Trout Salmo Research Articles

Impacts of reduced Lepidurus arcticus availability on brown trout life history traits in a mountain reservoir

Lepidurus arcticus (the Arctic tadpole shrimp) is a vulnerable keystone species in Arctic and alpine water bodies where its occurrence and population size may influence the viability and life history traits of resident salmonids. Using data from a Norwegian mountain hydropower reservoir, Aursjoen, we illustrate how reduced availability of L. arcticus as prey resulted in the reduced condition, growth and delayed maturation of resident brown trout (Salmo trutta). We further link changes in the relative abundance of L. arcticus as prey to changing reservoir conditions, e.g. water level changes in the spring period, thereby establishing an indirect link between reservoir operation regimes and brown trout population traits. While no evidence for decreased brown trout survival was found, the results indicate that alternative brown trout prey resources, i.e. the small chydorid cladoceran Eurycercus lamellatus, do not appear to have successfully offset the caloric loss from reduced consumption of large-sized L. arcticus. Although the fundamental explanation for the observed L. arcticus collapse remains largely unknown, the present findings provide strong evidence that this vulnerable crustacean species can affect the abundance, viability and life history traits of valued resident salmonid populations in oligotrophic alpine lakes and reservoirs exposed to climate- and hydropower-driven changes in water levels and temperature.

Mysterious syndrome causing high mortality in wild brown trout in Eastern Switzerland, pathology and search for a possible cause.

Since 2016, annually occurring species-specific die-offs of brown trout (Salmo trutta fario) occurred in the Thur River, situated in the Eastern part of Switzerland. These events lead to drastically reduced population densities in the impacted river regions. Clinical signs in brown trout and mortality were restricted to few weeks in August/September. To characterize the syndrome and to find possible causes, from end of March to November 2018, one-year-old brown trout (Salmo trutta fario) and rainbow trout (Oncorhynchus mykiss) were exposed to water from Thur River, fish were sampled regularly and screened for infectious agents, including viral metagenomics, and pathology was described. Starting approximately four months post-exposure, brown trout showed severe lymphohistiocytic pancarditis and necrotizing and haemorrhagic hepatitis. These lesions were recorded until the end of the experiment in November. Rainbow trout were not affected at any point in time. No infectious agents could be identified so far as cause of disease, especially no viral aetiology. Even if pathogenesis and pathology point in the direction of an infectious agent, a causative relationship could not be confirmed and aetiology remains unclear.

Anthropogenic and natural size-related selection act in concert during brown trout (Salmo trutta) smolt river descent

By hindering migration and inducing direct turbine mortality during downstream migration, hydropower is regarded as one of the most serious threats to anadromous salmonids. Yet, little attention has been paid to long-term turbine-induced selection mechanisms effecting fish populations. This work evaluates turbine and post-turbine survival of PIT-tagged wild brown trout smolts. By estimating individual river and sea survival rates, we were able to compare survival rates of smolts that had migrated through the turbine with smolts that had bypassed the turbine, as well as investigate both natural and anthropogenic size-selective mechanisms operative on the population. Total river-descent survival probability was 0.20 for turbine migrants and 0.44 for bypass migrants. The surviving turbine migrants were significantly smaller than their bypass counterparts and more exposed to predation from Northern pike. The estimated mean-adjusted selection gradient was − 0.76 for turbine migrants and + 1.85 for the bypass migrants. The resulting disruptive selection may ultimately lead to increased phenotypic smolt size variation provided sufficient additive genetic variance associated with smolt size. Mitigation measures at hydropower plants are thus essential for preserving sustainable populations of anadromous fish and maintaining population genetic variation.

Could mycotoxigenic Fusarium sp. play a role in ulcerative dermal necrosis (UDN) of brown trout (Salmo trutta morpha trutta)?

