Growth Of Juvenile Chinook Salmon Research Articles

Complex temperature mosaics across space and time in estuaries: implications for current and future nursery function for Pacific salmon

Water temperature is a key dimension of estuaries that can influence important biological processes including organismal growth, survival, and habitat use. For example, juvenile Pacific salmon rely on temperature-mediated growth opportunities in estuaries during seaward migration in the spring and summer. However, oncoming climate change is warming estuary temperatures and transforming growth potential. Yet, it is likely that estuary water temperatures are complex and dynamic across space and time. Here we investigated spatial and temporal patterns of water temperature across two contrasting estuaries on Vancouver Island, BC, and used these data to simulate juvenile Chinook salmon growth potential under both present conditions and a simplified scenario of climate warming. Summer temperatures were warmer and more spatially homogeneous across the Englishman River estuary relative to the Salmon River estuary. Within each system, temperature was variable across habitats and sites, appearing to be driven by a combination of local climate, river and ocean temperatures, tidal fluctuations, and habitat features. This shifting mosaic of temperatures generated a complex portfolio of growth opportunities for juvenile Chinook salmon. There were broad patterns of increasing growth potential across both systems as temperatures warmed in early summer. However, excessively hot late summer temperatures drove steep declines in growth potential across the Englishman River estuary, while positive trends continued through August in the Salmon River estuary. A simple climate change scenario revealed that estuaries and habitats may have differing vulnerabilities to increasing temperatures—with climate warming, favorable growth opportunities were even more constricted to early summer in the Englishman River estuary, but expanded across most habitats and times in the Salmon River estuary given lower baseline temperatures and higher habitat complexity. Collectively, this work underscores the importance of habitat complexity and local climate for maintaining diverse growth opportunities across estuaries, which may confer resilience to the nursery function of estuaries as temperature mosaics warm.

Size‐selective predation effects on juvenile Chinook salmon cohort survival off Central California evaluated with an individual‐based model

AbstractVariation in the recruitment of salmon is often found to be correlated with marine climate indices, but mechanisms behind environment–recruitment relationships remain unclear and correlations often break down over time. We used an ecosystem modeling approach to explore bottom‐up and top‐down mechanisms linking a variable environment to salmon recruitment variations. Our ecosystem model incorporates a regional ocean circulation submodel for hydrodynamics, a nutrient‐phytoplankton‐zooplankton submodel for producing planktonic prey fields, and an individual‐based model (IBM) representing juvenile Chinook salmon (Oncorhynchus tshawytscha), combined with observations of foraging distributions and diet of a seabird predator. The salmon IBM consists of modules, including a juvenile salmon growth module based on temperature and salmon–prey availability, a behavior‐based movement module, and a juvenile salmon predation mortality module based on juvenile salmon size distribution and predator–prey interaction probability. Seabird–salmon interactions depend on spatial overlap and juvenile salmon size, whereby salmon that grow past the size range of the prey distribution of the predator will escape predation. We used a 21‐year historical simulation to explore interannual variability in juvenile Chinook salmon growth and predation‐mediated survival under a range of ocean conditions for sized‐based mortality scenarios. We based a series of increasingly complex predation scenarios on seabird observational data to explore variability in predation mortality on juvenile Chinook salmon. We initially included information about the predator spatial distribution, then added population size, and finally the predator's diet percentage made up of juvenile salmon. Model agreement improves with added predator complexity, especially during periods when predator abundance is high. Overall, our model found that when the fraction of juvenile salmon in seabird diet increased relative to alternate prey (e.g., Northern anchovy Engraulis mordax, and juvenile rockfish Sebastes spp.), there was a concomitant decrease in salmon cohort survival during their first year at sea.

