Isolation and in vitro characteristics of chinook salmon (Oncorhynchus tshawytscha) rosette agent

Isolation and in vitro characteristics of a significant obligate intracellular eukaryotic pathogen (rosette agent) of chinook salmon (Oncorhynchus tshawytscha) are described. The pathogen was isolated by primary culture of infected spleen tissue followed by inoculation onto the chinook salmon embryo CHSE-214 cell line. Chicken and rabbit antibody was produced to a soluble antigen derived from the rosette agent. Mortalities and the disease were experimentally reproduced by inoculating chinook salmon with the isolate. The pathogen was reisolated 27 days post-inoculation from moribund fish. Identity of the isolate and the disease-causing organism was further supported by documenting antigenic and morphologic identity. Tested in CHSE-214 cells, the organisms are sensitive to the antibiotics oxytetracycline, chloramphenicol, amphotericin-B and nystatin. A ratio of about one organism per tissue culture cell at inoculation was required to develop an easily detectable infection. Adsorption of organisms to tissue culture cells at inoculation did not enhance the pathogen's infectivity. The pathogen replicated at an increasing rate from 5°C to 20°C. It could not be stored by freezing but storage in tissue culture at 5°C for at least 5 months proved possible. Early passages of the organism showed a high degree of host specificity in tissue culture which decreased in later passages. The taxonomic affinities of the organism are discussed.     

Distribution,abundance and growth of juvenile salmonids off the coast of Oregon and Washington,summer (1980)

Juvenile salmonids (< 50 cm) were sampled by purse seine off the Pacific coast from Tillamook Bay, Oregon, to Copalis Head, Washington, during the period May through September (1980). Temporal distribution and abundance of the major Columbia River species were determined. Spring chinook salmon (Oncorhynchus tshatuytscha) and steel head (Salmo gairdneri) were present only during early cruises and were distributed almost entirely in the Columbia River plume and the sample area to the north. Coho salmon (O. kisutch) and fall chinook salmon were distributed more uniformly throughout the sampling area and were relatively abundant throughout the sampling period. Concentrations of fish were found only within 28 km of the shore. A number of fish that had been marked before or during their outmigration from the Columbia River system were recaptured. It appeared possible that with concentrated sampling in areas with high fish abundances, sufficient numbers of marked juvenile salmonids could be captured to provide relative survival estimates between different stocks of Columbia River fish.     

Survival of smaller sport caught chinook, Oncorhynchus tshawytscha (Walbaum), and coho, Oncorhynchus kisutch (Walbaum), salmon from Lake Michigan and its management implications

Abstract The survival of small-sized (<50.8 cm) chinook salmon, Oncorhynchus tshawytscha (Walbaum), and coho salmon, Oncorhynchus kitsutch (Walbaum), caught by sport fishing was determined to assess the potential for increasing the size limit for these fish. Fishermen were recently catching smaller salmon than in the 1970s, but salmon growth rates had not changed. To be an effective management option, the survival rate of hooked and returned fish must be high. The overall survival rates were high: 76% for chinook salmon and 70% for coho salmon. There was no significant difference in survival of the coho salmon with size of hook used ( P = 0.31). Any mortality among fish was generally acute; fish hooked deep in the mouth or gills generally bled and died shortly after capture. Fish hooked in the gills had a significantly greater mortality ( P = 0.0002). The overall high survival rate for these species was the result of a small proportion of fish being hooked in the gills or deep in the mouth. Since the survival rate of the salmon was high, the size limit could be increased to allow smaller fish to grow to sizes preferred by sport fishermen.     

