Resident Trout Research Articles

Parallel evolution of profundal Arctic charr morphs in two contrasting fish communities

A population of small-sized Arctic charr living their entire life cycle in deep water was recently found in Lake Skogsfjordvatn, Northern Norway. Their phenotype and life history closely resembles the profundal charr morph in another subarctic Norwegian lake, Fjellfrosvatn. Fjellfrosvatn has one littoral omnivorous and one small profundal benthivorous (PB) charr morph coexisting with brown trout, whereas Skogsfjordvatn has migratory salmon, trout, charr and eel populations in addition to threespined sticklebacks, resident trout and three resident charr morphs of which one is a large piscivorous profundal morph. We predicted that the small profundal charr morph in Skogsfjordvatn would have a similar trophic niche to the PB-morph in Fjellfrosvatn and tested this by using stomach content and stable isotope (δ13C and δ15N) analyses. Profundal benthic invertebrates, mainly chironomid larvae, Pisidium sp. mussels and small crustaceans, dominated the diets, and the δ13C and δ15N values indicated almost identical long-term trophic niches of the PB-morphs in both lakes. The prediction was therefore supported. This seems to indicate that the small profundal Arctic charr morphs in these lakes have evolved in parallel, in nearly identical and stable habitat and dietary niches in spite of likely different ecological interactions within the contrasting fish communities.

Variations in water temperature and implications for trout populations in the Upper Schoharie Creek and West Kill, New York, USA

Water temperature is a key component of aquatic ecosystems because it plays a pivotal role in determining the suitability of stream and river habitat to most freshwater fish species. Continuous temperature loggers and airborne thermal infrared (TIR) remote sensing were used to assess temporal and spatial temperature patterns on the Upper Schoharie Creek and West Kill in the Catskill Mountains, New York, USA. Specific objectives were to characterize (1) contemporary thermal conditions, (2) temporal and spatial variations in stressful water temperatures, and (3) the availability of thermal refuges. In-stream loggers collected data from October 2010 to October 2012 and showed summer water temperatures exceeded the 1-day and 7-day thermal tolerance limits for trout survival at five of the seven study sites during both summers. Results of the 7 August 2012 TIR indicated there was little thermal refuge at the time of the flight. About 690,170 m2 of water surface area were mapped on the Upper Schoharie, yet only 0.009% (59 m2) was more than 1.0 °C below the median water surface temperature (BMT) at the thalweg and no areas were more than 2.0 °C BMT. On the West Kill, 79,098 m2 were mapped and 0.085% (67 m2) and 0.018% (14 m2) were BMT by 1 and 2 °C, respectively. These results indicate that summer temperatures in the majority of the study area are stressful for trout and may adversely affect growth and survival. Validation studies are needed to confirm the expectation that resident trout are in poor condition or absent from the downstream portion of the study area during warm-water periods.

Spatial extent of biotic interactions affects species distribution and abundance in river networks: the freshwater pearl mussel and its hosts

AbstractAimAt what spatial extent are biotic interactions discernible influences on the distribution and abundance of species in river networks? We address this question with analyses of data from river networks for Margaritifera margaritifera, a freshwater mussel that passes its larval stage attached to a host fish.LocationTwenty river networks in Galicia, north‐western Spain.MethodsA maximum‐entropy approach was implemented to model the species’ distribution. Geostatistical mixed models were used to analyse the mussel's abundance in dendritic river networks. Predictor variables included the abundance and biomass of host fish (biotic interactions) and abiotic predictors for climate, geology and land‐form.ResultsMaxent models of species distribution were improved by 4.5% in terms of the area under the receiver‐operating characteristic curve (AUC) by the inclusion of biotic interactions. Host‐fish predictors contributed 63% of the Maxent model prediction of mussel presence. A geostatistical mixed model explained 52% of the variance in mussel abundance when including all the mussel abundance sites in the study area; abiotic predictors had no significance and salmonid biomass and resident trout population density were the only significant biotic predictors, together explaining 2.4% of the variance. An autocovariate representing biotic interactions between mussels and fish explained 11.7% of the variance. Using only sites where migratory host fish were present (n = 149), a mixed model explained more variance (78%) and the contribution from the autocovariate for parasite–host interactions was about three times larger than for the model including all sites (n = 419). The spatial autocorrelation from mussel–fish interactions had a spatial extent (geostatistical range) greater than 15 km.Main conclusionsInteractions between mussels and their larval hosts in river networks are manifested in spatial patterns of species distribution and abundance in this region, encompassing 20 river networks. The directional topology of dendritic river networks strongly supports the upstream dispersal of mussels by parasitized host fish as a component of spatial autocorrelation in mussel abundance.

