A genetic analysis of the London strain of rainbow trout

The London strain of rainbow trout (Oncorhynchus mykiss) was created by interbreeding three other strains of rainbow trout and therefore was expected to have higher levels of genetic variation than other strains of rainbow trout. We examined 129 London strain rainbow trout from Indiana by allozyme electrophoresis to assess levels of genetic variation and to examine the relationship between the London strain and other hatchery strains. When using the same loci to compare with other hatchery strains the London strain showed levels of genetic variation within the range of other hatchery strains: mean heterozygosity of 0.053 (0.031-0.099), 1.27 (1.20-1.60) alleles per locus and 20.0% (20.0-40.0%) of the loci were polymorphic. The London strain is somewhat distinct from other hatchery strains (D=0.009-0.072), in part because of the high frequency of the sIDHP*40 allele.     

The effect of water-soluble aromatic hydrocarbons on some haematological parameters of rainbow trout, Salmo gairdneri Richardson, during acute exposure

Rainbow trout were exposed to a model mixture of water-soluble aromatic hydrocarbons (AH) at concentrations of 0.3–15.2 ppm for periods of 1–72 h. None of the parameters monitored except RBC changed at 0.3 ppm AH, while all constituents, except blood glucose, changed at 7.2 ppm AH. At 15.2 ppm, all fish died within 1 h. Haematological changes included increased haematocrit, haemoglobin concentration, erythrocyte counts, and blood clotting (prothrombin) times. Plasma Cl⁻ concentration decreased significantly. Changes resembled a stress response except for the increased clotting times, which typically decrease, and lack of increase in blood glucose.     

A genetically distinct wild redband trout (<Emphasis Type="Italic">Oncorhynchus mykiss gairdneri</Emphasis>) population in Crane Prairie Reservoir,Oregon, persists despite extensive stocking of hatchery rainbow trout (<Emphasis Type="Italic">O. m. irideus</Emphasis>)

Crane Prairie Reservoir in the upper Deschutes River Basin has historically supported a wild population of migratory Deschutes River redband trout. Owing to its status as a premier destination for recreational angling in Oregon, the reservoir has been stocked with domesticated hatchery rainbow trout since 1955. In recent years the wild redband trout population has experienced a substantial decline. Effects on productivity related to genetic interaction with naturally spawning hatchery-origin fish (fitness risks) have not been determined. The species Oncorhynchus mykiss has been characterized with substantial genetic diversity throughout the Deschutes River Basin that further heightens the challenge of identifying specific conservation needs of wild populations. A conservation plan for Crane Prairie wild redband trout requires a better understanding of the natural reproductive success of out-of-basin hatchery trout in the reservoir tributaries, and the similarity between Crane Prairie redband trout with other extant redband trout populations in the basin. Using a suite of 17 microsatellite nuclear DNA markers, we evaluated the genetic structure among Crane Prairie Reservoir redband trout, hatchery rainbow trout, and two adjacent populations of redband trout from within the Upper Deschutes River Basin. We observed significant heterogeneity between the hatchery and wild Crane Prairie populations that may reflect differences in life histories, differential productivity and assortative mating. The genetic distinctions observed among the three redband trout populations suggest restricted gene flow and genetic drift within the upper basin. Temporally stratified sampling and larger numbers of samples will be necessary to confirm these conclusions.     

Microsatellite and mitochondrial DNA polymorphism reveals life-history dependent interbreeding between hatchery and wild brown trout (Salmo trutta L.)