Evaluation of an experimental Aeromonas salmonicida epidemic in chinook salmon, Oncorhynchus tshawytscha (Walbaum)

To determine the dynamics of the transmission of Aeromonas salmonicida ssp. salmonicida infection, chinook salmon, Oncorhynchus tshawytscha, were exposed to bacteria by cohabitation. The latent period (time between exposure and infectivity) was determined by exposing a group of chinook salmonid fingerlings to A. salmonicida by bath, then, at daily intervals, by holding five exposed (donor) fish with approximately 50 naive fish for 24 h. The latent period was 3 days post-infection and the time period between the initial exposure to bacteria and the beginning of bacterial shedding was 4.5 days for the same animals. The prevalence and intensity of infection in the donor fish, to which recipient fish were exposed, i.e. the level of exposure, was highly correlated with the development of disease in recipient (susceptible) chinook salmon (r2 = 0.57). An experiment was conducted to determine the daily progress of infection and development of a furunculosis epidemic among recipient fish by cohabiting a single exposed fish with 43 unexposed salmon. At daily intervals, all fish (in seven treatment tanks and one control tank daily) were sacrificed and tested for the presence of A. salmonicida in the kidney (n = 3520). Over 10 days, mean prevalence among recipient fish reached 75% and disease related mortality exceeded 50%. Bacterial concentrations in the water continued to increase over the duration of the experiment in concert with the number of infected animals present in the population.     

Importance of river migration to the development of seawater tolerance in Columbia River anadromous salmonids

Gradually increasing levels of gill Na⁺K⁺ ATPase activity were observed in juvenile chinook, Oncorhynchus tshawytscha, and coho, Oncorhynchus kisutch, salmon and steelhead trout, Salmo gairdneri, undergoing parr-smolt transformation in artificial rearing facilities on the Columbia River. Portions of the same populations released to migrate seaward, however, generally showed much greater increases in enzyme activity with time and distance from the release point. After migrating 714 km to the Columbia River estuary, spring chinook salmon had a mean gill Na⁺K⁺ ATPase activity 2.5 times greater than fish retained at the hatchery and 1.9 times greater than fish adapted to 28 ppt seawater for 208 days. Similar observations were made on coho salmon.     

Prey selectivity and diel feeding chronology of juvenile chinook (Oncorhynchus tshawytscha) and coho (O. kisutch) salmon in the Columbia River plume

We studied salmon feeding selectivity and diel feeding chronology in the Columbia River plume. Juvenile chinook and coho salmon were caught by trawling at 2–3 h intervals throughout a diel period on three consecutive days (21–23 June 2000) at stations located 14.8 and 37 km offshore from the mouth of the Columbia River. A total of 170 chinook salmon were caught at the inshore and 79 chinook and 98 coho salmon were caught at the offshore station. After each trawl, potential prey were sampled at different depths with 2–3 different types of nets (1‐m diameter ring net, bongo net, neuston net). Despite the variability in zooplankton abundance, feeding selectivity was surprisingly constant. Both salmon species fed selectively on larger and pigmented prey such as hyperiid amphipods, larval and juvenile fish, various crab megalopae, and euphausiids. Hyperiid amphipods were abundant in the salmon diets and we hypothesize that aggregations of gelatinous zooplankton may facilitate the capture of commensal hyperiid amphipods. Small copepods and calyptopis and furcilia stages of euphausiids dominated the prey field by numbers, but were virtually absent from salmon diet. Juvenile chinook salmon, with increasing body size, consumed a larger proportion of fish. Stomach fullness peaked during morning hours and reached a minimum at night, suggesting a predominantly diurnal feeding pattern. In general, both chinook and coho salmon appear to be selective, diurnal predators, preying mostly on large and heavily pigmented prey items, in a manner consistent with visually oriented, size‐selective predation.     

Effects of dietary canola oil level on growth, fatty acid composition and osmoregulatory ability of juvenile fall chinook salmon (Oncorhynchus tshawytscha)

