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.     

Retention of suspended sediment and phosphorus on a freshwater delta, South Lake Tahoe, California

The Upper Truckee River and Trout Creek, two major tributaries inflowing to Lake Tahoe, join to form what was historically the largest wetland in the Sierra Nevada mountain range that separates California and Nevada (USA). In the 1950s the delta floodplain of the Upper Truckee River was greatly reduced in area (38%) by urban development and the diversion of the river into a single excavated channel. Conversely, Trout Creek still flows through a wide marsh system with significant overbank flooding before entering Lake Tahoe. This study hypothesized that river channel reaches that are not incised within the delta floodplain retain more sediment and nutrients as a result of greater floodplain connectivity, compared to more incised and excavated reaches. Suspended sediment (SS) and total phosphorus (TP) load data from the delta formed by the Upper Truckee River and Trout Creek were collected using flow stage sensors, turbidometers and depth-integrated samples. During the spring snowmelt flow events monitored in 2003, SS load was reduced by 13–41% for the Upper Truckee River and by 68–90% for Trout Creek. Similar reductions in TP load were observed: 13–32% for the Upper Truckee River and 61–84% for Trout Creek. Monitoring of Trout Creek indicated a reduction in load per unit volume of 20–34% in a moderately incised reach versus a reduction of 51–77% in a non-incised marsh reach containing lagoons, braided channels and backwater areas created by a beaver dam. Smaller particle sizes, <10 μm, were retained in the lower marsh reach with similar efficiencies as larger particle sizes. If retention rates from the Trout Creek portion of the marsh are applied to the Upper Truckee River, sediment loading to Lake Tahoe for 2003 would have been reduced by 917 tons of SS.     

Formation and structural organization of the egg�Csperm bundle of the scleractinian coral Montipora capitata

The majority of scleractinian corals are hermaphrodites that broadcast spawn their gametes separately or packaged as egg–sperm bundles during spawning events that are timed to the lunar cycle. The egg–sperm bundle is an efficient way of transporting gametes to the ocean surface where fertilization takes place, while minimizing sperm dilution and maximizing the opportunity for gamete encounters during a spawning event. To date, there are few studies that focus on the formation and structure of egg–sperm bundle. This study explores formation, ultrastructure, and longevity of the egg–sperm bundle in Montipora capitata, a major reef building coral in Hawai‘i. Our results show that the egg–sperm bundle is formed by a mucus layer secreted by the oocytes. The sperm package is located at the center of each bundle, possibly reflecting the development of male and female gametes in different mesenteries. Once the egg–sperm bundle has reached the ocean surface, it breaks open within 10–35 min, depending on the environmental conditions (i.e., wind, water turbulence). Although the bundle has an ephemeral life span, the formation of an egg–sperm bundle is a fundamental part of the reproductive process that could be strongly influenced by climate change and deterioration of water quality (due to anthropogenic effects) and thus requires further investigation.     

The effects of environmental factors on the migratory movement patterns of Sacramento River yearling late-fall run Chinook salmon (Oncorhynchus tshawytscha)

Understanding smolt migration dynamics is a critical step in the preservation and conservation of imperiled salmonids in California’s Sacramento River system. Late-fall run Chinook salmon yearling smolts were acoustically tagged and tracked during their outmigration through California’s Sacramento River and San Francisco Estuary during 2007–2009. Migration rates were 14.3 km·day^-1 (± 1.3 S.E.) to 23.5 km?day^-1 (± 3.6 S.E.), similar to rates published for other West Coast yearling Chinook salmon smolt emigrations. Region-specific movement rates were fastest through the upper river regions, and slowest in the Sacramento/San Joaquin River Delta. River travel times were recorded for smolts travelling through a series of ten monitor-delimited reaches. Using these, a smolt travel time model determined by two parameters (movement rate and rate of population spreading) was then used to determine the influence of different factors on the model’s fit, using model selection with Akaike’s Information Criterion. The model that allowed for both year and reach to be expressed additively for both travel time and population spreading rate estimates, while accounting for a “release” effect, was the best supported model. Finally, several models incorporated environmental data as a linear predictor of movement rates. The addition of the environmental variables, in order of importance, river width to depth ratio, river flow, water turbidity, river flow to mean river flow ratio, and water velocity all resulted in improved model fit. Water temperature did not improve model fit. These environmental associations are discussed and potential improvements on the travel time model are suggested.     

