Juvenile Steelhead Oncorhynchus Mykiss Research Articles

Flexible‐Pass‐Through Antennas for Half‐Duplex Passive Integrated Transponder Tags in Medium to Large Rivers Used to Study Size‐Selective Reservoir Passage

AbstractThe use of passive integrated transponder (PIT) tags for studying fish behavior and demography in medium‐large rivers is hindered by antenna designs that are susceptible to damage in those environments and low detection efficiencies. We designed a vertically oriented, flexible PIT tag antenna for half‐duplex tags that is simple to construct and deploy, resilient to the dynamic river environment, retains the higher detection efficiency of pass‐through antennas, and is suitable for rivers up to 30 m wide and 1.5 m deep. These “flexible‐pass‐through antennas” are economical to build and cost ~US$435 to $680 for a one‐ or two‐winding antenna, respectively. We used two flexible‐pass‐through antennas to test for size‐selective reservoir passage in juvenile steelhead Oncorhynchus mykiss (anadromous Rainbow Trout) and Rainbow Trout. We found that flexible‐pass‐through antennas withstood high flows and debris during a spring out‐migration season and provided data to reject the hypothesis of size‐selective fish passage at the Los Padres Dam and Reservoir on the Carmel River (California, USA).

Comparison of staircase and asymmetrical before–after, control–impact (aBACI) experimental designs to test the effectiveness of stream restoration at increasing juvenile steelhead density

Before–after, control–impact (BACI) experimental designs are commonly used in large-scale environmental experiments, but these designs can be confounded by location and time interactions. Staircase designs, where replicate treatments are staggered temporally, have been suggested as an alternative to BACI designs. We performed a simulation study based on data from an ongoing watershed-scale restoration experiment within three streams to test the effectiveness of adding large wood to increase habitat complexity and abundance and productivity of juvenile steelhead (Oncorhynchus mykiss). We compared the power of two asymmetric BACI (aBACI) designs with two staircase designs for detecting changes in the density of steelhead (fish·m–2). A staircase design where treatments were temporally staggered in one treatment section in each stream had the highest power and best precision, especially when the innate spatial and temporal variances of steelhead density were large. A traditional BACI performed the worst, and a variation on another BACI and staircase design had intermediate performance. Multistream staircase designs are also more logistically and economically feasible and can maximize learning by replicating experiments across different stream types.

Advances in our understanding of the parr–smolt transformation of juvenile salmon: a summary of the 10th International Workshop on Salmon Smoltification

Advances in our understanding of the parr–smolt transformation of juvenile salmon: a summary of the 10th International Workshop on Salmon Smoltification

Indexing Salmonid Abundance in Small Streams Using Reduced Effort Electrofishing

A paradox in managing threatened and endangered species is the increased need for documenting population status, which in many instances requires capturing and handling individuals. Electrofishing is a widely used method for sampling fish in small streams, but the potential for detrimental effects call for its careful use. Methods that reduce exposure for individual fish, yet still provide useful abundance estimates, are therefore desirable. Using data from a juvenile steelhead (Oncorhynchus mykiss) monitoring program in Idaho, we quantified variation and bias in capture probability from three-pass depletion electrofishing, and evaluated a method for indexing abundance based on single-pass electrofishing. Capture probability varied primarily at the level of sampling visits, with little spatial variation (i.e. at the level of the study sites). Water temperature, channel depth and, to a lesser degree, discharge best described the variation in capture probability between sampling events. We found no effect of previous capture and handling on individual capture probability, and the capture probability did not differ between subyearling and overyearling fish. Finally, a simple mixed-effects model with study site as a random effect, which related first-pass catch to the associated multiple-pass removal estimate, explained 91% of the variation in our data. The main limitation of the approach is that it does not provide error propagation and confidence intervals to the abundance estimates. However, the approach can be useful where multiple depletion electrofishing data exist, and when a single point estimate is sufficient to monitor major population trends in small streams.