This study assessed refined canola oil (CO) as a supplemental dietary lipid source for juvenile fall chinook salmon, Oncorhynchus tshawytscha, parr with respect to possible effects on their growth and osmoregulatory performance and body composition. Diets with equal protein ( 57%) and lipid ( 19%) content (dry weight basis) were supplemented with lipid from either anchovy oil (AO) or CO with AO so that CO comprised 0 (0CO), 11% (11CO), 22% (22CO), 33% (33CO), 43% (43CO) or 54% (54CO) of the dietary lipid content. Triplicate groups of juvenile chinook salmon were fed their prescribed diets for 104 days in freshwater (FW) and 31 days in seawater (SW) after a 4-day transition period. Dietary fatty acid compositions reflected the different proportions of AO and CO in the supplemental lipid. Diet treatment had no effect on fish growth, feed intake, feed efficiency, protein utilization, fish mortality or terminal whole body water and ash percentages. Whole body lipid percentages were higher in 11CO and 43CO fish than in 33CO fish and in 11CO fish versus 22CO fish. Whole body protein percentages were highest in 33CO, 43CO and 54CO fish and lowest in 0CO and 22CO fish. Terminal whole body fatty acid compositions were influenced strongly by the dietary fatty acid compositions. Haematocrit and muscle water percentages were not affected consistently and plasma Na⁺ and Cl⁻ concentrations were unaffected by diet treatment in FW or 24-h seawater challenges during FW residency. Also, diet treatment had no effect on the physiological parameters after SW residency. We conclude that dietary treatment had no effect on fish growth performance under our experimental conditions. Also, the dietary inclusion of CO neither facilitated nor impaired the transfer of chinook salmon parr to seawater. Thus, CO was found to be an excellent and cost-effective source of supplemental dietary lipid for culture of juvenile fall chinook salmon during freshwater residency.     

The effects of the 1983 El Niño on Oregon's Coho (Oncorhynchus kisutch) and Chinook (O. tshawytscha) Salmon

The 1983 El Niño event off the Pacific Coast of North America resulted in increased adult mortality and decreased average size for Oregon's coho and chinook salmon. Actual return of adult coho salmon to the Oregon Production Area in 1983 was only 42% of the pre-season prediction. Coho smolts entering the ocean in the spring of 1983 also survived poorly, resulting in low adult returns again in 1984. Abundance of chinook stocks in southern Oregon was also reduced, as was abundance of Columbia River chinook stocks that show localized ocean distribution. Northerly migrating chinook stocks from the Columbia River showed little or no decline in abundance. The average weight of coho and chinook salmon landed in 1983 by Oregon's commercial troll fishery was the lowest recorded since statistics were first recorded in 1952. Comparison of the length-weight relationship for these fish indicated coho and chinook were in poorer condition in 1983 than in non-El Niño years. Because adult coho salmon returned to hatcheries at a smaller size, the fecundity (eggs per female) in 1983 was reduced from the 1978–1982 average by 24% at coastal hatcheries and by 27% at Columbia River hatcheries. The fecundity of chinook salmon was unchanged at most hatcheries.     

Ichthyophonus hoferi Plehn & Mulsow in British Columbia stocks of Pacific herring, Clupea pallasi Valenciennes, and its infectivity to chinook salmon, Oncorhynchus tshawytscha (Walbaum)

Pacific herring, Clupea pallasi Valenciennes, collected from three areas of coastal British Columbia were screened for Ichthyophonus by histological examination. The infectivity of Ichthyophonus to juvenile chinook salmon, Oncorhynchus tshawytscha (Walbaum), was examined in laboratory studies. Ichthyophonus was detected in a total of 82 of 356 herring from all three areas. Prevalence in 2000 and 2001 ranged from 10.5 to 52.5% and was significantly lower in more northern (Hecate Strait) samples. Ichthyophonus was detected by histological examination in chinook salmon following oral or intraperitoneal (i.p.) exposure to homogenates of infected herring tissue. Infections in Yukon stock chinook salmon were occasionally associated with mortality and with inflammation in all tissues examined. Infections were detected significantly more frequently in the caecal mesenteries of i.p.-infected compared with oral-infected chinook salmon. The distribution and prevalence of Ichthyophonus isolates among diverse host species may assist in stock identification and in an improved understanding of trophic interactions.     

