Stocked Rainbow Trout Research Articles

Mechanical Removal of Minnows (Gila bicolor) to Improve Water Quality in a Hydropower Impoundment, Lemolo Lake, Oregon, USA

A project to mechanically remove an over-abundant population of cyprinids (Gila bicolor) was tested in a hydropower impoundment, Lemolo Lake (USA). The netting program removed 29.7 tons of fish (163 kg/ha) over a seven-year period that resulted in a reduction in the intensity and duration of cyanobacteria blooms. However, the suppression of the tui chub population apparently allowed for a residual population of kokanee (Oncorhynchus nerka) to proliferate. The resurgence of the kokanee, combined with the initiation of a rainbow trout stocking program, offset some of the gains in water quality achieved with the reduction of tui chub biomass. The dominant cyanobacterium present in Lemolo Lake transitioned to Gloeotrichia echinulata in 2013, but the majority of these colonies were apparently derived from an upstream lake. The changes in water quality from 2005–2013 were simulated for Lemolo Lake using the two-dimensional, hydrodynamic model, CE-QUAL-W2. The model reproduced many aspects of the water quality in Lemolo Lake and was used, in combination with a simple Bayesian model, to evaluate a number of potential scenarios for further improving water quality in the impoundment. Promoting a deeper drawdown of the hydroelectric impoundment during the winter to selectively increase entrainment of tui chub and kokanee, combined with continued netting targeted at tui chub, was judged to offer the greatest opportunity to manage the facility for water quality, fisheries and hydropower objectives.

Natal Origin Differentiation Using Eye Lens Stable Isotope Analysis

AbstractNatal origin determination can be critical to understanding the movement, recruitment, and stock discrimination of fish populations. We investigated the utility of eye lens stable isotope analysis as a natural marker to determine natal origins, specifically the differentiation of stocked versus naturally reproduced individuals. We compared isotopic values for stocked and naturally reproduced Rainbow Trout Oncorhynchus mykiss in the North Platte River (Wyoming, USA) and found that the metabolically inert core of the eye lens retained hatchery feed values as the fish grew poststocking. The stable isotope values of eye lens cores from stocked Rainbow Trout were >15‰ higher for δ34S and >4‰ higher for δ13C than naturally reproduced fish. The values for δ15N proved to be a less useful indicator for natal origin than sulfur and carbon. These high carbon and sulfur isotopic values were retained in the core, despite subsequent lens layers showing a dietary shift to natural prey items poststocking. Rainbow Trout formed over 10 layers at age 0, decreasing to an estimated one to two layers during subsequent years. Eye lens stable isotope analysis represents an additional tool for researchers and managers to use natural isotopic markers to accurately assess natal origins and hatchery contributions to fish populations.

Comparing dominance relationships and movement of native marble trout (Salmo marmoratus) and introduced rainbow trout (Oncorhynchus mykiss).

Behaviour observations of the endangered native marble trout (Salmo marmoratus, Cuvier, 1829) and introduced rainbow trout (Oncorhynchus mykiss, Walbaum, 1792) in the laboratory and in a tributary to the Idrijca River in Slovenia were combined to study the movements and dominance relationships between individuals of the two species in an open field test. Under laboratory conditions, no difference between the species was detected for neither time spent actively or distance moved. In species paired tests, rainbow trout initiated more aggressive behaviours towards marble trout than vice versa, and rainbow trout were clearly the dominant individuals. After simultaneous release in the river, marble trout immediately left the release area and spent twice as long time as rainbow trout until they settled in an area of the river; therefore, the release site was immediately occupied exclusively by rainbow trout. Thus, the dominant and aggressive behaviour of rainbow trout seen in the laboratory before release might have influenced marble trout's subsequent behaviour in the river, by marble trout leaving the areas occupied by rainbow trout and moving to locations further away from the release site. In the field, the marble trout occupied sites individually, whereas rainbow aggregated at a few locations. Rainbow trout showed higher movement activity in the morning compared to marble trout. There was a positive correlation between swimming speed in the laboratory and movement in the field for marble trout but not for rainbow trout. In conclusion, the results in this study support the need to end stocking of rainbow trout in rivers with native marble trout. To better understand the interaction between the species, and to develop efficient management plans to protect the native marble trout, reference behaviours should first be understood, and future research in sites where the two species do not co-exist is needed. This is especially important for marble trout for which behavioural research and data are lacking.