Size and growth relationships in juvenile steelhead: The advantage of large relative size diminishes with increasing water temperatures

In territorial stream salmonids, asymmetric competition can perpetuate individual size differences over time, but the extent to which this is manifested can be environmentally mediated. Here we study the variation in juvenile steelhead (Oncorhynchus mykiss) growth rates to identify the conditions (population density and water temperature) under which an individual’s size relative to its conspecifics conferred an advantage. Among steelhead rearing in the same stream section we found that relatively larger individuals on average grew faster than smaller conspecifics. However, comparing across stream sections there was a negative interaction between relative size and water temperature. The effect of an individual’s relative size on its growth rate decreased as temperatures were increasing, indicating that the advantages of being large diminished during periods of high temperatures or in locations with relatively higher temperatures. Compared to temperature, the effects of population density on the growth rate were less substantial. The results suggest that larger individuals on average acquire more resources than smaller individuals, and demonstrate that water temperature exerts an important, modulating control over growth performance in heterogeneous environments.

An Assessment of the Geographic Closure Assumption in Mark–Recapture Abundance Estimates of Anadromous Steelhead Populations

AbstractClosed population models are commonly used to estimate stream salmonid abundances using mark–recapture information collected during electrofishing surveys. To meet the model assumption of geographic closure, block nets are often used to prevent emigration and immigration of fish during the survey. Increased sampling and tagging efforts in an open site may be an appealing trade‐off given the time it takes to properly deploy block nets, but it also increases an abundance estimate's vulnerability to bias. We assessed the extent of geographic closure violation from emigration in open sites between mark and recapture passes utilizing PIT antennas as virtual block nets. This allowed us to quantify emigration rates of juvenile steelhead Oncorhynchus mykiss from 60 fish surveys across multiple seasons and watersheds. Our goals were to determine how season and site length influence emigration, examine how the life history of an anadromous salmonid may induce bias on mark–recapture abundance estimates, and provide recommendations on minimizing the bias associated with violating the geographic closure assumption. Average emigration rate was low across all surveys and watersheds (5.1%), with higher emigration rates correlating with larger fish, shorter site length, and the season in which the site was sampled. We concluded that the bias associated with violating the geographic closure assumption in an open site can be minimized by avoiding times of migration and by sampling sites up to 650 m long depending on fish density and capture efficiency. Our findings provide useful information for planning mark–recapture studies that have multiple sampling objectives.Received July 27, 2016; accepted May 24, 2017Published online August 4, 2017

Juvenile Steelhead Locate Coldwater Refugia in an Intermittently Closed Estuary

AbstractMany coastal estuaries in Mediterranean climates are susceptible to inlet closures resulting from barrier beach formation. These closures are ecologically important because they eliminate tidal exchange and connectivity of nekton movement to the coastal ocean and, depending on closure duration, can convert a dynamic estuary into a quiescent lagoon. Although closures can create lethal or stressful conditions for nekton and benthic communities, especially obligate diadromous species, under some conditions they can enhance survival of juvenile steelhead Oncorhynchus mykiss. However, the mechanisms explaining how closed conditions enhance the growth of juvenile steelhead and how inhabitants avoid physiologically stressful conditions remain unknown. In the present study, recent technological advances in sensor‐encoded acoustic telemetry provided the ability to simultaneously locate and determine the temperature of juvenile steelhead as small as 93 mm FL by using mobile and stationary tracking. In the Russian River estuary, an intermittently closed estuary in northern California, we used acoustic telemetry to infer water quality exposure by linking the temperature of individually tagged fish with water quality profiles collected in close proximity. Under open‐inlet conditions, juvenile steelhead experienced primarily brackish and saline water in the lower and middle reaches and warm freshwater in the upper reach, whereas under closed‐inlet conditions they experienced warm freshwater in the middle and upper reaches. During closed conditions, juvenile steelhead displayed behavior that suggested the ability to mediate stressful environmental conditions; specifically, they responded to closed conditions by moving greater distances and aggregating near thermal refugia. Our findings show the importance of recognizing these strategies when contemplating changes to estuary management and highlight the significance of tributary hydrogeomorphic processes and groundwater linkages in subwatersheds that are sources of cool water for thermal refugia in intermittently closed estuaries.Received August 5, 2016; accepted February 27, 2017 Published online May 11, 2017