A comparative tissue distribution study of oxytetracycline in rainbow trout, Oncorhynchus mykiss (Walbaum), and chinook salmon, Oncorhynchus tshawytscha (Walbaum)

Oxytetracycline (OTC), a broad-spectrum antibiotic, is used widely to treat bacterial diseases in farmed fish. In the present study, the time course of OTC concentrations in freshwater rainbow trout, Oncorhynchus mykiss (Walbaum), and seawater chinook salmon, Oncorhynchus tshawytscha (Walbaum), were compared, tissue by tissue, after receiving a bolus dose of the antibiotic (5 mg kg^–1 or 50 mg kg^–1) intra-arterially (i.a.). The OTC concentration–time profiles of rainbow trout tissues were found to be very similar to those of the corresponding tissues in chinook salmon. Therefore, neither water salinity nor fish species seemed to play an important role in the disposition and elimination of OTC in these salmonids. In a separate experiment, rainbow trout were implanted surgically with a urinary cannula and received a single dose of OTC (50 mg kg^–1) i.a. Urine was collected from the cannula daily for 13 days. The amount of OTC excreted into the bile was found to be larger than that eliminated by the urine. These results show the similarity of OTC pharmacokinetics in freshwater rainbow trout and seawater chinook salmon and render support in using a single fish species to study the pharmacokinetics of a drug for other species in the same taxon.     

Efficacy of manual removal and ivermectin gavage for control of Salmincola californiensis (Wilson) infestation of chinook salmon, Oncorhynchus tshawytscha (Walbaum), captive broodstocks

Captive broodstocks of spring chinook salmon, Oncorhynchus tshawytscha , were initiated from collections of naturally produced parr from the Lemhi River, a tributary of the Salmon River, ID, USA. These fish were subsequently demonstrated to be infested with the copepod parasite Salmincola californiensis . The initial prevalence of visible adult parasites for 4 years of observations made shortly after collection varied from 19.7 to 71.6%. Both the prevalence and intensity of the infestation increased in the freshwater culture of these fish. Manual removal was initiated as a means of control and practiced at monthly intervals. The number of Salmincola removed decreased in the ensuing 5 months, but the prevalence was not greatly affected. Ivermectin (22,23 dihydroavermectin), was diluted with saline and delivered by gavage at the rate of 0.20 mg kg⁻¹ body weight when the groups were being handled for the manual removal of parasites. Either two or three ivermectin treatments were given to four broodstocks of chinook salmon depending on the severity of the infestation and on the extent of gill pathology. The combination of manual removal and ivermectin gavage eliminated live Salmincola and resolved all associated necrosis of the gill tissues. There was no trend to indicate that individual chinook salmon possessed a natural resistance to reinfestation.     

Chronic glomerulonephritis in returning fall chinook salmon, Oncorhynchus tshawytscha (Walbaum), from the Rogue River, Oregon

Epizootic mortalities of returning adult Rogue River fall chinook salmon, Oncorhynchus tshawytscha (Walbaum), were first observed in 1978. These mortalities reached approximately 76% in 1979 and declined to 30--50% in 1980 and less than 10% in 1981 and 1982. Histopathological examination of kidneys from moribund returns in 1981 and 1982 showed a diffuse chronic glomerulonephritis (GN) in 83% and 30% of the fish examined, respectively. In most cases glomerular damage was severe enough to cause morbidity from osmoregulatory failure. The lesion was markedly similar to immune complex-mediated glomerular disease in higher vertebrates, but tissues for further ultrastructural examination and diagnosis were not processed. Giemsa-stained sections also revealed unidentified sporozoan-like vegetative stages encysted within the extracellular mesangial matrix of glomeruli in both GN and normal kidneys. Although probably incidental to the glomerular lesion, the causal relationship of the organism to GN remains unknown until further investigation. Glomeruli of outmigrating Rogue River fall chinook salmon smolts collected in 1983 were normal and free of the organism.     

