐based redd counts. This study describes RPV methods and can help agencies decide how to use RPVs in redd counting and incorporate RPV methods into long‐term datasets.

Remote sensing of salmonid spawning sites in freshwater ecosystems: The potential of low-cost UAV data.

Salmonids are especially vulnerable during their embryonic development, but monitoring of their spawning grounds is rare and often relies on manual counting of their nests (redds). This method, however, is prone to sampling errors resulting in over- or underestimations of redd counts. Salmonid spawning habitat in shallow water areas can be distinguished by their visible reflection which makes the use of standard unmanned aerial vehicles (UAV) a viable option for their mapping. Here, we aimed to develop a standardised approach to detect salmonid spawning habitat that is easy and low-cost. We used a semi-automated approach by applying supervised classification techniques to UAV derived RGB imagery from two contrasting lakes in Iceland. For both lakes six endmember classes were obtained with high accuracies. Most importantly, producer's and user's accuracy for classifying spawning redds was >90% after applying post-classification improvements for both study areas. What we are proposing here is an entirely new approach for monitoring spawning habitats which will address some the major shortcomings of the widely used redd count method e.g. collecting and analysing large amounts of data cost and time efficiently, limiting observer bias, and allowing for precise quantification over different temporal and spatial scales.

Abundance, cover use, and clustering of brown trout spawning redds during stream habitat rehabilitation

AbstractAbundance, placement beneath cover and clustering of brown trout spawning redds were monitored throughout five spawning seasons (2016–2020) in a 4.8‐km reach of a Minnesota coldwater stream to assess the potential impacts of concurrent stream habitat rehabilitation projects. We anticipated that redd abundance (redds/100 m stream segments) and placement beneath cover (e.g. logs and branches, boulders, overhanging bank vegetation, aquatic macrophytes) would increase and redd clustering (placement within 1.5 m of other redds) would decline as stream sections were rehabilitated. Repeated redd counts were conducted for the entire stream reach during each of the five spawning seasons, and redd cover and clustering were documented. Before‐after comparison of impact (BACI) tests were used to compare redd abundances in control versus rehabilitated stream sections, and before‐after distributional tests compared redd placement beneath cover and redd clustering within rehabilitated sections. In total, 1895 brown trout spawning redds were documented within the study reach during the five spawning seasons combined. Redd abundance increased nearly fivefold throughout the 5‐year study period (from 3.5 to 15.5 redds/100 m), but no significant difference was detected in redd abundances between rehabilitated versus control stream sections. Redd placement beneath cover (63% beneath cover in total) also increased throughout the study, but at a rate nine to 45 times greater in stream sections rehabilitated during the study than in control or previously rehabilitated sections. Redd clustering remained unchanged (5‐year means = 35 to 46% of all redds clustered) in rehabilitated sections, but increased significantly through time (0% to 35%) in the control section. Stream habitat rehabilitation did not increase spawning redd abundance or reduce redd clustering in rehabilitated stream sections, but redd placement beneath cover increased after rehabilitation to levels present in other stream sections.

Annual redd counts across two decades are indicators of changing migratory bull trout population sizes in two proximate Montana creeks

AbstractRedd counts are the most widely used method for bull trout (Salvelinus confluentus Suckley) population monitoring. Redd counts are an assumed indicator of population size (N), particularly for female spawners, but previous studies rarely considered the measurement uncertainty of N. The present study of two proximate, migratory bull trout populations across two decades examined statistical correlations between annual redd counts and concurrent N (predominantly fish ages 3–7) that included measurement uncertainty. Results showed that redd counts explained about one‐third of the concurrent N variation and, in one case, portended N 7 years later. But changing spawning distributions across years, as likewise shown here and as a probable effect of ongoing climate change, can confound the use of fixed index reaches for redd counts. Instead, abundance monitoring based on redd counts should be over a bull trout population's or metapopulation's entire spatial extent and have counts in all potential spawning areas. Such total‐annual redd counts may be the most cost‐effective and precise means of bull trout population monitoring.

