Gill Morphometry in Growth Hormone Transgenic Atlantic Salmon

We show that many of the morphological features of the respiratory system of growth enhanced transgenic salmon are greater than those of similarly sized control salmon. Growth hormone transgenic Atlantic salmon, Salmo salar were the F₂ generation produced using eggs from a transgenic F₁ female and milt from a nontransgenic male. At the time the gill tissues were sampled, the transgenic salmon were growing 2.1 times more rapidly than the nontransgenic control salmon, and they had oxygen uptake rates that were about 1.6 times greater than control salmon. In the present study we show that the gill surface area available for respiratory exchange in the transgenic salmon is about 1.24 times that in control salmon which does not parallel the 1.6 elevation in oxygen uptake. The increase in gill exchange area was due largely to a relatively uniform increase in length of each gill filament.     

Rearing in Seawater Mesocosms Improves the Spawning Performance of Growth Hormone Transgenic and Wild-Type Coho Salmon

Growth hormone (GH) transgenes can significantly accelerate growth rates in fish and cause associated alterations to their physiology and behaviour. Concern exists regarding potential environmental risks of GH transgenic fish, should they enter natural ecosystems. In particular, whether they can reproduce and generate viable offspring under natural conditions is poorly understood. In previous studies, GH transgenic salmon grown under contained culture conditions had lower spawning behaviour and reproductive success relative to wild-type fish reared in nature. However, wild-type salmon cultured in equal conditions also had limited reproductive success. As such, whether decreased reproductive success of GH transgenic salmon is due to the action of the transgene or to secondary effects of culture (or a combination) has not been fully ascertained. Hence, salmon were reared in large (350,000 L), semi-natural, seawater tanks (termed mesocosms) designed to minimize effects of standard laboratory culture conditions, and the reproductive success of wild-type and GH transgenic coho salmon from mesocosms were compared with that of wild-type fish from nature. Mesocosm rearing partially restored spawning behaviour and success of wild-type fish relative to culture rearing, but remained lower overall than those reared in nature. GH transgenic salmon reared in the mesocosm had similar spawning behaviour and success as wild-type fish reared in the mesocosm when in full competition and without competition, but had lower success in male-only competition experiments. There was evidence of genotype×environmental interactions on spawning success, so that spawning success of transgenic fish, should they escape to natural systems in early life, cannot be predicted with low uncertainty. Under the present conditions, we found no evidence to support enhanced mating capabilities of GH transgenic coho salmon compared to wild-type salmon. However, it is clear that GH transgenic salmon are capable of successful spawning, and can reproduce with wild-type fish from natural systems.     

Genotype-Temperature Interaction in the Regulation of Development,Growth, and Morphometrics in Wild-Type,and Growth-Hormone Transgenic Coho Salmon

Background

The neuroendocrine system is an important modulator of phenotype, directing cellular genetic responses to external cues such as temperature. Behavioural and physiological processes in poikilothermic organisms (e.g. most fishes), are particularly influenced by surrounding temperatures.

Methodology/Principal Findings

By comparing the development and growth of two genotypes of coho salmon (wild-type and transgenic with greatly enhanced growth hormone production) at six different temperatures, ranging between 8° and 18°C, we observed a genotype-temperature interaction and possible trend in directed neuroendocrine selection. Differences in growth patterns of the two genotypes were compared by using mathematical models, and morphometric analyses of juvenile salmon were performed to detect differences in body shape. The maximum hatching and alevin survival rates of both genotypes occurred at 12°C. At lower temperatures, eggs containing embryos with enhanced GH production hatched after a shorter incubation period than wild-type eggs, but this difference was not apparent at and above 16°C. GH transgenesis led to lower body weights at the time when the yolk sack was completely absorbed compared to the wild genotype. The growth of juvenile GH-enhanced salmon was to a greater extent stimulated by higher temperatures than the growth of the wild-type. Increased GH production significantly influenced the shape of the salmon growth curves.

Conclusions

Growth hormone overexpression by transgenesis is able to stimulate the growth of coho salmon over a wide range of temperatures. Temperature was found to affect growth rate, survival, and body morphology between GH transgenic and wild genotype coho salmon, and differential responses to temperature observed between the genotypes suggests they would experience different selective forces should they ever enter natural ecosystems. Thus, GH transgenic fish would be expected to differentially respond and adapt to shifts in environmental conditions compared with wild type, influencing their ability to survive and interact in ecosystems. Understanding these relationships would assist environmental risk assessments evaluating potential ecological effects.     

