Ocean-type Chinook Research Articles

Variation in estuary use patterns of juvenile Chinook salmon in the Fraser River, BC

Juvenile Pacific salmon ( Oncorhynchus spp.) use estuary habitats to varying degrees with some species and populations thought to rely heavily on these areas for early growth. In the Fraser River, British Columbia, there are 18 distinct conservation units of Chinook salmon ( O. tshawytscha), and all but one is of conservation concern. Our study compares the outmigration timing, size, and habitat use of juvenile Chinook salmon in the Fraser River estuary. Over 5 years (2016–2020), we captured 6493 juvenile Chinook salmon, with 3318 sampled for stock identification. Fraser River Chinook salmon extensively used estuary habitats, but patterns varied considerably by population. Juvenile Chinook salmon from the Lower Fraser River were most abundant and present the longest, arriving the smallest in late March and early April, and captured until July. South Thompson ocean-type Chinook salmon entered the estuary later, starting to arrive in late May or early June and remaining present until mid-August. Overall, juvenile Chinook salmon varied considerably in their estuary use across populations. Understanding this variation can inform differences in productivity and guide recovery actions.

Exposure of juvenile Chinook salmon to effluent from a large urban wastewater treatment plant. Part 2. Metabolomic profiling

Targeted metabolomic analysis was conducted on juvenile Chinook salmon exposed for 10 days to wastewater effluent (WWE) from a large urban treatment plant. Exposures included five dilutions of WWE (20%, 5.3%, 1.4%, 0.4%, and 0.1%) and a control with 7 replicates per treatment. Liver was extracted from fish and analyzed via liquid chromatography – mass spectrometry (LC-MS) for 361 endogenous metabolites, of which 185 were detected. Control-versus-treatment comparisons identified several metabolites that were associated with altered biochemical pathways observed for all treatments, including several that are important for energy generation and utilization. These altered pathways are crucial for fish health and may be an early indicator of potential adverse effects on growth, reproduction, behavior, and immune function. Juvenile ocean-type Chinook salmon spend several days to weeks in the nearshore estuary where they may encounter high concentrations of WWE contaminants. They are exposed to a wide range of potent pharmaceuticals, personal care products, and industrial compounds from WWE that have the potential to affect physiological homeostasis and disrupt their normal life cycle.

Discovery and validation of candidate smoltification gene expression biomarkers across multiple species and ecotypes of Pacific salmonids.

Early marine survival of juvenile salmon is intimately associated with their physiological condition during smoltification and ocean entry. Smoltification (parr–smolt transformation) is a developmental process that allows salmon to acquire seawater tolerance in preparation for marine living. Traditionally, this developmental process has been monitored using gill Na+/K+-ATPase (NKA) activity or plasma hormones, but gill gene expression offers the possibility of another method. Here, we describe the discovery of candidate genes from gill tissue for staging smoltification using comparisons of microarray studies with particular focus on the commonalities between anadromous Rainbow trout and Sockeye salmon datasets, as well as a literature comparison encompassing more species. A subset of 37 candidate genes mainly from the microarray analyses was used for TaqMan quantitative PCR assay design and their expression patterns were validated using gill samples from four groups, representing three species and two ecotypes: Coho salmon, Sockeye salmon, stream-type Chinook salmon and ocean-type Chinook salmon. The best smoltification biomarkers, as measured by consistent changes across these four groups, were genes involved in ion regulation, oxygen transport and immunity. Smoltification gene expression patterns (using the top 10 biomarkers) were confirmed by significant correlations with NKA activity and were associated with changes in body brightness, caudal fin darkness and caudal peduncle length. We incorporate gene expression patterns of pre-smolt, smolt and de-smolt trials from acute seawater transfers from a companion study to develop a preliminary seawater tolerance classification model for ocean-type Chinook salmon. This work demonstrates the potential of gene expression biomarkers to stage smoltification and classify juveniles as pre-smolt, smolt or de-smolt.

