Fluvial rainbow trout contribute to the colonization of steelhead (Oncorhynchus mykiss) in a small stream

Life history polymorphisms provide ecological and genetic diversity important to the long term persistence of species responding to stochastic environments. Oncorhynchus mykiss have complex and overlapping life history strategies that are also sympatric with hatchery populations. Passive integrated transponder (PIT) tags and parentage analysis were used to identify the life history, origin (hatchery or wild) and reproductive success of migratory rainbow/steelhead for two brood years after barriers were removed from a small stream. The fluvial rainbow trout provided a source of wild genotypes to the colonizing population boosting the number of successful spawners. Significantly more parr offspring were produced by anadromous parents than expected in brood year 2005, whereas significantly more parr offspring were produced by fluvial parents than expected in brood year 2006. Although hatchery steelhead were prevalent in the Methow Basin, they produced only 2 parr and no returning adults in Beaver Creek. On average, individual wild steelhead produced more parr offspring than the fluvial or hatchery groups. Yet, the offspring that returned as adult steelhead were from parents that produced few parr offspring, indicating that high production of parr offspring may not be related to greater returns of adult offspring. These data in combination with other studies of sympatric life histories of O. mykiss indicate that fluvial rainbow trout are important to the conservation and recovery of steelhead and should be included in the management and recovery efforts.     

Arteriosclerosis in steelhead trout, Oncorhynchus mykiss (Walbaum): a developmental analysis

Arteriosclerosis in Oncorhynchur mykiss has been examined histologically in parr, smolts, spawning adults and post-spawning adults captured during downstream migration. All stages of fish from the Clearwater River in Idaho are characterized by the presence of lesions in the main vessel on the bulbus arteriosus. The incidence is lowest in parr, increases significantly in smolts, and is present in 100% of spawning fish. Regression of lesions was not observed in either hatchery or wild fish after spawning.     

Rhamnose-binding lectins from steelhead trout (Oncorhynchus mykiss) eggs recognize bacterial lipopolysaccharides and lipoteichoic acid

The interaction between bacteria and three L-rham-nose-binding lectins, named STL1, STL2, and STL3, from steelhead trout (Oncorhynchus mykiss) eggs was investigated. Although STLs bound to most Gram-negative and Gram-positive bacteria, they agglutinated only Escherichia coli K-12 and Bacillus subtilis among the bacteria tested. The binding was inhibited by L-rhamnose. STLs bound to distinct serotypes of lipopolysaccharides (LPSs), and showed much higher binding activities to smooth-type LPSs of Escherichia coli K-12 and Shigella flexneri 1A than to their corresponding rough-type LPSs. STLs also bound to lipoteichoic acid (LTA) of Bacillus subtilis. These results indicate that STLs bound to bacteria by recognizing LPSs or LTA on the cell surfaces.     

Individual condition indicators of thermal habitat quality in field populations of redband trout (Oncorhynchus mykiss gairdneri)

We monitored redband trout (Oncorhynchus mykiss gairdneri) physiological status in multiple streams across eastern Oregon, USA that exhibit various summer thermal profiles. Summer river temperatures were higher overall in 2009 than in 2010. Heat shock protein 70 (hsp70) levels in both liver and fin tissue from redband trout increased with temperature regardless of location. Lipid levels were lower in fish from waters above 23 °C (in terms of degree-day exposure), and lipids were also related to food availability in 2010. Overall, fish ate more in 2009 than in 2010. Fish size increased more rapidly, and lipids were higher and hsp70 was lower in the cooler Crooked River relative to streams in the John Day River system, though the Crooked River did not have the highest food available. While food availability plays a role in physiological health, our data suggest that temperature is the most influential factor driving growth and energy reserves in stream salmonids during high temperature months; we argue that monitoring physiological indicators in individual fish can serve as a useful proxy for overall thermal habitat quality and population status.     