Fusarium infections have been reported in aquatic animals, but are still poorly investigated in wild salmonids. The aim of the study was to determine the impact of the fungi and their toxins on the health status of brown trout (Salmo trutta morpha trutta) migrating from the Baltic Sea to the freshwater. Individuals from the wild brown trout population exhibiting ulcerative skin lesions were collected from the Słupia River in Poland and subjected to microbiological, histopathological, and hematological examinations, as well as toxicological analysis for a presence of mycotoxins. The results of microflora isolation from the brown trout skin samples revealed the presence of conditionally pathogenic bacteria and fungi classified by molecular techniques as Fusarium spp. Toxicological analysis allowed for detection of zearalenone (ZEN) in the liver, kidney, and gastrointestinal tract of the fish. In several cases, there was α-zearalenone (α-ZEL) identified at trace levels in the liver, as well as sterigmatocystin and enniatin B at low levels in the kidney and the liver. Histopathological examination revealed the presence of fungal hyphae disrupting the epidermis and penetrating into the necrotic dermis and hypodermis. The decreased values of the blood parameters, i.e., hemoglobin concentration (HGB), packed cell volume (PCV), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and white blood cell count (WBC), were indicative of osmoregulation failure being a consequence of the skin damage. The results of the study provide new information regarding Fusarium sp. infection in brown trout and serve as the basis for further research on the potential impact of the fungi and their mycotoxins on the Baltic salmonid population, including their role in ulcerative dermal necrosis.

Exposure to carbamate fungicide iodocarb does not affect reproductive behavior or milt volumes in precocious male brown trout (Salmo trutta L.) parr

Previous studies with olfactory-disturbing pesticides resulted after exposure in disturbed behavior and physiology in fish. In the present experiment, reproductive behavior and milt volumes of precocious brown trout (Salmo trutta L.) male parr were studied in a large stream aquarium after exposure to the olfactory-disturbing fungicide 15 μg l−1 IPBC (iodocarb; 3-iodo-2-propynyl butyl carbamate) for 96 h. The statistical analyses did not reveal any significant differences for time attending females between controls and IPBC-exposed males. Furthermore, there were no significant differences in milt volumes. However, when taking all fish into consideration, there were significant differences in milt volumes between parr that had been attending females and those had not been attending females. Controls that had attended females had significantly higher milt volumes than controls or IPBC-exposed males that had not attended females. Taking all control and IPBC parr into consideration, there was a statistically significant positive correlation between time attended females and volume of milt and gonadosomatic index (GSI), respectively. In summary, 15 μg l−1 IPBC did not have any significant effects on mature male parr reproductive behavior and milt volumes.

ПРИМЕНЕНИЕ НОВОГО МЕТОДА ИСКУССТВЕННОЙ ИНКУБАЦИИ ИКРИНОК СЕВАНСКОЙ ФОРЕЛИ (SALMO ISCHCHAN, KESSLER) В ЕСТЕСТВЕННЫХ УСЛОВИЯХ ПРИТОКОВ ОЗ. СЕВАН (АРМЕНИЯ)

Endemic fish species of Armenian ichthyofauna – the Sevan trout (Salmo ischchan, Kessler, 1877) is registered in the Red Book of Animals of the Republic of Armenia as critically endangered (corresponds to IUCN category: CR A2cd). Its natural reproduction hardly occurs as a result of numerous problems related to the use and management of water and bio-resources in the drainage basin of Lake Sevan. The Lichq River is formerly known as a spawning river for two ecological races of Sevan trout, but because of different factors there are no more wild populations of Sevan trout spawning. The aim of the study was checking the efficiency of two methods of egg incubation in the Lichq River for restoration of Gegarkuni population. The experiments were carried out with newly fertilized and eyed eggs of Gegarkuni in the same periods and in the same areas to avoid the unforeseen effects of environmental factors on assessment. The results showed that the mortality rate of green eggs in the natural conditions is quite high before reaching eyed egg stage (65-68%), while the mortality rate of eyed eggs planted in the natural conditions is only 9-17%. This means that the effectiveness of planting eyed eggs is higher for the artificial restoration of Gegarkuni stocks in the nature.

Stairs pipe culverts: flow simulations and implications for the passage of European and Neotropical fishes

Culvert fishways can improve upstream fish passage in brooks and have been studied in relation to North American and Australian species. Research focusing on fish species from other world regions and, in parallel, effect of baffles on flow turbulence is rare. In this paper we present computational fluid dynamics of a sloped baffle culvert, called “stairs pipe”. We aimed at evaluating if: (1) the flow met the requirements of Neotropical and European species; (2) the flow turbulence was acceptable for fish passage; (3) the flow limited fish movements. The average flow velocities for 5% slope and discharge rates of 5–13 L/s were lower than the prolonged speeds of three Neotropical species, namely, piau (Leporinus reinhardti), mandi (Pimelodus maculatus), and lambari (Piabarchus stramineus) and higher than the sustainable speeds of three European species, namely, dace (Leuciscus leuciscus), barbel (Barbus barbus), and brown trout (Salmo trutta). The turbulence flow characteristics i.e. levels of turbulence kinetic energy were similar to those produced by comparable culverts but higher than those in a fish ladder. The water jet created by baffles at an angle of 30° can limit fish movements, restraining them from jumping. The stairs pipe might improve upstream fish movement in different regions of world; nevertheless, further experimental research should concern the use of different swimming modes in the culvert flow.

