Subyearling Chinook Salmon Research Articles

Diet, feeding patterns, and prey selection of subyearling Atlantic salmon (Salmo salar ) and subyearling chinook salmon (Oncorhynchus tshawytscha ) in a tributary of Lake Ontario

Summary Since juvenile Atlantic salmon (Salmo salar) and Chinook salmon (Oncorhynchus tshawytscha) occupy a similar habitat in Lake Ontario tributaries, we sought to determine the degree of diet similarity between these species in order to assess the potential for interspecific competition. Atlantic salmon, an historically important but currently extirpated component of the Lake Ontario fish community, are the focus of a bi-national restoration effort. Presently this effort includes the release of hatchery produced juvenile Atlantic salmon in Lake Ontario tributaries. These same tributaries support substantial numbers of naturally reproduced juvenile Pacific salmonids including Chinook salmon. Subyearling Atlantic salmon and subyearling Chinook salmon had significantly different diets during each of the three time periods examined. Atlantic salmon fed slightly more from the benthos than from the drift and consumed mainly chirononmids (47.0%) and ephemeropterans (21.1%). The diet of subyearling Chinook salmon was more closely associated with the drift and consisted mainly of chironomids (60.2%) and terrestrial invertebrates (16.0%). Low diet similarity between subyearling Atlantic salmon and subyearling Chinook salmon likely minimizes competitive interactions for food between these species in Lake Ontario tributaries. However, the availability of small prey such as chironomids which comprise over 50% of the diet of each species, soon after emergence, could constitute a short term resource limitation. To our knowledge this is the first study of interspecific diet associations between these two important salmonid species.

Comparing the survival rate of juvenile Chinook salmon migrating through hydropower systems using injectable and surgical acoustic transmitters

Acoustic telemetry is one of the primary technologies for studying the behavior and survival of fishes throughout the world. The size and performance of the transmitter are key limiting factors. The newly developed injectable transmitter is the first acoustic transmitter that can be implanted via injection instead of surgery. A two-part field study was conducted to evaluate the performance of the injectable transmitter and its effect on the survival of implanted fish. The injectable transmitter performed well and similarly to the proceeding generation of commercially-available JSATS transmitters tested concurrently. Snake River subyearling Chinook salmon smolts implanted with the injectable transmitter had a higher survival probability from release to each of eleven downstream detection arrays, because reach-specific survival estimates were significantly higher for the injectable group in three of the eleven reaches examined. Overall, the injectable group had a 0.263 (SE = 0.017) survival probability over the entire 500 km study area compared to 0.199 (0.012) for the surgically implanted group. The reduction in size and ability to implant the new transmitter via injection has reduced the tag or tagging effect bias associated with studying small fishes. The information gathered with this new technology is helping to evaluate the impacts of dams on fishes.

Trophic Interactions and Consumption Rates of Subyearling Chinook Salmon and Nonnative Juvenile American Shad in Columbia River Reservoirs

AbstractWe used a large lampara seine coupled with nonlethal gastric lavage to examine the diets and estimate consumption rates of subyearling Chinook Salmon Oncorhynchus tshawytscha during July and August 2013. During August we also examined the diet and consumption rates of juvenile American Shad Alosa sapidissima, a potential competitor of subyearling Chinook Salmon. Subyearling Chinook Salmon consumed Daphnia in July but switched to feeding on smaller juvenile American Shad in August. We captured no juvenile American Shad in July, but in August juvenile American Shad consumed cyclopoid and calanoid copepods. Stomach evacuation rates for subyearling Chinook Salmon were high during both sample periods (0.58 h−1 in July, 0.51 h−1 in August), and daily ration estimates were slightly higher than values reported in the literature for other subyearlings. By switching from planktivory to piscivory, subyearling Chinook Salmon gained greater growth opportunity. While past studies have shown that juvenile American Shad reduce zooplankton availability for Chinook Salmon subyearlings, our work indicates that they also become important prey after Daphnia abundance declines. The diet and consumption data here can be used in future bioenergetics modeling to estimate the growth of subyearling Chinook Salmon in lower Columbia River reservoirs.Received July 27, 2016; accepted November 22, 2016 Published online February 1, 2017