Toxicity of PHOS-CHEK LC-95A and 259F fire retardants to ocean- and stream-type Chinook salmon and their potential to recover before seawater entry

Long-term fire retardants are used to prevent the spread of wildland fire, but have inadvertently entered aquatic habitats and resulted in fish kills. We examined the toxicity of two fire retardant products; PHOS-CHEK 259F and LC-95A, on Chinook salmon with two different life histories, ocean-type and stream-type, at different stages of their development. Ocean-type Chinook outmigrate to the ocean as subyearlings; while, stream-type salmon overwinter in freshwater and outmigrate as yearlings. Ocean-type and stream-type salmon were exposed to the fire retardants prior to their parr to smolt transition (presmolts) as subyearlings (stream-type and ocean-type) and yearlings (stream-type only), as well as during their transition (smolts). The salmon were exposed to eight concentrations of each retardant and a control for 96h to determine acute toxicity. Lethal concentration curves were modeled by logistic regression for each life history and life stage exposed to the two fire retardants. Among all life histories and life stages tested, PHOS-CHEK 259F was most toxic to stream-type salmon at smolt stage and PHOS-CHEK LC-95A was most toxic to ocean-type salmon at smolt stage. To determine the delayed effects of product exposures on fish health as well as for the potential of recovery, 24-hour seawater challenges were performed immediately after fire retardant exposure, as well as after a recovery period. Previous PHOS-CHEK exposure reduced survival during seawater challenge among salmon from both life histories undergoing the parr-smolt transition and was more pronounced after PHOS-CHEK LC-95A exposure. However, this delayed effect was not observed 34 or more days after either PHOS-CHEK exposure. We conclude that accidental PHOS-CHEK LC-95A or 259F drops during salmon outmigration would have adverse impacts that extend beyond the acute mortality that occurs within the immediate drop and dilution areas.

A modeled comparison of direct and food web-mediated impacts of common pesticides on Pacific salmon.

In the western United States, pesticides used in agricultural and urban areas are often detected in streams and rivers that support threatened and endangered Pacific salmon. Although concentrations are rarely high enough to cause direct salmon mortality, they can reach levels sufficient to impair juvenile feeding behavior and limit macroinvertebrate prey abundance. This raises the possibility of direct adverse effects on juvenile salmon health in tandem with indirect effects on salmon growth as a consequence of reduced prey abundance. We modeled the growth of ocean-type Chinook salmon (Oncorhynchus tshawytscha) at the individual and population scales, investigating insecticides that differ in how long they impair salmon feeding behavior and in how toxic they are to salmon compared to macroinvertebrates. The relative importance of these direct vs. indirect effects depends both on how quickly salmon can recover and on the relative toxicity of an insecticide to salmon and their prey. Model simulations indicate that when exposed to a long-acting organophosphate insecticide that is highly toxic to salmon and invertebrates (e.g., chlorpyrifos), the long-lasting effect on salmon feeding behavior drives the reduction in salmon population growth with reductions in prey abundance having little additional impact. When exposed to short-acting carbamate insecticides at concentrations that salmon recover from quickly but are lethal to invertebrates (e.g., carbaryl), the impacts on salmon populations are due primarily to reductions in their prey. For pesticides like carbaryl, prey sensitivity and how quickly the prey community can recover are particularly important in determining the magnitude of impact on their predators. In considering both indirect and direct effects, we develop a better understanding of potential impacts of a chemical stressor on an endangered species and identify data gaps (e.g., prey recovery rates) that contribute uncertainty to these assessments.

Do chemically contaminated river estuaries in Puget Sound (Washington, USA) affect the survival rate of hatchery-reared Chinook salmon?

This study examined the rate of survival for hatchery-reared, ocean-type juvenile Chinook salmon (Oncorhynchus tshawytscha) to the adult life stage in relation to contamination status for estuaries where they temporarily reside. The hypothesis tested here is that juvenile Chinook from Puget Sound (Washington, USA) area hatcheries exhibit differential survival as categorized by the state of contamination in their respective natal estuaries. Data were examined from 20 hatcheries that released fish to 14 local estuaries in the Greater Puget Sound area over 37 years (1972–2008). A parallel analysis was also conducted for coho salmon (Oncorhynchus kisutch) outmigrating from many of the same hatcheries. For all years combined, juvenile Chinook transiting contaminated estuaries exhibited an overall rate of survival that was 45% lower than that for Chinook moving through uncontaminated estuaries, which was confirmed when tested year by year. The results for coho originating from the same hatcheries and sharing a similar marine distribution indicated no substantial differences among estuaries. These observations have important implications for wild juvenile Chinook that spend more time in the estuary compared with hatchery-reared fish.

