Coastal Cutthroat Research Articles

Estimation of forest harvesting-induced stream temperature changes and bioenergetic consequences for cutthroat trout in a coastal stream in British Columbia, Canada

Data from a paired-catchment study in south coastal British Columbia, Canada, were analyzed to assess the thermal effects of clearcut harvesting with no riparian buffer on a fish-bearing headwater stream. The approach used time series of daily mean water temperatures for East Creek (control) and A Creek (treatment), both before and after harvest. Statistical models were developed to predict (a) what the temperatures would have been in the post-harvest period had harvesting not occurred, and (b) what temperatures would have been in the pre-harvest period had harvesting already occurred. The Wisconsin Bioenergetics Model was used to simulate growth of coastal cutthroat trout (Oncorhynchus clarki clarki) for the first year following fry emergence using the predicted and observed stream temperatures to generate scenarios representing with-harvest and no-harvest thermal regimes. A Monte Carlo approach was used to quantify the effects of uncertainty associated with the regression models on predicted stream temperature and trout growth. Summer daily mean temperatures in the with-harvest scenario were up to $$5^{\circ}\hbox{C}$$ higher than those for the no-harvest scenario. Harvesting-induced warming reduced growth rates during summer, but increased growth rates during autumn and spring. In the with-harvest scenario, trout were 0.2–2.0 g (absolute weight) smaller throughout the winter period than in the no-harvest scenario. However, the bioenergetic simulations suggest that trout growth may be more sensitive to potential changes in food supply following harvesting than to direct impacts of stream temperature changes.

A riverscape perspective of Pacific salmonids and aquatic habitats prior to large‐scale dam removal in the Elwha River, Washington, USA

Abstract Dam removal has been increasingly proposed as a river restoration technique. In 2011, two large hydroelectric dams will be removed from Washington State’s Elwha River. Ten anadromous fish populations are expected to recolonise historical habitats after dam removal. A key to understanding watershed recolonisation is the collection of spatially continuous information on fish and aquatic habitats. A riverscape approach with an emphasis on biological data has rarely been applied in mid‐sized, wilderness rivers, particularly in consecutive years prior to dam removal. Concurrent snorkel and habitat surveys were conducted from the headwaters to the mouth (rkm 65–0) of the Elwha River in 2007 and 2008. This riverscape approach characterised the spatial extent, assemblage structure and patterns of relative density of Pacific salmonids. The presence of dams influenced the longitudinal patterns of fish assemblages, and species richness was the highest downstream of the dams, where anadromous salmonids still have access. The percent composition of salmonids was similar in both years for rainbow trout, Oncorhynchus mykiss (Walbaum), coastal cutthroat trout, Oncorhynchus clarkii clarkii (Richardson) (89%; 88%), Chinook salmon, Oncorhynchus tshawytscha (Walbaum) (8%; 9%), and bull trout, Salvelinus confluentus (Suckley) (3% in both years). Spatial patterns of abundance for rainbow and cutthroat trout (r = 0.76) and bull trout (r = 0.70) were also consistent between years. Multivariate and univariate methods detected differences in habitat structure along the river profile caused by natural and anthropogenic factors. The riverscape view highlighted species‐specific biological hotspots and revealed that 60–69% of federally threatened bull trout occurred near or below the dams. Spatially continuous surveys will be vital in evaluating the effectiveness of upcoming dam removal projects at restoring anadromous salmonids.

Seasonal Changes in Habitat Availability and the Distribution and Abundance of Salmonids along a Stream Gradient from Headwaters to Mouth in Coastal Oregon

AbstractVisual estimation techniques were used to quantify seasonal habitat characteristics, habitat use, and longitudinal distribution of juvenile steelhead Oncorhynchus mykiss, coastal cutthroat trout O. clarkii clarkii and coho salmon O. kisutch in a coastal Oregon basin. At the channel unit scale, fish distribution was not proportional to habitat type availability. Pool habitats contained a disproportionate percentage of the salmonid assemblage, and the percentage of fish in pools increased as flow decreased. Large woody debris formed 57–68% of pool habitats and was significantly correlated with pool volume, maximum pool depth, slow surface velocity in pools, and pieces of small woody debris. At the reach and basin scales, longitudinal distribution of the total salmonid assemblage generally did not differ from habitat distribution seasonally or between years. Abundance in the reaches varied annually, and the fish species were longitudinally segregated within the basin: coastal cutthroat trout occurred in the uppermost reaches, steelhead occupied the lowest reaches, and coho salmon inhabited the middle reaches. This study demonstrates that the basinwide distribution of salmonids varies among species, age‐classes, seasons, and years. These results suggest that our understanding of salmonid distribution and abundance could be greatly enhanced by adopting a basinwide, community, seasonal perspective. In addition, the methods described here offer one way to assess the seasonal distribution and abundance of salmonids in a relatively quick, inexpensive, nondestructive manner.