Ontogenetic shifts in the diets of juvenile Chinook Salmon: new insight from stable isotopes and fatty acids

Variations in marine prey availability and nutritional quality can affect juvenile salmon growth and survival during early ocean residence. Salmon growth, and hence survival, may be related to the onset of piscivory, but there is limited knowledge on the interplay between the prey field, environment, and salmon ontogeny. Subyearling Chinook Salmon (Oncorhynchus tshawytscha) and their potential prey were sampled in coastal waters off Willapa Bay, USA to explore this issue. Three seasonal prey assemblages were identified, occurring in spring (May), early summer (June – July), and late summer (August – September). The onset of piscivory, based on salmon stomach contents, fatty acids, and stable isotopes occurred later in 2011 compared to 2012, and coincided with the appearance of Northern Anchovy (Engraulis mordax). Salmon fork length (FL) and carbon isotope values (δ13C) increased with a fatty acid biomarker for marine phytoplankton and decreased with a freshwater marker, indicating dietary carbon sources changed as salmon emigrated from the Columbia River. Salmon FL also increased with nitrogen isotope ratios (δ15N), trophic position, and a fatty acid marker for piscivory – a consequence of the ontogenetic shift in diet to fish. Salmon grew faster and obtained larger size and condition by September 2011 compared to 2012, which was related to inter-annual differences in ocean conditions and the duration over which Northern Anchovy were available. Our results support the idea that juvenile salmon growth depends on the onset and duration of piscivory, suggesting both of these factors may be important components of lifetime growth and fitness.

Establishing and using study criteria to ensure the rigor and robustness of survival compliance testing at hydroelectric dams

An elaborate set of criteria have been developed by fish managers and regulators to assure the accuracy, precision, representativeness, and robustness of survival compliance studies coordinated within the Federal Columbia River Power System in the northwestern USA. Dam passage survival, defined as survival from the dam face to the tailrace mixing zone, must be ≥96 % for spring out migrating juvenile salmonids [i.e., yearling Chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss)] and ≥93 % for summer outmigrants (i.e., subyearling Chinook salmon). Survival must be estimated with a standard error ≤1.5 %. However, these quantitative benchmarks are only part of a multifaceted set of criteria, including representative dam operations, river discharge levels, and fish selection, along with tests of model validity that must be satisfied. These criteria are illustrated using acoustic-tag survival compliance studies conducted at The Dalles Dam on the Columbia River, 2010–2012. The results suggest evaluation criteria for survival compliance tests must balance the needs for rigor and robustness with the ability to reasonably perform the tests in naturally varying riverine systems.

Interannual variability in the feeding and condition of subyearling Chinook salmon off Oregon and Washington in relation to fluctuating ocean conditions

AbstractChinook salmon (Oncorhynchus tshawytscha) is one of several economically‐important species of salmon found in the Northeast Pacific Ocean. The first months at sea are believed to be the most critical for salmon survival, with the highest rate of mortality occurring during this period. In the present study, we examined interannual diet composition and body condition trends for late‐summer subyearling Chinook salmon caught off Oregon and Washington from 1998 to 2012. Interannual variability was observed in juvenile salmon diet composition by weight of prey consumed. Juvenile subyearling Chinook salmon were mainly piscivorous, with northern anchovy (Engraulis mordax) being especially important, making up half the diet by weight in some years. Annual diets clustered into two groups, primarily defined by their proportion of invertebrate prey (14% versus 39% on average). Diet composition was found to influence adult returns, with salmon from high‐invertebrate years returning in significantly larger numbers 2–3 yrs later. However, years that had high adult returns had overall lower stomach fullness and poorer body condition as juveniles, a counterintuitive result potentially driven by the enhanced survival of less fit individuals in better ocean conditions (top‐down effect). Ocean conditions in years with a higher percentage of invertebrates in salmon diets were significantly cooler from May to August, and bottom‐up processes may have led to a fall plankton community with a larger proportion of invertebrates. Our results suggest that the plankton community assemblage during this first fall may be critical in predicting adult returns of Chinook salmon in the Pacific Northwest.