Rates of straying by hatchery-produced Pacific salmon (Oncorhynchusspp.) and steelhead (Oncorhynchus mykiss) differ among species, life history types, and populations

Here we ask whether straying differs among species, life history types, and populations of adult hatchery-produced Pacific salmon (Oncorhynchus spp.) and steelhead (Oncorhynchus mykiss) in the Columbia River basin. Previous estimates of straying have been confounded by various factors influencing the probability of individuals returning to non-natal sites (e.g., off-station releases), whereas analyses undertaken here of nearly a quarter million coded-wire tag recoveries control for these factors. Our results revealed large and generally consistent differences in the propensity to stray among species, life history types within species, and populations. Paired releases indicated that (i) Chinook salmon (Oncorhynchus tshawytscha) strayed more (mean population range 0.11%–34.6%) than coho salmon (Oncorhynchus kisutch) (0.08%–0.94%); (ii) ocean-type Chinook (5.2%–18.6%) strayed more than stream-type Chinook (0.11%–10%); and Chinook salmon (0.90%–54.9%) strayed more than steelhead (0.30%–2.3%). We conclude these patterns are largely the result of species-specific behavioral and endocrine factors during the juvenile life stages, but analyses also suggest that environmental factors can influence straying during the adult upstream migration.

Linkages between life history type and migration pathways in freshwater and marine environments for Chinook salmon, Oncorhynchus tshawytscha

Chinook salmon, Oncorhynchus tshawytscha, are commonly categorized as ocean-type (migrating to the ocean in their first year of life) or stream-type (migrating after a full year in freshwater). These two forms have been hypothesized to display different ocean migration pathways; the former are hypothesized to migrate primarily on the continental shelf whereas the latter are hypothesized to migrate off the shelf to the open ocean. These differences in migration patterns have important implications for management, as fishing mortality rates are strongly influenced by ocean migration. Ocean-type Chinook salmon predominate in coastal rivers in the southern part of the species' range, whereas stream-type predominate in the interior and northerly rivers. This latitudinal gradient has confounded previous efforts to test the hypothesis regarding ocean migration pathways. To address this problem, we used a pair-wise design based on coded wire tagging data to compare the marine distributions of stream- and ocean-type Chinook salmon from a suite of rivers producing both forms. Both forms of Chinook salmon from the lower Columbia River, Oregon coast, lower Fraser River, and northern British Columbia rivers followed similar migration paths, contradicting the hypothesis. In contrast, recoveries of tagged Chinook salmon from the upper Columbia River, Snake River, and the upper Fraser River revealed migration patterns consistent with the hypothesis. These findings have important implications for our understanding of these life history types, and also for the conservation and management of declining, threatened, or endangered stream-type Chinook salmon populations in the US and Canada.

Use of Nonnatal Tributaries for Lake-Rearing Juvenile Chinook Salmon in the Lake Washington Basin, Washington

Abstract Ocean-type juvenile Chinook salmon (Oncorhynchus tshawytscha) can be present in the nearshore areas of Lake Washington and Lake Sammamish, WA for 4–5 months (January–June) and may encounter nonnatal tributaries. The use of these tributaries is not well known. We determined the presence of juvenile Chinook salmon in 12 tributaries through nighttime snorkel surveys from March to June. At one heavily-used tributary, we determined their temporal and spatial distribution by conducting surveys every two to three weeks from February to June. Additionally, we determined whether delta areas of tributaries are used by juvenile Chinook salmon by comparing their density and diet to other lakeshore sites. Of 12 streams surveyed, juvenile Chinook salmon were observed in eight. The abundance of Chinook salmon appeared to be related to a variety of factors including proximity to the natal stream, stream gradient, and stream size. In an intensively-monitored stream, juvenile Chinook salmon were found primarily in...

