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.     

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.     

Susceptibility of rainbow trout Oncorhynchus mykiss,Atlantic salmon Salmo salar and coho salmon Oncorhynchus kisutch to experimental infection with sea lice Lepeophtheirus salmonis

Physiological, immunological and biochemical parameters of blood and mucus, as well as skin histology, were compared in 3 salmonid species (rainbow trout Oncorhynchus mykiss, Atlantic salmon Salmo salar and coho salmon O. kisutch) following experimental infection with sea lice Lepeophtheirus salmonis. The 3 salmonid species were cohabited in order to standardize initial infection conditions. Lice density was significantly reduced on coho salmon within 7 to 14 d, while lice persisted in higher numbers on rainbow trout and Atlantic salmon. Lice matured more slowly on coho salmon than on the other 2 species, and maturation was slightly slower on rainbow trout than on Atlantic salmon. Head kidney macrophages from infected Atlantic salmon had diminished respiratory burst and phagocytic capacity at 14 and 21 d post-infection (dpi), while infected rainbow trout macrophages had reduced respiratory burst and phagocytic capacities at 21 dpi, compared to controls. The slower development of lice, coupled with delayed suppression of immune parameters, suggests that rainbow trout are slightly more resistant to lice than Atlantic salmon. Infected rainbow trout and Atlantic salmon showed increases in mucus lysozyme activities at 1 dpi, which decreased over the rest of the study. Mucus lysozyme activities of infected rainbow trout, however, remained higher than controls over the entire period. Coho salmon lysozyme activities did not increase in infected fish until 21 dpi. Mucus alkaline phosphatase levels were also higher in infected Atlantic salmon compared to controls at 3 and 21 dpi. Low molecular weight (LMW) proteases increased in infected rainbow trout and Atlantic salmon between 14 and 21 dpi. Histological analysis of the outer epithelium revealed mucus cell hypertrophy in rainbow trout and Atlantic salmon following infection. Plasma cortisol, glucose, electrolyte and protein concentrations and hematocrit all remained within physiological limits for each species, with no differences occurring between infected and control fish. Our results demonstrate that significant differences in mucus biochemistry and numbers of L. salmonis occur between these species.     

Season- and size-dependent risk taking in juvenile coho salmon: experimental evaluation of asset protection

Using juvenile coho salmon, Oncorhynchus kisutch, we tested predictions arising from dynamic optimization models of foraging under predation risk. Coho juveniles from two size groups raised in the laboratory were individually fed varying food rations. Their willingness to risk predation was measured as the time to resume foraging after presentation of a predator model. Small fish (mean weight 1.5 g) resumed feeding earlier than larger fish (3.5 g) as predicted by dynamic models under summer photoperiod but not under autumn photoperiod. Contrary to predictions, larger fish did not increase risk taking and small fish decreased risk taking between summer and autumn treatments. Food ration significantly influenced time to resume feeding only in small coho. A simple mechanistic model we proposed to explain feeding motivation under risk as a function of body size and prior growth rate was not sufficient to explain observed variation in risk taking. This study suggests that coho salmon use photoperiod and their own body size as cues for long-term, state-dependent adjustments of feeding behaviour. The lower risk taking of larger fish is probably an example of asset protection, whereby larger animals accept less predation risk to protect their greater accumulated fitness value. The decrease of risk taking in small fish in the autumn was possibly caused by a switch of life history trajectory towards delayed smolting. Copyright 1999 The Association for the Study of Animal Behaviour.     

DNA homologies between the rainbow trout, chum salmon and coho salmon

The thermal stabilities of hybrid duplexes between the DNAs from three salmonid fish species were monitored as measures of DNA homology. The chum salmon, Oncorhynchus keta, and coho salmon, Oncorhynchus kisutch, had more DNA homology with each other than either had with the rainbow trout, Salmo gairdnerii. Morphological, ecological and protein similarities between the coho salmon and the rainbow trout may be due to parallel or convergent evolution.     