Processing and macroinvertebrate colonization of detritus in an Alaskan subarctic stream

Monument Creek, a second-order Alaskan subarctic stream, experiences a severe thermal regime, with water temperatures of 0°C for six months. Leaching, long-term decomposition and macroinvertebrate colonization of 5 g leaf packs were studied using leaves of the major riparian tree species, alder Alnus tenuifolia Nutt., birch Betula papyrifera Marsh. and willow Salix alaxensis (Anderss.) Cov. subsp. alaxensis and Salix arbucolides Anders. var. glabra Anderss. The processing rates of birch (k = 0.0080) and willow (k = 0.0063) were moderate while alder was processed very rapidly (k = 0.0513). Leaching rates did not differ significantly among the three leaf species, and accounted for 10–15% of total weight loss. Densities and biovolume of colonizing insects increased from fall to winter. Shredders dominated insect biovolume by midwinter, but were a small fraction of total biovolume in leaf packs that had frozen over winter and were sampled following spring thaw. This may be a function of low food quality in spring and/or the timing of shredder life histories. Limited allochthonous input and constriction of the stream channel due to freezing, with resultant high shredder densities, may explain the relatively rapid detritus processing rates in Monument Creek, despite cold stream temperatures.     

Synchrony in the downstream migration of smolts and upstream migration of adult Atlantic salmon in the subarctic River Utsjoki

Seasonal and diel migration timing of wild Atlantic salmon Salmo salar smolts and adults were investigated annually (2001–2004) in the subarctic River Utsjoki, a tributary of the large River Teno (70° N), using underwater video monitoring. Submerged video cameras provided information on the exact timing and intensity of both migrations in a natural river channel, without disturbing the fish. In contrast to the mainly nocturnal migration pattern described from temperate rivers, the River Utsjoki smolts and adults migrated throughout the day. Smolts migrated most intensively during hours of rising (0300–0900 hours) and high sun (0900–1500 hours), while adults favoured the period of low sun (2100–0300 hours). Smolt migrations started in June and lasted on average 42 days. Adults usually ascended the site 2–3 weeks before the first descending smolts were observed and the adult migrations extended to the end of August. Seasonal synchrony was observed between smolt and adult migrations in years of slowly warming water, whereas in a year of exceptionally warm early summer (2002), smolts migrated earlier than adults. Thus, water temperature seemed to be an important environmental factor triggering the smolt migration, while the migration of adults was probably more fixed to a certain season. Weak positive correlations between fish counts and water temperature were observed, indicating that increasing water temperature may have promoted both smolt and adult migrations. The influence of discharge was negligible, although increasing discharge late in the season may have activated the remaining individuals in both groups.     

Using otolith chemical and structural analysis to investigate reservoir habitat use by juvenile Chinook salmon Oncorhynchus tshawytscha

Isotopic composition of ⁸⁷Sr:⁸⁶Sr and natural elemental tracers (Sr, Ba, Mg, Mn and Ca) were quantified from otoliths in juvenile and adult Chinook salmon Oncorhynchus tshawytscha to assess the ability of otolith microchemistry and microstructure to reconstruct juvenile O. tshawytscha rearing habitat and growth. Daily increments were measured to assess relative growth between natal rearing habitats. Otolith microchemistry was able to resolve juvenile habitat use between reservoir and natal tributary rearing habitats (within headwater basins), but not among catchments. Results suggest that 90% (n = 18) of sampled non‐hatchery adults returning to the Middle Fork Willamette River were reared in a reservoir and 10% (n = 2) in natal tributary habitat upstream from the reservoir. Juveniles collected in reservoirs had higher growth rates than juveniles reared in natal streams. The results demonstrate the utility of otolith microchemistry and microstructure to distinguish among rearing habitats, including habitats in highly altered systems.     