Renibacterium salmoninarum and Mycobacterium spp.: two bacterial pathogens present at low levels in wild brown trout (Salmo trutta fario) populations in Austrian rivers

BackgroundRenibacterium salmoninarum and Mycobacterium sp. are important bacterial pathogens of fish. R. salmoninarum is the causative agent of bacterial kidney disease, a Gram-positive bacterium mostly known for causing chronic infections in salmonid fish, while multiple species belonging to the Mycobacterium genus have been associated with mycobacteriosis in fish as well as in human. The objective of this study was to determine the prevalence of these two bacterial pathogens in populations of wild brown trout (Salmo trutta fario) in four rivers (Kamp, Wulka, Traun and Ybbs) in Austria.ResultsA total of 457 kidney samples were examined for both bacterial agents using nested and conventional PCR as well as bacterial cultivation on KDM-2, histological examination and immunohistochemistry. Molecular evidence showed an estimated prevalence level of 0.94% for R. salmoninarum in 2017 while the bacterium could not be detected in 2018 and histology showed signs consistent with a low-level chronic inflammation in the kidney of infected fish. Similarly, no fish were found positive for Mycobacterium in 2017 but in 2018, the prevalence was found to be 37.03% in the Kamp river (4.08% across all rivers). The sequencing data confirmed that these fish carried Mycobacterium sp. although the precise species of Mycobacterium could not be ascertained.ConclusionsThis survey constitutes the first insight into the prevalence rate of R. salmoninarum and Mycobacterium sp. in wild brown trout (Salmo trutta fario) populations in Austria. Both of these pathogens were only detected in the summer months (June and July), which might suggest that the stress linked to increased water temperature could act as stressor factor and contribute to the outbreak of these diseases. The age of the fish might also play a role, especially in the case of Mycobacterium sp. as all the infected fish were in their first summer (June).

Suture-less Implantation of Acoustic Transmitters in Two Salmonids

The surgical incision required to insert acoustic transmitters into fish peritoneal cavities is typically closed with sutures, staples, or cyanoacrylate adhesive. This study evaluated a novel tag insertion technique that did not use any method of wound closure. Twenty-six adult rainbow trout (Oncorhynchus mykiss) and 26 adult brown trout (Salmo trutta) received a small, 6 mm abdominal incision. A dummy acoustic transmitter tag was inserted into half (N = 13) of the fish in each species. Neither group received any wound closure; no sutures, staples, nor adhesives were used. Tag retention, wound healing, and fish survival were monitored weekly for 13 weeks post-surgery. No fish died over the course of the trial. Tag retention was 100% and 85% in the tagged rainbow trout and brown trout, respectively. The wounds in the untagged rainbow trout were all completely healed by six weeks post-surgery, while wounds of the tagged rainbow trout were completely healed at nine weeks post-surgery. Similarly, wounds in the untagged and tagged brown trout groups were completely healed by the fifth and eighth weeks respectively. This study documents the effectiveness of using a small incision with no wound closure methods during insertion of acoustic tags into rainbow trout and brown trout peritoneal cavities.

Variability of morphological characteristics and the population status of brook trout (Salmo trutta morpha fario Linnaeus, 1758) at the headwaters of the rivers Samur and Sulak

Variability of morphological characteristics and the population status of brook trout (Salmo trutta morpha fario Linnaeus, 1758) at the headwaters of the rivers Samur and Sulak

Proliferative Kidney Disease and Proliferative Darkening Syndrome are Linked with Brown Trout (Salmo trutta fario) Mortalities in the Pre-Alpine Isar River.