Growth of juvenile steelhead Oncorhynchus mykiss under size-selective pressure limited by seasonal bioenergetic and environmental constraints.

Increased freshwater growth of juvenile steelhead Oncorhynchus mykiss improved survival to smolt and adult stages, thus prompting an examination of factors affecting growth during critical periods that influenced survival through subsequent life stages. For three tributaries with contrasting thermal regimes, a bioenergetics model was used to evaluate how feeding rate and energy density of prey influenced seasonal growth and stage-specific survival of juvenile O. mykiss. Sensitivity analysis examined target levels for feeding rate and energy density of prey during the growing season that improved survival to the smolt and adult stages in each tributary. Simulated daily growth was greatest during warmer months (1 July to 30 September), whereas substantial body mass was lost during cooler months (1 December to 31 March). Incremental increases in annual feeding rate or energy density of prey during summer broadened the temperature range at which faster growth occurred and increased the growth of the average juvenile to match those that survived to smolt and adult stages. Survival to later life stages could be improved by increasing feeding rate or energy density of the diet during summer months, when warmer water temperatures accommodated increased growth potential. Higher growth during the summer period in each tributary could improve resiliency during subsequent colder periods that lead to metabolic stress and weight loss. As growth and corresponding survival rates in fresh water are altered by shifting abiotic regimes, it will be increasingly important for fisheries managers to better understand the mechanisms affecting growth limitations in rearing habitats and what measures might maintain or improve growth conditions and survival.

Avian Predation on Juvenile Salmonids: Spatial and Temporal Analysis Based on Acoustic and Passive Integrated Transponder Tags

AbstractWe evaluated the impact of predation on juvenile steelhead Oncorhynchus mykiss and yearling and subyearling Chinook Salmon O. tshawytscha by piscivorous waterbirds from 11 different breeding colonies in the Columbia River basin during 2012 and 2014. Fish were tagged with both acoustic tags and PIT tags and were tracked via a network of hydrophone arrays to estimate total smolt mortality (1 – survival) at various spatial and temporal scales during out‐migration. Recoveries of PIT tags on bird colonies, coupled with the last known detections of live fish passing hydrophone arrays, were used to estimate the impact of avian predation relative to total smolt mortality. Results indicated that avian predation was a substantial source of steelhead mortality, with predation probability (proportion of available fish consumed by birds) ranging from 0.06 to 0.28 for fish traveling through the lower Snake River and the lower and middle Columbia River. Predation probability estimates ranged from 0.03 to 0.09 for available tagged yearling Chinook Salmon and from 0.01 to 0.05 for subyearlings. Smolt predation by gulls Larus spp. was concentrated near hydroelectric dams, while predation by Caspian terns Hydroprogne caspia was concentrated within reservoirs. No concentrated areas of predation were identified for double‐crested cormorants Phalacrocorax auritus or American white pelicans Pelecanus erythrorhynchos. Comparisons of total smolt mortality relative to mortality from colonial waterbirds indicated that avian predation was one of the greatest sources of mortality for steelhead and yearling Chinook Salmon during out‐migration. In contrast, avian predation on subyearling Chinook Salmon was generally low and constituted a minor component of total mortality. Our results demonstrate that acoustic and PIT tag technologies can be combined to quantify where and when smolt mortality occurs and the fraction of mortality that is due to colonial waterbird predation relative to non‐avian mortality sources.Received November 4, 2015; accepted February 1, 2016 Published online June 27, 2016

Estimating Riverwide Abundance of Juvenile Fish Populations: How Much Sampling is Enough?