Riverine and early ocean migration and mortality patterns of juvenile steelhead trout (<Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>) from the Cheakamus River,British Columbia

High mortality (65–73%) occurred in the first month of the smolt migration in a population of wild steelhead trout. We used acoustic telemetry to monitor the downstream, estuarine, and early ocean migration of tagged smolts and estimate their mortality rates. After entering the Strait of Georgia most smolts migrated north through Johnstone and Queen Charlotte Straits rather than south through the Strait of Juan de Fuca. Of 51 smolts tagged in 2004 (49 in 2005), 36–38 (41–42) survived to leave freshwater and 14–19 (13–14) survived to leave the Strait of Georgia system. Mortality rates in separate segments of the migration were correlated with segment distances. An additional component of mobile sampling showed that few smolts died during the migration through Howe Sound. Migration rates averaged 0.7–0.9 body lengths per second (BL s⁻¹) downstream and 1.0–2.6 BL s⁻¹ in ocean waters. Aggregated detection probabilities of 92–96% on lines of ocean receivers suggest that migration routes of small fishes can be quantified over several hundred kilometres, and survival rates can be estimated for even a modest number of tagged fish. Quantifying mortality patterns during the smolt migration could help to determine causes of low marine survival rates observed in recent years.     

胡家溪摇蚊优势种的生产量动态和物质通量

2006年4月-2007年3月间,对长江支流清江二级支流——胡家溪大型底栖动物群落结构和生产量进行为期一年的调查.结果表明,主要优势摇蚊Conchapelopia sp.的生活史为一年两代,波特真开氏摇蚊(Eukiefferiella potthasti)为一年一代;Conchapelopia sp.的年均密度和年均生物量分别为105 ind·m-2、0.1g·m-2,波特真开氏摇蚊为280ind·m-2、0.3g·m-2.采用龄期频率法测算的周年生产量(g·m-2湿重)和生产量/生物量(P/B),Conchapelopia sp.分别为1.2g·m-2、2.3,波特真开氏摇蚊为1.4g·m-2、2.5.2种摇蚊的生产量时间重叠比例相似系数为0.7,重叠主要出现在冬春季节.营养基础分析发现,无形态碎屑为2种摇蚊的主要食物,占其组成的84％以上,对生产量的贡献率分别为80.1％、68.6％.2个优势种的年均物质通量分别为7.2和7.1g·m-2.     

Relative availability and utilization of algae in two subarctic rivers

The ash free dry weight of algae in the plankton, epilithon and gut of predominant herbivores was determined between June 1975 and July 1976 in 2 subarctic Canadian rivers (Baker Creek, Yellowknife River). Algae usually represented < 1% of suspended solids in both rivers and up to 8o% of material attached to rocks. While they were never found in the gut of rotifers and copepod nauplii in Baker Creek, they accounted for 0.5% of the contents of the planktonic conchostracon Lynceus brachyurus. Due to the inpalatable colonial structure of most attached algal species, the zoobenthos (mainly mayfly nymphs) contained only a few cells. Size selection against large fragments of detritus resulted in increased consumption of algae (5–13%) by Simulium venustum, S. decorum and S. arcticum. In the Yellowknife River, Diaptomus ashlandi (CIII–CVI stages) and Holopedium gibberum contained only a few (30–50) algal cells during summer. Because of size selection, large algae were not ingested by these species. The inpalatability of colonial algae greatly reduced consumption in the predominant zoobenthic species, Ephemerella coxalis and E. margarita. While all species digested diatoms, the Chlorophyta usually passed through the gut unharmed. It is concluded that algae are not an important energy source for invertebrates in either stream.     

Return of Salmon-Derived Nutrients from the Riparian Zone to the Stream during a Storm in Southeastern Alaska

Spawning salmon deliver nutrients (salmon-derived nutrients, SDN) to natal watersheds that can be incorporated into terrestrial and aquatic food webs, potentially increasing ecosystem productivity. Peterson Creek, a coastal watershed in southeast Alaska that supports several species of anadromous fish, was sampled over the course of a storm during September 2006 to test the hypothesis that stormflows re-introduce stored SDN into the stream. We used stable isotopes and PARAFAC modeling of fluorescence excitation–emission spectroscopy to detect flushing of DOM from salmon carcasses in the riparian zone back into a spawning stream. During the early storm hydrograph, streamwater concentrations of NH₄–N and total dissolved phosphorus (TDP), the fluorescent protein tyrosine and the δ¹⁵N content of DOM peaked, followed by a rapid decrease during maximum stormflow. Although δ¹⁵N has previously been used to track SDN in riparian zones, the use of fluorescence spectroscopy provides an independent indicator that SDN are being returned from the riparian zone to the stream after a period of intermediate storage outside the stream channel. Our findings further demonstrate the utility of using both δ¹⁵N of streamwater DOM and fluorescence spectroscopy with PARAFAC modeling to monitor how the pool of streamwater DOM changes in spawning salmon streams.     