For many years, brown trout (Salmo trutta fario) mortalities within the pre-alpine Isar River in Germany were reported by the Bavarian Fisheries Association (Landesfischereiverband Bayern e.V.) and local recreational anglers during August and September. Moribund fish seemed to be affected by proliferative darkening syndrome (PDS). In addition, proliferative kidney disease (PKD) caused by Tetracapsuloides bryosalmonae was discussed. To investigate this phenomenon, the present field study monitored brown trout mortalities by daily river inspection in 2017 and 2018. Moribund brown trout (n = 31) were collected and examined using histology, immunohistochemistry, qPCR, and quantitative stereology. Our investigations identified 29 (93.5%) brown trout affected by PKD. Four brown trout (12.9%) displayed combined hepatic and splenic lesions fitting the pathology of PDS. The piscine orthoreovirus 3, suspected as causative agent of PDS, was not detectable in any of the samples. Quantitative stereological analysis of the kidneys revealed a significant increase of the renal tissue volumes with interstitial inflammation and hematopoietic hyperplasia in PKD-affected fish as compared to healthy brown trout. The identified T. bryosalmonae strain was classified as part of the North American clade by phylogenetical analysis. This study highlights PKD and PDS as contributing factors to recurrent autumnal brown trout mortalities.

Fishery-exteriors characteristics of the brooders of brown trout, rainbow trout and brook trout

This study aims to determine the reproductive indices of three types of salmon fish: brown trout (Salmo trutta morfa fario Linnaeus (1758), rainbow trout (Oncorhynchus mykiss Walbaum, 1792) and brook trout (Salvelinus fontinalis Mitchill, 1814) in aquaculture. For this purpose, 10 females and 10 males of each species were selected on an analogous basis and the main fishery-exteriors and reproductive characteristics were identified. The researching of relative fertility permit to establish that this indicator in brook trout females was 18.36% higher compared to rainbow trout, however, by 7.5% lower compared to the brown trout. It was found that brook trout eggs had less weight and size in comparison with other studied species. In particular, the weight and diameter of the eggs were 2.89 and 9.7% respectively lower than of rainbow trout, as well as 8.1 and 3.44% respectively lower compared to the brown trout. When studying the quantity of ejaculate, it was found that on average, brook trout males had 16.95% less of ejaculates compared to rainbow trout males, but 79.6% more than of brown trout males. When comparing the main indices of the brooders body of the studied species, it was found that the condition factor of the brook trout and rainbow trout females was quite high at 1.55–1.57, the brown trout one was 1.35, and the profile index respectively was 3.5, 3.52 and 4.0. The males' condition factor of brook trout, rainbow trout and brown trout was respectively 1.64, 1.52 and 1.35. Fishery-exteriors indicators met the requirements of brooders species and age specificity norms of the studied fish species.

How to preserve and handle fish liver samples to conserve RNA integrity

As transcriptomic studies are becoming more and more common, it is important to ensure that the RNA used in the analyses is of good quality. The RNA integrity may be compromised by storage temperature or freeze-thaw cycles, but these have not been well studied in poikilothermic fishes. This work studied the effects of tissue storage time and temperature, and freeze-thaw cycles of tissue and extracted RNA on RNA integrity in brown trout (Salmo trutta L.) liver. The storage time and temperature had an effect on RNA integrity, but RNA suitable for quantitative reverse transcription PCR (RT-qPCR) (RIN > 7) was still obtained from samples preserved at − 20 °C for 6 months. Freeze-thaw cycles of tissue or RNA did not compromise the integrity of RNA. RNA degradation had an effect on RT-qPCR results, and the effect depended on gene. The RT-qPCR analysis of historical samples from a bleached kraft pulp mill effluent exposure in 1984 revealed no significant cyp1a induction. Recommendations are given for the preservation and handling procedures of samples designated for transcriptomic analyses.

The effect of static magnetic field on melanophores in the sea trout (Salmo trutta m. trutta Linnaeus, 1758) embryos and larvae