AbstractEstimating riverwide abundance of juvenile fish populations is challenging because detection probability is typically low and juveniles can be patchily distributed over large areas. We used a hierarchical Bayesian model to estimate the abundance of juvenile steelhead Oncorhynchus mykiss in two rivers in British Columbia over 3 years based on a multigear, two‐phase sampling design. These estimates were used to drive a simulation model to evaluate how the precision of abundance estimates varied with the number of single‐pass index and mark–recapture sites that were sampled, the proportion of shoreline sampled, and the mean and variation of detection probability and fish density across sites. The extent of variation in fish densities across index sites was the most important factor influencing the precision of river‐wide abundance estimates, and increasing the number of index sites was the best approach to reduce variability in abundance estimates. River size, which controls the proportion of habitat sampled for a given level of sampling effort, had a moderate effect on precision, but only when the extent of site‐to‐site variation in fish density was high. Factors affecting detection probability, such as the number of mark–recapture sites, the mean detection probability, or the extent of variation in detection probability across sites, had much less influence on precision of abundance estimates unless the proportion of river sampled was high. Hierarchical Bayesian models are no substitute for collecting informative data, but they improve our understanding of variance structure, which is critical for providing realistic estimates of uncertainty and designing informative and efficient sampling programs.Received July 16, 2015; accepted October 7, 2015 Published online March 8, 2016

Seasonal habitat use of brook trout and juvenile steelhead in a Lake Ontario tributary

Brook trout (Salvelinus fontinalis) are generally restricted to headwaters in New York tributaries of Lake Ontario. In only a few streams are brook trout abundant in lower stream reaches that are accessible to adult Pacific salmonids migrating from the lake. Consequently, because of the rarity of native brook trout populations in these lower stream reaches it is important to understand how they use stream habitat in sympatry with juvenile Pacific salmonids which are now naturalized in several Lake Ontario tributaries. In this study, we examined the seasonal (spring, summer, and fall) habitat use of brook trout and juvenile steelhead (Oncorhynchus mykiss) in Hart Brook, a tributary of eastern Lake Ontario. We found interspecific, intraspecific, and seasonal variation in habitat use. Subyearling steelhead were associated with faster water velocities than subyearling brook trout and, overall, had the least habitat similarity to the other salmonid groups examined. Overyearling brook trout and yearling steelhead exhibited the greatest degree of habitat selection and habitat selection by all four salmonid groups was greatest in summer. The availability of pool habitat for overyearling salmonids may pose the largest impediment to these species in Hart Brook.

Seasonal Rearing Habitat in a Large Mediterranean-Climate River: Management Implications at the Southern Extent of Pacific Salmon (Oncorhynchusspp.)

Pacific salmon (Oncorhynchus) use a variety of rearing environments prior to seaward migration, yet large river habitats and their use have not been well defined, particularly at the southernmost salmon range where major landscape-level alterations have occurred. We explored juvenile Chinook salmon (Oncorhynchus tshawytscha) and steelhead (Oncorhynchus mykiss) presence along the river continuum and in main-channel and off-channel habitats of a regulated California Mediterranean-climate river. Over an 8-year period, off-channels of the lower Mokelumne River exhibited slower and warmer water than the main-channel. Probability of salmonid presence varied by stream reach and habitat types. Steelhead and Chinook salmon both demonstrated transitional responses to the dry season, with juveniles leaving off-channels by midsummer. This corresponded to flow recession, increasing water temperatures, salmonid growth and end of emigration period. Main-channel steelhead observations continued until the following storm season, which brought cool flood flows to reconnect off-channels and the next juvenile cohort of both species to the river. Within arid climates, low-gradient off-channels appear more transiently used than in cooler and more northern humid climate systems. Within a highly regulated Mediterranean-climate river, off-channel habitats become increasingly scarce, disconnected or temperature limiting in low-gradient reaches both seasonally and due to anthropogenic modifications. These observations may provide guidance for future management within large salmon streams. Copyright © 2015 John Wiley & Sons, Ltd.