Effect of triploidy on growth and ionoregulatory performance in ocean-type Chinook salmon: A quantitative genetics approach

The use of triploid salmon for aquaculture is attractive as they do not mature sexually, limiting losses associated with poor flesh quality in mature fish and posing less genetic risk to wild stocks if they escape. Inconsistent survival and growth performance in triploid fish, however, has limited their implementation. In our study, ocean-type Chinook salmon (Oncorhynchus tshawytscha) were bred using replicated 3×3 factorial mating designs to create 18 families to test whether triploidization resulted in changes in growth and ionoregulation performance in freshwater and seawater. Eggs were pressure shocked after fertilization to create triploid offspring in a subset of each family. In spring, freshwater fish were sampled for size and gill Na+, K+-ATPase activity. Plasma chloride and cortisol were measured following a 24-h saltwater challenge. Growth performance was monitored for a further four months following transfer to sea water. We found significant effects of ploidy and sire (paternal effect) on smolt weight, as well as on gill Na+, K+-ATPase activity, although the latter did not correspond with performance in a 24-h saltwater challenge. Following four months of ocean growth, diploid animals were consistently larger, with greater circulating levels of insulin-like growth factor-1 than triploid sibs, although specific growth rates did not differ. Conversely, gill Na+, K+-ATPase activity at that time was significantly higher in triploid than diploid fish. When the phenotypic variance for the various traits was partitioned, triploids exhibited significantly greater additive genetic variance and maternal effects across all traits relative to diploid fish — indicating that gene dosage effects were primarily additive. The strong family effect indicates that genotype has a substantial role in determining the effects of ploidy manipulation on ionoregulatory and growth performance in Chinook salmon.

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.

Effect of daily oscillation in temperature and increased suspended sediment on growth and smolting in juvenile chinook salmon, Oncorhynchus tshawytscha

We examined the effect of temperature oscillation and increased suspended sediment concentration on growth and smolting in juvenile ocean-type chinook salmon (Oncorhynchus tshawytscha). Fish were ponded on February 26; each treatment group had three replicates of 250 fish. Mean temperatures for the entire experiment were 12.3 °C for all tanks with a total of 1348 and 1341 degree days for the constant temperature and oscillating temperature tanks, respectively. Daily fluctuation in temperature averaged 7.5 °C in the variable temperature groups and less than 1 °C for the constant temperature group. Starting on April 5, bentonite clay was added each day to tanks as a pulse event to achieve a suspended sediment concentration of 200 mg l−1; clay cleared from the tanks within approximately 8 h. Fish were sampled at approximately two week intervals from ponding until mid-June. On the last sample date, June 12, a single gill arch was removed and fixed for histological examination of gill morphology. By early May, significant differences were seen in size between the groups; control>temperature=sediment>(temperature×sediment). This relationship was consistent throughout the experiment except for the last sample date when the temperature group had a mean weight significantly greater than the sediment group. Gill Na+,K+-ATPase activity was not affected by daily temperature oscillations, but groups subjected to increased suspended sediment had significantly lower enzyme activities compared to controls. Mean cell size for gill chloride cells did not differ between groups. Plasma cortisol increased significantly during the spring, but there were no significant differences between groups.

Biochemical and molecular differences in diploid and triploid ocean-type chinook salmon (Oncorhynchus tshawytscha) smolts

There is increasing evidence for complex dosage effects on gene expression, enzyme activity and phenotype resulting from induced ploidy change. In this study, ocean-type chinook salmon were bred using a 2 × 2 factorial mating design to create four families and test whether triploidization resulted in changes in growth performance and smolting. Eggs were pressure shocked after fertilization to create triploid fish from a subset of each family. In June, fish were sampled for size, plasma insulin-like growth factor 1 (IGF-1), gill Na+–K+-ATPase activity, and expression of two Na+–K+-ATPase α subunits in the gill. Diploids were significantly heavier than triploids, and there were significant differences due to family. Despite a significant positive correlation between plasma IGF-1 and fish size, plasma IGF-1 did not differ between diploid and triploid smolts. Diploids also had significantly greater gill Na+–K+-ATPase enzyme activities than triploids and there was a strong family effect. Gill Na+–K+-ATPase α1b isoform expression differed significantly by family, but not ploidy, and generally families with lower Na+–K+-ATPase enzyme activity had higher α1b isoform gene expression. Na+–K+-ATPase α1a isoform expression did not differ among any of the groups. Although diploids were larger and had higher specific activities of Na+–K+-ATPase in the gills, there was no difference in gene expression or circulating hormone levels. The strong family effect, however, suggests that strain selection may be useful in improving performance of triploids for aquaculture.