Gastric evacuation rates and daily ration of piscivorous coho salmon, Oncorhynchus kisutch Walbaum

Effects of temperature and meal size on gastric evacuation rates of juvenile coho salmon, Oncorhynchus kisutch , consuming sockeye salmon, O. nerka , fry were examined and used in the estimation of daily meal, daily ration and number of fry consumed by coho in Chignik Lake, Alaska. Evacuation of fry by coho was best described by a negative exponential model (average R² = 0.93). A square root model also provided a good fit (average R² = 0·93), but the y-intercepts deviated more from the expected value than did the y-intercepts of the exponential model. The effect of temperature ( T , 5–13° C) and meal size (MS, 0·166–0·367 g) on the exponential evacuation rate (r_e, h^-1) could be described as
In the lake, coho fed continuously during the 24-h period in early June 1986 and 1987. Estimates of daily meal and ration of coho calculated by the Eggers method and the geometric mean of prey weight ranged from 0·224 to 0·435 g (2.1–4.4% body wt) depending on location and year. The Elliott & Persson method provided similar estimates of food consumption, whereas estimates based on the Pennington method and square root evacuation of prey differed from the exponential models. Sockeye fry represented 93% of the total prey weight. The average number of sockeye fry consumed per coho per 24 h, based on the arithmetic mean of prey weight, was 3·0–3·9 fry.     

An analysis of the distribution of juvenile Atlantic salmon (Salmo salar) in nature as a function of relatedness using microsatellites

The major objective of this study was to test the hypothesis that juvenile Atlantic salmon kin occupy adjacent territories in their natural habitat in order to profit from the benefits associated with kin-biased behaviours, as has been observed under controlled laboratory conditions. Microsatellites were used to establish the relatedness of salmon fry (in their first summer of life) and parr (in their second and third summer of life) captured in adjacent territories. We did not observe a relationship between the proximity and the relatedness of either parr of the same cohort or fry in their natural habitat. Although many pairs of fry were identified as being related when sampled immediately after emergence, most family groups did not occupy adjacent territories. The high dispersal potential in rivers, the low occupation rate of the habitat and the incidence of half-sibs in nature most probably reduce the opportunity and advantage of kin-biased behaviour, in contrast to laboratory studies conducted in artificial, high-density conditions.     

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

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

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.     

Selection on increased intrinsic growth rates in coho salmon, Oncorhynchus kisutch

Substantial evidence from the animal kingdom shows that there is a trade-off between benefits and costs associated with rapid somatic growth. One would therefore expect growth rates under natural conditions to be close to an evolutionary optimum. Nevertheless, natural selection in many salmonid species appears to be toward larger size and earlier emergence from spawning redds, indicating a potential for increased growth rate to evolve. We tested how selection for genetic variants (growth hormone transgenic coho salmon, Oncorhynchus kisutch, with more than doubled daily growth rate potential relative to wild genotypes) depended on predator timing and food abundance during the early period of life (fry stage). In artificial redds, fry of the fast-growing genotypes showed a highly significant developmental shift, emerging from gravel nests approximately two weeks sooner, but with an 18.6% reduced survival, relative to wild-genotype fry. In seminatural streams, fry of the fast-growing genotypes suffered higher predation than those of wild genotypes when predators were present at the time of fry emergence, but this difference was less pronounced when food was scarce. In streams where predators were introduced after emergence, fry survived equally well regardless of food availability. Surviving fry grew faster in habitats provided with more food, and fast-growing genotypes also grew faster than wild genotypes when predators arrived late and food was abundant. Fewer fish migrated downstream past a waterfall when food availability was high and in the presence of predators, and wild-genotype fry were more likely to migrate than fry of the fast-growing genotypes. After being returned to the experimental streams after migration, fast-growing genotypes survived equally well as those of the same genotypes that did not migrate, whereas migrating wild genotypes experienced higher mortality relative to those of the same genotypes that did not migrate. Comparisons of growth rates between siblings retained under hatchery conditions and those from habitats with the fastest growth in the experimental stream revealed that growth rates were similar for wild genotypes in both environments, whereas the fast-growing genotypes in the streams only realized 90% of their growth potential. The present study has shown that a major shift in developmental timing can alter critical early stages affecting survival and can have a significant effect on fitness. Furthermore, ecological conditions such as food abundance and predation pressure can strongly influence the potential for fast-growing variants to survive under natural conditions. The large-scale removal of many predatory species around the world may augment the evolution of increased intrinsic growth rates in some taxa.     