Variation in Juvenile Steelhead Density in Relation to Instream Habitat and Watershed Characteristics

AbstractPhysical habitat characteristics can affect the abundance and distribution of organisms, and are frequently used to predict the standing crop of stream fish for purposes of understanding their ecology and better direct management. However, the spatial scale of the investigation and the resolution of the data can affect the outcome of such analyses. In this study we coupled watershed‐level characteristics with instream habitat variables to model the density of two age‐classes of juvenile steelhead Oncorhynchus mykiss in a watershed in the Clearwater River basin, Idaho. Density varied considerably across time and space. Variance partitioning showed that 41–50% of the variances in density were due to unexplained differences between sampling occasions (residual variance), and the rest resulted from variation at the site and site‐and‐year levels, depending on the age‐class. Instream habitat variables better explained the variation in density than did models that included watershed‐level characteristics. The density of subyearling steelhead was best explained by stream discharge, with a negative relationship. The density of yearling steelhead was best explained by a negative relationship with average weekly temperature; however, this relationship was statistically indistinguishable from zero. Finally, total density (subyearlings and yearlings combined) was best explained by discharge and average daily temperature. We believe that our approach is useful for identifying the physical factors associated with the density of stream salmonids, but we stress that findings from correlative studies should be interpreted in concert with detailed knowledge about life history variation in the study system.

Relationship between Juvenile Fish Condition and Survival to Adulthood in Steelhead

AbstractUnderstanding how individual characteristics are associated with survival is important to programs aimed at recovering fish populations of conservation concern. To evaluate whether individual fish characteristics observed during the juvenile life stage were associated with the probability of returning as an adult, juvenile steelhead Oncorhynchus mykiss from two distinct population segments (DPSs; Snake River and upper Columbia River) were captured, photographed to determine external condition (body injuries, descaling, signs of disease, fin damage, and ectoparasites), measured, classified by rearing type (hatchery, wild), marked with a PIT tag, and released to continue out‐migration to the Pacific Ocean during 2007–2010. The PIT tags of returning adults were interrogated in fishways at hydroelectric dams on the lower Columbia River 1–3 years following release as juveniles. Juvenile‐to‐adult survival models were investigated independently for each DPS and indicated that similar individual fish characteristics were important predictors of survival to adulthood for both steelhead populations. The data analysis provided strong support for survival models that included explanatory variables for fish length, rearing type, and external condition, in addition to out‐migration year and timing. The probability of a juvenile surviving to adulthood was positively related to length and was higher for wild fish compared with hatchery fish. Survival was lower for juveniles with body injuries, fin damage, and external signs of disease. Models that included variables for descaling and ectoparasite infestation, however, had less support than those that incorporated measures of body injuries, fin damage, and disease. Overall, results indicated that individual fish characteristics recorded during the juvenile life stage can be used to predict adult survivorship in multiple steelhead populations.Received August 20, 2013; accepted February 14, 2014

Juvenile Anadromous Salmonid Production in Upper Columbia River Side Channels with Different Levels of Hydrological Connection