Effects of Hatchery Fish Density on Emigration, Growth, Survival, and Predation Risk of Natural Steelhead Parr in an Experimental Stream Channel

AbstractHatchery supplementation of steelhead Oncorhynchus mykiss raises concerns about the impacts on natural populations, including reduced growth and survival, displacement, and increased predation. The potential risks may be density dependent. We examined how hatchery stocking density and the opportunity to emigrate affect the responses of natural steelhead parr in an experimental stream channel and after 15 d found no density‐dependent effects on growth, emigration, or survival at densities ranging from 1–6 hatchery parr/m2. The opportunity for steelhead parr to emigrate reduced predation by coastal cutthroat trout O. clarkii clarkii on both hatchery and natural steelhead parr. The cutthroat trout exhibited a type‐I functional response (constant predation rate with increased prey density) for the hatchery and composite populations. In contrast, the predation rate on natural parr decreased as hatchery stocking density increased. Supplementation with hatchery parr at any experimental stocking density reduced the final natural parr density. This decline was explained by increased emigration from the supplemented groups. Natural parr had higher mean instantaneous growth rates than hatchery parr. The proportion of parr emigrating decreased as parr size increased over successive experimental trials. Smaller parr had lower survival and suffered higher predation. The final density of the composite population, a measure of supplementation effectiveness, increased with the hatchery steelhead stocking rate. Our results indicate that stocking larger hatchery parr (over 50 d postemergence) at densities within the carrying capacity would have low short‐term impact on the growth, survival, and emigration of natural parr while increasing the density of the composite population; in addition, a stocking density greater than 3 fish/m2 might be a good starting point for the evaluation of parr stocking in natural streams.Received June 28, 2010; accepted December 30, 2010

Predation on Juvenile Pacific Salmon Oncorhynchus spp. in Downstream Migrant Traps in Prairie Creek, California

AbstractDownstream migrant traps are a widely applied fishery management tool for sampling anadromous Pacific salmon Oncorhynchus spp. and steelhead O. mykiss smolts along the West Coast of North America and elsewhere, yet predation on juvenile salmonids in traps has not been studied quantitatively. We assessed the frequency of occurrence and abundance of juvenile salmonids in the stomachs of coastal cutthroat trout O. clarkii clarkii, coho salmon O. kisutch, steelhead, and prickly sculpin Cottus asper (>70 mm fork length) captured in traps and in nearby stream habitats. All four predator species took juvenile salmonids with much greater frequency in traps than in stream habitats. Among free‐swimming predators, only coastal cutthroat trout were observed with salmonid fry in their stomachs, but they took fewer salmonid prey and appeared to rely more heavily on insect prey than did coastal cutthroat trout captured in traps. Predators consumed up to 25% of the available prey over a broad range of prey abundances. Over the course of the study, predators consumed 2.5% of all salmonid fry captured in traps, but this fraction ranged from less than 1% to more than 10% in any given year. The number of prey taken in traps increased with predator length and with prey abundance in traps, and predation in traps peaked during the period of most intense downstream migration by salmon fry. In contrast, live‐box design and trap location had little or no effect on the total number of prey taken by individual predators. We estimated that the predation mortality of juvenile salmon increased by 0.5–1.0% due to in‐trap predation (i.e., a 9–10% relative increase over natural predation rates). We found no evidence that predators selected for prey on the basis of species. These results should motivate additional research on methods that reduce or eliminate predation in trap live‐boxes and protocols for efficiently measuring predation associated with the trapping of downstream migrants.Received May 11, 2010; accepted November 29, 2010

A suite of twelve single nucleotide polymorphism markers for detecting introgression between cutthroat and rainbow trout