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

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

Status after 5 Years of Survival Compliance Testing in the Federal Columbia River Power System (FCRPS)

AbstractSurvival studies of juvenile Pacific salmon Oncorhynchus spp. implanted with acoustic tags have been conducted at hydroelectric dams within the Federal Columbia River Power System in the Columbia and Snake rivers between 2010 and 2014 to assess compliance with the dam passage survival standards stipulated in the 2008 Biological Opinion. For juvenile yearling Chinook Salmon O. tshawytscha and steelhead O. mykiss that migrate downstream in the spring, dam passage survival (defined as survival from the upstream dam face to the tailrace mixing zone) must be ≥ 96%, and for subyearling Chinook Salmon that migrate downstream in summer, dam passage survival must be ≥ 93%. Precision requirement stipulates a standard error ≤ 1.5% (i.e., a 95% confidence interval of ± 3%). A total of 29 compliance tests have been conducted at six of eight main‐stem dams in the Federal Columbia River Power System, using over 109,000 acoustic‐tagged salmonid smolts. Of these 29 compliance studies, 23 met the survival standards and 26 met the precision requirements. Of the six dams evaluated to date, individual survival estimates range from 0.9597 to 0.9868 for yearling Chinook Salmon, from 0.9534 to 0.9952 for steelhead, and from 0.9076 to 0.9789 for subyearling Chinook Salmon. Averages across the six dams exceed the survival standards for all three migrant populations.Received November 9, 2015; accepted February 16, 2016 Published online June 27, 2016

Residency and Movement of Juvenile Chinook Salmon at Multiple Spatial Scales in a Tidal Marsh of the Columbia River Estuary

AbstractUse of the Columbia River estuary by juvenile Pacific salmon Oncorhynchus spp. is garnering more attention as managers look to improve salmon survival through estuary restoration. Studies have shown that juvenile salmon are abundant in shallow‐water habitats within the Columbia River estuary, but information on how juveniles exploit specific estuarine habitats is lacking. We used a combination of physical marks and PIT tag technology to record residence time, movement, and growth of juvenile Chinook Salmon O. tshawytscha, particularly subyearlings, within an emergent marsh of the Columbia River estuary during 2005, 2006, and 2008. We documented marsh‐scale residency and movement within the marsh complex and channel‐scale residency and movement within two small secondary channels. Many juvenile Chinook Salmon remained in the marsh for 2–4 weeks and increased in FL by 10–20 mm, with an average growth rate of 0.53 mm/d. Chinook Salmon entered secondary channels most frequently in late afternoon and occasionally did so against the tide. Our results indicate that subyearling Chinook Salmon take advantage of shallow estuarine habitat in the Columbia River to a greater extent than previously documented.Received January 8, 2016; accepted March 21, 2016 Published online June 21, 2016

Comparative Use of Shallow and Deepwater Habitats by Juvenile Pacific Salmon in the Columbia River Estuary Prior to Ocean Entry