AbstractWe examined the contribution of three types of side channels based on their hydrologic connectivity (seasonally disconnected, partially connected, and connected) to production of juvenile anadromous salmonids. Juvenile steelhead Oncorhynchus mykiss and Chinook Salmon O. tshawytscha were found in all three of these side channel types and in each year of the study. Upon connection with the main stem at high flows, the seasonally disconnected side channels experienced an emptying out of the previous year's fish while filling with young‐of‐year fish during the 2‐ to 4‐month period of hydrologic connection. There were no differences between the densities of juvenile steelhead and Chinook Salmon and the rate of smolts produced among the three types of side channels. Recently reintroduced Coho Salmon O. kisutch had sporadic presence and abundance in partially and connected side channels, but the smolt production rate was over two times that of steelhead and Chinook Salmon in seasonally disconnected side channels. Within seasonally disconnected side channels, young‐of‐year salmonids in deep pools (≥100cm) had greater survival than those in shallow pools (<100cm). Densities of juvenile steelhead in all side channel types were similar to those in tributaries and were higher than in main‐stem lateral margins. Juvenile Chinook Salmon densities were higher in side channels than in both tributary and main‐stem lateral margins. Our results suggest that improving quality of pool habitat within seasonally disconnected side channels can result in improved survival for juvenile anadromous salmonids during the period of disconnection. Habitat improvement in these seasonally disconnected side channels should be recognized as a worthy restoration strategy, especially when full connectivity of side channels may not be a feasible target (e.g., through lack of water availability) or when full connectivity may present too high a risk (e.g., flooding, stream capture, bank destabilization).

Using scale characteristics and water temperature to reconstruct growth rates of juvenile steelhead Oncorhynchus mykiss

Juvenile steelhead Oncorhynchus mykiss from a northern California Central Valley population were reared in a controlled laboratory experiment. Significantly different rates of growth were observed among fish reared under two ration treatments and three temperature treatments (8, 14 and 20°C). Wider circulus spacing and faster deposition was associated with faster growth. For the same growth rate, however, circulus spacing was two-fold wider and deposited 36% less frequently in the cold compared to the hot temperature treatment. In a multiple linear regression, median circulus spacing and water temperature accounted for 68% of the variation in observed O. mykiss growth. These results corroborate previous research on scale characteristics and growth, while providing novel evidence that highlights the importance of water temperature in these relationships. Thus, this study establishes the utility of using scale analysis as a relatively non-invasive method for inferring growth in salmonids.

A Bioenergetics Approach to Assessing Potential Impacts of Avian Predation on Juvenile Steelhead during Freshwater Rearing

AbstractAvian predation on juvenile salmonids is an important source of mortality in freshwater and estuarine habitats when birds and salmonids overlap spatially and temporally. We assessed the potential impact of avian predation upon juvenile steelhead Oncorhynchus mykiss in a coastal watershed in central California. We conducted stream surveys between 2008 and 2010 to determine the composition, distribution, and density of piscivorous birds in areas that provide rearing habitat for juvenile steelhead. The most commonly sighted bird species were common mergansers Mergus merganser and belted kingfishers Megacyrle alcyon. The density of avian predators varied spatially and temporally but was greatest in the estuary regardless of season and decreased with increasing distance from the estuary. In the absence of local predator diet data, we applied a bioenergetics model to estimate the potential predation on juvenile steelhead by mergansers and kingfishers in the Scott Creek estuary. Model parameters included (1) published values of bird energetic requirements and steelhead energy density, (2) the number of birds present in the estuary during the closure period (from stream surveys), and (3) the size frequency and abundance of steelhead present in the estuary during closure. We predicted the extent of predation for different values of steelhead in bird diets, accounting for uncertainty in the estimates using a Monte Carlo simulation approach. With the assumed contribution of steelhead to the diet ranging from 20% to 100%, the population of kingfishers foraging in the Scott Creek estuary had the potential to remove 3–17% of annual production, whereas mergansers had the potential to remove 5–54% of annual steelhead production. Our results suggest that predation by avian species, particularly mergansers, is an important source of mortality for threatened steelhead populations in central California and should be addressed in future salmonid research and recovery planning.Received February 13, 2013; accepted June 4, 2013

The Effects of Semichronic Thermal Stress on Physiological Indicators in Steelhead