A suite of 12 subspecies and species-specific single nucleotide polymorphism (species-specific SNP) markers was developed to distinguish rainbow trout (RT) Oncorhynchus mykiss from the four major subspecies of cutthroat trout: westslope cutthroat trout (WCT) Oncorhynchus clarki lewisi, Yellowstone cutthroat trout (YCT) Oncorhynchus clarki bouvieri, coastal cutthroat trout (CCT) Oncorhynchus clarki clarki, Lahontan cutthroat trout (LCT) Oncorhynchus clarki henshawi, and their hybrids. Several of the markers were linked to help strengthen hybrid determinations, and sex-specific species-specific SNP assays were also developed.

Genetic variation and effective population size in isolated populations of coastal cutthroat trout

Following glacial recession in southeast Alaska, waterfalls created by isostatic rebound have isolated numerous replicate populations of coastal cutthroat trout (Oncorhynchus clarkii clarkii) in short coastal streams. These replicate isolated populations offer an unusual opportunity to examine factors associated with the maintenance of genetic diversity. We used eight microsatellites to examine genetic variation within and differentiation among 12 population pairs sampled from above and below these natural migration barriers. Geological evidence indicated that the above-barrier populations have been isolated for 8,000–12,500 years. Genetic differentiation among below-barrier populations (FST = 0.10, 95% C.I. 0.08–0.12) was similar to a previous study of more southern populations of this species. Above-barrier populations were highly differentiated from adjacent below-barrier populations (mean pairwise FST = 0.28; SD 0.18) and multiple lines of evidence were consistent with asymmetric downstream gene flow that varied among streams. Each above-barrier population had reduced within-population genetic variation when compared to the adjacent below-barrier population. Within-population genetic diversity was significantly correlated with the amount of available habitat in above-barrier sites. Increased genetic differentiation of above-barrier populations with lower genetic diversity suggests that genetic drift has been the primary cause of genetic divergence. Long-term estimates of Ne based on loss of heterozygosity over the time since isolation were large (3,170; range 1,077–7,606) and established an upper limit for Ne if drift were the only evolutionary process responsible for loss of genetic diversity. However, it is likely that a combination of mutation, selection, and gene flow have also contributed to the genetic diversity of above-barrier populations. Contemporary above-barrier Ne estimates were much smaller than long-term Ne estimates, not correlated with within-population genetic diversity, and not consistent with the amount of genetic variation retained, given the approximate 10,000-year period of isolation. The populations isolated by waterfalls in this study that occur in larger stream networks have retained substantial genetic variation, which suggests that the amount of habitat in headwater streams is an important consideration for maintaining the evolutionary potential of isolated populations.

Early marine migration patterns of wild coastal cutthroat trout (Oncorhynchus clarki clarki), steelhead trout (Oncorhynchus mykiss), and their hybrids.

BackgroundHybridization between coastal cutthroat trout (Oncorhynchus clarki clarki) and steelhead or rainbow trout (Oncorhynchus mykiss) has been documented in several streams along the North American west coast. The two species occupy similar freshwater habitats but the anadromous forms differ greatly in the duration of marine residence and migration patterns at sea. Intermediate morphological, physiological, and performance traits have been reported for hybrids but little information has been published comparing the behavior of hybrids to the pure species.Methodology/Principal FindingsThis study used acoustic telemetry to record the movements of 52 cutthroat, 42 steelhead x cutthroat hybrids, and 89 steelhead smolts, all wild, that migrated from Big Beef Creek into Hood Canal (Puget Sound, Washington). Various spatial and temporal metrics were used to compare the behavior of the pure species to their hybrids. Median hybrid residence time, estuary time, and tortuosity values were intermediate compared to the pure species. The median total track distance was greater for hybrids than for either cutthroat or steelhead. At the end of each track, most steelhead (80%) were located near or north of the Hood Canal, as expected for this seaward migrating species, whereas most cutthroat (89%) were within 8 kilometers of the estuary. Most hybrids (70%) were detected leaving Hood Canal, though a substantial percentage (20%) remained near the Big Beef Creek estuary. More hybrids (7.5%) than pure cutthroat (4.5%) or steelhead (0.0%) were last detected in the southern reaches of Hood Canal.Conclusions/SignificanceGiven the similarity in freshwater ecology between the species, differences in marine ecology may play an important role in maintaining species integrity in areas of sympatry.