Migration, Size, and Age Structure of Brook Trout (Salvelinus fontinalis) from Two Lake Superior Tributaries

Abstract Brook trout (Salvelinus fontinalis) from Lake Superior and its tributaries exhibit a variety of life histories and different growth characteristics; however, life history-specific effects on population dynamics are not well understood. The goal of this study was to characterize the movement patterns of brook trout in two streams with access to Lake Superior and compare age and growth metrics between fish that moved into Lake Superior (coasters) and resident brook trout that remained within the streams (residents). There were no significant differences in length, condition factor, or relative weight between coasters and residents from a given stream. Less than 5% of PIT tagged fish were detected emigrating from the study streams, although there was a peak in migration during the fall. The length distribution of fish differed among years for both streams. The data demonstrate that migratory brook trout are likely present, though rare, in the study streams and multiple life histories are present in ...

Divergence in expression of candidate genes for the smoltification process between juvenile resident rainbow and anadromous steelhead trout.

Rainbow and steelhead trout (Oncorhynchus mykiss), among other salmonid fishes, exhibit tremendous life history diversity, foremost of which is variation in migratory propensity. While some individuals possess the ability to undertake an anadromous marine migration, others remain resident in freshwater throughout their life cycle. Those that will migrate undergo tremendous physiological, morphological, and behavioral transformations in a process called smoltification which transitions freshwater-adapted parr to marine-adapted smolts. While the behavior, ecology, and physiology of smoltification are well described, our understanding of the proximate genetic mechanisms that trigger the process are not well known. Quantitative genetic analyses have identified several genomic regions associated with smoltification and migration-related traits within this species. Here we investigate the divergence in gene expression of 18 functional and positional candidate genes for the smoltification process in the brain, gill, and liver tissues of migratory smolts, resident parr, and precocious mature male trout at the developmental stage of out-migration. Our analysis reveals several genes differentially expressed between life history classes and validates the candidate nature of several genes in the parr-smolt transformation including Clock1α, FSHβ, GR, GH2, GHR1, GHR2, NDK7, p53, SC6a7, Taldo1, THRα, THRβ, and Vdac2.

Potential impacts of climate change on brook trout (Salvelinus fontinalis) populations in streams draining the Laurel Hill in Pennsylvania

To establish a baseline for assessment of climate change effects on populations of brook trout (Salvelinus fontinalis) in southwestern Pennsylvania, we surveyed a collective of headwater streams for fish population status and the development of annual air and stream temperature profiles. The 20 streams selected, 10 located on each facing slope of an anticlinal fold on the Appalachian Plateau, Laurel Hill, historically supported self-sustaining brook trout populations. In these watersheds, anthropogenic activity was minimal and riparian canopy intact. Three, from each facing slope along with the cool-water receiving stream for the respective tributary network, received riparian air and in-stream temperature data loggers. Data loggers were retrieved after one calendar year and temperature profiles for all eight streams downloaded. From these, we analyzed the air/stream temperature relationship for the calendar year and during the growing season for each headwater stream. Wild brook trout were present in all streams along with mottled sculpins (Cottus bairdi) and occasional representatives of six other species. Streams varied in the number of days experiencing thermal stress for brook trout (>18°C) ranging from a minimum of 0 to a maximum of 67, and the slope of each individual air/in-stream temperature relationship correlated with temporal stress. Local differences in riparian cover and groundwater inputs appear to be responsible for the deviation from climate model predictions and suggest resiliency in these headwater ecosystems. Of more immediate concern is the potential for the interruption of connectivity among tributary networks. Thermal profiles of the cool-water receiving streams revealed air temperature as a strong correlate of in-stream temperature. A warming climate may progressively constrict these avenues of gene flow among resident brook trout tributary populations leading eventually to their genetic isolation.