Survival and behavior of juvenile steelhead trout (Oncorhynchus mykiss) in two estuaries in Oregon, USA

Anadromous salmonids are viewed as a prized commodity and cultural symbol throughout the Pacific coast of North America. Unfortunately, several native salmonid populations are threatened or at risk of extinction. Despite this, little is known about the behavior and survival of these fish as the juveniles transition from freshwater to the ocean. Our primary objectives were to estimate survival of juvenile steelhead migrating between trapping sites and the ocean and evaluate whether survival in the estuary varies temporally (within a year) or spatially (within and between estuaries) within the same distinct population segment. We also evaluated whether flow or fork length were correlated with survival and collected information on variables that have been demonstrated to affect smolt survival in other studies to lend insight regarding differences in survival estimates between basins. We compared run timing, migration rate, survival, condition factor, age composition and time of residence in the estuary for steelhead outmigrants from each basin and measured parasite loads in outmigrating steelhead to evaluate potential differences in parasite density and parasite community between basins. In 2009, we implanted acoustic transmitters in 139 wild steelhead smolts in two small rivers on the Oregon Coast. In general, only 40–50 % of the wild steelhead smolts tagged at upstream smolt traps were detected entering the ocean. The majority of mortality occurred in the lower estuary near the ocean. Wild steelhead smolts typically spent less than 1 day in the estuary in both basins. Using similar data from previous studies in the Nehalem and Alsea basins, we showed that survival appears to be negatively correlated with flow in most releases, and in 2009 fork length was not correlated with survival. Our observations provide baseline information on factors that could influence smolt survival through the estuary as well as smolt to adult survival in these basins, and emphasize the importance of monitoring smolt survival in the estuary.     

Diel movements of out-migrating Chinook salmon (Oncorhynchus tshawytscha) and steelhead trout (Oncorhynchus mykiss) smolts in the Sacramento/San Joaquin watershed

We used ultrasonic telemetry to describe the movement patterns of late-fall run Chinook salmon (Oncorhynchus tshawytscha) and steelhead trout (O. mykiss) smolts during their entire emigration down California’s Sacramento River, through the San Francisco Bay Estuary and into the Pacific Ocean. Yearling hatchery smolts were tagged via intracoelomic surgical implantation with coded ultrasonic tags. They were then released at four upriver locations in the Sacramento River during the winters of 2007 through 2010. Late-fall run Chinook salmon smolts exhibited a nocturnal pattern of migration after release in the upper river. This is likely because individuals remain within a confined area during the day, while they become active at night and migrate downstream. The ratio between night and day detections of Chinook salmon smolts decreased with distance traveled downriver. There was a significant preference for nocturnal migration in every reach of the river except the Estuary. In contrast, steelhead smolts, which reside upriver longer following release, exhibited a less pronounced diel pattern during their entire migration. In the middle river, Delta, and Estuary, steelhead exhibited a significant preference for daytime travel. In the ocean Chinook salmon preferred to travel at night, yet steelhead were detected on the monitors equally during the night and day. These data show that closely related Oncorhynchus species, with the same ontogenetic pattern of out-migrating as yearlings, vary in migration tactic.     

Exhaustive exercise, animal stress, and environmental hypercapnia on motility of sperm of steelhead trout (Oncorhynchus mykiss)