Contrasting effects of cross-ecosystem subsidies and predation on benthic invertebrates in two Pacific coastal streams

Cross-ecosystem subsidies, such as terrestrial invertebrates and leaf litter falling into water as resources for aquatic communities, can vary across environmental gradients. We examined whether the effect of terrestrial subsidy inputs on benthic invertebrates was mediated by resident coastal cutthroat trout (Oncorhynchus clarki) in two representative streams. We experimentally manipulated the input rates (reduced, ambient) of terrestrial subsidies (terrestrial invertebrates and leaf litter) as well as the presence or absence of cutthroat trout in the two streams. The hypothesis that the reduction of terrestrial subsidies to the stream influences benthic invertebrate assemblages was supported by experimental results. The treatments of terrestrial subsidy reduction and cutthroat trout presence had a significant negative effect on benthic invertebrate community biomass and shredder biomass in East Creek with high natural terrestrial subsidy input and small amount of large wood in channel. In contrast, results from Spring Creek with low subsidy input and large amount of large wood in channel showed that only the terrestrial subsidy reduction significantly reduced the biomass of shredders. The effects of the terrestrial subsidy input and trout predation on benthic invertebrate communities varied between the two streams. Our results indicate that a subsidy effect on benthic communities can vary between nearby streams differing in canopy and habitats. This study, with the major finding of highly context-dependent effects of spatial subsidies, suggests that the interplay of resource subsidies and predators on invertebrate community assemblages can be site-specific and context-dependent on habitat features.

Evidence of Panmixia between Sympatric Life History Forms of Coastal Cutthroat Trout in Two Lower Columbia River Tributaries

AbstractCoastal cutthroat trout Oncorhynchus clarkii clarkii exhibit resident and migratory life history strategies that often occur sympatrically, but the relationship between these forms within a population is poorly characterized. Through use of passive integrated transponder technology, migratory and resident coastal cutthroat trout were identified in two lower Columbia River tributaries (Abernathy Creek and the Chinook River) separated by more than 80 km. Genetic data from 17 highly variable microsatellite loci were used to ascertain the genetic population structure of these life history forms within and between streams. No distinct genetic separation was observed between the life history forms within a stream, as assessed by four different statistical approaches: permutation tests based on the genetic differentiation index FST, principal components analysis of individuals, analysis of molecular variance, and contingency tests of allele frequency heterogeneity. Genetic differences were an order of magnitude higher between stream samples (FST > 0.03) than between life history forms within a stream (FST < 0.003). The contingency test detected allele frequency differences between migratory and resident life history forms in Abernathy Creek (P = 0.001), but this result was influenced more by age‐class structure than by reproductive isolation between life history forms. Results are consistent with a single, randomly mating population in each stream producing both migratory and resident life history forms. These data suggest that individual life history strategy in coastal cutthroat trout is predominantly determined by phenotypic plasticity rather than genotype.

Environmental factors associated with reproductive barrier breakdown in sympatric trout populations on Vancouver Island.

The incidence of hybridization between coastal cutthroat (Oncorhynchus clarki clarki) and rainbow trout (Oncorhynchus mykiss) varies widely among populations. The breakdown of reproductive isolation is of concern to managers, and raises the question: how have the two species retained their genetic and morphological divergence? Using a combination of mitochondrial DNA and nuclear DNA markers coupled with watershed attribute and disturbance data, we determined the distribution and frequency of trout hybridization on Vancouver Island, BC and the environmental factors associated with the hybridization. We found 284 hybrids (among 1004 fish) in 29 of 36 sampled populations. High variation in levels of hybridization was observed among populations, and no single environmental factor was found to dominate in determining hybridization levels. However, logging activity, urban infrastructure development, and stocking of hatchery rainbow trout played significant roles in determining hybridization levels, and populations in small watersheds are more at risk of reproductive barrier breakdown. This study illustrates that cutthroat–rainbow trout reproductive barrier breakdown is widespread on Vancouver Island and that anthropogenic disturbance plays a role in the process. As similar environmental disturbance is common in much of coastal trout habitat, large-scale hybridization may be occurring elsewhere and thus may represent a critical management issue for Pacific trout species.