Future climate change-related decreases in food quality may affect juvenile Chinook salmon growth and survival

Global climate change is projected to raise global temperatures by 3.3–5.7 °C by 2100, resulting in changes in species composition, abundance, and nutritional quality of organisms at the base of the marine food web. Predicted increases in prey availability and reductions in prey nutritional quality under climate warming in certain marine systems are expected to impact higher trophic levels, such as fish and humans. There is limited knowledge of the interplay between food quantity and quality under warming, specifically when food availability is high, but quality is low. Here, we conducted an experiment assessing the effects of food quality (fatty acid composition and ratios) on juvenile Chinook salmon's (Oncorhynchus tshawytscha) body and nutritional condition, specifically focusing on RNA:DNA ratio, Fulton's K, growth, mortality and their fatty acid composition. Experimental diets represented three different climate change scenarios with 1) a present-day diet (Euphausia pacifica), 2) a control diet (commercial aquaculture diet), and 3) a predicted Intergovernmental Panel on Climate Change (IPCC) worst-case scenario diet with low essential fatty acid concentrations (IPCC SSP5-8.5). We tested how growth rates, RNA:DNA ratio, Fulton's K index, fatty acid composition and mortality rates in juvenile Chinook salmon compared across diet treatments. Fatty acids were incorporated into the salmon muscle at varying rates but, on average, reflected dietary concentrations. High dietary concentrations of DHA, EPA and high DHA:EPA ratios, under the control and present-day diets, increased fish growth and condition. In contrast, low concentrations of DHA and EPA and low DHA:EPA ratios in the diets under climate change scenario were not compensated for by increased food quantity. This result highlights the importance of considering food quality when assessing fish response to changing ocean conditions.

Detrital food web contributes to aquatic ecosystem productivity and rapid salmon growth in a managed floodplain.

Similar to many large river valleys globally, the Sacramento River Valley has been extensively drained and leveed, hydrologically divorcing river channels from most floodplains. Today, the former floodplain is extensively managed for agriculture. Lack of access to inundated floodplains is recognized as a significant contributing factor in the decline of native Chinook Salmon (Oncorhynchus tshawytscha). We observed differences in salmon growth rate, invertebrate density, and carbon source in food webs from three aquatic habitat types—leveed river channels, perennial drainage canals in the floodplain, and agricultural floodplain wetlands. Over 23 days (17 February to 11 March, 2016) food web structure and juvenile Chinook Salmon growth rates were studied within the three aquatic habitat types. Zooplankton densities on the floodplain wetland were 53x more abundant, on average, than in the river. Juvenile Chinook Salmon raised on the floodplain wetland grew at 0.92 mm/day, 5x faster than fish raised in the adjacent river habitat (0.18 mm/day). Two aquatic-ecosystem modeling methods were used to partition the sources of carbon (detrital or photosynthetic) within the different habitats. Both modeling approaches found that carbon in the floodplain wetland food web was sourced primarily from detrital sources through heterotrophic pathways, while carbon in the river was primarily photosynthetic and sourced from in situ autotrophic production. Hydrologic conditions typifying the ephemerally inundated floodplain—shallower depths, warmer water, longer water residence times and predominantly detrital carbon sources compared to deeper, colder, swifter water and a predominantly algal-based carbon source in the adjacent river channel—appear to facilitate the dramatically higher rates of food web production observed in the floodplain. These results suggest that hydrologic patterns associated with seasonal flooding facilitate river food webs to access floodplain carbon sources that contribute to highly productive heterotrophic energy pathways important to the production of fisheries resources.

English

This study investigated the effects of coloured structures on the growth of juvenile landlocked fall Chinook salmon (Oncorhynchus tshawytscha, Walbaum) during hatchery rearing. Structures consisted of an array of four aluminum angles painted one of four colours: Silver, red, black and green, which were vertically suspended in circular tanks. After 25 days, mean total lengths and weights of individual salmon reared in tanks with the green arrays were significantly (P 0.05) among the colour treatments. Final total tank biomass and gain were also not significantly different (P > 0.05) among the colour treatments. The results of this study indicate that structural colours could be considered to maximize juvenile Chinook salmon growth during hatchery rearing. Key words: Chinook salmon, colour, Oncorhynchus tshawytscha, hatchery rearing.