An Evaluation of the Efficacy of a Targeted Supplemental Stocking Using a Multistate Capture–Recapture Model

AbstractStocking is a management tool that provides fish directly to anglers and can partly address concerns over low catch rates. Although stocking is recognized as an effective management tool for addressing low catch rates, stocking fish represents a considerable investment. Stocking may not be a viable option if fish do not persist within targeted areas, are not accessible to anglers, or pose a potential risk to sensitive species. The goal of this study was to assess the survivorship, movement, and persistence of stocked Rainbow Trout Oncorhynchus mykiss (RBT) within the walk‐in section of Lees Ferry, Colorado River, Arizona, in order to evaluate the efficacy of targeted stocking for this fishery. To meet this goal, we used acoustic telemetry to monitor stocked trout and assess movement out of the management area. A multistate model was used to distill spatiotemporal telemetry data into a multinomial state response, the spatial zones of our study area, to evaluate the efficacy of targeted stocking. The estimated 3‐month survival of RBT that remained within the walk‐in section without transitioning was 37.3–68.1% based on the 95% confidence interval (CI). Persistence of RBT, which accounts for fish movement, within the walk‐in section after 3 months was estimated to be 29.6–33.5% of the original stocked population. Three‐month transition rates downstream out of the walk‐in section (95% CI = 12.8–15.8%) were higher than upstream transition rates out of the walk‐in section (95% CI = 7.7–10.0%). These findings suggest that targeted stocking can effectively supplement the fishery at the walk‐in section of Lees Ferry based on the relatively high persistence of stocked RBT within the targeted area. However, managers will need to weigh stocking densities, transition probabilities, and catch rate goals to determine an acceptable level of risk to the native populations while addressing management goals.

Trout Responses to Stocking Rates and River Discharge within a Southeastern U.S. Hydropeaking Tailwater

AbstractFreshwater fish populations often exist in systems characterized by novel ecological processes resulting from human alteration. Salmonid populations embedded within coldwater sections of warmwater rivers are spatially constrained by habitat availability. Tailwater fish contend with fluctuating river discharges and density‐dependent processes associated with fish stocking and exploitation. Salmonid populations sustained through stocking versus natural reproduction may respond differently to changes in hydrologic patterns (e.g., hydropeaking) as well as declines in fish abundance. We assessed differences between stocked (Rainbow Trout Oncorhynchus mykiss) and naturalized (Brown Trout Salmo trutta) trout populations in Greers Ferry tailwater, Arkansas, regarding (1) spatial and temporal patterns of mean length, electrofishing catch rates, and relative condition following reduced number of stocked Rainbow Trout and (2) evidence that hydrologic characteristics and fish stocking intensity influenced relative condition. A 56% reduction in Rainbow Trout stocking did not result in systemwide change in mean length or relative abundance for Rainbow Trout or Brown Trout over the 16‐year study period. Hydrologic variability, where river discharge spanned both reduced and elevated levels, positively influenced condition of both Rainbow Trout and Brown Trout. Assessment of survival of stocked Rainbow Trout may aid in further refining the timing and amount of stocking needed to sustain the population at a desired abundance. Further, assessing the influence of stocking fewer but perhaps larger (in terms of mean length) fish to meet management goals may be warranted. The persistent differences in relative abundance among river sections can inform management actions directed at Brown Trout, including harvest regulations. Such regulations may aid in reaching desired management goals, including abundance and mean length targets not observed after reduced stocking.