Patterns of migration and residency in coastal cutthroat trout Oncorhynchus clarkii clarkii from two tributaries of the lower Columbia River

Coastal cutthroat trout Onchorhynchus clarkii clarkii life-history variants, migration and freshwater residency were monitored using stationary passive integrated transponder (PIT) tag arrays in two tributaries of the Columbia River from 2001 to 2005 (Abernathy Creek, river kilometre, rkm 76) and from 2002 to 2005 (Chinook River, rkm 6). In 2001-2003 and 2002-2003 (Abernathy and Chinook, respectively), 300-500 coastal O. c. clarkii were captured in each tributary by electrofishing and implanted with 23 mm PIT tags. PIT arrays monitored movements from the initiation of tagging through the spring of 2005. Rotary screw traps were also operated on both tributaries. In Abernathy Creek, 28% of tagged individuals were observed through either active capture or passive interrogation. Of these, 32% were identified as migrants and 68% were identified as residents. In the Chinook River, 48% of tagged fish were observed subsequent to tagging; 92% of these fish were migrants and only 8% were resident. In both tributaries, a greater proportion of resident fish were in the upper reaches. The majority of migrants (78-93%) moved the spring following tagging. Migrants leaving at age 2+ years tended to grow faster than those that migrated at age 3+ years or residents. Patterns of growth or growth opportunities may influence both patterns of life-history expression and the timing of migration.

Passive Integrated Transponder Tag Retention Rates in Headwater Populations of Coastal Cutthroat Trout

AbstractPassive integrated transponder (PIT) tags have desirable qualities (e.g., unique identification, indefinite tag life, and capacity for remote detection) that make them useful for evaluating survival, growth, and movement of fish, but low tag retention rates can confound data interpretation. Although the effects of PIT tags on short‐term growth and survival have been minimal and tag retention rates in laboratory and field studies using only juvenile individuals have been high, tag retention rates for fish at different ontological stages (including reproductively active males and females) remain unknown. We evaluated tag retention rates in wild populations of coastal cutthroat trout Oncorhynchus clarkii clarkii in three catchments of western Oregon using a double‐marking approach (i.e., the adipose fin was removed from all fish that were PIT‐tagged). Tags were inserted into the body cavities of fish 100 mm or more in length (fork length; range = 100–250 mm). In the study catchments, this size range includes both juvenile and mature fish. Tag retention rates were approximately 25% lower than those reported by previous studies of juvenile salmonids alone. A number of PIT tags were recovered in redds, indicating that mature individuals eject tags during spawning. Although some coastal cutthroat trout retained PIT tags for up to 4 years, others expelled them repeatedly and were implanted with a minimum of three different PIT tags during the same period. These data are concordant with those of other studies in which PIT tags had been inserted into the body cavity of salmonids and the population of tagged fish comprised both juvenile and mature individuals. Therefore, for multiple‐year studies, it may be prudent to double‐mark fish when PIT tags are to be inserted in the body cavity.

Estimating the proportions of closely related species: Performance of the two-phase ratio estimator

In many ecological field contexts, accurately classifying closely related species based on phenotypic characteristics may be difficult. In such cases, definitive classification of species may require expensive genetic analysis of tissues or extensive quantitative measurements. If inexpensive phenotype-based species classifications are highly correlated with expensive but definitive classifications, however, then estimating the proportion of a target species in a mixed species complex using a two-phase ratio estimator may prove cost-efficient. In two-phase ratio estimation, a first-phase sample is randomly selected, and phenotype is used to classify all individuals in the sample. A smaller, second-phase sample is then randomly selected from the first-phase sample, and a definitive method is used to classify the individuals in this subsample. Net relative efficiency (i.e., cost-effectiveness) of the optimally allocated two-phase ratio estimator of a proportion depends on the relative costs of classification at the first and second phases, on the phenotypic classification accuracy for the target species (sensitivity) and nontarget species (specificity), and on the magnitude of the target species proportion. Results are presented that allow assessment of the circumstances in which this two-phase estimation approach can be recommended over an equal-cost single-phase approach based only on expensive but definitive classification. An illustrative application of these methods is provided using an example of two closely related, sympatric, anadromous salmonid species: steelhead (Oncorhynchus mykiss) and coastal cutthroat trout (O. clarki clarki).