How Small Can We Go? Evaluating Survival, Tag Retention, and Growth of Juvenile Chinook Salmon Implanted with a New Acoustic Microtag

AbstractRecently, an acoustic microtag, called the Eel–Lamprey Acoustic Tag (ELAT), was developed for use in juvenile American Eel Anguilla rostrata and Pacific Lamprey Entosphenus tridentatus. The ELAT (12.0 × 2.0 mm; 0.088 g in air) has the potential to be used to study other small‐bodied fishes. In this pilot study, juvenile Chinook Salmon Oncorhynchus tshawytscha 52–99 mm fork length (FL) were surgically implanted with an ELAT (n = 99) or remained untagged (control; n = 101). Fish were held for 30 d to evaluate survival, tag retention, and growth. Survival did not differ significantly between control (n = 98 or 97.0%) and tagged fish (n = 95 or 95.9%). Logistic regression analysis suggested that survival was influenced by initial FL and tag burden. Mortalities only occurred in tagged fish with initial FLs of ≤65 mm (FL range = 57–65 mm; tag burdens = 3.8–5.9%). However, a tagged fish with an initial FL of 62 mm or an initial tag burden of 4.1% (FL range = 58–64 mm) was predicted to have a 90% chance of survival, and the smallest fish that survived the study was 54 mm FL (1.8 g; tag burden = 4.9%). Tag retention was high (99%), with only one fish expelling an ELAT during the study. Control fish grew slightly larger than the tagged individuals, but differences in final pooled FLs and weight gains were not significant. Our results demonstrate the successful surgical implantation of the ELAT into Chinook Salmon as small as 54 mm FL (1.8 g).

GrowChinook: an optimized multimodel and graphic user interface for predicting juvenile Chinook salmon growth in lentic ecosystems

Linked foraging and bioenergetics models allow for increased understanding of fish growth potential and behavior by incorporating prey availability coupled to environmental conditions including temperature and prey visibility. To inform our understanding of growth and vertical migration patterns of Chinook salmon (Oncorhynchus tshawytscha) inhabiting lentic ecosystems, we linked foraging and bioenergetics models to create GrowChinook ( http://growchinook.fw.oregonstate.edu/ ). This multimodel design and optimization routine has broad applications in examining growth potential and predicting habitat use in stratified environments. We demonstrate the use of GrowChinook for the spring–summer rearing period in three Willamette River basin reservoirs, Oregon, USA. These reservoirs support juvenile spring Chinook salmon that exhibit a novel reservoir-reared life history that includes larger juvenile fish compared with nearby stream-reared subyearlings. Model outputs of predicted growth and depth use patterns based on observed prey distributions and environmental conditions were corroborated by observed empirical size and growth data from other years. Our simulations support diel vertical migration as a tactic that increases growth potential and contribute to understanding juvenile Chinook salmon growth in stratified systems.

Differential north–south response of juvenile Chinook salmon (Oncorhynchus tshawytscha) marine growth to ecosystem change in the eastern Bering Sea, 1974–2010

Abstract Chinook salmon (Oncorhynchus tshawytscha, Salmonidae) returns to western Alaska were historically high and variable but recently reached record lows. Understanding the differential influence of climatic and oceanic conditions on the growth of juvenile Chinook salmon in the north and south eastern Bering Sea is key to understanding mechanisms and factors affecting the production dynamics of Chinook salmon from western Alaska and the Arctic. Summer growth was lower and more variable among years for Chinook salmon in the south than the north eastern Bering Sea. Summer growth decreased with a rise in sea temperatures in the north and south and increased with more sea ice coverage and a later time of ice retreat in the south but not in the north. Capelin (Mallotus villosus), an important prey for juvenile Chinook salmon in the north and during cold years may link increased growth to cooler sea temperatures. Reduced and more variable summer growth of juvenile Chinook salmon from the eastern Bering Sea with warming may have implications on overwintering survival.

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

Environmental conditions impacting juvenile Chinook salmon growth off central California: An ecosystem model analysis

AbstractA fully coupled ecosystem model is used to identify the effects of environmental conditions and upwelling variability on growth of juvenile Chinook salmon in central California coastal waters. The ecosystem model framework consists of an ocean circulation submodel, a biogeochemical submodel, and an individual‐based submodel for salmon. Simulation results indicate that years favorable for juvenile salmon growth off central California are characterized by particularly intense early season upwelling (i.e., March through May), leading to enhanced krill concentrations during summer near the location of ocean entry (i.e., Gulf of the Farallones). Seasonally averaged growth rates in the model are generally consistent with observed values and suggest that juvenile salmon emigrating later in the season (i.e., late May and June) achieve higher weight gains during their first 90 days of ocean residency.