Effects of species, culture history, size and residency on relative competitive ability of salmonids

The relative competitive ability of juvenile farm and wild salmonids was investigated to provide insight into the potential effects of introduction of cultured salmon on wild Pacific salmonid ( Oncorhynchus ) species. Aquarium experiments involving equal contests ( i.e. size matched, simultaneously introduced individuals) indicated that two wild coho salmon Oncorhynchus kisutch populations were competitively equal to a farm coho salmon population. In equal contests between farm Atlantic salmon Salmo salar (Mowi strain) and these wild coho salmon populations or coastal cutthroat trout Oncorhynchus clarki clarki , Atlantic salmon were subordinate in all cases. When Atlantic salmon were given a residence advantage, however, they were competitively equal to both wild coho salmon populations, but remained subordinate to coastal cutthroat trout. When Atlantic salmon were given a 10–30% length advantage, they were competitively equal to one wild coho salmon population but remained subordinate to the other. In equal contests in semi-natural stream channels, both wild coho and farm Atlantic salmon grew significantly more in the presence of the other species than when alone. It appears that coho salmon obtain additional food ration by out competing Atlantic salmon, whereas Atlantic salmon were stimulated to feed more in the presence of coho salmon competitors. These results suggest that wild coho salmon and cutthroat trout should out compete farm Atlantic salmon of a similar size in nature. As the relative competitive ability of Atlantic salmon improves when they have a size and residence advantage, should feral populations become established, they may exist on a more equal competitive footing owing to the long freshwater residence of Atlantic salmon.     

Vertical Position Reflects Increased Feeding Motivation in Growth Hormone Transgenic Coho Salmon (Oncorhynchus kisutch)

Growth hormone (GH) gene transgenesis has allowed the production of salmon with an inherently increased growth potential, on average two to threefold higher compared with daily specific growth rates observed in normal, non‐transgenic fish. This difference quickly results in animals of very different sizes at age, and is associated with specific morphological effects and enhanced appetites in transgenic animals. However, less is known of the feeding and antipredator behaviour of GH‐transgenic fish, information that can help with predictions of potential ecological consequences of release or escape of transgenic fish into the wild. In a series of experiments, transgenic (T) and normal (N) coho salmon of varying age and size (from 0.5 to 40 g, 3.5–21 mo) were studied singly, in pairs, and in groups during feeding and simulated predation threat. Vertical position generally did not differ between T and N fry, but at larger size (>4 g) T fish remained closer to the surface than N fish both during feeding and predatory attacks, probably as a consequence of inherent differences in feeding motivation and later reinforcement by associative learning. This difference in vertical position was not the result of competition as it remained even after either fish in the pair had been removed. In nature, where predators may attack from above (birds) or below (fish), this kind of behaviour may translate into higher risk of predation, which could increase mortality and lower the fitness of transgenic fish, unless their increased growth rate can compensate for the increased risk‐taking.     

Disease resistance, stress response and effects of triploidy in growth hormone transgenic coho salmon

Diploid and triploid coho salmon Oncorhynchus kisutch transgenic for growth hormone (GH) and control coho salmon were compared for differences in disease resistance and stress response. Resistance to the bacterial pathogen Vibrio anguillarum was not affected in transgenic fish relative to their non‐transgenic counterparts when they were infected at the fry stage, but was lower in transgenic fish when infected near smolting. Vaccination against vibriosis provided equal protection to both transgenic and non‐transgenic fish. Triploid fish showed a lower resistance to vibriosis than their diploid counterparts. Diploid transgenic fish and non‐transgenic fish appeared to show similar physiological and cellular stress responses to a heat shock. These studies provide information useful for both performance and ecological risk assessments of growth‐accelerated coho salmon.     

The importance of genetic verification for determination of Atlantic salmon in north Pacific waters

Genetic analyses of two unknown but putative Atlantic salmon Salmo salar captured in the Copper River drainage, Alaska, demonstrated the need for validation of morphologically unusual fishes. Mitochondrial DNA sequences (control region and cytochrome b) and data from two nuclear genes [first internal transcribed spacer (ITS-1) sequence and growth hormone (GH1) amplification product] indicated that the fish caught in fresh water on the Martin River was a coho salmon Oncorhynchus kisutch , while the other fish caught in the intertidal zone of the Copper River delta near Grass Island was an Atlantic salmon. Determination of unusual or cryptic fish based on limited physical characteristics and expected seasonal spawning run timing will add to the controversy over farmed Atlantic salmon and their potential effects on native Pacific species. It is clear that determination of all putative collections of Atlantic salmon found in Pacific waters requires validation. Due to uncertainty of fish identification in the field using plastic morphometric characters, it is recommended that genetic analyses be part of the validation process.     