Motility of salmonid sperm is inhibited by the presence of carbon dioxide (CO2) in vitro; however, whether this occurs in response to challenges to the adult in vivo is not known. To determine whether CO2 negatively impacts sperm function in vivo, mature males were exposed to exhaustive exercise as well as to acute stress, chronic stress, tricaine anesthesia and environmental hypercapnia and sperm motility and semen CO2 tensions and pH values assessed. Semen CO2 rose and pH decreased significantly only in response to exhaustive exercise and environmental hypercapnia (13 kPa CO2). These changes in semen CO2 and pH were associated with reductions in numbers of sperm becoming motile upon water activation. Chronic and acute stress and tricaine anesthesia were without effect on sperm motility or on semen CO2 or pH. The time course of CO2 inhibition and recovery was evaluated in vitro. At least 50 min was required to note 50% of the inhibitory effect of low CO2 tensions on motility when sperm were exposed to 1.6-3.1 kPa CO2. At higher CO2 levels sperm motility displayed 50% of the inhibitory effect of these tensions within about 30 min. Sperm recovered maximal motility within 1 h of being placed in a nominally CO2-free environment. This study demonstrates sperm vulnerability to not only in vitro CO2 exposure but also in vivo exposure during exhaustive exercise and as result of environmental hypercapnia.     

Discovery and characterization of single-nucleotide polymorphisms in steelhead/rainbow trout, Oncorhynchus mykiss

Single-nucleotide polymorphisms (SNPs) have several advantages over other genetic markers, including lower mutation and genotyping error rates, ease of inter-laboratory standardization, and the prospect of high-throughput, low-cost genotyping. Nevertheless, their development and use has only recently moved beyond model organisms to groups such as salmonid fishes. Oncorhynchus mykiss is a salmonid native to the North Pacific rim that has now been introduced throughout the world for fisheries and aquaculture. The anadromous form of the species is known as steelhead. Native steelhead populations on the west coast of the United States have declined and many now have protected status. The nonanadromous, or resident, form of the species is termed rainbow, redband or golden trout. Additional life history and morphological variation, and interactions between the forms, make the species challenging to study, monitor and evaluate. Here, we describe the discovery, characterization and assay development for 139 SNP loci in steelhead/rainbow trout. We used EST sequences from existing genomic databases to design primers for 480 genes. Sanger-sequencing products from these genes provided 130 KB of consensus sequence in which variation was surveyed for 22 individuals from steelhead, rainbow and redband trout groups. The resulting TaqMan assays were surveyed in five steelhead populations and three rainbow trout stocks, where they had a mean minor allele frequency of 0.15-0.26 and observed heterozygosity of 0.18-0.35. Mean F(ST) was 0.204. The development of SNPs for O. mykiss will help to provide highly informative genetic tools for individual and stock identification, pedigree reconstruction, phylogeography and ecological investigation.     

Ocean migration and behavior of steelhead Oncorhynchus mykiss kelts from the Situk River,Alaska

Environmental Biology of Fishes - Although steelhead (Oncorhynchus mykiss) is an iconic species found throughout the North Pacific rim, little is known about its ocean ecology. To provide insights...     

Growth, survival, and tag retention of steelhead trout (Oncorhynchus mykiss) and its application to survival estimates

Steelhead trout, Oncorhynchus mykiss, are known to expel acoustic tags which can negatively bias population survival estimates. Tag burden may also affect the development and behavior of smolts, thereby impacting the results of population and behavioral studies. We monitored the growth, condition, and tag expulsion rate of steelhead trout in similar-sized individuals and used these data to adjust survival rates from an acoustic telemetry study conducted in the Sacramento River. Eighty fish were surgically implanted with tags—40 with cylindrical tags of 9 mm diameter and 21 or 24 mm length (V9, Vemco Ltd) and 40 with a 7 mm diameter and 20 mm length tag (V7, Vemco Ltd)—to examine the impact of tag size on peritoneal retention and survival rate of juvenile steelhead trout. A total of 20 % (16/80) of all tags were expelled by smolts during the 143-day study. Ten V9 tags were expelled between day 18 and day 66. Six V7 tags were expelled between day 21 and day 143. A statistical difference was found for retention rate by surgeon even though the surgeons were of equal experience and received the same training. There were no significant differences in the tag retention rate in relation to the tag/body weight ratio, or in growth (weight or fork length) among the control, V7 or V9 treatment groups over the duration of the study. All individuals survived throughout the experiment. Two methods were used to adjust the survival estimates of an acoustic telemetry data set from the Sacramento River based on the tag retention study. First, a simple individual censorship approach in Program MARK was utilized and next ATLAS, a software program designed to compensate for bias in survival estimates caused by tag failures was used. The results of the adjusted survival estimates were not significantly different from the unadjusted rates suggesting that it may be more important to focus on improving surgical techniques to reduce tag expulsion rather than adjusting survival estimates dependent on the study. The surgical techniques utilized in this study did not have significant impacts on the growth rates of either of the tag treatment groups compared to the control. However, tag retention was an issue regardless of the size and weight of the implanted tag and the size of the steelhead.     