Redd superimposition mediates the accuracy, precision, and significance of redd counts for cutthroat trout

Redd counts are commonly applied to estimate spawning population size for salmonids and allow for broad spatial and temporal coverage in monitoring efforts. However, the utility of redd counts may be compromised by observation error, particularly with respect to superimposition, where later arriving spawners construct redds overlapping existing redds. Here, we provide a mechanistic evaluation of the effects of superimposition on the error structure and biological significance of redd count data for Yellowstone cutthroat trout ( Oncorhynchus clarkii bouvieri) spawning within tributaries to the Snake River, Wyoming. We used a Bayesian framework to parse observation error into distinct components and found low detection of redd clusters (i.e., areas of superimposition) was offset by overestimates of the number of redds per cluster, such that observed counts accurately reflected census redd abundance. However, a saturating relationship between redd counts and spawner abundance indicated that counts is best interpreted as effective reproductive effort rather than spawner abundance. Our results provide a mechanistic understanding of redd count data that can be used to assess their application and interpretation for monitoring.

Recovery of a headwater stream population of brown trout after a fish kill in southeastern Minnesota, USA

After a complete fish kill on the 2-km-long, spring-fed headwater section of Garvin Brook, the brown trout (Salmo trutta) population was studied over the next 29 months to examine the relative roles of spawning migration of adults and natural reproduction in the population recovery process. Spawning redd counts during two spawning seasons post-kill, and repeated population surveys in kill and reference zones, were conducted to assess adult and total population changes and shifting age structures. One month after the kill, spawning adult trout were observed in the kill zone. An estimated 100 to 200 adult trout spawned in the kill zone during each of the two spawning seasons after the fish kill, 40% fewer than spawned in the reference reach. Total trout populations were similar between kill and reference zones 12 to 16 months after the kill due to proportionally (80% versus 40 to 60%) larger numbers of young trout spawned within the kill zone, but adult trout remained 4X more abundant within the reference reach than in the kill reach until 25 months after the kill. Age structures still differed between kill and reference zones 29 months after the kill, due to few large adults within the kill reach. Ultimately, natural recovery of the brown trout population in the headwaters of Garvin Brook was accomplished through a combination of spawning migrations of adult fish from downstream unimpacted reaches, reproduction producing large numbers of young fish, and subsequent recruitment of those young fish to the adult age classes.

Wild trout and hydrologic response to restoration of incised streams with improved water use for ranching and fisheries

AbstractIn the face of climate change and dwindling water supplies, this long‐term monitoring study examines the effectiveness of restoring two incised and degraded streams (Crystal Creek and Black Slough) to benefit livestock ranching and wild trout populations in a spring creek and wetland complex, central Idaho, USA. By raising the incised streambeds and connecting to historical floodplains, the restoration aimed to raise groundwater levels to create subsurface saturation conditions to sustain grazing forage and hay production without flood irrigation; diverted flows from the Big Wood River previously used for flood irrigation were instead routed through the restored systems. The restoration used natural channel design methods to reconstruct 5.2 km of Black Slough and 8.4 km of Crystal Creek to emulate a downstream reference reach (Willow Creek); within the relic incised channels, off‐channel oxbow ponds and wetlands were created while filling other segments. Additionally, screened gravels were placed in the reconstructed channels. We monitored before–after restoration changes in channel morphology, groundwater levels, wetlands, instream flows, spawning substrates and redds, fish populations, and invertebrates for the restored creeks and reference reach over 10+ years. Following restoration, average groundwater levels increased by 0.35 m, mean annual flows at Willow Creek (receiving stream) increased from 0.64 to 1.11 m3/s, and average days per year that baseflows were below 0.28 m3/s decreased from 44.5 to 5.4 despite a decrease in average annual precipitation (16.97 to 11.53 cm). Moreover, residual pool depths increased (<0.18 to >0.90 m), spawning substrate sizes increased (0.1 to 19.0 mm), and egg‐to‐fry trout survival rates increased from <20% to >90%. Six years after restoration, fish diversity increased from three to seven species, matching reference conditions. We also found dramatic increases in Brown Trout and Rainbow Trout redd counts (7 to 161 and 17 to 143, respectively) and relative abundance (0.013 to 1.166 trout/m and 0.029 to 0.222 trout/m, respectively). Invertebrate indices of restoration effectiveness were positive for both restored streams but negative for the reference reach. Overall, this study exemplifies how restoration reduced consumptive water use, created sustainable streams with hydrological connections and ponds and wetlands, and improved trout populations.