A historical perspective on drift foraging models for stream salmonids

Thirty years ago, Fausch (Can J Zool 62:441–451, 1984) proposed a simple model of optimal positions for drift-feeding salmonids in streams, whereby fish maximize their net energy intake (NEI) by selecting focal points in low water velocity near faster currents that deliver abundant drifting invertebrates. The theory was based on earlier observations in artificial and natural streams describing characteristics of salmonid positions and a conceptual model by Chapman (Am Nat 100:345–357, 1966). A test of this simple drift foraging model in a laboratory stream showed that the growth rate of juvenile trout and salmon increased with NEI, and that the rank of NEI at positions held by coho salmon (Oncorhynchus kisutch) correlated nearly perfectly with their rank in the dominance hierarchy. Fausch (1984) inferred from these findings that positions that optimize NEI, within the constraints of the dominance hierarchy, are the resource for which these stream salmonids compete. In turn, the model was used to test the effects of interspecific competition by coho salmon on the foraging positions held by brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta), and these results were used to infer potential effects of the introduced salmon on resident trout in Lake Michigan tributaries. Though the goals for this model were originally modest, it was tested in the field and further refined by Hughes and Dill (Can J Fish Aquat Sci 47:2039–2048, 1990) and others. During the last 20 years, the general theory has been incorporated into other models, which have been applied widely to analyze salmonid distribution and abundance in streams and rivers and used for management and restoration of habitat and flow regimes to benefit these fishes.

Linking spatial patterns of bed surface texture, bed mobility, and channel hydraulics in a mountain stream to potential spawning substrate for small resident trout

In this paper we examined spatial patterns of bed texture, bed mobility, and channel hydraulics in four reaches of a mountain stream with moderate gradient (<0.02) and linked them to availability and disturbance of potential spawning substrate for small resident cutthroat trout (body size 200mm). Within-reach spatial distribution and extent of potential substrate and disturbance risk enabled us to identify between-reach differences, which pointed to existence of two distinct domains within the study area. In two cobble–gravel reaches bed was generally too coarse for spawning and potential substrate appeared to be limited to small areas protected by flow obstructions. These hydraulically sheltered sites promoted accumulation of large quantities of fine sediment that seemed to pose high disturbance risk to the associated substrate patches. In contrast, general bed fining resulted in development of extensive areas of spawning substrate, unaffected by excess fine sediment deposition, in the unobstructed portions of gravel-dominated reaches. In all study reaches bed mobility and risk of scour disturbance was generally limited, even though its significance was sensitive to the adopted value of eggs burial depth. Abrupt transition between the two contrasting domains in response to relatively modest changes in channel texture led us to hypothesize that small-bodied salmonids in similar mountain streams may be vulnerable to habitat fragmentation that prevent fish movement between reaches but also to climate and land use changes that alter inputs of water and sediment into the channels. Overall, our within-reach analyses illustrated that between-reach differences in potential substrate availability and disturbance risk may be strongly dependent on small-scale hydro-geomorphic processes and their configuration in space relative to one another.