A novel rhamnose-binding lectin family from eggs of steelhead trout (Oncorhynchus mykiss) with different structures and tissue distribution

An L-rhamnose-binding isolectin named STL3 (subunit Mr, 21.5 k) was isolated from eggs of the steelhead trout (Oncorhynchus mykiss) in addition to STL1 (subunit Mr, 31.4 k) and STL2 (subunit Mr, 21.3 k) that had been already isolated. STLs were composed of noncovalently linked subunits. The primary structures of STL1 and STL3 were analyzed by the combined use of protein sequencing and cDNA sequencing. A cDNA encoding STL2, of which the protein sequence had been previously studied, was also analyzed. The STL1 subunit (289 amino acid residues) had different structural properties compared to those of the STL2 subunit (195 amino acid residues) and the STL3 subunit (195 amino acid residues); e.g., the number of repeated domain (three for STL1, and two for STL2 and STL3), although all of them were composed of tandemly repeated homologous domains (40 to 53% identities). The lectin levels in various tissues and during the embryonic development showed that STL1 had different distribution and expression profiles from those of STL2 and STL3. Although STL1 could be detected in several tissues and serum of both male and female steelhead trout, STL2 and STL3 were only abundant in the ovary. STL2 and STL3 levels dramatically decreased just after hatching, however, the STL1 level increased temporarily. These results indicate that the multiple lectins from eggs of the steelhead trout form a novel rhamnose-binding lectin family with different structures and tissue distribution to share distinct functions in eggs.     

Individual condition and stream temperature influence early maturation of rainbow and steelhead trout, Oncorhynchus mykiss

Alternative male phenotypes in salmonine fishes arise from individuals that mature as larger and older anadromous marine-migrants or as smaller and younger freshwater residents. To better understand the processes influencing the expression of these phenotypes we examined the influences of growth in length (fork length) and whole body lipid content in rainbow trout (Oncorhynchus mykiss). Fish were sampled from the John Day River basin in northeast Oregon where both anadromous (“steelhead”) and freshwater resident rainbow trout coexist. Larger males with higher lipid levels had a greater probability of maturing as a resident at age-1+. Among males, 38% were maturing overall, and the odds ratios of the logistic model indicated that the probability of a male maturing early as a resident at age-1+ increased 49% (95% confidence interval (CI) = 23–81%) for every 5 mm increase in length and 33% (95% CI = 10–61%) for every 0.5% increase in whole body lipid content. There was an inverse association between individual condition and water temperature as growth was greater in warmer streams while whole body lipid content was higher in cooler streams. Our results support predictions from life history theory and further suggest that relationships between individual condition, maturation, and environmental variables (e.g., water temperature) are shaped by complex developmental and evolutionary influences.     