AbstractThe degree to which fine‐scale habitat use by salmonid species and stocks varies within habitat types such as estuaries is not fully resolved. We sampled shallow shoreline and deeper main‐stem channel habitats in the Columbia River estuary over 3 years to compare salmon species composition, migration timing, density, size, and production type (hatchery or natural). Results indicated a high degree of spatial heterogeneity in habitat occupancy by the five salmonid species that are native to the basin. Salmonid communities at two channel habitat sites were much more similar to each other than to the community at a shoreline site. Salmonids sampled at the shoreline site were primarily subyearling Chinook Salmon Oncorhynchus tshawytscha and Chum Salmon O. keta and yearling Coho Salmon O. kisutch, with few other salmonids present. In contrast, channel habitat contained a higher diversity of salmon species, with samples representing all species of anadromous salmonids, including Sockeye Salmon O. nerka and steelhead O. mykiss. Salmonids in deeper channel habitat were generally larger than salmonids found along the shore, and the proportion of hatchery‐origin salmon was also higher in deep channel habitats. On a per‐area basis, we also found much higher densities of salmon along the shoreline than in channel habitats. For Chinook Salmon, habitat use also differed by genetic stock of origin: upper‐river stocks primarily used deeper channels, while lower‐river populations used both channel and shoreline areas. We concluded that sampling at both habitat types is required to fully encompass the migration patterns of all salmon evolutionarily significant units in the Columbia River basin. These spatial and temporal variations in salmon timing and density have ramifications for feeding, growth, and competitive interactions. This study provides information that is relevant for conservation efforts targeting specific fish populations and efforts to evaluate the potential impacts of in‐water activities in the Columbia River estuary.Received May 12, 2016; accepted August 19, 2016

Residence times of juvenile salmon and steelhead in off-channel tidal freshwater habitats, Columbia River, USA

We documented two life history strategies for juvenile salmonids as expressed in off-channel tidal freshwater habitats of the Columbia River: (i) active migrations by upper river Chinook salmon (Oncorhynchus tshawytscha) and steelhead (Oncorhynchus mykiss) during the primary spring and summer migration periods and (ii) overwinter rearing in tidal freshwater habitats by coho salmon (Oncorhynchus kisutch) and naturally produced Chinook salmon mostly from lower river sources. During spring–summer 2007–2008, acoustic-tagged fish originating above Bonneville Dam (rkm 234) had short residence times in off-channel areas (rkm 192–203): median 2.5 and 2.6 h for yearling (mean lengths 134 and 158 mm) and 3.0 and 3.4 h for subyearling (104 and 116 mm) Chinook salmon and 2.5 h for yearling steelhead (215 mm). The percentage of fish in off-channel areas out of the total in the main- and off-channels areas was highest for yearling Chinook salmon (8.1% and 9.3% for 2007 and 2008, respectively) and lowest for steelhead (4.0% for 2008) and subyearling Chinook salmon (3.6% and 6.1% for 2007 and 2008, respectively). In late January and early February 2010, 2011, and 2012, we captured and tagged yearling Chinook and coho salmon occupying off-channel tidal freshwater habitats. Median residence times in off-channel areas were 11.6–25.5 days for juvenile Chinook (106, 115, and 118 mm, respectively by year) and 11.2 days for coho salmon (116 mm). This study is the first to estimate residence times for juvenile salmonids specifically in off-channel areas of tidal fresh water and, most importantly, residence times for Chinook salmon expressing a life history of overwintering in tidal fresh water. The findings support restoration of shallow off-channel habitats in tidal freshwater portions of the Columbia River.

Re‐Estimating Temperature‐Dependent Consumption Parameters in Bioenergetics Models for Juvenile Chinook Salmon

AbstractResearchers have cautioned against the borrowing of consumption and growth parameters from other species and life stages in bioenergetics growth models. In particular, the function that dictates temperature dependence in maximum consumption (Cmax) within the Wisconsin bioenergetics model for Chinook Salmon Oncorhynchus tshawytscha produces estimates that are lower than those measured in published laboratory feeding trials. We used published and unpublished data from laboratory feeding trials with subyearling Chinook Salmon from three stocks (Snake, Nechako, and Big Qualicum rivers) to estimate and adjust the model parameters for temperature dependence in Cmax. The data included growth measures in fish ranging from 1.5 to 7.2 g that were held at temperatures from 14°C to 26°C. Parameters for temperature dependence in Cmax were estimated based on relative differences in food consumption, and bootstrapping techniques were then used to estimate the error about the parameters. We found that at temperatures between 17°C and 25°C, the current parameter values did not match the observed data, indicating that Cmax should be shifted by about 4°C relative to the current implementation under the bioenergetics model. We conclude that the adjusted parameters for Cmax should produce more accurate predictions from the bioenergetics model for subyearling Chinook Salmon.Received May 20, 2014; accepted November 5, 2014