AbstarctMagnetic field affects many living organisms, fishes included. The fish reactions are observed from the early stage of development, during embryogenesis and larval period, and into adult stage. Magnetic field affects, among other processes, water exchange in the eggs, embryo heart rate and respiration, directional reactions and spatial orientation. The few papers in this field published so far present melanophores morphology and development. Melanophores developing in embryos and in newly hatched sea trout (Salmo trutta m. trutta L.) larvae which had been incubated and kept under optimal conditions constituted the material for the study. The effect of static, generated magnetic field of intensity of 1, 3 and 5 mT on melanophores of sea trout embryos and larvae was studied. The control setting was kept under geomagnetic field. The appearance of melanophores in the trout embryos exposed to magnetic field was delayed. The number of melanophores was smaller, and the movement of melanin within the melanophores, visible as aggregation of pigment in the cells. Static magnetic field of relatively low intensity influences the reaction of melanophores of developing trout embryos and newly hatched larvae. Most probably, magnetite (Fe3O4) compounds, detected in many fish species, mainly migratory fishes such as trout, are responsible for such behaviour of melanophores.HighlightsStatic magnetic field delays the appearance of melanophores in the trout embryos and larvae.The largest aggregation of melanin in melanophores was found in larvae incubated in a 5 mT magnetic field.The largest number of melanophores was found in the embryos incubated in the geomagnetic field.

Impact of glacial refugia on present-day diversity of the Black Sea trout (Salmo labrax Pallas, 1814) in Croatia

Impact of glacial refugia on present-day diversity of the Black Sea trout (Salmo labrax Pallas, 1814) in Croatia

Do fish get wasted? Assessing the influence of effluents on parasitic infection of wild fish.

Many ecosystems are influenced simultaneously by multiple stressors. One important environmental stressor is aquatic pollution via wastewater treatment plant (WWTP) effluents. WWTP effluents may contribute to eutrophication or contain anthropogenic contaminants that directly and/or indirectly influence aquatic wildlife. Both eutrophication and exposure to anthropogenic contaminants may affect the dynamics of fish-parasite systems. With this in mind, we studied the impact of WWTP effluents on infection of brown trout by the parasite Tetracapsuloides bryosalmonae, the causative agent of proliferative kidney disease (PKD). PKD is associated with the long-term decline of wild brown trout (Salmo trutta) populations in Switzerland. We investigated PKD infection of brown trout at two adjacent sites (≈400 m apart) of a Swiss river. The sites are similar in terms of ecology except that one site receives WWTP effluents. We evaluated the hypothesis that fish inhabiting the effluent site will show greater susceptibility to PKD in terms of prevalence and disease outcome. We assessed susceptibility by (i) infection prevalence, (ii) parasite intensity, (iii) host health in terms of pathology, and (iv) estimated apparent survival rate. At different time points during the study, significant differences between sites concerning all measured parameters were found, thus providing evidence of the influence of effluents on parasitic infection of fish in our study system. However, from these findings we cannot determine if the effluent has a direct influence on the fish host via altering its ability to manage the parasite, or indirectly on the parasite or the invertebrate host via increasing bryozoa (the invertebrate host) reproduction. On a final note, the WWTP adhered to all national guidelines and the effluent only resulted in a minor water quality reduction assessed via standardized methods in this study. Thus, we provide evidence that even a subtle decrease in water quality, resulting in small-scale pollution can have consequences for wildlife.

Plasma and tissue disposition of danofloxacin in brown trout (Salmo trutta fario) after intravenous and intramuscular administrations

ABSTRACTPlasma and muscle pharmacokinetics of danofloxacin were investigated after 10 mg/kg intravenous (IV, caudal vein) and intramuscular (IM, right epaxial muscles) administrations in 168 healthy brown trout (Salmo trutta fario) at 10°C–13°C. High-performance liquid chromatography was used to determine its plasma and muscle concentrations. Pharmacokinetic parameters were analysed with a non-compartmental model. After IV administration, elimination half-life (t1/2ʎz), area under the concentration–time curve (AUC0–∞), mean residence time (MRT0–∞), volume of distribution at steady state, total body clearance in plasma and AUCMuscle/AUCPlasma ratio were 22.22 h, 140.66 h*µg/mL, 23.15 h, 2.28 L/kg, 0.07 L/h/kg and 3.79, respectively. After IM administration, t1/2ʎz, AUC0–∞, MRT0–∞, peak concentration (Cmax), time to reach Cmax, bioavailability in plasma and AUCMuscle/AUCPlasma ratio were 28.28 h, 84.39 h*µg/mL, 37.31 h, 4.79 µg/mL, 1 h, 59.99% and 8.46, respectively. Danofloxacin exhibited long t1/2ʎz and good bioavailability after IM administration. Therefore, 10 mg/kg IM administration of danofloxacin in brown trout can provide AUC0–24/MIC of > 125 and Cmax/MIC of > 10 to treat diseases caused by susceptible bacteria with ≤ 0.336 µg/mL MIC.