Using a Laboratory‐Based Growth Model to Estimate Mass‐ and Temperature‐Dependent Growth Parameters across Populations of Juvenile Chinook Salmon

AbstractTo estimate the parameters that govern mass‐ and temperature‐dependent growth, we conducted a meta‐analysis of existing growth data from juvenile Chinook Salmon Oncorhynchus tshawytscha that were fed an ad libitum ration of a pelleted diet. Although the growth of juvenile Chinook Salmon has been well studied, research has focused on a single population, a narrow range of fish sizes, or a narrow range of temperatures. Therefore, we incorporated the Ratkowsky model for temperature‐dependent growth into an allometric growth model; this model was then fitted to growth data from 11 data sources representing nine populations of juvenile Chinook Salmon. The model fit the growth data well, explaining 98% of the variation in final mass. The estimated allometric mass exponent (b) was 0.338 (SE = 0.025), similar to estimates reported for other salmonids. This estimate of b will be particularly useful for estimating mass‐standardized growth rates of juvenile Chinook Salmon. In addition, the lower thermal limit, optimal temperature, and upper thermal limit for growth were estimated to be 1.8°C (SE = 0.63°C), 19.0°C (SE = 0.27°C), and 24.9°C (SE = 0.02°C), respectively. By taking a meta‐analytical approach, we were able to provide a growth model that is applicable across populations of juvenile Chinook Salmon receiving an ad libitum ration of a pelleted diet.Received May 20, 2014; accepted December 4, 2014

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.

Functions of restored wetlands for juvenile salmon in an industrialized estuary

The Duwamish estuary is an industrialized waterway located in Seattle, WA, USA. Despite a history of habitat loss, naturally produced juvenile Chinook salmon use the estuary. In addition to experiencing degraded habitat in the estuary, wild salmon growth may be affected by competition with more than three million hatchery fish released yearly into the river. Restoring habitat to benefit salmon in the Duwamish River is a priority for trustees of public resources, and a number of wetland restoration sites have been created there. We tested the function of restored sites in the Duwamish estuary for juvenile Chinook salmon by comparing fish densities from enclosure nets or beach seines at three paired restored/un-restored sites and by applying environmental and diet data to a bioenergetics model. We also examined temporal and diet overlap of wild juvenile Chinook salmon with other salmon species and with hatchery-reared Chinook salmon using non-metric multidimensional scaling (NMDS). At a brackish upstream site with a relatively large opening to the river, we found higher densities of juvenile Chinook salmon at the restored site. NMDS results indicated that juvenile Chinook salmon fed on different taxa at the restored sites than at the reference sites. However, modeled growth was similar at restored and reference sites. Co-occurring juvenile chum and Chinook salmon fed differently, with chum eating smaller prey, and Chinook salmon eating larger prey. Co-occurring hatchery and wild juvenile Chinook salmon had similar diets, indicating that they may compete for prey. However, modeled growth was positive and did not differ between hatchery and wild fish, suggesting that food was not limiting. Bioenergetics models indicated that overall juvenile Chinook salmon growth potential at the brackish water site was consistently higher than at more saline sites. Our results suggest that restoration sites in the Duwamish estuary that have larger access openings and are located in brackish water may have increased function over other configurations.