The mucous coat on gill lamellae of rainbow trout (Oncorhynchus mykiss)

The existence of a layer of mucus covering the gill lamellae of healthy rainbow trout (Oncorhynchus mykiss) was investigated. Using cryo-scanning electron microscopy, a smooth, undulating, thin layer was observed which completely covered gill filaments and lamellae, thereby obscuring epithelial microridges. After processing cryopreserved gill arches in glutaraldehyde for conventional scanning electron microscopy, the layer was no longer present and epithelial microridges were clearly visible. The identity of this layer was investigated using cryopreserved gills which were treated in one of two ways. First, gills were incubated with a rabbit antiserum to gill mucus, with normal rabbit serum, or with phosphate-buffered saline. Following fixation in glutaraldehyde and processing, only the gill tissue incubated with the mucus-specific antiserum was still covered with the smooth layer. The layer was also retained on the gills of fish anesthetized in a solution containing mucusspecific antiserum and then processes in glutaraldehyde for conventional scanning electron microscopy. The tenacious nature of the mucous layer was demonstrated by its stability following exposure to formalin and a cationic detergent. Second, the presence of this layer was confirmed on gill tissue which was cryopreserved, followed by freeze-substitution and vapor fixation, and then examined by transmission electron microscopy.     

Peripheral serotonin dynamics in the rainbow trout (Oncorhynchus mykiss)

Serotonin (5-hydroxytryptamine, 5-HT) occurs in a wide range of tissues throughout the body of the rainbow trout. Results reported here indicate that the main peripheral sources of serotonin are the intestinal tract and the gill epithelium (levels above 1500 ng/g). The high intestinal serotonin concentration is mostly due to serotoninergic nerve fibres, which are present at high density in the intestinal wall. Only about 2% of serotonin is associated with mucosal enterochromaffin cells. In the remaining tissues studied serotonin concentration was below 160 ng/g: the highest concentrations were seen in the anterior and posterior kidneys, followed by the liver, heart, and spleen. 5-Hydroxyindolacetic acid (5-HIAA) levels, except in plasma, were generally lower than serotonin levels, and were below our detection limits in heart, spleen and posterior kidney. Acute d-fenfluramine treatment (5 or 15 mg/kg i.p.) significantly increased 5-HIAA/5-HT ratio in the anterior intestine, pyloric caeca and plasma. Serotonin released from intestinal serotoninergic fibres in response to d-fenfluramine treatment is metabolized locally, and only a small part reaches the blood, from where it can be taken up and metabolized by other peripheral tissues, such as the liver and gill epithelium. The non-metabolized serotonin pool in the blood appears to be located extracellularly, not intracellularly as in mammals. In view of these findings, we present an overview of peripheral serotonin dynamics in rainbow trout.     

Genetic variation of resistance to mercury poisoning in steelhead (Oncorhynchus mykiss) alevins

Newly hatched steelhead alevins were obtained from the factorial breeding of 24 male and 10 female steelhead trout, Oncorhynchus mykiss. Each set of offspring were in a separate cell. They were tested for resistance to intoxication by methylmercuric chloride (CH3HgCl) in water at a nearly constant mercury concentration of 8 microg l(-1). High mortality (81% of the tested alevins) occurred within 2 weeks. Resistance to intoxication, as measured by the time to death, as well as by the survival rate, shared high paternal and maternal variation with negligible interaction. Heritability of time to death was 0.59 +/- 0.17; heritability of survival (all-or-none trait) was lower (0.26 +/- 0.09). Mercury in dead alevins increased with time to death, exhibiting a large environmental variation and (comparatively) negligible genetic influence. At the end of the bioassay, the mercury content in survivors varied widely (3-21 microg g(-1) wet weight). The content was greater than, but correlated with that of dead alevins from the same cells, and it showed little relation with survival rate. Thus, it seems that resistance to poisoning implies a tolerance to high levels of mercury rather than a limitation of its accumulation.     