Branchitis and mortality in rainbow trout Oncorhynchus mykiss exposed to iron oxidizing bacteria: Diagnostics and management in a Colorado hatchery

AbstractA public trout hatchery in Colorado recorded repeat episodes of morbidity and mortality in early life stages of Rainbow Trout (Oncorhynchus mykiss) stocks. The water source for the affected tanks is a groundwater well with a pH of 6.3 and a ferrous iron concentration of 120 µg/L. Iron oxidizing bacteria (FeOB) were suspected as the underlying cause due to insoluble red particulate matter (iron oxide precipitate) in the water, and the presence of thick red slime (biofilm) that coated tanks and pipes of the facility and adhered to gills, opercula, and fins of fry. On necropsy, gill and fin clippings from moribund fish had mats of adherent red‐orange material. Histopathology revealed severe diffuse proliferative branchitis. There was abundant debris including granular to sheath‐like and stalk‐like pigmented material within the oral and branchial chambers, intimately associated with gill filaments. A Gram preparation demonstrated florid Gram‐negative bacteria within this debris. A Perls’ Prussian blue preparation revealed colocalization of abundant iron. Bacteria were identified morphologically to be of the iron oxidizing bacterial genera Gallionella and Leptothrix. This is the first known report of disease in fish associated with iron‐oxidizing bacteria.

Gonadal Maturity of Rainbow Trout (Oncorhynchus mykiss) Naturally Inhabited in Balkhila Stream of Uttarakhand, India

In this study an attempt has been made for evaluating the gonadal maturity in the natural stock of rainbow trout inhabited in Balkhila stream of Alakhnanda river at Chamoli district of Uttarakhand, India. In collected stock, about 80% specimen showed full gonadal maturity, which reflects the conducive conditions for the gonadal maturity. The Gonadosomatic index (GSI) values of female rainbow trout ranged from 0.42 to 22.84 with a peak in January and reached its lowest level in March. In case of males, observed minimum value of GSI was calculated as 0.27 during the month of May, whereas the maximum value was 3.44 in the month of December and persisted in month of January and February also with the almost similar values. GSI values reflected the maturity of females during the month of January and February, while males became mature one month advance and remained mature during the December to early March. Hence, non- synchronization situation was observed in the gonadal maturity of both the sexes, which adversely affected the duration of breeding season and is a subject to a further study. In this study, the ovarian and testicular development of rainbow trout showed distinct stages. The oogonia and spermatogonia pass through a number of maturation stages finally leading to the formation of ripe ova and sperms.

Length-Weight Relationship and Well Being of the Natural Stock of Rainbow Trout Inhabited in Balkhila Stream in Uttarakhand, India

In this study an attempt has been made for evaluating the length weight relationship and well being of the natural stock of rainbow trout inhabited in Balkhila stream of Alakhnanda river at Chamoli district of Uttarakhand, India. In collected stock, about 80% specimen showed full gonadal maturity, which reflects the conducive conditions for the gonadal maturity. In present study coefficient correlation (r) for male was found to be 0.84257 which is positively significant (P

Seasonal Diet and Habitat Use of Large, Introduced Rainbow Trout in an Ozark Highland Stream