Exploring the Persistence of Stream‐Dwelling Trout Populations under Alternative Real‐World Turbidity Regimes with an Individual‐Based Model

AbstractWe explored the effects of elevated turbidity on stream‐resident populations of coastal cutthroat trout Oncorhynchus clarkii clarkii using a spatially explicit individual‐based model. Turbidity regimes were contrasted by means of 15‐year simulations in a third‐order stream in northwestern California. The alternative regimes were based on multiple‐year, continuous monitoring in two streams. Turbidity affected model fish by reducing both their risk of predation and their reactive distance to drifting prey. It did not affect their ability to locate nondrifting food, such as invertebrates on the stream bottom. Under a calibration scenario that assumed trout predominantly consume drifting prey, the less‐turbid real‐world regime produced relatively stable abundance across years (similar to field observations) whereas the more‐turbid regime (under otherwise identical physical conditions) resulted in extinction within the 15‐year simulation period. Additional simulations revealed sensitivity to the relative amounts of prey available via drift versus search feeding and showed that seasonal variation in food availability or strong positive relationships between streamflow and food concentration would not prevent extinction in the high‐turbidity regime under a drift‐feeding‐based food calibration. Extinction of predominantly drift‐feeding trout populations in our simulations contrasts with field observations of salmonid populations that have persisted in moderately turbid regimes. The results highlight the need for better understanding of patterns in the availability of food under turbid conditions and the capability of stream salmonids to use nonvisual cues in feeding.

Intraspecific taxonomy and ecology characterize morphological divergence among cutthroat trout (Oncorhynchus clarkii ssp. Richardson) populations

We compared the proportion of morphological variation accounted for by subspecies categories with the proportion encompassed by ecologically based categories in cutthroat trout (Oncorhynchus clarkii ssp.), as a means of assessing the relative importance of each approach in identifying intraspecific diversity. We used linear and geometric morphometrics to compare measures of body shape, fin length, and head features between and within subspecies of cutthroat trout. Both categories accounted for a significant proportion of the variation between and within the subspecies; however, the larger proportion was explained by subspecific differences, with the greatest morphological divergence between coastal cutthroat trout (Oncorhynchus clarkii clarkii) and interior subspecies. Ecotypic categories within each subspecies also explained significant morphological differences: stream populations had longer fins and deeper, more robust bodies than lake populations. The largest ecotypic differences occurred between stream and lake populations of Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri). Given that many cutthroat trout subspecies are of conservation concern, our study offers a better understanding of intraspecific variation existing within the species, providing precautionary evidence of incipient speciation, and a framework of describing phenotypic diversity that is correlated with ecological conditions. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96, 266–281.

Morphological, Physiological, and Genetic Techniques for Improving Field Identification of Steelhead, Coastal Cutthroat Trout, and Hybrid Smolts

AbstractIn streams with sympatric populations of steelhead Oncorhynchus mykiss and coastal cutthroat trout O. clarkii clarkii (hereafter, cutthroat trout), life history descriptions and smolt production estimates may be hampered by misclassification of hybrids as steelhead or cutthroat trout. Additionally, important morphological and physiological differences between hybrid and non‐hybird smolts are often unknown. Therefore, we assessed field classification and created classification models to quantify and reduce misclassification rates among migrating steelhead, cutthroat trout, and hybrid smolts. Field misclassifications of smolts with steelhead or cutthroat trout genotypes were low (1% and 2%, respectively). However, field misclassification of fish with hybrid genotypes was high, with 11% of the hybrids being misclassified as steelhead and 42% of the hybrids being misclassified as cutthroat trout. Hybrid smolts were larger, had lower gill Na+, K+‐ATPase activities, and lower condition factors than steelhead but were similar to cutthroat trout smolts in these same measurements. Additionally, statistical classification analyses using morphological traits including subterminal jaw slash intensity, hyoid teeth presence, maxillary length, breaks of pigment along outer margin of adipose fin, condition factor, and migration date improved classification error rates of hybrids from 53% to 21%. In systems with sympatric populations of steelhead and cutthroat trout, we recommend a thorough evaluation of field‐based identification methods with genetic techniques to assess the effectiveness of field‐based classification in addition to examining important life history differences among steelhead, cutthroat trout, and their hybrids.