The effects of stocking hatchery trout into wild populations were studied in a Danish river, using microsatellite and mitochondrial DNA (mtDNA) markers. Baseline samples were taken from hatchery trout and wild trout assumed to be unaffected by previous stocking. Also, samples were taken from resident and sea trout from a stocked section of the river. Genetic differentiation between the hatchery strain and the local wild population was modest (microsatellite FST = 0.06). Using assignment tests, more than 90% of individuals from the baseline samples were classified correctly. Assignment tests involving samples from the stocked river section suggested that the contribution by hatchery trout was low among sea trout (< 7%), but high (46%) among resident trout. Hybrid index analysis and a high percentage of mtDNA haplotypes specific to indigenous trout observed among resident trout that were assigned to the hatchery strain suggested that interbreeding took place between hatchery and wild trout. The latter result also indicated that male hatchery trout contributed more to interbreeding than females. We suggest that stronger selection acts against stocked hatchery trout that become anadromous compared to hatchery trout that become resident. As most resident trout are males this could also explain why gene flow from hatchery to wild trout appeared to be male biased. The results show that even despite modest differentiation at neutral loci domesticated trout may still perform worse than local populations and it is important to be aware of differential survival and reproductive success both between life-history types and between sexes.     

Role of genetic background in the introgressive hybridization of rainbow trout (<Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>) with Westslope cutthroat trout (<Emphasis Type="Italic">O. clarkii lewisi</Emphasis>)

Conservation and management of endemic species may increasingly involve efforts to prevent hybridization with other species. Native westslope cutthroat trout (Oncorhynchus clarkii lewisi) management in western North America is based largely on admixture estimates with introduced rainbow trout (O. mykiss), with the highest conservation priority given to cutthroat populations that do not exhibit admixture. This study examined the hypothesis that such ancestry quotients are dependent upon the genetic background of reference rainbow trout populations. We used 10 microsatellite loci to estimate admixture within westslope cutthroat trout collected from 39 sites from Alberta, Canada, using three genetically distinct (pairwise F_ST = 0.100–0.281) rainbow trout genetic backgrounds: a wild (introduced) population from Alberta, two wild (native) populations from British Columbia, and a present-day hatchery broodstock line. Ancestry quotients were significantly impacted by genetic background, whereby the extent of admixture was highest with locally introduced (wild, naturalized) rainbow trout lines and lowest with the hatchery lines. Our results suggest that future studies ought to explore the possibility that local adaptation or drift in introduced rainbow trout populations may contribute to decreased reproductive isolation with geographically proximal cutthroat trout populations.     

Long-term captive breeding does not necessarily prevent reestablishment: lessons learned from Eagle Lake rainbow trout

Captive breeding of animals is often cited as an important tool in conservation, especially for fishes, but there are few reports of long-term (<50 years) success of captive breeding programs, even in salmonid fishes. Here we describe the captive breeding program for Eagle Lake rainbow trout, Oncorhynchus mykiss aquilarum, which is endemic to the Eagle Lake watershed of northeastern California. The population in Eagle Lake has been dependent on captive breeding for more than 60 years and supports a trophy fishery in the lake. Nevertheless, the basic life history, ecological, and genetic traits of the subspecies still seem to be mostly intact. Although management has apparently minimized negative effects of hatchery rearing, reestablishing a wild population would ensure maintenance of its distinctive life history and its value for future use as a hatchery fish. An important factor that makes reestablishment possible is that the habitat in Eagle Lake is still intact and that Pine Creek, its major spawning stream, is recovering as habitat. With the exception of an abundant alien brook trout (Salvelinus fontinalis) population in Pine Creek, the habitat factors that led to the presumed near-extinction of Eagle Lake rainbow trout in the early twentieth century have been ameliorated, although the final stages of reestablishment (eradication of brook trout, unequivocal demonstration of successful spawning migration) have still not been completed. The Eagle Lake rainbow trout story shows that long-term captive breeding of migratory salmonid fishes does not necessarily prevent reestablishment of wild populations, provided effort is made to counter the effects of hatchery selection and that natural habitats are restored for reintroduction. Long-term success, however, ultimately depends upon eliminating hatchery influences on wild-spawning populations. Extinction of Eagle Lake rainbow trout as a wild species becomes increasingly likely if we fail to act boldly to protect it and the Eagle Lake watershed.     