Migration and rearing histories of chinook salmon (Oncorhynchus tshawytscha) determined by ion microprobe Sr isotope and Sr/Ca transects of otoliths

Strontium isotope and Sr/Ca ratios measured in situ by ion microprobe along radial transects of otoliths of juvenile chinook salmon (Oncorhynchus tshawytscha) vary between watersheds with contrasting geology. Otoliths from ocean-type chinook from Skagit River estuary, Washington, had prehatch regions with 87Sr/86Sr ratios of ~0.709, suggesting a maternally inherited marine signature, extensive fresh water growth zones with 87Sr/86Sr ratios similar to those of the Skagit River at ~0.705, and marine-like 87Sr/86Sr ratios near their edges. Otoliths from stream-type chinook from central Idaho had prehatch 87Sr/86Sr ratios ≥0.711, indicating that a maternal marine Sr isotopic signature is not preserved after the ~1000- to 1400-km migration from the Pacific Ocean. 87Sr/86Sr ratios in the outer portions of otoliths from these Idaho juveniles were similar to those of their respective streams (~0.708–0.722). For Skagit juveniles, fresh water growth was marked by small decreases in otolith Sr/Ca, with increases in Sr/Ca corresponding to increases in 87Sr/86Sr with migration into salt water. Otoliths of Idaho fish had Sr/Ca radial variation patterns that record seasonal fluctuation in ambient water Sr/Ca ratios. The ion microprobe's ability to measure both 87Sr/86Sr and Sr/Ca ratios of otoliths at high spatial resolution in situ provides a new tool for studies of fish rearing and migration.

Patterns of gametic investment by female stream- and ocean-type chinook salmon

Patterns of gametic investment by female stream- and ocean-type chinook salmon

The importance of influence diagnostics: examples from Snake River chinook salmon spawner-recruit models

The rapid decline of some salmonid populations in the Columbia River Basin led investigators to analyze spawner-recruit (SR) data in order to understand the potential gains of improving main-stem passage conditions and quantify the effectiveness of the juvenile transportation program. Direct measurements of passage survival and transportation were not always available, so instead, the researchers attempted to tease out the passage or transportation effects by using trends in production estimated from SR models. Small subsets of data, or even single observations, highly influenced the estimates of passage survival and transportation effectiveness derived from these models. For stream-type chinook salmon, deleting 1 of 13 stocks changed the estimate of main-stem passage survival from 11 to 34%. For ocean-type chinook salmon, the conclusion that transportation should be immediately halted hinged on a single observation. The Snake River salmon SR models starkly illustrate the importance of using influence diagnostics to temper inferences.

Use of otolith microstructure to study life history of juvenile chinook salmon in the Strait of Georgia in 1995 and 1996

The pattern of otolith daily increments was used to identify hatchery-reared, wild ocean-type and wild stream-type chinook and study their life history in the Strait of Georgia. In 1995 and 1996, almost all of hatchery-reared and wild stream-type chinook entered the Strait of Georgia in May and June, while wild ocean-type chinook entered from April to August. Upon ocean entry stream-type fish were the largest and wild ocean-type the smallest. Hatchery-reared and wild ocean-type chinook leaving fresh water later in the year were larger than those leaving earlier. The mean length of wild stream-type chinook was not related to the time of ocean entry. All the life history types maintained their size differences throughout the summer and fall despite some differences in rates of size increase. Hatchery-reared and wild stream-type chinook grew faster in 1995 than in 1996. Wild ocean-type grew at about the same rate. During early summer, the percentage of wild ocean-type chinook was low in contrast to the percentage of hatchery-reared and wild stream-type chinook in 1995 and 1996, respectively. In September of both years, the percentage of wild ocean-type chinook increased and the percentage of hatchery-reared and wild stream-type chinook dropped. By November in both years, the percentage of hatchery-reared chinook increased, especially in 1996 when the percentage exceeded 80%. After the first ocean winter, the percentage of hatchery-reared fish remained high.