Unintended consequences of restoration: Loss of riffles and gravel substrates following weir installation

We used pre- and post-restoration channel surveys of the Donner und Blitzen River, Oregon, to evaluate the effects of grade-control structures on channel morphology and baseflow habitat conditions for native redband trout and other aquatic biota. Six years after installation, we found that the channel had a smaller proportion of riffles and pools and less gravel substrate, combined with an increase in the proportion of flat waters and consolidated clay on the bed surface. Both local scour downstream from weirs and backwater effects upstream from weirs appear to have caused the general flattening and fining of the channel. A direct-step backwater calculation indicates that backwaters extended to the upstream weir at both low and high flows, creating long sections of flat water separated by short, steep drops. Despite backwater effects, a comparison of longitudinal profiles before and six years after weir installation showed bed erosion downstream of nearly all weirs, likely a consequence of the cohesive clay material that dominates the channel bed and banks. A deep inner channel reflects the cohesive nature of the clay and the mechanisms of abrasion, and indicates that sediment load is low relative to the transport capacity of the flow. Unfortunately, weirs were problematic in this system because of the cohesive clay substrate, limited sediment supply, and low channel gradient. Although deeper flows due to backwaters might be more favorable for resident trout, less gravel and fewer riffles are likely to negatively impact trout spawning habitat, macroinvertebrate communities, and biofilm productivity. Our results demonstrate the potential limitations of a single-feature approach to restoration that may be ineffective for a given geomorphic context and may overlook other aspects of the ecosystem. We highlight the need to incorporate geomorphic characteristics of a system into project design and predictions of system response.

Movement of Resident Rainbow Trout Transplanted below a Barrier to Anadromy

AbstractWe tracked the movement of resident coastal rainbow trout Oncorhynchus mykiss irideus that were experimentally transplanted below a migration barrier in a northern California stream. In 2005 and 2006, age‐1 and older rainbow trout were captured above a 5‐m‐high waterfall in Freshwater Creek and individually marked with passive integrated transponder tags. Otolith microchemistry confirmed that the above‐barrier trout were the progeny of resident rather than anadromous parents, and genetic analysis indicated that the rainbow trout were introgressed with cutthroat trout O. clarkii. At each of three sampling events, half of the tagged individuals (n = 22 and 43 trout in 2005 and 2006, respectively) were released 5 km downstream from the waterfall (approximately 10 km upstream from tidewater), and an equal number of tagged individuals were released above the barrier. Tagged individuals were subsequently relocated with stationary and mobile antennae or recaptured in downstream migrant traps, or both, until tracking ceased in October 2007. Most transplanted individuals remained within a few hundred meters of their release location. Three individuals, including one rainbow trout released above the waterfall, were last detected in the tidally influenced lower creek. Two additional tagged individuals released above the barrier were found alive in below‐barrier reaches and had presumably washed over the falls. Two of seven tagged rainbow trout captured in downstream migrant traps had smolted and one was a presmolt. The smoltification of at least some individuals, coupled with above‐barrier “leakage” of fish downstream, suggests that above‐barrier resident trout have the potential to exhibit migratory behavior and to enter breeding populations of steelhead (anadromous rainbow trout) within the basin.