The relationship between hydroregime and coho salmon (Oncorhynchus kisutch) redd construction in the Smith River, Oregon

Abstract Adult salmonid migration to natal habitats and spawning are affected both by physiological factors and environmental conditions. While research has focused on physiological thresholds that influence the initiation of migration, few studies have investigated the relationship between both hydrological and thermal conditions and salmon spawning throughout the course of the reproductive period. We examined whether Oregon coastal coho salmon (Oncorhynchus kisutch) returning to three tributaries of the Smith River watershed (in the central Oregon Coast Range) responded to the stream hydroregime throughout their spawning period. Generalised linear mixed models were constructed to evaluate relationships between various stream discharge metrics, water temperature and redd count data collected between 2010 and 2016. Across the three sub‐basins we analysed, discharge and temperature metrics were important in explaining redd construction timing. However, the same parameters were not consistently important for every sub‐basin. Water temperature was important in explaining redd construction in the largest sub‐basin, North Sister Creek, and discharge metrics were consistently important in Beaver Creek, the smallest sub‐basin. At the sub‐basin scale, peak redd construction by coho salmon starts after increase in discharge (25th percentile) occurs but before the highest discharge events take place. However, synchrony among sub‐basins was evident in delayed redd construction during 2013 when the autumn rains were delayed. Understanding relationships between the timing of redd construction and characteristics of the hydrological regime are critical to analysis of potential future climate impacts on all life stages of Oregon Coastal coho salmon.

Is Marine Survival for Puget Sound’s Wild Steelhead Really That Bad? A Nisqually River Case Study Evaluating Estimates of Productivity and Survival of Oncorhynchus mykiss

AbstractWe highlight the uncertainty that exists around estimating productivity and survival for one population of threatened steelhead Oncorhynchus mykiss (anadromous Rainbow Trout) with the goal of strengthening our understanding of the well‐documented poor marine survival of Puget Sound steelhead. We evaluated how sensitive estimates of productivity and survival were to the uncertainty associated with smolt trap enumerations, redd survey methodology, estimates of fish per redd, and estimates of smolt production by resident Rainbow Trout in order to clarify causes of poor steelhead survival in this area. We show that from 2004 to 2014, estimates of both freshwater productivity and marine survival were highly sensitive to estimates of fish per redd used to expand redd counts, as well as error around smolt abundance estimates. Regardless, uncertainty associated with these inputs did not explain the low survival and high productivity observed for Nisqually River steelhead relative to other populations. In addition, we identified progeny from anadromous mothers upstream of what was previously considered a barrier, and we also documented that a proportion of steelhead smolts (N = 4/43) originated from resident mothers (Rainbow Trout). While these results indicated an underestimation in the total number of steelhead redds counted each year and an overestimation of steelhead progeny enumerated at smolt traps, they had little impact on estimates of survival and productivity. Results from the current study support previous work reporting poor marine survival for populations of Puget Sound steelhead and highlight the sympatric relationship between resident and anadromous life histories of O. mykiss. Overall, our study supports a management strategy that protects both the anadromous and fluvial forms of O. mykiss, prioritizes habitat improvements that promote freshwater productivity, and increases research focused on identifying causes of poor marine survival.

In a warming river, natural-origin Chinook salmon spawn later but hatchery-origin conspecifics do not

Median timing of reproduction in salmonid populations is generally consistent among years, reflecting long-term patterns of natural selection from characteristics of the local environment. However, altered selection from factors related to climate change or human intervention might shift timing over generations, with implications for the population’s persistence. To study these processes, we modeled median timing of redd (nest) counts as an index of spawning timing by natural-origin Chinook salmon (Oncorhynchus tshawytscha) in the Skagit River system in Washington State, USA. Over the last 2–6 decades, natural-origin salmon have been spawning later by 0.03–0.52 days·year–1, while a naturally spawning group that is influenced by strays from a hatchery has been spawning earlier by 0.19 days·year–1. Trends in the spawning timing of hatchery-origin strays may reflect opposing selection from the hatchery, where egg take for propagation has become earlier by 0.58 days·year–1. As mean August river temperatures have risen over the period of record, hatchery timing trends may be moving in the opposite direction from the plastic or adaptive patterns expressed by natural-origin fish.