AbstractStocking of Rainbow TroutOncorhynchus mykisscommonly provides seasonal or mitigation fisheries; however, these fish are usually small and ecosystem effects are spatially or temporally limited. Yet agencies receive requests to stock Rainbow Trout in relatively natural settings (i.e., not tailwater or mitigation fisheries), where introductions may have greater ecosystem consequences. The size of introduced fish is an important factor in determining biotic interactions with native species; therefore, our objectives were to assess the seasonal feeding ecology and microhabitat use of large (265–530 mm TL) nonnative Emmerson strain Rainbow Trout in a relatively unaltered, groundwater‐influenced, warmwater stream of the Ozark Highlands. Rainbow Trout consumed a variety of prey; however, diets differed between cool (winter and spring) and warm (summer) seasons. Cool‐season Rainbow Trout exhibited a mixed feeding strategy, with individual specialization on crayfishes and fishes and generalist feeding on Ephemeroptera and Diptera, but Gastropoda were the dominant prey. Feeding strategy in the warm season switched to individual specialization on numerous prey types. Overall, larger prey resources were important components of Rainbow Trout diets. Piscivory was relatively high in both seasons, and crayfishes were one of the most important prey types across seasons. Selection of coarse substrates and deeper‐water microhabitats (>0.95 m) was similar between seasons. Rainbow Trout selected the lowest‐velocity microhabitats available during the warm season and moderate velocities in the cool season. Rainbow Trout were five times more likely to be associated with cover in the warm season. Due to their higher temperature tolerance, Emmerson strain Rainbow Trout may persist in Ozark Highland streams, where they disrupt local food webs and occupy habitat otherwise selected by native fish, such as Neosho Smallmouth BassMicropterus dolomieu velox. If native species conservation is a priority for agencies, then caution regarding Rainbow Trout stockings may be warranted.

Ecological Assessment and Evaluation of Potential Eradication Approaches for Introduced Redside Shiners in a Montane Lake

AbstractIntroduced species can alter the dynamics of food webs, species interactions, populations, and communities and can reduce a system’s recreational value. Several reports have documented the establishment of Redside Shiners Richardsonius balteatus in mountain lakes, resulting in declines in sizes and abundance of salmonid species. An illegal bait bucket release introduced Redside Shiners into Green Canyon Lake, Montana, USA, in the 1980s. Recently, local anglers began complaining of decreases in size and catch rates of Westslope Cutthroat Trout (WCT) Oncorhynchus clarkii lewisi. Our first objective was to understand the diet overlap between Redside Shiners and WCT in Green Canyon Lake. Based on δ15N and δ13C isotopic signatures and diet contents, we found little evidence that WCT were substantially preying upon Redside Shiners. We found moderate diet overlap between WCT and Redside Shiners, with both species predominately feeding on Diptera. For our second objective, we developed a population model to assess potential options for eradicating Redside Shiners. We observed Redside Shiners up to 7 years of age and with a maximum TL of 159 mm, high annual survival (50%), and mature females at age 2. We used an age‐structured model to evaluate two hypothetical suppression actions: piscicide application and stocking of Rainbow Trout O. mykiss. Four consecutive piscicide treatments had a 20% probability of eradicating Redside Shiners, increasing to a 100% eradication probability after seven treatments. Bioenergetic simulations indicated that an individual Rainbow Trout (500 g) would consume 1,759 g of Redside Shiners annually. Incorporating this predation mortality and a compensatory response (50% increase in age‐1 survival), we estimated that maintained abundances of up to 700 Rainbow Trout would not cause a decline in the Redside Shiner population. Our results suggest that eradication of an introduced prey fish like the Redside Shiner would be difficult and that effective removal will require dedicated effort and resources by managers.

Persistence and Dispersal of Stocked Rainbow Trout in an Alabama Tailwater

AbstractTailwater trout fisheries provide popular and unique opportunities for anglers across the southeastern USA but often suffer from high mortality and low returns to creel. The Sipsey Fork tailwater in Alabama provides such an opportunity and is stocked monthly with Rainbow Trout Oncorhynchus mykiss. However, a recent creel survey found that less than 25% of fish stocked each month were harvested, with the fate of remaining fish unknown. Thus, movement patterns and fates of four cohorts of stocked Rainbow Trout were examined in the Sipsey Fork using radiotelemetry. Ninety‐two tagged Rainbow Trout travelled an average of 5.8 km (SE = 0.7 km) away from their stocking location. Dam operation rarely appeared to influence trout movement. However, tagged Rainbow Trout rapidly dispersed below where most angler access was located; more than 40% of fish locations were found in this area by 3 weeks poststocking in three of four cohorts. Overall, only 30% of tagged Rainbow Trout remained alive 5 weeks after stocking. Rainbow Trout survival during this 5‐week period ranged from 11% to 74% across cohorts, and those stocked in January exhibited greater survival than those stocked in June or October. Factors affecting trout movement and mediating Rainbow Trout survival may include Striped Bass Morone saxatilis predation, water temperature, and dissolved oxygen concentrations during certain times of the year. Efforts to mitigate such factors by altering stocking schedules could increase longevity of stocked Rainbow Trout and therefore maximize availability of fish to anglers. However, the propensity of stocked fish to migrate below the area of public access will continue to limit catch and harvest from this fishery.