Immunostimulant effects of Cotinus coggyria on rainbow trout (Oncorhynchus mykiss)

In this study, non-specific immune effects of tetra (Cotinus coggyria) on rainbow trout (Oncorhynchus mykiss) by dietary intake were investigated. Fish were fed daily ad libitum with diets containing 0.5% and 1.0% tetra for 3 weeks. After this period, fish were switched back to the basal diet for additional 6 weeks. Extracellular and intracellular respiratory burst activities, phagocytosis in blood leukocytes, lysozyme activities, and total plasma protein levels were evaluated at the end of the tetra feeding period and every 3 weeks during the basal diet period. Extracellular and intracellular respiratory burst activities, phagocytic activity, lysozyme activity and total protein level parameters of the groups containing 0.5% and 1.0% tetra were higher than the control group at the end of the 3rd, 6th and 9th weeks, respectively (P < 0.05). The highest values of the non-specific immune parameters were observed in the group fed with 1.0% tetra. Tetra groups did not show any significant difference (P > 0.05) in terms of specific growth rate and average weight of the fish.     

The ontogeny of drinking in the rainbow trout, Oncorhynchus mykiss (Walbaum)

Drinking was measured by the rate of uptake of tritiated dextran in fresh water by the alevins and fry of rainbow trout at various stages of development during a 40-day post-hatch period. Drinking increased almost S-fold during the initial 16 day yolk-sac stage. Drinking rate increased most between 16 and 23 days, the transitional period between yolk-sac absorption and first feeding. The maximum weight-specific drinkingrateof 3.24μlg⁻¹ h⁻¹ recorded for 40-day-old fry was higher than previously recorded for adults. Abrupt transfer from an adaptation temperature of 10 to 19° C increased drinking significantly (Q₁₀= 1.2), but sudden transfer to 1 1‰ salinity sea water caused a substantial fall in drinking rate in 23-day fry. A 24-h period of adaptation to 1 1‰ and 15‰ salinity restored drinking to a rate similar to that in fresh water. The water drunk by 31-day fry fed to satiation was initially higher than by unfed fry, but drinking rates subsequently fell below the control level. The results are discussed in terms of osmoregulation and the uptake of dissolved or suspended substances by the intestinal tract for the purpose of immunization.     

Studies on lipid metabolism in trout (Oncorhynchus mykiss) branchial cultures

Cultured branchial cell epithelia from freshwater rainbow trout were incubated with ((32)P)phosphate and ((14)C)acetate as lipid precursors under both symmetrical (L15 media apical/L15 media basolateral) and asymmetrical (freshwater apical/L15 media basolateral) culture conditions. Epithelia composed of pavement cells alone, or containing both pavement cells and chloride cells, were examined. Lipids (labeled with (32)P and (14)C) were isolated and assayed by thinlayer chromatography, and fatty acids (labeled with (14)C) were isolated and assayed by paper chromatography. The main goal was to see whether the loss of a major incorporation into ((32)P)phosphatidylethanolamine [((32)P)PE], previously seen in eel gills in vivo when the fish were transferred from an osmotic steady state to more dilute media, was the result of a hormonal regulation, i.e., did it only apply to gill tissue in vivo or could it also be seen in the absence of hormonal modulation after incorporation of ((32)P)phosphate in vitro? We likewise wished to see whether a major incorporation into ((32)P)PE was dependent upon the presence of chloride cells. Results show that it is possible to obtain a ((32)P)PE dominated incorporation pattern, even in the pavement cells alone, provided that ((32)P)phosphate is added specifically to freshwater on the apical side of epithelia bathed asymmetrically (freshwater/L15). This is identical to the pattern seen in vivo in trout adapted to freshwater. However, this pattern is not seen under symmetrical conditions (L15/L15) or when ((32)P)phosphate is added to the basolateral media. The shift from symmetrical (L15/L15) to asymmetrical (freshwater/L15) culture conditions thus leads to the establishment of a major incorporation into ((32)P)PE and not to the equivalent loss as seen in vivo in more dilute apical media. We conclude that hormonal control is not needed to change the pattern of short-term lipid formation but, nevertheless, the responses are not altogether the same in vitro and in vivo. Furthermore, cultured trout gill epithelia, in contrast to gills in vivo, do not exhibit a marked incorporation of ((14)C)acetate into palmitoleic acid.