EFFECTS OF PASSAGE BARRIERS ON DEMOGRAPHICS AND STABILITY PROPERTIES OF A VIRTUAL TROUT POPULATION

ABSTRACTHabitat fragmentation is widely assumed to have negative effects on populations and communities, but some effects of fragmentation are subtle, difficult to measure and not always negative. For stream fish, barriers to upstream passage, such as waterfalls or culverts with perched outlets, are a common cause of fragmentation. We explored the effects of barriers on a virtual stream trout population occupying a network of reaches in the inSTREAM individual‐based trout population model. The model includes daily habitat selection by individual fish, with barriers represented by assuming trout cannot move upstream past a barrier and move downstream over a barrier only if habitat above it offers low potential fitness. In 78‐year simulations of a resident trout population occupying the stream network of a 25‐km2 catchment, we varied barrier density and observed effects on population stability properties and demographics. Increasing barrier density decreased subpopulation persistence in first‐order tributaries but not larger streams. Barriers at tributary mouths reduced the population's resistance to extreme flows, but additional barriers caused no further loss of resistance. Barriers reduced overall abundance and biomass at intermediate and high densities and caused a small but surprising increase in biomass at low density. In comparison with fish in the remainder of the network, fish in isolated first‐order tributaries had higher survival early in life, smaller sizes at age and reproduced as smaller, younger individuals. Fish that passed over barriers contributed relatively little to downstream populations but, had they not moved, would have increased persistence and abundance of the tributaries they moved from. Relationships such as these would be very difficult to elucidate without a model representing individual behaviour. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Influences of Habitat and Hybridization on the Genetic Structure of Redband Trout in the Upper Snake River Basin, Idaho

AbstractThe genetic structure of redband trout Oncorhynchus mykiss gairdnerii in the upper Snake River basin was investigated at various scales using 13 microsatellite loci. The majority of the genetic variation was partitioned between streams, although differentiation among watersheds was significant. This diversity was probably historically partitioned at the watershed scale when steelhead O. mykiss (anadromous rainbow trout) were present, with the exception of small, isolated, headwater streams where there may have been only resident trout. Genetic structure appears to have been altered by a combination of factors, including habitat fragmentation and hybridization with hatchery trout. Redband trout populations in the desert and montane environments both experienced reduced gene flow, but the desert populations displayed higher degrees of genetic differentiation. There was also a significant inverse relationship between the degree of genetic differentiation and the level of allelic diversity. Interspecific hybrids with cutthroat trout O. clarkii were detected within 9% of the sampled sites, but they made up only 2% of fish and were mostly confined to one sample location. In contrast, intraspecific hybrids with coastal rainbow trout O. m. irideus were detected within 31% of the samples sites and were more than twice as likely to be found where historical records indicated that stocking of hatchery rainbow trout occurred. The inclusion of intraspecific hybridized populations altered genetic structure by creating an artificial shared ancestry among populations from different drainages and led to higher levels of genetic variation in each of the populations. The threats of fragmentation and hybridization will need to be considered in developing conservation and management policies for redband trout in Idaho.

Biological and water quality responses to hydrologic disturbances in third‐order forested streams

AbstractWe compared stream ecosystem responses to two types of disturbances: flood and debris flows. A large storm in February 1996 disturbed four similarly sized sub‐watersheds of the Calapooia River, in the Cascade Mountains of Oregon, USA. All sub‐watersheds had flood and in two, 31 and 81% of the perennial channel, a debris flow affected the channel. For 8 years, we used a suite of approaches: stream temperature, nutrient regime, periphyton and macroinvertebrate assemblages, and resident trout abundance and habitat to evaluate the persistence of instream impacts. Differences in stream temperatures and nitrate‐nitrogen concentrations were evident, probably, due to the removal of vegetation and modification of riparian soils at the debris flow sites. Instream biological responses varied. After the event, fish, including trout, were rare with no fish at the debris flow sites. Within 6 years, trout densities (Oncorhynchus mykiss and Oncorhynchus clarki) were similar and young‐of‐the‐year trouts were common. In contrast, periphyton and macroinvertebrate assemblages differed. Periphyton biomass was lower and nitrogen‐fixing periphyton was more abundant at the debris flow sites. Macroinvertebrate assemblage diversity was higher at the debris flow sites due to fewer dominant taxa. Macroinvertebrate functional feeding groups also differed with fewer gatherers and more scrapers at the debris flow sites. Debris flow impacts related to loss of riparian canopy will probably persist until mature red alder stands are re‐established along stream‐reaches affected by debris flows to provide nitrogen input and shade. Copyright © 2011 John Wiley &amp; Sons, Ltd.

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.