Effects of stress on growth, cortisol and glucose levels in non-domesticated Eurasian perch (Perca fluviatilis) and domesticated rainbow trout (Oncorhynchus mykiss)

Eurasian perch (Perca fluviatilis) is a promising aquaculture candidate, but the growth performance of this non-domesticated species may be negatively affected by its stress responsiveness to intensive culture conditions. To evaluate this potential problem, juvenile Eurasian perch were exposed to a standardized handling stressor twice a week for an 8-week period. A similar study was conducted on domesticated rainbow trout (Oncorhynchus mykiss) for comparison of intra- and inter-specific differences. The stressed fish of both species showed lower body growth than the non-stressed control fish, however, the final mean body mass was 35.4% lower in the stressed Eurasian perch than in the non-stressed controls, compared to 22.8% difference between the two groups in rainbow trout. The stress responsiveness was examined by comparing the post-stress cortisol and glucose levels in repeatedly stressed fish and fish exposed to the stressor only once. The cortisol stress response in both species strongly indicated a habituation to the repeated stressor. Thus, repeatedly stressed Eurasian perch reached maximum cortisol levels of 130 ng/mL after 0.5 h compared to 200 ng/mL in the fish stressed once, while considerably smaller differences in cortisol levels were shown between the repeatedly and single stressed rainbow trout. Rainbow trout also showed lower post-stress glucose levels in the repeatedly stressed fish compared to the single stressed fish. In contrast, the glucose levels in both groups of Eurasian perch increased abruptly after stress treatment and remained elevated at approximately 19 mM for 6 h; levels were three times as high as the peak levels 3 h post-stress in rainbow trout. Together, the habituation of the stress response shown in both species did not eliminate the growth difference found in the repeatedly stressed fish versus the control fish. Further, the lower growth performance of Eurasian perch compared to rainbow trout could partly be due to the increased energy consumption in the more stress responsive Eurasian perch.     

Deviation from morphological intermediacy in interstrain hybrids of rainbow trout,Salmo gairdneri

Synopsis Deviations from morphological intermediacy in six first generation hybrids between three hatchery strains of rainbow trout, raised in a common environment, are reported. Hybrids have higher mean counts of four meristic characters than their maternal parental strain in a significantly greater number of cases (18 out of 24). Furthermore, eight of eleven hybrid indices are not intermediate. These results are discussed in reference to several mechanisms and models proposed to account for observed responses of meristic characters to environmental and genetic influences.     

Identifying footprints of selection in stocked brown trout populations: a spatio‐temporal approach

Studies of interactions between farmed and wild salmonid fishes have suggested reduced fitness of farmed strains in the wild, but evidence for selection at the genic level is lacking. We studied three brown trout populations in Denmark which have been significantly admixed with stocked hatchery trout (19–64%), along with two hatchery strains used for stocking. The wild populations were represented by contemporary samples (2000–2006) and two of them by historical samples (1943–1956). We analysed 61 microsatellite loci, nine of which showed putative functional relationships [expressed sequence tag (EST)‐linked or quantitative trait loci]. F_ST‐based outlier tests provided support for diversifying selection at chromosome regions marked by three loci, two anonymous and one EST‐linked. Patterns of differentiation suggested that the loci were candidates for being under diversifying hitch‐hiking selection in hatchery vs. wild environments. Analysis of hatchery strain admixture proportions showed that in one wild population, two of the loci showed significantly lower admixture proportions than the putatively neutral loci, implying contemporary selection against alleles introduced by hatchery strain trout. In the most strongly admixed population, however, there was no evidence for selection, possibly because of immigration by stocked trout overcoming selection against hatchery‐derived alleles or supportive breeding practices allowing hatchery strain trout to escape natural selection. To our knowledge, this is the first study demonstrating footprints of selection in wild salmonid populations subject to spawning intrusion by farmed fish.     