Survival and Movement Patterns of Rainbow Trout Stocked in a Groundwater‐Influenced Warmwater Stream

AbstractStocking Rainbow Trout Oncorhynchus mykiss to create additional angling opportunities is common; however, the spatial and temporal dynamics of such stocking practices are unclear in groundwater‐influenced, warmwater streams. Our objectives were to determine Rainbow Trout dispersal from a stocking location on Spavinaw Creek, Oklahoma and to quantify apparent survival of two cohorts of Rainbow Trout stocked in November 2018 and February 2019. Rainbow Trout were PIT‐tagged prior to autumn (N = 495) and winter (N = 605) stocking and located poststocking using both an active floating array and two passive fixed arrays. We actively tracked a 6‐km extent every 2–3 weeks poststocking and found that PIT‐tagged Rainbow Trout dispersal ranged from 0 to 4 km with ~90% of detected tagged fish remaining within 1 km of the stocking location. Directional movement by stocked Rainbow Trout was evident with upstream movements related to increases in daily water discharge and downstream movements related to decreases in daily discharge. Estimated apparent weekly survival of Rainbow Trout was lowest during the first 2 weeks poststocking (91% and 75% for autumn and spring cohorts, respectively). Apparent weekly survival rates for both autumn and spring stocked fish increased through winter and spring when maximum water temperatures were below 25℃, reaching survival rates of 99% by the end of spring. Rainbow Trout persisted in Spavinaw Creek throughout the summer; however, the weekly apparent survival rate declined for both cohorts (93%) when maximum daily water temperatures exceeded 25°C. Our results can be used to guide stocking decisions in similar warmwater streams and suggest that Rainbow Trout persist through spring and summer poststocking under the conditions encountered during our study.

Genetic structure of rainbow trout Oncorhynchus mykiss (Salmoniformes, Salmonidae) from aquaculture by DNA-markers

The rational use of valuable fish species from aquaculture is difficult to implement without knowledge of the state of the genetic structure of local stocks. Different types of DNA markers can be used to achieve the goals of selection and breeding work. The genetic structure of a local stock of rainbow trout (Oncorhynchus mykiss Walbaum, 1792) (Salmoniformes, Salmonidae) farmed in Ukraine was studied using DNA-markers: microsatellite (SSR-markers – simple-sequence repeats-markers) and intermicrosatellite (ISSR – inter-simple sequence repeat). Five fragments of trinucleotide microsatellite motifs with a single anchor nucleotide at the 3'-end were used as a primer for analysis by the ISSR-PCR method. Totally, 85 amplicons were obtained across the five loci, of which 92.9% were polymorphic. The total number of alleles ranged from 10 (marker (ACC)₆G) to 23 (marker (AGC)₆G). The following monomorphic amplicons were determined for the studied local stock of rainbow trout: according to marker (CTC)₆C – 770 and 520 bp bands, for the marker (GAG)₆C – 345, 295 and 260 bp, and for the marker (AGC)₆C – 350 bp. The average number of polymorphic bands per locus was 15.8. The selected ISSR primers had a level of polymorphic information content above the average. The most effective markers for molecular-genetic analysis of rainbow trout were (AGC)₆G and (AGC)₆C according to the percentage of polymorphic bands, marker index, effective multiplex ratio and resolving power. The selected ISSR loci allow the genetic structure of the studied local stock to be characterized using the total and the effective number of alleles per locus (Na and Ne were 1.9 and 1.4, respectively), the Shannon index (average value I was 0.4) and the unbiased expected heterozygosity (mean uHe = 0.3). Microsatellite-based analysis showed features of the genetic structure of the local stock of rainbow trout at six microsatellite loci (OMM 1032, OMM 1077, OMM 1088, Str 15, Str 60, Str 73). Allelic diversity was established and alleles with the highest frequency and most typical for the given stock were identified. The Shannon index and unbiased expected heterozygosity were determined using SSR-markers and were 1.42 and 0.79, respectively. This depicts the complexity of the population structure, a high level of genetic diversity and indicates a high level of heterozygosity of local stock. The “gene pool profile” established as a result of ISSR-PCR in the future will help to differentiate local stocks of rainbow trout in aquaculture of Ukraine. Microsatellite markers provide the ability to determine individual features of genetic variation of local populations and to conduct the management of genetic resources on fish farms.