Metabolic recovery in Atlantic salmon fry and parr following forced activity

Atlantic salmon Salmo salar fry and parr were subjected to 5 min of forced activity and the subsequent changes in oxygen consumption and ammonia excretion rates were evaluated over a 24 h period. In a second experiment, individual Atlantic salmon fry and parr were freeze‐clamped in liquid nitrogen, before, immediately following a 5 min activity period, or after periods of recovery up to 2 h. Samples were analysed for whole body phosphocreatine (PCr), ATP and lactate. Five minutes of forced activity resulted in significant increases in both oxygen consumption and ammonia excretion rates. Changes in the oxygen consumption rates were greater in the parr compared with the fry. In contrast, the post‐exercise ammonia excretion rates were nearly twice as high for the fry compared with the parr. Exercise also caused a marked decrease in PCr levels (c. 47 and 65% in fry and parr, respectively), no change in ATP levels and a significant increase in lactate levels in Atlantic salmon fry and parr. Recovery of PCr occurred quickly (between 15 and 30 min) in fry and parr. Although the post‐activity levels of lactate were lower in fry (c. 3 μmol g^?1) compared with parr (c. 14 μmol g^?1), lactate levels returned to control levels within 60 min in fry, but it took >2 h for this metabolite to recover in parr. Compared with parr, these findings show that Atlantic salmon fry possess a reduced anaerobic capacity, and these results are consistent with the theoretical and experimental evidence that smaller fish support burst swimming through aerobic processes.     

Growth and survival of Atlantic salmon (Salmo salar) in Llyn Dwythwch, North Wales

SUMMARY. Atlantic salmon fry have been annually stocked into Llyn Dwythwch, North Wales, since 1969, in an attempt to increase the natural stocks of the area. The growth and survival of 1- and 2-year-old salmon were investigated, and compared with that of other lake-reared populations and also with salmon in the natural stream environment. Lake-reared salmon follow the same patterns of slow and rapid growth as found for river fish, but the growth rate was superior in the former. The variation in length – weight relationship with age and sex was investigated. Survival rates in general compared favourably with the survival in rivers, with high mortality rates of salmon in Llyn Dwythwch resulting from predation at spring stocking by the resident brown trout. This was later avoided by stocking larger fish in the autumn.     

The accumulation of 137-caesium from fresh water by alevins and fry of Atlantic salmon and brown trout

The accumulation of 137-caesium from water by alevins and fry of Atlantic salmon and brown trout was studied, At 'normal' pH (∼7.4), input rates (k_WF) and equilibrium concentration factors (CF_eq) of 137-caesium were four to five times greater in both species of alevins than those in the fry. Input rates and equilibrium concentration factors were consistently greater in brown trout than in Atlantic salmon. The input rate of 137-caesium was most rapid in kidney, gill and gut of fry. The majority of the radiocaesium was, however, deposited in muscle tissue which had consistently the longest biological half-life of 50–90 days. 137-Caesium input was significantly reduced at low pH (∼5.0) but output rates (k_FW) were little affected. It is concluded that juvenile fish are more susceptible than adults to radiocaesium accumulation from freshwater but that food is the major source of 137-caesium in freshwater fish. The behaviour of 137-caesium is discussed with respect to potassium.     

The effects of intra- and inter-specific competition on the distribution of stocked juvenile Atlantic salmon, Salmo salar L., in relation to depth and gradient in an upland trout, Salmo trutta L., stream

The relative effects of inter- and intra-specific competition on the distribution of stocked salmon in relation to depth and gradient were investigated in an upland stream during two summer sampling periods. The stream was divided into two areas by an impassable fish barrier, and trout were removed from the upstream section prior to 2 years of salmon stocking. A small amount of trout re-immigration to the cleared area occurred. Under sympatric conditions in the downstream section the fry of both species were significantly more abundant in shallow water, whereas the yearling and older fish tended to inhabit the deeper, slow-flowing areas. Under near allopatric conditions in the cleared section salmon fry changed their distribution to include deeper areas, with shallow, fast-flowing water becoming the least preferred habitat. This trend for a wide distribution of salmon fry over all the available habitat in the upstream section was maintained in the second year when parr were present but trout remained at very low densities. It was therefore concluded that high inter-specific competition from trout was responsible for restricting the distribution of salmon fry to shallow habitat in the control area. Intra-specific competition from older salmon apparently only affected the growth and survival of salmon fry. The regulating mechanisms involved in these inter- and intra-specific effects are discussed in terms of competition for stream resources and predation. Recommendations based on the findings are made for stocking and habitat management of salmonid waters.     