Effects of Hatchery Rearing on Brain Structures of Rainbow Trout, Oncorhynchus mykiss

In this study, we contrast brain morphology from hatchery and wild reared stocks to examine the hypothesis that in salmonid fishes, captive rearing produces changes in brain development. Using rainbow trout, Oncorhynchus mykiss, as a model, we measured eight regions of the salmonid brain to examine differences between wild and hatchery reared fish. We find using multiple analysis of covariance (MANCOVA), analysis of covariance (ANCOVA) and discriminant function analysis (DFA) that the brains of hatchery reared fish are relatively smaller in several critical measures than their wild counterparts. Our work may suggest a mechanistic basis for the observed vulnerability of hatchery fish to predation and their general low survival upon release into the wild. Our results are the first to highlight the effects of hatchery rearing on changes in brain development inbreak fishes.     

Microsatellite DNA analysis of rainbow trout (Oncorhynchus mykiss) from western Alberta, Canada: native status and evolutionary distinctiveness of “Athabasca” rainbow trout

Molecular genetic assays can contribute to conservation of aquatic taxa by assessing evolutionary and taxonomic distinctiveness, levels of genetic variation within and between populations, and the degree of introgression with introduced taxa. The Athabasca River drainage of␣western Alberta, Canada is one of only three (and the largest) drainages flowing east of the continental divide that contain native populations of rainbow trout (Salmonidae: Oncorhynchus mykiss). The “Athabasca” rainbow trout has been considered a preglacial relict worthy of special conservation measures. In addition, the native range of Athabasca rainbow trout has seen many instances of introductions of non-native populations since the beginning of the 20th century. We assayed rainbow trout from the Athabasca River drainage, from hatchery populations, and from representative populations in adjacent regions (N = 49 localities) for variation at 10 microsatelite loci to assess the level of evolutionary distinctiveness of Athabasca rainbow trout, and to assess the levels of introgression with non-native hatchery fish. We found that native Athabasca rainbow trout did not form a distinctive genetic assemblage and that the greatest amount of allele frequency variation was attributable to contemporary drainage systems (29.3%) rather than by a Athabasca/non-Athabasca distinction (12.6%). We found that 78% of all fish were confidently assigned to a “wild” rather than a “hatchery” genetic grouping and that most of the inferred introgression with hatchery fish was restricted to a few localities (N = 6). Our results suggest that: (i)␣Athabasca River rainbow trout are likely postglacial immigrants from adjacent populations of the Fraser River, and (ii) that there is no evidence of widespread introgression of hatchery alleles into native Athabasca River drainage rainbow trout.     

Who are the missing parents? Grandparentage analysis identifies multiple sources of gene flow into a wild population

In order to increase the size of declining salmonid populations, supplementation programmes intentionally release fish raised in hatcheries into the wild. Because hatchery-born fish often have lower fitness than wild-born fish, estimating rates of gene flow from hatcheries into wild populations is essential for predicting the fitness cost to wild populations. Steelhead trout (Oncorhynchus mykiss) have both freshwater resident and anadromous (ocean-going) life history forms, known as rainbow trout and steelhead, respectively. Juvenile hatchery steelhead that 'residualize' (become residents rather than go to sea as intended) provide a previously unmeasured route for gene flow from hatchery into wild populations. We apply a combination of parentage and grandparentage methods to a three-generation pedigree of steelhead from the Hood River, Oregon, to identify the missing parents of anadromous fish. For fish with only one anadromous parent, 83% were identified as having a resident father while 17% were identified as having a resident mother. Additionally, we documented that resident hatchery males produced more offspring with wild anadromous females than with hatchery anadromous females. One explanation is the high fitness cost associated with matings between two hatchery fish. After accounting for all of the possible matings involving steelhead, we find that only 1% of steelhead genes come from residualized hatchery fish, while 20% of steelhead genes come from wild residents. A further 23% of anadromous steelhead genes come from matings between two resident parents. If these matings mirror the proportion of matings between residualized hatchery fish and anadromous partners, then closer to 40% of all steelhead genes come from wild trout each generation. These results suggest that wild resident fish contribute substantially to endangered steelhead 'populations' and highlight the need for conservation and management efforts to fully account for interconnected Oncorhynchus mykiss life histories.     