Efficacy of a Modified Health Assessment Utilized on Two Genetically Distinct Stocks of Rainbow Trout.

The Rainbow Trout Oncorhynchus mykiss is a salmonid fish that is maintained in aquaculture facilities and used as a source of protein around the world. Veterinarians need accurate, efficient tools and straightforward assessments to evaluate the health status of fish, like Rainbow Trout, that are cultivated on farms. In this study, two genetically distinct broodstock lines of Rainbow Trout originally obtained from two different U.S. states but propagated at Rushing Waters Trout Farm (Palmyra, Wisconsin) were evaluated. Groups originating from both states (State A: n=20; State B: n=20) were surveyed with a modified health assessment in a prospective, randomized, single-blind study. The adapted health assessment included external and internal gross evaluations, skin scrapes, gill clips, intestinal scrapes, and calculated indices, including condition factor and the hepatosomatic index. Generally, there were few significant differences in health assessment values between State A and State B fish. When this information was presented to the producer, it was discovered that perceived differences between groups were due to management errors in calculated growth rates. The results of this study add validity to the use of standardized health assessments for evaluating fish within aquaculture.

Genetic Differentiation and Origin of Naturalized Rainbow Trout Populations From Southern Chile, Revealed by the mtDNA Control Region Marker.

Numerous self-sustaining naturalized or introduced populations of rainbow trout (Oncorhynchus mykiss) are widely distributed throughout the freshwaters of southern Chile. In this study, analysis of the mitochondrial DNA control region (CR) marker was conducted to investigate the level of genetic divergence among populations and their phylogenetic relationships with respect to native lineages. This information provided a framework to interpret the genetic structure and origin that was shaped during historical trout introduction efforts. To this end, we analyzed eleven naturalized populations of lakes and rivers from five basins. The CR marker revealed five haplotypes. The overall haplotype (H) and nucleotide (Π) diversities were 0.684 ± 0.030 and 0.00460 ± 0.00012, respectively. Global F ST was 0.169, with several pairwise F ST estimates showing significant differences (P < 0.05). The exact test of population differentiation corroborated this result (P < 0.001). Significant geographic structure was found (P < 0.05), with variations explained primarily by differences within populations (61.65%) and among group basins (20.82%). Maximum likelihood phylogenetic analysis resolved two distinct clades with medium bootstrap support when naturalized populations were aligned in conjunction with reference native lineages. The haplotype network revealed a close association between naturalized populations and four main haplotypes representative of three native ecotypes or lineages from western North America (rainbow trout, steelhead trout and redband trout). These results indicate a genetic population structuring for naturalized rainbow trout from southern Chile and an origin probably represented by multiple lineages sources. Thus, mitochondrial DNA data strongly suggest that stocking of rainbow trout from different origins may have occurred during or after the initial introduction efforts.