Stock‐Specific Size and Timing at Ocean Entry of Columbia River Juvenile Chinook Salmon and Steelhead: Implications for Early Ocean Growth

AbstractJuvenile salmon transitioning from freshwater to marine environments experience high variation in growth and survival, yet the specific causes of this variation are poorly understood. Size at and timing of ocean entry may contribute to this variation because they influence both the availability of prey and vulnerability to predators. To explore this issue, we used stock assignments based on genetic stock identification and internal tags to document the stock‐specific size and timing of juvenile hatchery and presumed wild Columbia River Chinook SalmonOncorhynchus tshawytschaand steelheadO. mykissat ocean entry during 2007–2011. We found that juvenile salmon and steelhead had consistent stock‐specific capture dates, with lower‐river stocks typically having earlier timing than those originating farther upstream. Mean size also varied among stocks and was related to hatchery practices. Hatchery yearling Chinook Salmon and steelhead were consistently larger than wild fish from the same stocks, although timing in the estuary was similar. In contrast, hatchery subyearling Chinook Salmon were of similar size to wild fish but entered the ocean up to a month earlier. We evaluated the potential importance of these traits on early marine growth by estimating stock‐specific growth rates for Chinook Salmon caught in estuarine and ocean habitats. Growth rates were related to relative ocean entry timing, with lower growth rates for stocks that had only recently arrived in marine waters. Our results demonstrate that stocks within a single basin can differ in their size and timing of ocean entry, life history traits that contribute to early marine growth and potentially to the survival of juvenile salmon. Our results also highlight the necessity of considering stock‐specific variation in life history traits to understand salmon ecology and survival across the entire life cycle.Received January 16, 2015; accepted April 28, 2015

Diet variability of forage fishes in the Northern California Current System

As fisheries management shifts to an ecosystem-based approach, understanding energy pathways and trophic relationships in the Northern California Current (NCC) will become increasingly important for predictive modeling and understanding ecosystem response to changing ocean conditions. In the NCC, pelagic forage fishes are a critical link between seasonal and interannual variation in primary production and upper trophic groups. We compared diets among dominant forage fish (sardines, anchovies, herring, and smelts) in the NCC collected in May and June of 2011 and June 2012, and found high diet variability between and within species on seasonal and annual time scales, and also on decadal scales when compared to results of past studies conducted in the early 2000s. Copepoda were a large proportion by weight of several forage fish diets in 2011 and 2012, which differed from a preponderance of Euphausiidae found in previous studies, even though all years exhibited cool ocean conditions. We also examined diet overlap among these species and with co-occurring subyearling Chinook salmon and found that surf smelt diets overlapped more with subyearling Chinook diets than any other forage fish. Herring and sardine diets overlapped the most with each other in our interdecadal comparisons and some prey items were common to all forage fish diets. Forage fish that show plasticity in diet may be more adapted to ocean conditions of low productivity or anomalous prey fields. These findings highlight the variable and not well-understood connections between ocean conditions and energy pathways within the NCC.

Density and Condition of Subyearling Chinook Salmon in the Lower Columbia River and Estuary in Relation to Water Temperature and Genetic Stock of Origin