Stability in reproductive timing and habitat usage of Chinook salmon across six years of varying environmental conditions and abundance

AbstractThe spatial and temporal distribution of spawning activity by autumn‐run Chinook salmon Oncorhynchus tshawytscha (Walbaum) was examined across multiple years. The study period included two years of extreme drought conditions when water temperatures in the spawning reach of the Stanislaus River were considered sub‐optimal for spawning and egg incubation. Despite varying levels of superimposition, redd counts and densities remained stable and positively associated with river location, indicating that superimposition may be driven by habitat preference rather than the absence of suitable spawning locations. Spawning occurred slightly later during drought years (6–10 days later compared with wetter years). This delay was attributable to deferred migration instead of deferred spawning, and the majority of redds were constructed at water temperatures exceeding the optimal temperature range. As a consequence, estimates of juvenile production during 2014 and 2015 were among the lowest on record. These findings may be related in part to the high hatchery contribution to the population, above‐average temperatures during spawning and incubation, and superimposition rates. Management recommendations include adequate cold‐water storage in the upstream reservoir, refined spawning habitat restoration techniques in the light of superimposition rates and, on a broader system scale, actions that reduce the amount of stray hatchery Chinook salmon.

Wild salmon and the shifting baseline syndrome: application of archival and contemporary redd counts to estimate historical Chinook salmon (Oncorhynchus tshawytscha) production potential in the central Idaho wilderness

The “shifting baseline syndrome” (SBS) is the paradigm whereby recent species abundances and environmental conditions are accepted as reflecting historical conditions. This leads to false impressions of the past, inaccurate baselines, and unrealistic recovery goals. Idaho biologists have counted Chinook salmon redds for >60 consecutive years, generating an invaluable database; however, inaccurate historical baselines compromise the utility of even such high-quality, long-term databases. To develop an accurate baseline, we integrated archival (1951–1964), maximum redd counts with contemporary (1995–2017), continuous counts and spawn timing datasets to estimate historical (1950s–1960s) wild Chinook salmon production potential. Current salmon populations average 3% of 1950s–1960s abundances, which may have been 30% of precommercial fishery (1880s) populations. Notably, the SBS has influenced contemporary managers as reflected in minimum viable abundance, sustainable escapement, and adequate seeding objectives equaling 10.4%, 17.9%, and 20.4%, respectively, of 1950s–1960s potential. Our approach provides a framework for utilizing archival and contemporary data to reconstruct historical baselines and repress SBS. Contrasting contemporary goals with maximum production potential provides new reference points and perspectives for managers to consider.

Identification of Summer‐Run Chinook Salmon Spawning Areas in the Main‐Stem Columbia River Upstream of Wells Dam, Washington

AbstractSummer‐run Chinook Salmon Oncorhynchus tshawytscha historically spawned in the Columbia River as far upstream as British Columbia, Canada, but the upstream extent of main‐stem spawning in the contemporary Columbia River has been documented in the tailrace of Wells Dam, Washington. We utilized radiotelemetry and video monitoring equipment between 2011 and 2013 to track wild summer‐run Chinook Salmon upstream of Wells Dam and document any main‐stem spawning sites. Two small spawning areas totaling 3,389 m2 were identified within 1.76 km downstream of Chief Joseph Dam and were used by spawning salmon in each year of the study. Redd deposition totaled 70, 59, and 134 redds in 2011, 2012, and 2013, respectively. When considered with tributary redd counts upstream of Wells Dam from the Methow and Okanogan rivers, main‐stem spawning represented between 1.6% and 2.9% of the annual redd deposition upstream of Wells Dam within each spawning year. Redds were located in depths between 3.7 and 7 m, with spawning occurring between late October and mid‐November in each year. These novel results are the first to detail main‐stem spawning locations of summer‐run Chinook Salmon upstream of Wells Dam and should inform population monitoring metrics, such as annual escapement and prespawn mortality estimates.