The life cycle and drumming behavior of Zealeuctra claasseni (Frison) and Zealeuctra hitei ricker and ross (Plecoptera: Leuctridae) in Texas,USA 1

The life histories of Zealeuctra claasseni (Frison) and Zealeuctra hitei (Ricker and Ross) (Plecoptera: Leuctridae) were determined from observations and sampling on a Red River tributary in Cooke County, and N. Pecan Creek, Wise County, Texas, Dec., 1974‐June, 1978. Both species showed peak emergence when daily temperature was ca. 12°. Emergence was diurnal, with teneral adults and mating pairs found from dawn—late evening. Male‐female drumming signals were successfully recorded for both species. Incubation experiments at simulated stream temperatures indicated rapid embryonic development, so that larvae were visible through the chorion within ca. 4 weeks after deposition. Both species have highly flexible seasonal development patterns. Four types of life cycles seem possible, depending on field conditions: (1) slow univoltine, (2) fast univoltine, (3) slow semivoltine, and (4) fast semivoltine.     

Twenty years of hydropower regulation in the River Alta: long-term changes in abundance of juvenile and adult Atlantic salmon

The River Alta, northern Norway (70°N), was regulated for hydropower in 1987. Densities of juveniles and catches of adult Atlantic salmon have been studied since 1980–1981 to examine the effects of regulation. The need to control environmental variables during electrofishing was emphasized, as flow variables explained up to 42% of the variation in estimated juvenile densities. The number of spawning redds was counted along the river from 1996 to 2005. The annual number of spawning redds was correlated with the catch of multi-sea-winter salmon (predominantly females). In the upper 7 km section, just downstream of the power station outlet, juvenile densities were reduced by 80% from pre-regulation levels to minimum levels in 1992–1996. This was followed by partial recovery during 1997–2005, although not entirely back to pre-regulation levels. In contrast, the general trend in the middle part of the river was a linear increase in juvenile densities during 1981–2005. Decreased juvenile densities in the upper section was subsequently followed by reduced catches of adult salmon in this part of the river. The relative catches of smolt year classes migrating to sea in the upper section was reduced by up to 75% from 1991 onwards. Spawning and recruitment in the upper section have increased in recent years, probably back to the introduction of catch-and-release angling and an increase in salmon runs. However, present day smolt production in the upper section is still reduced compared to the middle part of the river, 18 years after regulation. The decreased densities of juvenile salmon in the upper section were probably caused by several factors, of which stranding mortality due to sudden drops in the water level and increased winter mortality due to changed environmental conditions, especially reduced ice-cover, may be the most important. In conclusion, the regulation caused a considerable reduction of the salmon production in the upper 16% of salmon reaches, but did not affect the salmon population negatively further downstream. This study illustrates that apparently small environmental disturbances can cause large changes in Atlantic salmon abundance in high latitude populations. Guest editors: R. L. Welcomme & G. Marmulla Hydropower, Flood Control and Water Abstraction: Implications for Fish and Fisheries     

Marine-derived nitrogen and carbon in freshwater-riparian food webs of the Copper River Delta,southcentral Alaska

After rearing to adulthood at sea, coho salmon (Oncorhynchus kisutch) return to freshwater to spawn once and then die on or near their spawning grounds. We tested the hypothesis that spawning coho salmon return marine N and C to beaver (Castor canadensis) ponds of the Copper River Delta (CRD), Cordova, southcentral Alaska, thereby enhancing productivity of the aquatic food webs that support juvenile coho salmon. We sampled three types of pond treatment: (1) natural enrichment by spawning salmon, (2) artificial enrichment via addition of salmon carcasses and eggs, and (3) ponds with no salmon enrichment. All ponds supported juvenile coho salmon. Seasonal samples of stable isotopes revealed that juvenile coho salmon, threespine sticklebacks (Gasterosteus aculeatus), caddisfly larvae, leeches, and chironomid midge larvae were enriched with marine N and C. The aquatic vascular plants bur reed (Sparganium hyperboreum), pondweed (Potamogeton gramineus), and mare’s tail (Hippuris vulgaris) were enriched with marine N only. Riparian vegetation (Sitka alder Alnus viridis ssp. sinuata and willow Salix spp.) did not show enrichment. Artificial additions of adult carcasses and eggs of coho salmon increased the δ¹⁵N and δ¹³C values of juvenile coho salmon. In this dynamic and hydrologically complex coastal environment, spawning coho salmon contributed marine N and C comprising 10–50% of the dietary needs of juvenile coho salmon through direct consumption of eggs and carcass material. Invertebrates that have assimilated marine N and C yield a further indirect contribution. This perennial subsidy maintains the productivity of the ecosystem of the coho salmon on the CRD.     