A Genetic and Phenetic Baseline before the Recolonization of Steelhead above Howard Hanson Dam, Green River, Washington

AbstractIn 2011, steelhead Oncorhynchus mykiss (anadromous rainbow trout) will be systematically transported above Howard Hanson Dam, Green River, Washington. We are interested in providing data to help manage the stock of fish that are selected to recolonize the upper river and to track how resident rainbow trout above the dam respond genetically after 80 years of isolation. We characterized relevant gene pools in the upper Green River before fish transportation with 11 microsatellite loci to evaluate the genetic variability within and among collections. We also examined morphometric and coloration patterns as potential indicators of adaptive variation. Hatchery steelhead are clearly different from wild steelhead (FST = 0.037); genetic assignment tests correctly distinguished 91% of the steelhead. While there was no reduction in the amount of genetic variability in the resident rainbow trout above Howard Hanson Dam compared with that of wild steelhead collections below the dam, the two groups had low but statistically significant differences (FST = 0.03). The transport of juvenile and adult steelhead above the dam in the last 20 years may have affected these genetic results. Two collections of hatchery rainbow trout were highly differentiated from all other collections, and a STRUCTURE analysis indicated that there was no introgression of their genes into Green River fish. Morphologically, significant differences were seen between juvenile resident rainbow trout and steelhead. Resident trout had a stout caudal peduncle and more parr marks, whereas juvenile steelhead had a more elongate tail and fewer parr marks. Given these genetic and phenetic measures of differentiation, managers can monitor and screen the upstream passage of steelhead, and will be able evaluate the level of participation of resident rainbow trout gene pools in the recolonization event.

Can the lost migratory Salmo trutta stocks be compensated with resident trout stocks in coastal rivers?

Abstract The migration patterns and recapture rates of migratory and resident trout stocks ( Salmo trutta ) were compared by tagging and releasing hatchery-reared smolts of both migratory types in the same coastal rivers, Merikarvianjoki (river A) and Viantienjoki (river B), flowing into the Gulf of Bothnia, the Baltic Sea. There is no longer native trout production in these open rivers. The resident trout stocks originated from closed streams. The resident Karvianjoki (R1_ A ) trout released in river A remained resident more often (70%) than the migratory Isojoki trout (M1_ A ) (16%) ( P B ) and migratory Iijoki trout (M2_ B ) and their crossed offspring, released in river B, did not differ; about 30% of the recaptures from each genetic group were caught in the river and the rest in the sea. A few fish from both migratory types had a long feeding migration into the main basin of the Baltic Sea. The growth rate to catch size did not differ between the resident and migratory trout (R1_ A /M1_ A or R2_ B /M2_ B ). The migratory M1 trout had somewhat better survival and gave a higher catch than the resident R1 trout ( P P

Are growth and recapture of hatchery‐reared and resident brown trout (Salmo truttaL.) density dependent after stocking?

Abstract – The population density of brown trout (Salmo trutta) in a small natural system was manipulated in six equal‐length stream sections by stocking hatchery‐reared 1+ brown trout (unstocked, tripled and quintupled) over two consecutive years. The results showed that hatchery‐reared trout grew more slowly and were more mobile than resident trout, and that their growth was inversely density dependent. In contrast, growth of the resident trout was density independent. The recapture of 1+ resident and hatchery‐reared trout was inversely density dependent. This is most likely a consequence of increased competition. However, after a single winter the population density returned to its base level prior stocking and older resident trout showed no density‐dependent recapture. Thus, the advantage of stocking, here higher biomass, may have the detrimental effect of decreasing resident stocks of the same size class during summer.