Intestinal morphology in growth hormone transgenic coho salmon

In two GH transgenic coho salmon Oncorhynchus kisutch , the surface area of the intestine was 2·2 times that of control salmon and the growth rate was about twice that of controls. It seems likely that the enhanced intestinal surface area is a compensatory feature that is manifested in fast growing salmonids.     

Interaction of growth hormone overexpression and nutritional status on pituitary gland clock gene expression in coho salmon,Oncorhynchus kisutch

Clock genes are involved in generating a circadian rhythm that is integrated with the metabolic state of an organism and information from the environment. Growth hormone (GH) transgenic coho salmon, Oncorhynchus kisutch, show a large increase in growth rate, but also attenuated seasonal growth modulations, modified timing of physiological transformations (e.g. smoltification) and disruptions in pituitary gene expression compared with wild-type salmon. In several fishes, circadian rhythm gene expression has been found to oscillate in the suprachiasmatic nucleus of the hypothalamus, as well as in multiple peripheral tissues, but this control system has not been examined in the pituitary gland nor has the effect of transgenic growth modification been examined. Thus, the daily expression of 10 core clock genes has been examined in pituitary glands of GH transgenic (T) and wild-type coho salmon (NT) entrained on a regular photocycle (12L: 12D) and provided either with scheduled feeding or had food withheld for 60?h. Most clock genes in both genotypes showed oscillating patterns of mRNA levels with light and dark cycles. However, T showed different amplitudes and patterns of expression compared with wild salmon, both in fed and starved conditions. The results from this study indicate that constitutive expression of GH is associated with changes in clock gene regulation, which may play a role in the disrupted behavioural and physiological phenotypes observed in growth-modified transgenic strains.     

Disease resistance and health parameters of growth-hormone transgenic and wild-type coho salmon,Oncorhynchus kisutch

To extend previous findings regarding fish health and disease susceptibility of growth-enhanced fish, hematological and immunological parameters have been compared between growth hormone (GH) transgenic and wild-type non-transgenic coho salmon (Oncorhynchus kisutch). Compared to non-transgenic coho salmon, transgenic fish had significantly higher hematocrit (Hct), hemoglobin (Hb), mean cellular hemoglobin (MCH), mean cellular volume (MCV), and erythrocyte numbers, and lower white cell numbers. In addition, resistance to the bacterial pathogen Aeromonas salmonicida (causal agent of furunculosis) has been assessed between the strains. Higher susceptibility of transgenic fish to this disease challenge was observed in two separate year classes of fish. The present findings provide fundamental knowledge of the disease resistance on GH enhanced transgenic coho salmon, which is of importance for assessing the fitness of transgenic strains for environmental risk assessments, and for improving our understanding effects of growth modification on basic immune functions.     

On-site Direct Detection of Astaxanthin from Salmon Fillet Using Raman Spectroscopy

A new technology employing Raman spectroscopy is attracting attention as a powerful biochemical technique for the detection of beneficial and functional food nutrients, such as carotenoids and unsaturated fatty acids. This technique allows for the dynamic characterization of food nutrient substances for the rapid determination of food quality. In this study, we attempt to detect and measure astaxanthin from salmon fillets using this technology. The Raman spectra showed specific bands corresponding to the astaxanthin present in salmon and the value of astaxanthin (Raman band, 1518 cm^?1) relative to those of protein/lipid (Raman band, 1446 cm^?1) in the spectra increased in a dose-dependent manner. A standard curve was constructed by the standard addition method using astaxanthin as the reference standard for its quantification by Raman spectroscopy. The calculation formula was established using the Raman bands typically observed for astaxanthin (i.e., 1518 cm^?1). In addition, we examined salmon fillets of different species (Atlantic salmon, coho salmon, and sockeye salmon) and five fillets obtained from the locations (from the head to tail) of an entire Atlantic salmon. Moreover, the sockeye salmon fillet exhibited the highest astaxanthin concentration (14.2 mg/kg), while coho salmon exhibited an intermediate concentration of 7.0 mg/kg. The Raman-based astaxanthin concentration in the five locations of Atlantic salmon was more strongly detected from the fillet closer to the tail. From the results, a rapid, convenient Raman spectroscopic method was developed for the detection of astaxanthin in salmon fillets.     