Immunofluorescence screening of Renibacterium salmoninarum in the tissues and eggs of farmed chinook salmon spawners

A simple and rapid on site-test method was used to screen farmed chinook salmon (Oncorhynchus tschawytscha) broodstock for bacterial kidney disease (BKD) at the time of spawning. Kidney tissue and ovarian fluid smears were examined, using the indirect fluorescent antibody technique (IFAT). To check the validity of our field screening procedure, intra-ovum fluid, taken from random samples of 300 surface-sterile, fertilized eggs, was examined in the laboratory for the presence or absence of Renibacterium salmoninarum, using the IFAT. Prior to sampling, eggs were incubated at 15°C for 40 days in enrichment KDM-2 broth. R. salmoninarum cells were found in one out of 10 samples of pooled contents of eggs from fish showing no R. salmoninarum in the kidney tissue (these fish were assumed to have R. salmoninarum-free ovarian fluid); three out of 10 pooled samples from fish in which R. salmoninarum was detected in the kidney but not the ovarian fluid; all of the 10 pooled samples from fish with R. salmoninarum in both the kidney tissue and the ovarian fluid. The number of R. salmoninarum in the pooled egg contents was low (2–23 per 100 fields). Using the IFAT technique it was not possible to determine whether the BKD-bacteria in the eggs were alive or dead. Methods of BKD treatment and screening are discussed as is the validity of using these techniques to prevent vertical transmission of BKD in farmed salmon.     

The effect of whole body lipid on early sexual maturation of 1+ age male chinook salmon (Oncorhynchus tshawytscha)

Early sexual maturation of male chinook salmon (maturation 1 to 4 years prior to females in the same age class) results in reduced effectiveness of stock enhancement programs and a financial loss to the salmon farming industry. Previous studies in Atlantic salmon have shown that the age of maturity in males is affected by growth and/or body energy stores, but the relative roles of these two factors are not well understood. Therefore, an experiment was designed to determine when spermatogenesis was initiated, to characterize the endocrine changes during the onset of puberty in male salmon, and to determine if the level of whole-body lipid affects the incidence of early male maturation in a wild stock (Yakima River) of 1+ spring chinook salmon. Fry were fed a commercial diet from February until August and were then divided into groups of 320 fish (mean weight, 5.6 g) and fed one of five experimental diets (two replicate groups/diet) containing 4%, 9%, 14%, 18% or 22% lipid and 82%, 77%, 73%, 69%, or 65% protein for 13 months. Fish were reared on natural photoperiod and ambient temperature (6°C to 16°C), and pair-fed to a level based on the tank with the lowest feed consumption. Fish were weighed monthly and sampled to determine body composition, pituitary follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels, plasma insulin-like growth factor I (IGF-I) levels, and stage of gonadal development.

Throughout the experimental period the mean fish weight was similar among treatment groups. However, from December through the end of the experiment in the following September, maturing males were significantly larger than nonmaturing fish. Initial lipid levels in 0-age experimental fish were near 6%, which is similar to wild fish of the same stock and age captured in the Yakima River during August. Fish fed diets containing more than 4% lipid increased in whole-body lipid content during the first 2 months of feeding and then maintained at relatively constant levels during the course of the experiment. Whole-body lipid levels for the dietary treatment groups averaged 5.6%, 7.1%, 8.2%, 9.4%, and 9.6% from October through the following September.

Based on histological examination of the testes of experimental fish, type B spermatogonia and primary spermatocytes were first observed in some of the yearling males during November. These were designated maturing males. Pituitary FSH levels were significantly higher in maturing than nonmaturing males at this time and for the remainder of the study. Pituitary FSH levels increased as spermatogenesis proceeded in maturing fish, whereas pituitary LH levels increased in maturing 1+ males only during July and August, when testes were in late stages of spermatogenesis and in September during spermiation. Plasma IGF-I levels were significantly higher in maturing males than nonmaturing fish from December through the end of experiment. Since maturing males were significantly larger than nonmaturing fish of both sexes from December through September, the difference in IGF-I levels could be due to differences in growth or due to maturation.

The percentage of maturing males was significantly influenced by whole-body lipid, increasing from 34% in fish fed the 4% lipid diet to 45% in fish fed the 22% lipid diet. These data suggest that whole-body lipid levels influenced the incidence of maturation of male spring chinook salmon. In addition, both endocrine and histological indicators suggest that maturation was initiated in males approximately a full year prior to the time the fish will spawn.     