ОСОБЕННОСТИ ФОРМИРОВАНИЯ МАТОЧНОГО СТАДА РАДУЖНОЙ ФОРЕЛИ В НАУЧНО-ПРОИЗВОДСТВЕННОМ ЦЕНТРЕ «ФОРЕЛЬ» (ОКОНСК, УКРАИНА)

The article presents the analysis of the data on developing brood stock and repair-brood stock of rainbow trout, as well as the data on individual reproductive indicators of females carried out in the Research and Production Center “Forel” (Okonsk Vlg., Ukraine). The stages of development of trout farming in the territories of the former USSR, Ukraine (the Carpathians, Ternopil and Volyn regions), and the Baltic states are considered. Currently, rainbow trout hatcheries need modern methods to increase the efficiency of artificial reproduction by reducing the loss of living material (especially at the early stages of ontogenesis), and increasing the vitality of juveniles. For the study, 20 trout females were selected from those participated in the spawning campaign in 2014, of which 18 females were from the main broodstock and 2 females were spawning for the first time in the repair broodstock. The females were divided into 3 groups depending on weight. In the course of the study there was stated the direct dependence of the main reproductive indicators on weight: the minimum amount of eggs was obtained from individuals with a minimum weight, and the maximum number of eggs was obtained from heavy individuals. It has been found that the greater the weight of individuals, the higher the percentage of egg fertilization. Females from the repair brood stock that spawned for the first time had low breeding and biological indicators.

Colonial Waterbird Predation and Angler Catch of Hatchery Rainbow Trout Stocked in Southern Idaho Fisheries

AbstractThe abundance of piscivorous colonial waterbirds such as double‐crested cormorants Phalacrocorax auritus and American white pelicans Pelecanus erythrorhynchos has increased dramatically in recent decades in North America, resulting in increased conflict between these birds and fisheries resources, which raises the need to quantify the effects of waterbird predation on fisheries. We estimated avian predation and angler use of two sizes of hatchery‐reared Rainbow Trout Oncorhynchus mykiss (250 mm and 300 mm on average; termed standard catchables and magnum catchables) at 15 Idaho waters with known cormorant and pelican presence. Fish were tagged with passive integrated transponder (PIT), radio, and T‐bar anchor tags prior to release; the tags were subsequently recovered from bird nesting, roosting, and loafing sites. Estimated angler use, which included fish harvested as well as those that were catch‐and‐release, averaged 17%, while avian predation averaged 35%. Angler catch at study waters declined exponentially as avian predation increased. Predation specifically attributed to double‐crested cormorants and American white pelicans was related to their relative abundance at the waterbodies where the fish were stocked. Avian predation rates were similar for standard and magnum catchables, suggesting that larger fish were not able to escape predation any better than smaller ones. Our results add to existing evidence indicating that in some southern Idaho fisheries, piscivorous birds, rather than anglers, are the dominant consumers of hatchery trout. This study further contributes to the larger issue of competition between humans, wildlife, and fisheries populations.

Genetic structure of the Chernivtsi local stock of rainbow trout (Oncorhynchus mykiss) as determined by SSR-markers

Aim. The trend towards the development of high-tech trout farms is observed in Ukraine. The analysis of the genetic structure of rainbow trout by molecular genetic methods is necessary for breeding work. Therefore, the purpose of the study was to study polymorphism at the microsatellite loci of Oncorhynchus mykis of the chernivetsk local herd. Me-thods. The determination of the genetic polymorphism was carried out according to indicators: the effective number of alleles on the locus (ne), the observed (Но) and expected (Hе) heterozygosity, indexes of fixation (Fis) and polymorphism (PIC). Results. Specific features of the structure of the gene pool of the local herd O. mykiss by the SSR loci OMM 1032, 1077, 1088, STR 15, 60, 73 were revealed. The average value of the effective number of alleles per locus was 3.87. The high average values of the heterozygosity of the local herd and the index of fixation were fixed (mean values of H e and F is: 0.73 and -0.14, respectively). А high index of polymorphism was recorded for all used DNA markers and was 0.69. Тhe possibility to analyse the genetic structure of salmon using these SSR-markers was substantiated. Conclusions. An analysis of the genetic structure of trout (O. mykiss) was performed by using of 6 microsatellite. It was shown that the investigated local herd has a high level of heterozygosity. The conducted studies have shown the effectiveness of using selected loci for individual identification and population-genetic analysis. Keywords: Oncorhynchus mykis, SSR-markers, heterozygotes, polymorphism.