Factors influencing coastal cutthroat trout (Oncorhynchus clarkii clarkii) seasonal survival rates: a spatially continuous approach within stream networks

Mark–recapture methods were used to examine watershed-scale survival of coastal cutthroat trout ( Oncorhynchus clarkii clarkii ) from two headwater stream networks. A total of 1725 individuals (≥100 mm, fork length) were individually marked and monitored seasonally over a 3-year period. Differences in survival were compared among spatial (stream segment, subwatershed, and watershed) and temporal (season and year) analytical scales, and the effects of abiotic (discharge, temperature, and cover) and biotic (length, growth, condition, density, movement, and relative fish abundance) factors were evaluated. Seasonal survival was consistently lowest and least variable (years combined) during autumn (16 September – 15 December), and evidence suggested that survival was negatively associated with periods of low stream discharge. In addition, relatively low (–) and high (+) water temperatures, fish length (–), and boulder cover (+) were weakly associated with survival. Seasonal abiotic conditions affected the adult cutthroat trout population in these watersheds, and low-discharge periods (e.g., autumn) were annual survival bottlenecks. Results emphasize the importance of watershed-scale processes to the understanding of population-level survival.

Landscape-scale evaluation of genetic structure among barrier-isolated populations of coastal cutthroat trout,Oncorhynchus clarkii clarkii

Relationships among landscape structure, stochastic disturbance, and genetic diversity were assessed by examining interactions between watershed-scale environmental factors and genetic diversity of coastal cutthroat trout ( Oncorhynchus clarkii clarkii ) in 27 barrier-isolated watersheds from western Oregon, USA. Headwater populations of coastal cutthroat trout were genetically differentiated (mean FST= 0.33) using data from seven microsatellite loci (2232 individuals), but intrapopulation microsatellite genetic diversity (mean number of alleles per locus = 5, mean He= 0.60) was only moderate. Genetic diversity of coastal cutthroat trout was greater (P = 0.02) in the Coast Range ecoregion (mean alleles = 47) than in the Cascades ecoregion (mean alleles = 30), and differences coincided with indices of regional within-watershed complexity and connectivity. Furthermore, regional patterns of diversity evident from isolation-by-distance plots suggested that retention of within-population genetic diversity in the Coast Range ecoregion is higher than that in the Cascades, where genetic drift is the dominant factor influencing genetic patterns. Thus, it appears that physical landscape features have influenced genetic patterns in these populations isolated from short-term immigration.

Seasonal Patterns of Predation on Juvenile Pacific Salmon by Anadromous Cutthroat Trout in Puget Sound

AbstractIn the marine environment, Pacific salmon Oncorhynchus spp. suffer the greatest natural losses during early marine residence, and predation is hypothesized to be the key source of mortality during this life history stage. In the face of recent declines in Puget Sound salmon populations, our goal was to determine the extent of predation mortality on salmon during early marine life. In spring and summer of 2001‐2003, we caught juvenile salmon and potential predators at nearshore areas in northern and southern regions of Puget Sound, Washington. We focused on the potential predation impact of coastal cutthroat trout O. clarkii clarkii, which were caught in low but consistent numbers in both regions and were the most abundant large‐bodied potential predators of juvenile salmon in our catches. Cutthroat trout consumed a diverse and dynamic array of diet items and became increasingly piscivorous with increasing fork length above 140 mm. Cutthroat trout consumed a greater biomass of Pacific herring Clupea pallasii than any other prey fish species, but juvenile salmon were particularly important prey between April and June, making up greater than 50% of the fish prey consumed. Cutthroat trout exhibited size‐selective predation, eating salmon that were smaller than the average size of conspecific prey available in the environment. For a hypothetical size‐structured population of 1,000 cutthroat trout, pink salmon O. gorbuscha and chum salmon O. keta contributed the greatest number of salmon to the diet but Chinook salmon O. tshawytscha contributed the greatest salmonid biomass. On an order‐of‐magnitude basis, these predation estimates represented a relatively minor amount of early marine mortality for Chinook salmon and lower rates for the other salmon species. Conversely, juvenile salmon contributed significantly to the spring energy budget for cutthroat trout in Puget Sound.