Induced ovulation of female chinook salmon (Oncorhynchus tshawytscha) at a production hatchery

A simple, low-cost method for inducing ovulation in hatchery-held adult chinook salmon was developed. Use of this method resulted in an increase in the number of eggs collected, by circumventing losses due to prespawning mortality.One hundred and twenty 4-year-old chinook salmon which had returned to the Capilano hatchery prior to 1 October were used as a stock population. Three weeks before the peak spawning period, 36 fish were divided into two groups. The control group received two injections of 0.65% saline spaced 3 days apart. The test group received a 0.1-mg/kg SG-G100 (salmon gonadotropin) injection, followed on the third day by a 2.5-mg/kg SG-G100 injection. By Day 8, 73% of the treated group and 0% of the control group had ovulated. Prespawning mortality accounted for 22% of the treated fish and 83.4% of the control fish. Treatment resulted in a net gain of 16,000 eggs over the control. The second experiment began 3 days before the peak spawning period. The 32 fish which remained in the stock population were divided into three groups and received two injections 3 days apart. The two treatment groups received either 0.1 mg SG-G100/kg followed by 2.0 mg SG-100/kg, or 0.1 mg SG-G100/kg followed by 50.0 mg salmon pituitary extract (SPE)/kg. The control group was injected with 0.65% saline. By Day 8, 73% of the SPE group, 82% of the SG-G100 group, and 40% of the control group had ovulated. Treatment resulted in a net gain of 16,000 eggs for the SG-G100 group and 19,000 eggs for the SPE group over the control. No significant differences in egg size or percent survival to the eyed stage were found between test and control groups. The misjudgment of maturity led to the stripping of several fish before their eggs were completely ovulated. The factors leading to these partial ovulations are discussed.     

Age,ration level,and exercise affect the fatty acid profile of chinook salmon (Oncorhynchus tshawytscha) muscle differently

This study was undertaken to investigate the effect of sustained physical exercise (SS, swimming speed) on the fatty acid profile of muscle in PIT-tagged all-female chinook salmon (Oncorhynchus tshawytscha) in relation to their age and ration level (RL; i.e., maximum ration, RL100 or 75% of maximum, RL75). Accordingly, body weight (BW), specific growth rate (SGR), and total muscle lipid content (TLC) were considered as covariates in data analyses. In addition, plasma levels of thyroid hormones (thyroxine, T₄ and 3,5,3′-triiodo-l-thyronine, T₃) were compared to the muscle fatty acid (FA) compositions of individual fish to determine if there were any associations between thyroidal status and FA percentages. During the 300-day study, body weight increased from 75 to 440 g. Fish age explained most of the changes found in muscle FA composition [that is, R² was 0.23 for saturated fatty acids (SAFAs), 0.65 for monounsaturated fatty acids (MUFAs), and 0.71 for polyunsaturated fatty acids (PUFAs) with p<0.0007 in all cases]. Reduction in RL had less influence on FA composition (p>0.15 for SAFA, and R² was 0.27 for MUFA and 0.34 for PUFA, with p<0.0001 in each of the latter cases). SS only affected MUFA significantly (p<0.07 with R²=0.09). The observed effects of fish age and SS on FA composition were to a large degree uninfluenced by BW, SGR, or TLC, while the effects of RL were markedly diminished when these covariates were included in the statistical model. Thus, the changes in body weight and/or lipid content were viewed as being the direct cause for the alteration in muscle FA profile seen with the main factor RL. The effect of SS was pronounced only if fish with a reduced RL were compared. In this situation, the level of PUFA, especially docosahexaenoic acid (DHA; 22:6 n-3), decreased and MUFA increased. Plasma T₄ was unrelated to muscle FA levels, but plasma T₃ was correlated positively with muscle MUFA and negatively with PUFA. We conclude that in order for exercise to affect chinook salmon fillet FA composition, it has to be combined with restricted feeding. The data also imply that in accordance with higher vertebrates, T₃ is also involved in the regulation of FA metabolism in fish.     