Changes in fish communities following recolonization of the Cedar River, WA, USA by Pacific salmon after 103 years of local extirpation

AbstractMigration barriers are a major reason for species loss and population decline of freshwater organisms. Significant efforts have been made to remove or provide passage around these barriers; however, our understanding of the ecological effects of these efforts is minimal. Installation of a fish passage facility at the Landsburg Dam, WA, USA provided migratory fish access to habitat from which they had been excluded for over 100 years. Relying on voluntary recruitment, we examined the effectiveness of this facility in restoring coho (Oncorhynchus kisutch) salmon populations above the diversion, and whether reintroduction of native anadromous species affected the distribution and abundance of resident trout (O. mykiss and O. clarki). Before the ladder, late summer total salmonid (trout only) density increased with distance from the dam. This pattern was reversed after the ladder was opened, as total salmonid density (salmon + trout) approximately doubled in the three reaches closest to the dam. These changes were primarily due to the addition of coho, but small trout density also increased in lower reaches and decreased in upper reaches. A nearby source population, dispersal by adults and juveniles, low density of resident trout and high quality habitat above the barrier likely promoted rapid colonization of targeted species. Our results suggest that barrier removal creates an opportunity for migratory species to re‐establish populations leading to range expansion and potentially to increased population size. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Seasonal variations in stable isotope ratios of juvenile coho salmon (Oncorhynchus kisutch) from western Washington rivers

Salmon carcasses provide a marine derived nutrient (MDN) subsidy to river systems, but the extent to which it affects juvenile salmon growth is unclear. To evaluate temporal and spatial nutrient contributions from watershed sources and MDNs using stable isotopes, Skagit River (Washington, USA) juvenile coho salmon (Oncorhynchus kisutch) were collected. Muscle samples were taken from fry through smolts to measure temporal changes in δ15N and δ13C. δ15N and δ13C levels declined from emergence until fall, when they approached values for resident cutthroat trout (Oncorhynchus clarkii) collected above anadromous barriers. Muscle δ13C was highly variable and did not increase subsequently. However, coho salmon δ15N increased during the winter. March coho salmon parr δ15N levels suggested high variability in carcass availability for consumption. During the next spring, δ15N levels again declined. In Griffin Creek, a Snoqualmie River tributary, a significant relationship between carcass density and δ15N and δ13C levels was found in March coho salmon parr. At high spawner densities, some parr δ15N exceeded carcass values; however, parr δ13C increased moderately. These findings show that stable isotope data provide insights on seasonal sources of nutrients. In addition, results indicate that March coho salmon parr δ15N levels would be a useful index of carcass availability for overwintering juvenile consumption.

Do dams increase genetic diversity in brown trout (Salmo trutta)? Microgeographic differentiation in a fragmented river

Abstract – Local genetic differentiation may potentially arise in recently fragmented populations. Brown trout is a polytypic species exhibiting substantial genetic differentiation, which may evolve in few generations. Movement (semi‐)barriers in rivers may cause fragmentation, isolation and genetic differentiation in fish. In the Måna River (28 km) flowing from the alpine Lake Møsvatn to the boreal Lake Tinnsjø, construction of four hydropower dams during the period 1906–1957 have fragmented the previously (since last Ice Age) continuous wild resident brown trout population. Samples from the two lakes (N = 40) and six sites in the river (N = 30) isolated at different times were analysed at nine microsatellite loci. All populations showed substantial genetic variation (mean number of alleles per locus 5.3–8.9, observed heterozygosity 0.57–0.65 per population, overall Fst = 0.032). Pairwise multilocus Fst estimates indicated no significant differentiation between populations in the two lakes, and no or little differentiation in the lower river (Fst = 0.0035–0.0091). The microgeographic differentiation among wild resident trout at these sites was less than expected based on similar previous studies. However, results from the upper river, in particular the site immediately below the Lake Møsvatn outlet and dam, indicated isolation (Fst &gt; 0.035). Calculation of genetic distances and assignment tests corroborated these results, as did a significant correlation between years of isolation (since dam construction) and Fst. The population structuring is most likely a result of fragmentation by dams, which has increased overall genetic diversity. This increased local differentiation may be caused by natural selection, but more likely by genetic drift in small, recently fragmented populations. Increased local genetic diversity by genetic drift does not justify conservation measures aiming at preserving genetic diversity.