Estimating observer error and steelhead redd abundance using a modified Gaussian area-under-the-curve framework

This study examined how a suite of habitat and environmental variables relate to the ability of a stream surveyor to identify (observer efficiency) and distinguish (observer accuracy) steelhead (Oncorhynchus mykiss) redds from other stream features. Two existing spawning survey protocols that included one or two redd observers were used to develop models to estimate redd observer error. In most cases, steelhead redd abundances using raw redd counts were underestimated. Mean annual rates of observer efficiency ranged from 0.44 to 0.57, and observer accuracy ranged from 0.67 to 0.83. Regardless of the observer error model used, adjusted annual redd abundance estimates were generally unbiased (range 1.6–0.6 redds). A Gaussian area-under-the-curve methodology that incorporates redd count data and observer error rates was used to generate unbiased estimates of steelhead redd abundance in the Wenatchee (170 redds, coefficient of variation (CV) = 44%) and Methow (106 redds, CV = 41%) rivers. Unbiased estimates of redd abundance will help inform new population viability analyses to better prioritize those populations with the greatest conservation need.

Long-term ecological responses of the River Spöl to experimental floods

The river Spöl flows from Livigno reservoir on the Swiss–Italian border in the central Alps. Flow regulation since 1970 has decreased and stabilized the mean annual discharge of 8.6 to 1.0 m3/s (winter: 0.55 m3/s, summer: 2.5 m3/s). Thirty experimental floods were released between 2000 and 2016 to improve the ecology of the river. A 100-m reach ~2.3 km downstream of the dam was used to monitor changes in water physicochemistry, periphyton, transported and benthic organic matter, and macroinvertebrates. Fish redds (native Brown Trout) were counted each autumn along the same stretch (~2.6 km) of river. Most physicochemical variables displayed strong seasonal cycles unrelated to the flow program, but N, P, and water temperature increased significantly over the 18-y study. Seston decreased after each high flow, but was seasonal and increased over the study. Periphyton biomass decreased after each high flow to low levels and increased between high flows. Benthic organic matter decreased 2× over the study but increased between floods. Macroinvertebrate densities decreased over the study. High flows reduced macroinvertebrate densities to <10,000 individuals (ind)/m2, with densities usually increasing to ~10,000 to 20,000 ind/m2 between floods. Floods reduced taxonomic richness with increases between floods. Ordination showed temporal shifts in macroinvertebrate assemblages over the study. Mayflies and stoneflies responded positively and chironomids and gammarids negatively to the floods over the study. Fish redds increased from 58 in 1999 to >200 by 2003. Redd counts decreased to <30 after an accidental sediment spill in 2013 but rebounded to 153 redds in 2016. Macroinvertebrate assemblages recovered within months after the spill. Long-term monitoring was essential for elucidating ecosystem dynamics and evaluating biological responses to pulse disturbances, such as the sediment spill, during the study.

Long-term population dynamics and conservation risk of migratory bull trout in the upper Columbia River basin

We used redd count data from 88 bull trout (Salvelinus confluentus) populations in the upper Columbia River basin to quantify local and regional patterns in population dynamics, including adult abundance, long-term trend, and population synchrony. We further used this information to assess conservation risk of metapopulations using eight population dynamic metrics associated with persistence. Local population abundances were generally low (&lt;20 redds annually) and the majority of trends were either stable (85%) or declining (13%). Evidence of synchrony among populations was apparent but not related to fluvial distance between streams. Variability in annual abundances was 1.4–2.5 times lower in metapopulations than local populations, indicating moderate portfolio effects across the regional stock complex. Importantly, most metrics of conservation risk were uncorrelated with one another, emphasizing that multiple statistics describing population dynamics at various scales are needed for monitoring and assessing recovery. We provide a composite description of conservation risk based on local and regional population dynamics that can help inform conservation management decisions for bull trout and other freshwater fishes.