Inheritance of Smolting Phenotypes in Backcrosses of Hybrid Stream-Type × Ocean-Type Chinook Salmon (Oncorhynchus tshawytscha)

Chinook salmon (Oncorhynchus tshawytscha) are ecologically and geographically differentiated into two life history types: “ocean-type,” which enter estuaries as fry or fingerlings in the first year of life and rear there for up to 6 wk before migrating to sea, and “stream-type,” which migrate seaward as smolts after one or more years in fresh water and are only transient residents in the outer portion of estuaries. Following exposure to a long-day priming photoperiod for 2 mo from the time of first feeding, progeny of stream-type chinook salmon undergo a reduction in growth during summer and remain as parr, whereas progeny of ocean-type chinook salmon and hybrids continue to grow during the summer and develop into underyearling smolts. Male purebred and hybrid F1 progeny of ocean-type and stream-type chinook salmon were backcrossed to females of both parental populations to examine the segregation of photoperiod-independent (smolt) and photoperiod-dependent (parr) phenotypes among progeny. Results of the backcrosses of the hybrid F1 males depended on female type. The backcross to ocean-type females produced progeny that were predominantly smolts, whereas progeny from the backcross to stream-type females segregated into approximately equal numbers of smolts and parr. These results are consistent with Mendelian inheritance of a single major gene with two alleles: a dominant allele for the photoperiod-independent phenotype of ocean-type chinook salmon and a recessive allele for the photoperiod-dependent phenotype of stream-type chinook salmon.

Life History of Juvenile Ocean-type Chinook Salmon (Oncorhynchus tshawytscha) in the Situk River, Alaska

Distribution, abundance, habitat preference, migration and residence timing, seawater tolerance, and size were determined for juvenile ocean-type (age 0) chinook salmon (Oncorhynchus tshawytscha) in the Situk River, Alaska. Chinook primarily occupied main-stem habitats (channel edges in spring, pools and willow edges in summer). Peak chinook densities in the upper and lower main stem were 96 and 76 fish/100 m2, respectively. Chinook migrated downstream in two phases: a spring dispersal of emergent fry and a summer migration. Chinook marked in the upper river in late June and early July were recaptured 20 km downstream in the lower river in late July. Marked chinook resided in the lower river up to 34 d. Mean fork length of chinook in the lower river increased from 40 mm in May to 80 mm in early August. By late August, chinook had emigrated from the lower river at a size of approximately 80 mm. Fish this size were seawater tolerant and had the physical appearance of smolts. Ocean-type chinook in the Situk River are unique because in most Alaskan streams, chinook are stream-type (rear in freshwater at least 1 yr).

Phenotypic Correlates of Life-History Variation in Juvenile Chinook Salmon, Oncorhynchus tshawytscha

SUMMARY (1) Chinook salmon, Oncorhynchus tshawytscha (Walbaum), commonly follow two life-history patterns as juveniles. 'Stream-type' juveniles reside in streams for a year or more before abandoning territoriality and migrating to marine habitats. By contrast, 'ocean-type' juveniles migrate to sea sometime during their first year of life, often as newly emerged fry. (2) At age 1, stream-type chinook showed a distinct reduction in positive rheotaxis during both 'diurnal' and 'nocturnal' current response tests, consistent with their downstream migration to the sea at this age in nature. By contrast, ocean-type chinook showed no pronounced shifts in rheotaxis after age 2-3 months. (3) Ocean-type fish that migrate seaward after 2-3 months in freshwater were more aggressive than those that migrate as fry, but both groups were less aggressive than stream-type chinook. (4) Ocean-type chinook grew at a faster rate than did stream-type.