AbstractWe examined the hypotheses that density and morphometric condition of subyearling juvenile Chinook Salmon Oncorhynchus tshawytscha would decline during periods of high water temperature in the lower Columbia River and estuary. The hypotheses were tested using salmon density measurements and a condition anomaly calculated from residuals of the length–weight linear regression based on 5,536 subyearlings collected from brackish estuarine and tidal freshwater (TFW) habitats. We captured Chinook Salmon at all temperatures encountered (4.2–23.5°C). In the TFW zone, densities were highest at optimal temperatures and lowest at suboptimal and supraoptimal temperatures; in the estuary, density did not differ among temperature regimes. Fish condition was lowest in winter, when temperatures were suboptimal, and highest in summer, when temperatures were supraoptimal. Pairwise comparisons of fish condition between periods of optimal temperature (spring) and supraoptimal or stressful temperature (summer) showed little change in the estuary but a large, positive increase with temperature in the TFW zone. Similarly, we examined seasonal differences in the condition of 50–60‐mm fry and again found condition to be lowest in winter and highest in summer. Finally, using genetic information, we identified stock‑specific differences in migration timing and concluded that most large yearlings and many subyearlings migrated in late winter or spring and therefore were never exposed to high temperatures. Other prevalent stocks persisted in the estuary during periods of elevated temperature; however, the condition of those fish also tended to be higher or neutral in summer than in spring. High temperatures appeared to influence migration timing, as evidenced by reduced density in TFW reaches during summer. However, we found little support for the hypothesis that condition of juvenile Chinook Salmon is reduced during periods of high water temperature in the lower Columbia River and estuary.Received April 5, 2014; accepted April 11, 2014

Assessment of Subyearling Chinook Salmon Survival through the Federal Hydropower Projects in the Main‐Stem Columbia River

AbstractHigh survival through hydropower projects is an essential element in the recovery of Pacific salmon Oncorhynchus spp. populations in the Columbia River. High dam passage survival is also a regulatory requirement under the 2008 Biological Opinion (BiOp; established under the Endangered Species Act) on Federal Columbia River Power System operation. The BiOp requires dam passage survival to be at least 0.96 and at least 0.93 for spring and summer out‐migrating juvenile salmonids, respectively, and to be estimated with an SE of 0.015 or lower. An innovative virtual/paired‐release design was used to estimate dam passage survival, which was defined as survival from the upstream face of a dam to the tailrace mixing zone. A coordinated four‐dam study was conducted during the 2012 summer out‐migration using 14,026 subyearling Chinook Salmon O. tshawytscha out‐migrants with surgically implanted acoustic micro‐transmitter tags. The release–recapture design consisted of 9 different release locations and 14 different detection arrays. Each of the four estimates of dam passage survival exceeded BiOp requirements, with values ranging from 0.9414 to 0.9747 (SE = 0.0031–0.0114). The virtual/paired‐release design illustrated here has potential applicability wherever dam passage survival of migrant juvenile fish stocks must be estimated.Received November 21, 2013; accepted March 13, 2014

Interspecific habitat associations of juvenile salmonids in Lake Ontario tributaries: implications for Atlantic salmon restoration

Summary Diel variation in habitat use of subyearling Chinook salmon (Oncorhynchus tshawytscha), subyearling coho salmon (O. kisutch), yearling steelhead (O. mykiss), and yearling Atlantic salmon (Salmo salar) was examined during the spring in two tributaries of Lake Ontario. A total of 1318 habitat observations were made on juvenile salmonids including 367 on steelhead, 351 on Chinook salmon, 333 on Atlantic salmon, and 261 on coho salmon. Steelhead exhibited the most diel variation in habitat use and Chinook the least. Juvenile salmonids were generally associated with more cover and larger substrate during the day in both streams. Interspecific differences in habitat use in both streams occurred with Atlantic salmon (fast velocities) and coho salmon (pools) using the least similar habitat. Chinook salmon and Atlantic salmon used similar habitat in both streams. These findings should help guide future management actions specific to habitat protection and restoration of Atlantic salmon in Lake Ontario tributaries.