Do tributaries affect loads and fluxes of particulate organic matter,inorganic sediment and wood? Patterns in an upland river basin in south-eastern Australia

Tributary junctions are points in river networks where there can be an influx of organic matter and inorganic sediment. Addition of materials at tributary junctions is likely to alter food availability and habitat for aquatic organisms. We surveyed junctions of upland cobble-bed streams (stream orders 1–4, 2.2–10.8 m wide) in the Acheron River catchment (or watershed) in Victoria, Australia, to determine whether tributaries were an important source of suspended particulate matter, and whether benthic organic matter and coarse wood density increased at tributary junctions. We conducted measurements in high (austral spring 2005) and low flows (austral summer 2006) seasons. There were no systematic patterns in concentrations of suspended particulate matter with respect to positions within confluences (tributaries, upstream mainstem, downstream mainstem and confluence). However, total exports of suspended particulate matter in high-flow appeared to be the summation of exports from the upstream mainstem and the tributary in an approximate ratio of 2:1. In low flow, the 2:1 ratio was similar but the downstream mainstem value was similar to the upstream mainstem value (i.e., no clear summation). The fraction of organic matter in the suspended particulate matter did not depend on position within the junction, but was about 19% higher in the low-flow season. Tributaries had lower amounts of benthic organic matter than any measured positions in the mainstem, which themselves were indistinguishable. However, benthic organic matter was positively related to discharge ratio (tributary:mainstem), which may indicate that smaller, upriver junctions, which tended to have higher discharge ratios, were associated with higher standing crops of benthic organic matter. The distribution of coarse wood (logs and branches ≥10 cm diameter) was asystematic with respect to position in the junction. Overall, tributaries had little effect in these junctions, with the most evident effect being an increase of about one-third in exports of suspended particulate matter when flows are high.     

Geometry of biodiversity patterning: assemblages of benthic macroinvertebrates at tributary confluences

We assessed whether tributaries in upland catchments (=watersheds) affected assemblages of benthic macroinvertebrates in mainstems, as has been reported in northern hemisphere systems. Eight confluences of small to medium streams (stream orders 1–4, 2.2–10.8 m wide) were studied in the Acheron River basin in Victoria, Australia. For each confluence, two transects were sampled at each of five zones relative to the confluence: two zones upstream in the mainstem, one zone upstream in the tributary, one zone at the confluence and one zone downstream in the mainstem. Surveys were conducted in both high-flow and low-flow conditions. In mainstems, there was no change in macroinvertebrate density, taxonomic richness or functional feeding group composition downstream relative to upstream of the confluences. While tributaries statistically had distinctive benthic macroinvertebrate assemblages compared to mainstems, these distinctions were small. In low flows, densities in tributaries were substantially lower than in mainstems, but densities during high flows were more similar (albeit only about one-third as high as in low flow) in tributaries and mainstems. An inverse pattern was evident for taxonomic richness, where richness in tributaries and mainstems was similar in low flows but was greater in mainstems than in tributaries in high flows. We found little evidence of tributary effects in macroinvertebrate assemblages in this basin, which is at odds with some previous results from other continents. To explain this divergence, we suggest a conceptual model outlining factors that control variation in effects of tributaries on assemblages of benthic macroinvertebrates in mainstems.     