AbstractThe physiological response of juvenile steelhead Oncorhynchus mykiss to prolonged heat stress was examined by exposing replicated groups of fish to three different temperatures, 15, 23, and 25°C, for 25 consecutive days followed by a 55‐d recovery period at 15°C. We found that at temperatures ≥ 25°C, steelhead consumed significantly less food per day and had elevated feed conversion rates but experienced slower growth, reduced body size, lower body fat, and elevated heat shock protein (hsp) 70 levels relative to fish exposed to 23°C and 15°C. Growth decreased 24.4% and 27.1% for length and mass, respectively, between 15°C and 23°C, and an additional 60% and 56.5%, respectively, between 23°C and 25°C during exposures. While growth increments and lipid levels recovered to control levels after water temperature was reduced, body size of the fish exposed to 25°C lagged throughout the experiment. Our results indicate a temperature threshold after which steelhead exposed to semichronic thermal stress incur a physiological debt. Heat shock protein 70 levels were detectable up to 25 d poststress in fin and liver tissues. This result provides evidence that hsp 70 is a useful metric for measuring thermal stress that can be assessed nonlethally and used to monitor thermal‐habitat restoration efforts for threatened and endangered salmonids.

Susceptibility of Juvenile Steelhead to Avian Predation: the Influence of Individual Fish Characteristics and River Conditions

AbstractIdentification of the factors that influence susceptibility to predation can aid in developing management strategies to recover fish populations of conservation concern. Predator–prey relationships can be influenced by numerous factors, including prey condition, prey size, and environmental conditions. We investigated these factors by using juvenile steelhead Oncorhynchus mykiss from the Snake River (Pacific Northwest, USA), a distinct population segment that is listed as threatened under the U.S. Endangered Species Act. During 2007–2009, steelhead smolts (n = 25,909) were captured, examined for external condition characteristics (e.g., body injuries, descaling, external signs of disease, fin damage, and ectoparasite infestations), marked with passive integrated transponder (PIT) tags, and released to continue their out‐migration. Recoveries of PIT tags on a downstream colony of Caspian terns Hydroprogne caspia (n = 913 tags) indicated that steelhead susceptibility to Caspian tern predation increased significantly with decreases in steelhead external condition, decreased water discharge, and decreased water clarity. Susceptibility to Caspian tern predation also increased with increasing steelhead fork length up to 202 mm but then decreased for longer steelhead. Recoveries of PIT tags on a downstream colony of double‐crested cormorants Phalacrocorax auritus (n = 493 tags) indicated that steelhead susceptibility to double‐crested cormorant predation increased significantly with declining external condition of steelhead, and that steelhead of hatchery origin were more susceptible than their wild counterparts. Results indicate that steelhead susceptibility to avian predation is dependent on fish condition and length and is influenced by river conditions and rearing environment.

Fine‐Scale Population Genetic Structure and Dispersal of Juvenile Steelhead in the Bulkley‐Morice River, British Columbia

AbstractA knowledge of fine‐scale population genetic structure and patterns of dispersal is an essential component of any action to conserve genetic diversity and maintain population viability. We genotyped 417 juvenile steelhead Oncorhynchus mykiss from the main stem and tributaries of the Bulkley–Morice River, British Columbia, at 10 microsatellite loci to assess fine‐scale population structure and the patterns and magnitude of juvenile dispersal and mixing. We detected significant genetic structuring among juvenile steelhead from seven tributaries of the Bulkley–Morice River (pairwise FST: 0.008–0.156) and found significant isolation by distance among the tributary populations (R2 = 0.198, P = 0.038). These results reflect the homing behavior of spawning adults as well as the temporal stability of those populations. Genotype assignment of tributary‐caught juveniles showed that rates of juvenile dispersal varied among tributaries. The assignment of juveniles sampled from the main stem of the river to source tributary populations suggested that long‐distance movement in juvenile steelhead is common and that juveniles are well mixed in the main stem. Dispersal and fine‐scale genetic structure in pristine steelhead populations are more complex than previously thought. Therefore, actions to conserve Bulkley–Morice River steelhead must strive to maintain the genetic diversity of tributary populations.