Infections by Renibacterium salmoninarum and Nanophyetus salmincola Chapin are associated with reduced growth of juvenile Chinook salmon, Oncorhynchus tshawytscha (Walbaum), in the Northeast Pacific Ocean

We examined 1454 juvenile Chinook salmon, Oncorhynchus tshawytscha (Walbaum), captured in nearshore waters off the coasts of Washington and Oregon (USA) from 1999 to 2004 for infection by Renibacterium salmoninarum, Nanophyetus salmincola Chapin and skin metacercariae. The prevalence and intensities for each of these infections were established for both yearling and subyearling Chinook salmon. Two metrics of salmon growth, weight residuals and plasma levels of insulin-like growth factor-1, were determined for salmon infected with these pathogens/parasites, both individually and in combination, with uninfected fish used for comparison. Yearling Chinook salmon infected with R.salmoninarum had significantly reduced weight residuals. Chinook salmon infected with skin metacercariae alone did not have significantly reduced growth metrics. Dual infections were not associated with significantly more severe effects on the growth metrics than single infections; the number of triple infections was very low and precluded statistical comparison. Overall, these data suggest that infections by these organisms can be associated with reduced juvenile Chinook salmon growth. Because growth in the first year at sea has been linked to survival for some stocks of Chinook salmon, the infections may therefore play a role in regulating these populations in the Northeast Pacific Ocean.

The impact of temperature stress and pesticide exposure on mortality and disease susceptibility of endangered Pacific salmon

Anthropogenic stressors, including chemical contamination and temperature stress, may contribute to increased disease susceptibility in aquatic animals. Specifically, the organophosphate pesticide malathion has been detected in surface waters inhabited by threatened and endangered salmon. In the presence of increasing water temperatures, malathion may increase susceptibility to disease and ultimately threaten salmon survival. This work examines the effect of acute and sublethal exposures to malathion on ocean-type subyearling Chinook salmon held under two temperature regimes. Chinook salmon were exposed to malathion at optimal (11°C) or elevated (19 and 20°C) temperatures. The influence of temperature on the acute toxicity of malathion was determined by generating 96-h lethal concentration (LC) curves. A disease challenge assay was also used to assess the effects of sublethal malathion exposure. The malathion concentration that resulted in 50% mortality (LC50; 274.1μgL−1) of the Chinook salmon at 19°C was significantly less than the LC50 at 11°C (364.2μgL−1). Mortality increased 11.2% in Chinook salmon exposed to malathion at the elevated temperature and challenged with Aeromonas salmonicida compared to fish held at the optimal temperature and exposed to malathion or the carrier control. No difference in disease challenge mortality was observed among malathion-exposed and unexposed fish at the optimal temperature. The interaction of co-occurring stressors may have a greater impact on salmon than if they occur in isolation. Ecological risk assessments considering the effects of an individual stressor on threatened and endangered salmon may underestimate risk when additional stressors are present in the environment.

Prey Availability, Consumption, and Quality Contribute to Variation in Growth of Subyearling Chinook Salmon Rearing in Riverine and Reservoir Habitats

AbstractWe examined prey availability, prey consumed, and diet energy content as sources of variation in growth of natural fall Chinook Salmon Oncorhynchus tshawytscha subyearlings rearing in riverine and reservoir habitats in the Snake River. Subyearlings in riverine habitat primarily consumed aquatic insects (e.g., Diptera, Ephemeroptera, Trichoptera), of which a high proportion was represented by adult, terrestrial forms. In the reservoir, subyearlings also consumed aquatic insects but also preyed heavily at times on nonnative lentic amphipods Corophium spp. and the mysid Neomysis mercedis, which were absent in riverine habitats. The availability of prey was typically much higher in the reservoir due to N. mercedis often composing over 90% of the biomass, but when this taxon was removed from consideration, biomass estimates were more often higher in the riverine habitat. Subyearling diets during 2009–2011were generally 17–40% higher in energy in the riverine habitat than in the reservoir. Observed growth in both length and weight were significantly higher in the riverine habitat than in the reservoir. Little is known about how temporal and spatial changes in the food web in large river landscapes influence populations of native anadromous fishes. Our results provide a glimpse of how the spread and establishment of nonnative prey species can reduce juvenile salmon growth in a large river impoundment, which in turn can affect migration timing and survival.