Emergence of Baltic salmon, Salmo salar L., in relation to temperature: a laboratory study

Emergence pattern and developmental status at emergence of Baltic salmon fry from the Umeälven hatchery stock (63°50'N, 20°25'E) were studied at 6, 10 and 12°C in the laboratory. The number of days and degree days from hatching to 50% emergence decreased exponentially with increasing temperature. Synchronization of emergence increased with temperature. Optimal temperature for incubation of yolk-sac alevins was 10°C, which resulted in the largest fry at emergence and the lowest death rate. Fry kept at 6°C had the lowest mean weight and at 12°C the highest death rate. The fry emerged at an earlier developmental state with more yolk at 12°C than at 6°C. The Baltic salmon had a faster developmental rate during the gravel-phase, as compared to more southern Atlantic salmon populations.     

Discrimination of family‐specific odours by juvenile coho salmon: roles of learning and odour concentration*

Free‐swimming coho salmon fry Oncorhynchus kisutch of some families showed preference (relative to the behaviour of naïve sibs) for the odours of similarly aged non‐sibs to whom they had been exposed during the post‐hatch (alevin) stage and the early free‐swimming (fry) stage, but not the embryo (egg) stage, indicating that odour‐learning had occurred during the later developmental periods. Recognition (i.e. preference) of sib‐ pecific odours was evident after a month, and in one case 5 months, of separation from those odours. Thus, young salmon incubating in their gravel nests in streams appear to have the capacity to learn the chemical characteristics of conspecifics and retain this memory for at least several months without reinforcement. However, in addition to the general attractiveness of sibs and familiar non‐sibs over unfamiliar non‐sibs, some non‐sibs were consistently more attractive than others. Preference between two different non‐sib odours could be reversed by changing their relative concentrations, indicating that relative attractiveness is a function of both familiarity and odour concentration. Therefore, although juvenile coho salmon learn, remember, and are subsequently attracted by sib‐specific odours in early life, familiar odours are not always preferred over unfamiliar conspecific odours. Preference in dyadic assays is therefore an insensitive measure of recognition.     

Morpho-ecological patterns and feeding of coho salmon Oncorhynchus kisutch fry in the period of downstream migration

The principal patterns of feeding of downstream-migrant and coastal young of the year of coho salmon are considered. The so-called potential downstream migrants and residents, which differ in size and feeding patterns, were distinguished among coastal fry. It is assumed that identification of particular coastal specimens to phenotypic groups of downstream migrants or nonmigrants of young of the year of coho salmon may be determined even in the daytime by such parameters as the body length, feeding intensity, and food spectrum.     

Light-mediated predation by northern squawfish on juvenile chinook salmon

Northern squawfish Ptychocheilus oregonensis cause significant mortality of juvenile salmon in the lower Columbia River Basin (U.S.A.). The effects of light intensity on this predator-prey interaction were examined with laboratory experiments and modelling studies. In laboratory experiments, the rate of capture of subyearling chinook salmon Oncorhynchus tshawytscha by northern squawfish was inversely related to light intensity. In a large raceway, about five times more salmon were captured during 4 h periods of relative darkness (0–03 Ix) than during periods with high light intensity (160 Ix). The rate of predation could be manipulated by increasing or decreasing light intensity.
A simulation model was developed for visual predators that encounter, attack, and capture juvenile salmon, whose schooling behaviour was light-sensitive. The model was fitted to laboratory results using a Monte Carlo filtering procedure. Model-predicted predation rate was especially sensitive to the visual range of predators at low light intensity and to predator search speed at high light. Modelling results also suggested that predation by northern squawfish on juvenile salmon may be highest across a narrow window of fight intensity.     

Trade-off between growth rate and aggression in juvenile coho salmon, Oncorhynchus kisutch

In juvenile salmon and trout, there seems to be a positive phenotypic correlation between individual aggression level and growth rate. Aggressive fish are dominant, and they obtain and defend territories, giving them access to good feeding sites. Being aggressive may increase predation risk, and may also carry costs such as increased metabolic demand, with effects on growth. To test the hypothesis that there is a trade-off between individual growth rate and aggression, we mated 12 female coho salmon with two unique males each, creating 24 full-sibling families. Growth of individually marked fish from each family was estimated in a situation where food could not be monopolized. Thereafter, individual fish were tested for mirror-elicited agonistic behaviour. We found significant variation between families in early growth rate, with a high heritability (1.04). There was also significant between-family variation in agonistic behaviour, but activity was generally low and heritability was low (0.25) and not significant. Growth rate and agonistic behaviour were negatively correlated. These results imply that aggressive behaviour has an energetic cost. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.