Alternate Directed Anthropogenic Shifts in Genotype Result in Different Ecological Outcomes in Coho Salmon Oncorhynchus kisutch Fry

Domesticated and growth hormone (GH) transgenic salmon provide an interesting model to compare effects of selected versus engineered phenotypic change on relative fitness in an ecological context. Phenotype in domestication is altered via polygenic selection of traits over multiple generations, whereas in transgenesis is altered by a single locus in one generation. These established and emerging technologies both result in elevated growth rates in culture, and are associated with similar secondary effects such as increased foraging, decreased predator avoidance, and similar endocrine and gene expression profiles. As such, there is concern regarding ecological consequences should fish that have been genetically altered escape to natural ecosystems. To determine if the type of genetic change influences fitness components associated with ecological success outside of the culture environments they were produced for, we examined growth and survival of domesticated, transgenic, and wild-type coho salmon fry under different environmental conditions. In simple conditions (i.e. culture) with unlimited food, transgenic fish had the greatest growth, while in naturalized stream tanks (limited natural food, with or without predators) domesticated fish had greatest growth and survival of the three fish groups. As such, the largest growth in culture conditions may not translate to the greatest ecological effects in natural conditions, and shifts in phenotype over multiple rather than one loci may result in greater success in a wider range of conditions. These differences may arise from very different historical opportunities of transgenic and domesticated strains to select for multiple growth pathways or counter-select against negative secondary changes arising from elevated capacity for growth, with domesticated fish potentially obtaining or retaining adaptive responses to multiple environmental conditions not yet acquired in recently generated transgenic strains.     

The glutathione antioxidant system is enhanced in growth hormone transgenic coho salmon (<Emphasis Type="Italic">Oncorhynchus kisutch</Emphasis>)

Insertion of a growth hormone (GH) transgene in coho salmon results in accelerated growth, and increased feeding and metabolic rates. Whether other physiological systems within the fish are adjusted to this accelerated growth has not been well explored. We examined the effects of a GH transgene and feeding level on the antioxidant glutathione and its associated enzymes in various tissues of coho salmon. When transgenic and control salmon were fed to satiation, transgenic fish had increased tissue glutathione, increased hepatic glutathione reductase activity, decreased hepatic activity of the glutathione synthesis enzyme γ-glutamylcysteine synthetase, and increased intestinal activity of the glutathione catabolic enzyme γ-glutamyltranspeptidase. However, these differences were mostly abolished by ration restriction and fasting, indicating that upregulation of the glutathione antioxidant system was due to accelerated growth, and not to intrinsic effects of the transgene. Increased food intake and ability to digest potential dietary glutathione, and not increased activity of glutathione synthesis enzymes, likely contributed to the higher levels of glutathione in transgenic fish. Components of the glutathione antioxidant system are likely upregulated to combat potentially higher reactive oxygen species production from increased metabolic rates in GH transgenic salmon.     

Skin morphology and humoral non-specific defence parameters of mucus and plasma in rainbow trout,coho and Atlantic salmon

Susceptibility to different diseases among related species, such as coho salmon (Oncorhynchus kisutch), rainbow trout (Oncorhyncus mykiss) and Atlantic salmon (Salmo salar), is variable. The prominence of these species in aquaculture warrants investigation into sources of this variability to assist future disease management. To develop a better understanding of the basis for species variability, several important non-specific humoral parameters were examined in juvenile fish of these three economically important species. Mucous protease, alkaline phosphatase and lysozyme, as well as plasma lysozyme activities and histological parameters (epidermal thickness and mucous cell density, and size) were characterized and compared for three salmonids: rainbow trout, Atlantic salmon and coho salmon. Rainbow trout had a thicker epidermis and significantly more mucous cells per cross-sectional area than the other two species. Rainbow trout also had significantly higher mucous protease activity than Atlantic salmon and significantly higher lysozyme (plasma and mucus) activities than coho and Atlantic salmon, in seawater. Atlantic salmon, on the other hand, had the lowest activities of mucous lysozyme and proteases, the thinnest epidermal layer and the sparsest distribution of mucous cells, compared with the two other salmonids in seawater. Only coho salmon had sacciform cells. Atlantic and coho salmon had higher mucous lysozyme activities in freshwater as compared to seawater. There was no significant difference between mucous lysozyme activities in any of the three species reared in freshwater; however, rainbow trout still had a significantly higher plasma lysozyme activity compared with the other two species. All three species exhibited significantly lower mucous alkaline phosphatase and protease activities in freshwater than in seawater. Our results demonstrate that there are significant histological and biochemical differences between the skin and mucus of these three salmonid species, which may change as a result of differing environments. Variation in these innate immune factors is likely to have differing influences on each species response to disease processes.     