Energy dynamics and growth of Chinook salmon (Oncorhynchus tshawytscha) from the Central Valley of California during the estuarine phase and first ocean year

The greatest rates of energy accumulation and growth in subyearling Chinook salmon ( Oncorhynchus tshawytscha ) occurred during the first month following ocean entry, supporting the importance of this critical period. Data from an 11-year study in the coastal ocean off California and the San Francisco Estuary revealed that juvenile salmon gained 3.2 kJ·day–1and 0.8 g·day–1, representing 4.3%·day–1and 5.2% day–1, respectively, relative to estuary exit values. Little gain in energy (0.28 kJ·day–1) or size (0.07 g·day–1) occurred in the estuary, indicating that the nursery function typically ascribed to estuaries can be deferred to initial ocean residence. Calculated northern anchovies ( Engraulis mordax ) equivalents to meet energy gains were one anchovy per day in the estuary (8% body weight·day–1) and about three per day immediately following ocean entry (15% body weight·day–1). Energy content in the estuary was positively related to higher salinity and lower freshwater outflow, whereas in the ocean, cooler temperatures, lower sea level, and greater upwelling resulted in greater gains. These results suggest that greater freshwater flows, warmer sea temperatures, and reduced or delayed upwelling, all of which are indicated by some (but not all) climate models, will likely decrease growth of juvenile Chinook salmon, leading to reduced survival.

A fish of many scales: extrapolating sublethal pesticide exposures to the productivity of wild salmon populations

For more than a decade, numerous pesticides have been detected in river systems of the western United States that support anadromous species of Pacific salmon and steelhead. Over the same interval, several declining wild salmon populations have been listed as either threatened or endangered under the U.S. Endangered Species Act (ESA). Because pesticides occur in surface waters that provide critical habitat for ESA-listed stocks, they are an ongoing concern for salmon conservation and recovery throughout California and the Pacific Northwest. Because pesticide exposures are typically sublethal, a key question is whether toxicological effects at (or below) the scale of the individual animal ultimately reduce the productivity and recovery potential of wild populations. In this study we evaluate how the sublethal impacts of pesticides on physiology and behavior can reduce the somatic growth of juvenile chinook salmon (Oncorhynchus tshawytscha) and, by extension, subsequent size-dependent survival when animals migrate to the ocean and overwinter in their first year. Our analyses focused on the organophosphate and carbamate classes of insecticides. These neurotoxic chemicals have been widely detected in aquatic environments. They inhibit acetylcholinesterase, an enzyme in the salmon nervous system that regulates neurotransmitter-mediated signaling at synapses. Based on empirical data, we developed a model that explicitly links sublethal reductions in acetylcholinesterase activity to reductions in feeding behavior, food ration, growth, and size at migration. Individual size was then used to estimate size-dependent survival during migration and transition to the sea. Individual survival estimates were then integrated into a life-history population projection matrix and used to calculate population productivity and growth rate. Our results indicate that short-term (i.e., four-day) exposures that are representative of seasonal pesticide use may be sufficient to reduce the growth and size at ocean entry of juvenile chinook. The consequent reduction in individual survival over successive years reduces the intrinsic productivity (lambda) of a modeled ocean-type chinook population. Overall, we show that exposures to common pesticides may place important constraints on the recovery of ESA-listed salmon species, and that simple models can be used to extrapolate toxicological impacts across several scales of biological complexity.

Immunocompetence of juvenile chinook salmon against Listonella anguillarum following dietary exposure to polycyclic aromatic hydrocarbons.

Juvenile chinook salmon (Oncorhynchus tschawytscha) were fed a mixture of 14 polycyclic aromatic hydrocarbon (PAH) compounds that reflected the PAH composition of salmon stomach contents in an urban estuary of Puget Sound, Washington (USA). Following a 28-d dietary exposure, a standardized Listonella anguillarum challenge model was used to determine whether PAH exposure (16, 64, and 252 mg/kg wet wt feed) causes reduced disease resistance under the conditions examined in this study. To assess innate immunity, five replicate groups of fish per dose were acclimated for one week, exposed to a lethal concentration 60 of bacteria, and monitored for 14 d. In a parallel experiment, the effects of PAH exposure on the acquired immune response were examined by immersion vaccinating fish against L. anguillarum and allowing specific immunity to develop for three weeks prior to challenge. All mortalities were aseptically sampled to confirm L. anguillarum infections. No significant differences in fish length, weight, or coefficient of condition were observed. These controlled laboratory experiments suggest that dietary exposures to an environmentally relevant mixture of PAH compounds do not alter the immunocompetence or growth of juvenile chinook salmon.