Toxic effect of heavy metals on ovarian deformities,apoptotic changes,oxidative stress,and steroid hormones in rainbow trout

BackgroundAs is well known, the pollution in the aquatic environment in which fish grow has a direct impact on aquaculture practices. Pollution in aquatic systems because of multiple adverse effects on fish metabolic processes, especially the reproductive systems.AimThe goal of this study was to assess the severity of pollution impact in two different hatcheries, Verinag hatchery, Site 1 (S1) and Panzath hatchery, Site 2 (S2) in Anantnag region, using histopathological, ultrastructural, oxidative stress, genotoxic, and hormonal analysis in rainbow trout gonad (ovary).M&M: Fish were collected between May 2018 and April 2019 from two locations, Verinag hatchery (S1) and Panzath hatchery (S2), which were affected by heavy metals.ResultsThe histological and ultrastructural examination of rainbow trout ovaries from the Verinag hatchery (S1) revealed normal structure in growing oocytes in rainbow trout at various stages based on morphological features while the fish ovaries in the Panzath hatchery (S2) showed various deformities and irregularly shaped oocytes. The surfaces of some of these oocytes were wrinkled, rough, or distorted. Apoptotic studies revealed that the frequency of apoptotic cells collected from S2 water was significantly increased in ovarian cells (P < 0.05). The activity of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) were found to be increased in fish collected from S1 but decreased in fish collected from S2. In S2 caught fish, malondialdehyde (MDA) levels were found to increase gradually, and the degree of heavy metal stress was positively correlated (p < 0.05). The comet assay was used to determine the induction of DNA damage in ovarian cells. The induction of DNA damage was found to be significantly higher (p < 0.05) in S2 fish specimens compared to fish from S1. On comparing the DNA damage of the rainbow trout from the two sampling sites, it was revealed that the fish is much more sensitive to aquatic contaminants. Regarding steroid hormones, higher levels of progesterone and estrogen were reported in the fish samples collected from S1 as compared to S2 captured fish.ConclusionIn conclusion, the comparative study of fish from two different sites viz. Verinag hatchery (S1) and Panzath hatchery (S2) revealed that S2 sampled fish suffered more heavy metal damage, including cellular deformities, apoptosis, oxidative damage, and altered steroid hormones.     

Lower fitness of hatchery and hybrid rainbow trout compared to naturalized populations in Lake Superior tributaries

We have documented an early life survival advantage by naturalized populations of anadromous rainbow trout Oncorhynchus mykiss over a more recently introduced hatchery population and outbreeding depression resulting from interbreeding between the two strains. We tested the hypothesis that offspring of naturalized and hatchery trout, and reciprocal hybrid crosses, survive equally from fry to age 1+ in isolated reaches of Lake Superior tributary streams in Minnesota. Over the first summer, offspring of naturalized females had significantly greater survival than offspring of hatchery females in three of four comparisons (two streams and 2 years of stocking). Having an entire naturalized genome, not just a naturalized mother, was important for survival over the first winter. Naturalized offspring outperformed all others in survival to age 1+ and hybrids had reduced, but intermediate, survival relative to the two pure crosses. Averaging over years and streams, survival relative to naturalized offspring was 0.59 for hybrids with naturalized females, 0.37 for the reciprocal hybrids, and 0.21 for hatchery offspring. Our results indicate that naturalized rainbow trout are better adapted to the conditions of Minnesota's tributaries to Lake Superior so that they outperform the hatchery-propagated strain in the same manner that many native populations of salmonids outperform hatchery or transplanted fish. Continued stocking of the hatchery fish may conflict with a management goal of sustaining the naturalized populations.     