Acute Physiological Stress Down-Regulates mRNA Expressions of Growth-Related Genes in Coho Salmon

Growth and development in fish are regulated to a major extent by growth-related factors, such as liver-derived insulin-like growth factor (IGF) -1 in response to pituitary-secreted growth hormone (GH) binding to the GH receptor (GHR). Here, we report on the changes in the expressions of gh, ghr, and igf1 genes and the circulating levels of GH and IGF-1 proteins in juvenile coho salmon (Oncorhynchus kisutch) in response to handling as an acute physiological stressor. Plasma GH levels were not significantly different between stressed fish and prestressed control. Plasma IGF-1 concentrations in stressed fish 1.5 h post-stress were the same as in control fish, but levels in stressed fish decreased significantly 16 h post-stress. Real-time quantitative PCR (qPCR) analysis showed that ghr mRNA levels in pituitary, liver, and muscle decreased gradually in response to the stressor. After exposure to stress, hepatic igf1 expression transiently increased, whereas levels decreased 16 h post-stress. On the other hand, the pituitary gh mRNA level did not change in response to the stressor. These observations indicate that expression of gh, ghr, and igf1 responded differently to stress. Our results show that acute physiological stress can mainly down-regulate the expressions of growth-related genes in coho salmon in vivo. This study also suggests that a relationship between the neuroendocrine stress response and growth-related factors exists in fish.     

Physiological and Behavioral Effects of Zinc and Temperature on Coho Salmon (Oncorhynchus kisutch)

Pacific salmon species including the U.S. federally endangered coho salmon (Oncorhynchus kisutch) and the U.S. federally threatened steelhead trout (Oncorhynchus mykiss) have declined at an alarming rate in the last 40 years. Two of the main causes for the decline in coastal coho populations include increases in temperature and contaminant loads in coastal watersheds. Zinc, in particular, is one of the most common contaminants in aquatic systems. Using an experimental mesocosm design, we examined physiological, biochemical, and behavioral responses of coho salmon to excess dietary zinc and increased temperatures, with the ultimate goal of relating results to wild populations of coho salmon and steelhead in the Navarro River, California. Fish were obtained from a hatchery and divided into four treatments: low water temperature-no dietary zinc, high temperature-no zinc, low temperature-zinc, and high temperature-zinc. Each treatment had four replicate tanks. Zinc concentrations in liver increased during exposure to a high zinc diet. Iron concentrations in liver increased during simultaneous exposure to high zinc diet and increased temperature, and growth was reduced in this experimental treatment. Expression of hsp-70 was not significantly different between treatments, but showed decreasing trends with high dietary zinc and high temperature. Feeding rate increased with exposure to a high zinc diet. Comparison with steelhead trout samples from the Navarro River, California, showed levels of zinc, iron, and hsp-70 greater than those found in the experimental Coho salmon. All comparisons between the hatchery coho salmon and wild steelhead should be viewed with caution due to the differences between species, the laboratory and natural environment, and the genetic differences between wild and hatchery fish.     

Disruption of seasonality in growth hormone-transgenic coho salmon (Oncorhynchus kisutch) and the role of cholecystokinin in seasonal feeding behavior