Comparative sensitivity of five fish cell lines to wild type infectious haematopoietic necrosis virus from two Oregon sources

Abstract. Five fish cell lines (CHSE-214, STE-137, RTG-2, EPC and FHM) were compared for sensitivity to infectious haematopoietic necrosis virus (IHNV) from samples obtained from naturally-infected fish. Infectious ovarian fluids were obtained from steelhead trout, Salmo gairdneri Richardson, at the Round Butte Hatchery in central Oregon and tissue homogenates were prepared from chinook salmon, Oncorhynchus tshawytscha (Walbaum), alevins during an IHN virus epizootic at the Elk River Hatchery in coastal Oregon. The only lines to show characteristic viral cytopathology by plaque or end-point dilution assay for the steelhead trout virus isolate were the EPC and FHM cell lines. The chinook salmon isolates produced CPE in CHSE-214, STE-137, FHM and EPC cells. The titre of the salmon virus isolate was 10-50-fold higher on FHM and EPC cells by both assay methods. Neither by end-point nor plaque assay did the Round Butte or Elk River isolates produce CPE on RTG-2 cells. With both virus isolants both cell lines showed that greater sensitivity was obtained with plaque assay than with end-point titration. Pre-treatment of the cells with the polycation, polybrene, did not increase the virus titre in either assay. However, a transient enhancement in virus titre was observed in polybrene-treated STE-137 and CHSE-214 cells.     

Relative importance of chinook salmon abundance on resident killer whale population growth and viability

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The vertical distribution of juvenile salmon (Oncorhynchus spp.) and associated fishes in the Columbia River plume

Simultaneous trawling at surface and at depth at one location off the Columbia River, Oregon, in June 2000 identified the depth distribution of juvenile salmonids and associated fishes. Juvenile salmon off the Columbia River were distributed primarily near the surface, within the upper 12 m. Highest densities of subyearling chinook salmon (Oncorhynchus tshawytscha) off the Columbia River were associated with high surface currents and decreasing tidal levels, with time of day possibly a co‐factor. Densities of yearling chinook salmon increased with higher turbidity. Pacific herring (Clupea pallasi) was the most abundant and commonly caught forage fish, with density increasing at night, probably related to diel vertical migration. Catches of juvenile salmonids were not associated with catches of forage fishes. Daytime surface trawling appears to be an appropriate method for assessing the distribution and abundance of juvenile salmonids in marine habitats.     

Growth Acceleration of Seawater-Adapted Female Chinook Salmon Oncorhynchus tshawytscha by Constant Infusion of Recombinant Bovine Growth-Hormone Under Ambient Summer Conditions

Seawater-adapted female chinook salmon ( Oncorhynchus tshawytscha ) in their second summer were treated with one of three doses of recombinant bovine growth-hormone (rbGH) administered in a continuous fashion by osmotic pump for approximately five weeks. Untreated fish, sham-treated fish and fish treated with the low dose of hormone (0.003 μg/g bwt/day) did not differ significantly in their growth performance over the 10 week course of the experiment. By the end of the treatment period (week 6) the mean increases in fork length and weight for these groups were 7.6% and 38.9%, respectively. A dose-dependent enhancement of growth was observed for the fish treated with the mid (0.016 μg/g bwt/day) and high (0.078 μg/g bwt/day) doses of hormone. After 6 weeks, the fish given the mid dose of rbGH had gained 9.6% in fork length and 45.3% in weight, while the fish treated with the high dose of rbGH had gained 13.9% in fork length and 52.9% in weight. A significant increase in specific growth rate was observed for these latter two groups during the treatment period while condition factor declined in response to the high dose of rbGH. This experiment indicates that chronic administration of growth-hormone to chinook salmon can result in a dose dependent elevation of growth rate. Results are discussed in relation to previous attempts to accelerate the growth of Pacific salmon with growth-hormone and the potential for using recombinant growth-hormone to improve the production characteristics of salmon being cultured in seawater.