Effect of aeration and oxygenation on growth and survival of rainbow trout in a commercial serial-pass, flow-through raceway system

Serial-pass raceway systems are characterized by reduced water quality as water passes from upper to lower raceways. We previously showed that rainbow trout reared in 1st use (source spring) and 3rd use (after 2 raceway passes) water exhibited significantly better growth and survival compared to those in 5th use water (after 4 raceway passes) and directly related to dissolved oxygen (DO) concentration. Our objective was to determine if increasing the DO in 5th use water by aeration or oxygenation to levels in 3rd use water could restore growth and survival of rainbow trout. We stocked rainbow trout (126 ± 9.3 g mean weight) into tanks receiving either 1st use, 3rd use, or 5th use water with and without supplemental air or oxygen (five treatments at four tanks each). Average DO concentration was highest in 1st use (92.0% saturation) followed by 3rd use (79.4%) and 5th use (52.1%) water. In 5th use water, use of aeration (68.4%) or oxygenation (71.7%) raised the DO levels similar to 3rd use water. Growth performance (SGR and FCR) and survival were significantly lower for trout in 5th use compared to 1st and 3rd use water. Aeration and oxygenation of 5th use water restored growth to that observed for trout in 3rd use water. Improvements were limited for survival with 3rd use water (88.8%) showing higher values than 5th use with oxygenation (74.2%) or aeration (53.1%). These treatments were still significantly better than survival observed for trout in untreated 5th use water (35.0%). Dissolved oxygen appears to be the primary water quality parameter limiting performance in this system. However, due to dissimilar survival in 3rd and 5th use water with aeration or oxygenation, other water parameters (e.g. total dissolved solids or turbidity), which increased in 5th use water, may also contribute to the reduced growth and survival.

Effects of a rainbow trout stocking moratorium on the Daphnia species composition and water quality of Square Lake (Minnesota)

Hembre, LK. 2019. Effects of a rainbow trout stocking moratorium on the Daphnia species composition and water quality of Square Lake (Minnesota). Lake Reserv Manage. 35:127–139.Square Lake is among the clearest lakes in the Minneapolis–St. Paul (Minnesota) metropolitan area, but its water clarity has decreased over the past several decades despite levels of total phosphorus (TP) remaining relatively stable. Predation by zooplanktivorous rainbow trout annually stocked since the early 1980s was hypothesized to be the cause for this eutrophication trend. To evaluate this hypothesis, a 3-year moratorium on trout stocking was imposed by the Minnesota Department of Natural Resources (MNDNR) and water quality and zooplankton monitoring data from 2 years prior to the moratorium (2010 and 2012) were compared to data from the 3 moratorium years (2013–2015). Significant changes observed during the moratorium years that support the hypothesis include (1) an increase in biomass concentrations of the large-bodied Daphnia pulicaria, (2) a coincident decrease in biomass concentrations of the smaller bodied D. mendotae, and (3) more pronounced spring clear-water phases when D. pulicaria reached peak densities. In addition, the volume of hypoxic water (dissolved oxygen [DO] < 1 mg/L) that developed in deep water was less in moratorium years compared to the premoratorium years. Unexpectedly, surface water TP concentrations were significantly lower during the moratorium years than in premoratorium years. Greater sequestration of P in the biomass of Daphnia during the moratorium years is the likely cause for the decrease in surface water TP levels in those years. Natural resource managers from the MNDNR have extended the moratorium and are using the conclusions from this study to determine the future fisheries management plan for Square Lake.