Elementary functions modified for seasonal effects to describe growth in freshwater fish✰

Two models were derived to describe fish growth while accounting for the effects of fluctuating water temperatures. The models were initially expressed in a rate:state form and subsequently integrated resulting in two analytical solutions, representing two distinct types of growth: exponential (Model 1) and asymptotic (Model 2). Both models share the assumptions that growth machinery works at a rate which varies with water temperature and that growth is irreversible. In addition, in Model 1 it is assumed that quantity of growth machinery is proportional to live body weight and substrate is non-limiting over the period of growth; whereas Model 2 is based on the assumption that quantity of growth machinery is proportional to available substrate. Effects of seasonal variations in water temperature on fish growth are represented in both models by a sinusoidal function. The potential of these models was investigated through their ability to describe growth in eight datasets encompassing three species: European bullhead (Cottus gobio), brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss). Models were evaluated using statistical measures of goodness-of-fit and through the analysis of residuals. Of the eight datasets, six displayed asymptotic growth while the other two exhibited exponential growth. Both models yield suitable simple growth functions with acceptable goodness-of-fit to fish growth curves under fluctuating water temperatures. However, Model 1, representing exponential growth, shows limited ability to predict fish size (length) when growth curves follow a clear asymptotic trend. This study enforces the idea that a given model is not always superior to another and that data structure and underlying model assumptions must be considered in model selection.

Population genetic structure after 125 years of stocking in sea trout (Salmo trutta L.)

Stocking can be an effective management and conservation tool, but it also carries the danger of eroding natural population structure, introducing non-native strains and reducing genetic diversity. Sea trout, the anadromous form of the brown trout (Salmo trutta), is a highly targeted species that is often managed by stocking. Here, we assess the present-day population genetic structure of sea trout in a backdrop of 125 years of stocking in Northern Germany. The study area is characterized by short distances between the Baltic and North Sea river watersheds, historic use of fish from both watersheds for stocking, and the creation of a potential migration corridor between the Baltic and North Sea with the opening of the Kiel Canal 120 years ago. A survey of 24 river systems with 180 SNPs indicates that moderate but highly significant population genetic structure has persisted both within and between the Baltic and North Sea. This genetic structure is characterized by (i) heterogeneous patterns of admixture between the Baltic and North Sea that do not correlate with distance from the Kiel Canal and are therefore likely due to historic stocking practises, (ii) genetic isolation by distance in the Baltic Sea at a spatial scale of < 200 km that is consistent with the homing behaviour of sea trout, and (iii) at least one genetically distinct Baltic Sea river system. In light of these results, we recommend keeping fish of North Sea and Baltic Sea origin separate for stocking, and restricting Baltic Sea translocations to neighbouring river systems.

Long-Term Trends in Abundance and Size of Sport Fishes in the Flathead River, Montana, Following Changes in Hydroelectric Dam Operations

We studied long-term trends in catches per unit effort (CPUE; fish h-1) of four sport fish taxa in the Flathead River, Montana, following changes in hydroelectric operations at the Slis Ksanka Qisp Project (SKQP; formerly Kerr Dam). Prior to 1997 SKQP operations caused frequent, unnatural flow fluctuations. In 1997 the dam was changed from a power-peaking and load-following facility to a base-load facility. The new operations included seasonal minimum flows and ramping rates that greatly reduced flow fluctuations. Autumn trends in CPUEs of two size classes (substock and stock) of northern pike (Esox lucius), Oncorhynchus spp., brown trout (Salmo trutta), and smallmouth bass (Micropterus dolomieu) were monitored using nighttime electrofishing during 1984–86 before operational changes and again at the onset of operational changes in 1998 and continuing through 2008. We observed little difference in CPUEs between 1984–86 and 1998, the first year after changes, but we documented strong increasing trends in CPUEs of both sizes of all taxa, except stock northern pike, over the long term following operational changes. We also examined long-term patterns in the size structure (total length; TL mm) of fishes following operational changes. All taxa had either initial downward shifts in median TL, decreases in minimum sizes of fish captured, or both, a pattern consistent with enhanced recruitment and survival of smaller fishes. We conclude that modifications in dam operations were associated with increases of four sport fish taxa in the Flathead River and that similar normative flow applications might benefit riverine fish populations elsewhere.