Effects of disturbance on contribution of energy sources to growth of juvenile chinook salmon (Oncorhynchus tshawytscha) in boreal streams

We used stable isotopes of carbon in a growth-dependent tissue-turnover model to quantify the relative contribution of autochthonous and terrestrial energy sources to juvenile chinook salmon (Oncorhynchus tshawytscha) in five small boreal streams tributary to the upper Yukon River. We used a tissue-turnover model because fish did not grow enough to come into isotopic equilibrium with their diet. In two streams, autochthonous energy sources contributed 23 and 41% to the growth of juvenile salmon. In the other three, fish growth was largely due to terrestrial (i.e., allochthonous) energy sources. This low contribution of autochthonous energy appeared to be related to stream-specific disturbances: a recent forest fire impacted two of the streams and the third was affected by a large midsummer spate during the study. These disturbances reduced the relative abundance of herbivorous macroinvertebrates, the contribution of autochthonous material to other invertebrates, and ultimately, the energy flow between stream algae and fish. Our findings suggest that disturbances to streams can be an important mechanism affecting transfer of primary energy sources to higher trophic levels.

Bovine placental lactogen invokes rapid growth in chinook salmon

The effect of recombinant bovine placenta) lactogen (rbPL) treatment upon growth of juvenile chinook salmon, Oncorhynchus tshawytscha (Walbaum) (14.0 ± 0.31 g wet wt), was examined over a period of 6 weeks. Experimental animals were either injected (5 μg rbPL g−1 week−1), implanted with a cholesterol pellet containing 0.4 mg rbPL (approximately 4 μg g−1 week−1), or orally and rectally intubated with 7.5 μg rbPL g−1 week−1. Control animals were injected with bovine serum albumen (BSA), 5 μg g−1 week−1, implanted with a placebo or orally intubated (7.5 μg BSA g−1 week−1). Significant (P < 0.05) growth acceleration was recorded for rbPL-injected and pellet-implanted groups from week 2 onwards when compared against all other groups. Oral or rectal intubation of rbPL (7.5 μg rbPL g−1 week−1), however, was without effect. Condition factor decreased in all groups, but was significantly lower in rbPL-injected and pellet-implanted fish at trial end when compared against controls. No differences were recorded between groups for per cent body moisture or relative gut length. Hepatosomatic index was significantly (P < 0.05) higher in rbPL-injected and rbPL-implanted chinook salmon versus other fish.

Estuarine Growth of Juvenile Chinook Salmon (Oncorhynchus tshawytscha) as Inferred from Otolith Microstructure

The growth of juvenile chinook salmon (Oncorhynchus tshawytscha) in the Sixes River Estuary, Oregon, was inferred from otolith microstructure. Otolith increments appeared to be formed daily, on average, and a transition between those produced during freshwater residence and estuary residence was apparent. Otolith microstructure was also examined with respect to time series of water temperature, population density, and benthic standing crop data. While the results supported the hypothesis of food-limited growth, elevated water temperatures in midsummer may have decreased food conversion efficiencies, thus also contributing to an observed growth rate decline. It was also found that from counts of otolith increments formed during freshwater life, fish that hatched relatively late tended to spend less time in freshwater before moving downstream to the estuary.

THE LETHAL AND SUB LETHAL EFFECTS OF A ZIRCONIUM PROCESS EFFLUENT ON JUVENILE SALMONIDS1

ABSTRACT: An effluent (ZPE), with high concentrations of ammonia was compared with solutions of ammonium chloride in both lethal and sublethal tests. The ZPE was more toxic than were solutions of ammonium chloride in acute toxicity tests, although ammonia was shown to be the major toxicant. The secondary toxicants present in the effluent were not identified. Growth was used as a sublethal test parameter and ZPE was again more toxic than ammonium chloride solutions at the same ammonia concentration. Concentrations of ammonia less than 3 mg/liter in ammonium chloride solutions actually stimulated the growth of juvenile chinook salmon. Concentrations of ZPE in the river at extreme low river flow could have sublethal effects on juvenile salmonids below the outfall.