Effects of fish age and parasite dose on the development of whirling disease in rainbow trout

We determined the ages at which juvenile rainbow trout Oncorhynchus mykiss became resistant to the effects of whirling disease following exposure to a range of parasite doses. Heretofore, the development and severity of whirling disease in salmonids was known to be generally dependent on the age or size of fish when first exposed to the triactinomyxon spores of Myxobolus cerebralis; larger, older individuals tended to be less diseased. However, no systematic determination had been made of the exact age at which fish become resistant to the development of the disease. We exposed rainbow trout at 9 ages (1 to 17 wk post-hatch) to 4 parasite dose levels (0, 100, 1000 and 10,000 triactinomyxons per fish). Disease severity was measured using mortality, clinical signs, microscopic pathology, and myxospore counts. Disease and mortality were substantially reduced when exposure to the parasite occurred for the first time at 9 wk post-hatch (756 degree-days at 12 degrees C) or older. High doses elicited more disease among the younger age groups, but the effect was dampened in groups exposed at about 9 to 11 wk post-hatch and absent thereafter. Rainbow trout reared in M. cerebralis-free waters for 9 wk post-hatch or longer, whether in the wild or in a hatchery situation, should experience greater survival and less disease than fish first exposed to the parasite at younger ages.     

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.     

Relationship between stress, feeding and plasma ghrelin levels in rainbow trout, Oncorhynchus mykiss

Sexually immature rainbow trout were acclimated to small-volume (1 m³) holding tanks and then exposed to short-term stress to examine the relationship between feeding, stress, plasma ghrelin levels and other plasma stress parameters. Plasma ghrelin levels showed an increase 24 h after a single feed, plasma lactate and glucose levels decreased over the same period and plasma cortisol levels were low and constant. One hour of confinement stress resulted in elevations of plasma cortisol, glucose and lactate and depression of plasma ghrelin levels. In a separate experiment, 2 h of confinement stress also depressed feeding immediately after stress, concomitant with increases in plasma cortisol, lactate and glucose; however, in this case there was no change in plasma ghrelin concentrations. A repeat of the 2-h confinement experiment using fish that had not been acclimated to small-volume holding tanks produced a more marked elevation in plasma cortisol and a stronger suppression of feeding post-stress but in this case also, there was no change in plasma ghrelin levels. The results of this study confirm that feeding in rainbow trout is suppressed by confinement stress although the effect is transitory in this domesticated stock. Similar to that in other fishes, plasma ghrelin levels appear to be modulated by feeding status and may be influenced by stress, suggesting an orexigenic role for ghrelin in rainbow trout.     

Mitochondrial haplotype variation in wild trout populations (Teleostei: Salmonidae) from northwestern Mexico

The variation and composition of Mexican wild trout mitochondrial DNA haplotypes throughout northwestern Mexico was determined by means of polymerase chain reaction–restriction fragment polymorphism analysis (PCR–RFLP), of one region of mitochondrial DNA between cytochrome b and the D-loop. This analysis was based on 261 specimens taken in 12 basins and four hatcheries from northwestern Mexico. From 23 haplotypes, 15 wild trout haplotypes were identified and classified in four groups: (1) one restricted to Nelson’s trout (Oncorhynchus mykiss nelsoni), (2) four restricted to Río Mayo and RíoYaqui trout (O. mykiss sspp.), (3) six to Mexican golden trout (O. chrysogaster) with two subgroups, and (4) one exclusive to Río Piaxtla trout. Distributions of native haplotypes broadly overlap the distribution of non-native hatchery rainbow trout reflecting the historical management of introductions of exotic rainbow trout and the artificial transference of these trout among basins.     

Predator avoidance behaviour in wild and hatchery‐reared brown trout: the role of experience and domestication

Juvenile brown trout Salmo trutta from natural populations reacted to the presence of piscivorous brown trout by increasing the use of refuges. In contrast, second‐generation hatchery fish and the offspring of wild fish raised under hatchery conditions were insensitive to predation risk. The diel pattern of activity also differed between wild and hatchery brown trout. Second‐generation hatchery fish were predominantly active during daytime regardless of risk levels. Wild fish, however, showed a shift towards nocturnal activity in the presence of predators. These findings emphasize the potential role of domestication in weakening behavioural defences. They support the idea that the behavioural divergence between wild and domesticated individuals can arise from a process of direct or indirect selection on reduced responsiveness to predation risk, or as a lack of previous experience with predators.