Exploring the Use of Environmental DNA to Determine the Species of Salmon Redds

AbstractAnnual redd counts are used to monitor the status and trends of salmonid populations, but methods to easily and reliably determine which of sympatric species made specific redds are lacking. We explored whether environmental DNA (eDNA) analysis might prove useful for assigning salmon redds to the species of origin. We collected eDNA samples from the interstitial spaces of redds constructed by Chinook Salmon Oncorhynchus tshawytscha, redds constructed by Coho Salmon O. kisutch, and areas of undisturbed gravel (n = 10 of each type) as well as from the water column adjacent to each of those sites in the Sandy River basin, Oregon, during fall 2013. The concentrations of Chinook Salmon and Coho Salmon eDNA were quantified within each sample by using real‐time PCR. The water in the interstitial spaces of redds contained significantly higher concentrations of eDNA from the species that made the redd than from the other species; concentrations of eDNA from the species that built the redd were also significantly higher in the redd than in the adjacent water column. In contrast, within samples of water from the interstitial spaces of undisturbed gravel, neither Chinook Salmon eDNA nor Coho Salmon eDNA was significantly more concentrated than the other. The interstitial water of undisturbed gravel contained significantly higher Coho Salmon eDNA concentrations than the adjacent water column. In contrast, Chinook Salmon eDNA concentrations in the interstitial water of undisturbed gravel and in the adjacent water column were similar. Both species’ redds had significantly higher concentrations of their respective eDNA than did undisturbed gravel, but conclusions were confounded by differences in the timing and locations of sampling. This initial investigation highlights the potential value and some of the complexities involved in using eDNA analysis to identify the species that constructed a given redd.Received November 18, 2016; accepted May 20, 2017Published online August 4, 2017

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.

Testing unmanned aircraft systems for salmon spawning surveys

Unmanned aircraft systems (UASs) were tested for counting Chinook salmon ( Oncorhynchus tshawytscha) redds as a more accurate, safer alternative to manned helicopter flights. Counting redds from the helicopter was less expensive and time consuming, but of the total redds counted at selected sites with a UAS, an average (± SD) of only 77% ± 14% was counted from the helicopter. A river-wide census of redds was not possible with a UAS because the study area was too large for the single field crew to survey. Simulation analyses were used to compare stratified random sampling (STRS) and sampling proportional to size (PPS) for estimating annual total redd counts from data collected with a UAS. The STRS estimates were more accurate and precise, whereas the PPS estimates, though biased, had 95% CIs that included the observed redd count more frequently. We strongly recommend that researchers conduct simulation analyses to evaluate alternative survey sampling methods if they are considering replacing census counts made from manned aircraft with counts estimated from data collected with a UAS. We conclude that UAS application reduces the risk inherent to manned aircraft flights, but the reduction in risk can come at the cost of estimates of population parameters that can sometimes be inaccurate and lack 95% CI coverage.

Quantifying long-term population growth rates of threatened bull trout: challenges, lessons learned, and opportunities

Temporal symmetry models (TSM) represent advances in the analytical application of mark–recapture data to population status assessments. For a population of char, we employed 10 years of active and passive mark–recapture data to quantify population growth rates using different data sources and analytical approaches. Estimates of adult population growth rate were 1.01 (95% confidence interval = 0.84–1.20) using a temporal symmetry model (λTSM), 0.96 (0.68–1.34) based on logistic regressions of annual snorkel data (λA), and 0.92 (0.77–1.11) from redd counts (λR). Top-performing TSMs included an increasing time trend in recruitment (f) and changes in capture probability (p). There was only a 1% chance the population decreased ≥50%, and a 10% chance it decreased ≥30% (λMCMC; based on Bayesian Markov chain Monte Carlo procedure). Size structure was stable; however, the adult population was dominated by small adults, and over the study period there was a decline in the contribution of large adults to total biomass. Juvenile condition decreased with increasing adult densities. Utilization of these different information sources provided a robust weight-of-evidence approach to identifying population status and potential mechanisms driving changes in population growth rates.