Seasonal variation in daily food intake is a well-documented phenomenon in many organisms including wild-type coho salmon where the appetite is noticeably reduced during periods of decreased day length and low water temperature. This reduction may in part be explained by altered production of cholecystokinin (CCK) and growth hormone (GH). CCK is a hormone produced in the brain and gut that mediates a feeling of satiety and thus has an inhibitory effect on food intake and foraging behaviour. Growth hormone (GH) enhances feeding behaviour and consequently growth, but its production is reduced during winter. The objectives of this study were: first, to compare the seasonal feeding behaviour of wild and GH-transgenic coho salmon; second, to determine the behavioural effect of blocking the action of CCK (by using devazepide) on the seasonal food intake; and third, to measure CCK expression in brain and gut tissues between the two genotypes across seasons. We found that, in contrast to wild salmon, food intake in transgenic salmon was not reduced during winter indicating that seasonal control of appetite regulation has been disrupted by constitutive production of GH in transgenic animals. Blocking of CCK increased food intake in both genotypes in all seasons. The increase was stronger in wild genotypes than transgenic fish; however blocking CCK in wild-type fish in winter did not elevate appetites to levels observed in the summer. The response to devazepide was generally faster in transgenic than in wild salmon with more rapid effects observed during summer than during winter, possibly due to a higher temperature in summer. Overall, a seasonal effect on CCK mRNA levels was observed in telencephalon with levels during winter being higher compared to the summer in wild fish, but with no seasonal effect in transgenic fish. No differences in seasonal CCK expression were found in hypothalamus. Higher levels of CCK were detected in the gut of both genotypes in winter compared to summer. Thus, CCK appears to mediate food intake among seasons in both wild-type and GH-transgenic salmon, and an altered CCK regulation may be responsible at least in part for the seasonal regulation of food intake.     

Feeding on Profitable and Unprofitable Prey: Comparing Behaviour of Growth-Enhanced Transgenic and Normal Coho Salmon (Oncorhynchus kisutch)

We compared the performance of normal and growth hormone‐transgenic coho salmon feeding on surface drifting edible and inedible novel prey items in various social environments. With an inherently higher appetite, we predicted that transgenic fish would be more willing to feed on novel prey, and that visual company with another fish would enhance this difference further. Transgenic and normal fish, of similar size and age, were equally willing to attack both the edible (live insects) and inedible (artificial angling lure flies) prey, but transgenic fish did so faster and were more likely to make repeated attacks. Transgenic fish managed to seize and consume the edible prey after fewer attacks than did normal fish. However, swallowing of prey took longer than for normal fish. More transgenic individuals interacted with the inedible prey compared with normal salmon, and initially, transgenic fish in visual company with another fish also interacted more with the prey than single transgenic or any constellation of normal focal fish. With repeated exposures, the number of individuals attacking and the number of interactions with the prey decreased. These responses were stronger in transgenic fish, partly explained by the initially low response in normal fish. The observed differences are most likely the consequences of elevated levels of growth hormone in transgenic fish generating enhanced feeding motivation and reinforcement capacity. In a natural environment, the performance of a growth hormone‐transgenic fish may therefore depend on the relative abundance of profitable vs. unprofitable prey, as well as the presence of other transgenic individuals.     

A global assessment of salmon aquaculture impacts on wild salmonids

Since the late 1980s, wild salmon catch and abundance have declined dramatically in the North Atlantic and in much of the northeastern Pacific south of Alaska. In these areas, there has been a concomitant increase in the production of farmed salmon. Previous studies have shown negative impacts on wild salmonids, but these results have been difficult to translate into predictions of change in wild population survival and abundance. We compared marine survival of salmonids in areas with salmon farming to adjacent areas without farms in Scotland, Ireland, Atlantic Canada, and Pacific Canada to estimate changes in marine survival concurrent with the growth of salmon aquaculture. Through a meta-analysis of existing data, we show a reduction in survival or abundance of Atlantic salmon; sea trout; and pink, chum, and coho salmon in association with increased production of farmed salmon. In many cases, these reductions in survival or abundance are greater than 50%. Meta-analytic estimates of the mean effect are significant and negative, suggesting that salmon farming has reduced survival of wild salmon and trout in many populations and countries.     

Interspecific relationships between emerging Atlantic salmon, Salmo salar, and coho salmon, Oncorhynchus kisutch, juveniles

Interspecific relationships between Atlantic salmon and coho salmon were studied at early life stages in laboratory and semi-natural stream channels. During emergence, the survival and dispersal patterns were similar for the two species in single or mixed populations. Survival of Atlantic salmon fry was reduced in the presence of older coho fry. However, no predation was observed. Microdistribution differed between the two species, with Atlantic salmon fry more numerous in riffles when coho were present.
Coho juveniles had a pelagic and gregarious distribution, in contrast to the benthic behaviour of the Atlantic salmon. In laboratory streams, Atlantic salmon fry moved out or adopted a subordinate cryptic behaviour which allowed them to escape predation while negatively affecting their growth.