Hemimyzon nanensis, a new balitorid fish from the chao phraya basin, thailand

A new balitorid fish,Hemimyzon nanensis, is described from the Mae Nam Nan River basin, a tributary of the Chao Phraya River, in northern Thailand. This species is distinguishable from its congeners by the following unique combination of characters: pectoral fin with 9–11 simple and 10–12 branched, rays; pelvic fin with 3–4 (usually 3) simple and 8–9 (usually 9) branched rays; 59–63 total lateral line scales; distance between pelvic fin bases 8.1–9,6% SL.H. nanensis is the first species of the genus recorded outside China and Taiwan.     

Physiological and behavioural differences of hatchery and wild‐reared steelhead Oncorhynchus mykiss smolts of the same genetic origin

In many parts of the world release of hatchery‐reared smolts has long been used to mitigate for the deleterious effects of habitat loss and overfishing on salmonid populations. Of increasing concern is whether this may cause harm by spreading non‐native stocks and potentially releasing incompetent smolts. The objective of this study was to determine if smolt physiology and behavior of juveniles produced from a recently founded native broodstock differ from their wild (naturally‐reared) counterparts. In the fall of 2002 and 2003 juvenile wild steelhead were captured, PIT tagged, and returned ( n  = 1360 in 2002 and n  = 2708 in 2003) to Abernathy Creek. In winter of 2003 and 2004 hatchery‐reared fish were PIT tagged and later released ( n  = 1100 in 2003 and n  = 1400 in 2004) into Abernathy Creek. Gill biopsies were collected from wild and hatchery fish throughout the rearing and out‐migration season. The timing and speed of outmigration was assessed using two stationary PIT tag antennas (92–97% efficient). Hatchery migrants in 2003 were larger, had significantly lower gill Na⁺, K⁺‐ATPase activities, and migrated slower than wild fish. Results from the 2004 migratory season will also be presented. This study shows that hatchery rearing can result in smolts which are physiologically and behaviourally different from genetically similar wild fish. Whether these differences are critical enough to affect the rate of adult returns will be determined in future years.     

Restricted gene flow between resident Oncorhynchus mykiss and an admixed population of anadromous steelhead

The species Oncorhynchus mykiss is characterized by a complex life history that presents a significant challenge for population monitoring and conservation management. Many factors contribute to genetic variation in O. mykiss populations, including sympatry among migratory phenotypes, habitat heterogeneity, hatchery introgression, and immigration (stray) rates. The relative influences of these and other factors are contingent on characteristics of the local environment. The Rock Creek subbasin in the middle Columbia River has no history of hatchery supplementation and no dams or artificial barriers. Limited intervention and minimal management have led to a dearth of information regarding the genetic distinctiveness of the extant O. mykiss population in Rock Creek and its tributaries. We used 192 SNP markers and collections sampled over a 5‐year period to evaluate the temporal and spatial genetic structures of O. mykiss between upper and lower watersheds of the Rock Creek subbasin. We investigated potential limits to gene flow within the lower watershed where the stream is fragmented by seasonally dry stretches of streambed, and between upper and lower watershed regions. We found minor genetic differentiation within the lower watershed occupied by anadromous steelhead (F_ST = 0.004), and evidence that immigrant influences were prevalent and ubiquitous. Populations in the upper watershed above partial natural barriers were highly distinct (F_ST = 0.093) and minimally impacted by apparent introgression. Genetic structure between watersheds paralleled differences in local demographics (e.g., variation in size), migratory restrictions, and habitat discontinuity. The evidence of restricted gene flow between putative remnant resident populations in the upper watershed and the admixed anadromous population in the lower watershed has implications for local steelhead productivity and regional conservation.     

Longitudinal movement of fish in response to a single-day flow pulse

We studied whether fish were displaced longitudinally downstream by a 1-day whitewater kayaking pulse flow release from Camino Dam on Silver Creek, a tributary of the South Fork American River, California. On 15 September 2004 flows were increased from a base flow of 0.48 m³·s^-1 to a peak of 18.48 m³·s^-1 by midday, and decreased back to base levels. Rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) were observed in snorkel surveys before and after the pulse. Counts of young-of-the-year and juvenile trout were 26% and 9% lower after the pulse, respectively. Counts of adult trout were 12% higher. Six adult trout were radio-tagged and were observed in the reach before, during, and after the pulsed flow. Our results suggest that most trout were able to remain in the study reach during the pulse, but that smaller fish may be more likely to be displaced downstream.