Wild Trout Research Articles

Alien Rainbow Trout Oncorhynchus mykiss in the Balkhash Basin (Kazakhstan, Central Asia): 50 Years of Naturalization.

Rainbow trout, or mykiss (Oncorhynchus mykiss), is one of the most popular species used in aquaculture and has been naturalized worldwide, including in the Central Asian Balkhash basin, which has unique aboriginal fish fauna. Both rainbow trout from European farms and wild mykiss from Kamchatka were introduced to some mountain lakes and rivers of the Balkhash basin about 50 years ago. This study investigates the current distribution and life history traits of the alien species and its possible impact on the local fish fauna. This study showed that the rainbow trout occupies various habitats in the Ili River basin: mountain lakes, fast-flowing mountain rivers, and lowland rivers with slow currents and warm water (up to +27 °C). Rainbow trout from European fish farms dominate the mountain Middle Kolsay Lake, while the wild trout from Kamchatka occupies the small Ulken Kokpak River. Both co-occur in the Chilik River. Contrary to that in other regions, the distribution of rainbow trout in the Balkhash basin remained almost the same after their introduction. Broad intrapopulation variability in terms of size, growth rate, and maturation age was revealed, apparently as a result of adaptation to the new environment and intrapopulation competition. In particular, the growth rate has decreased, but life span, surprisingly, has increased as compared to the originally introduced fish. Intrapopulation variation in growth and maturity patterns was also noted. Differences in skin coloration between highland (cold-water) and lowland (warm-water) populations were discovered. The feeding mode of naturalized trout is insectivorous (insect imago), indicating that it occupies its own niche in the local fish communities. The largest population of rainbow trout was recorded in the Lower Kolsay Lake, lowering the population of native fish species, while in other localities, no negative impact on local fish communities was recorded.

The myxozoans Myxobolus cerebralis and Tetracapsuloides bryosalmonae modulate rainbow trout immune responses: quantitative shotgun proteomics at the portals of entry after single and co-infections.

Little is known about the proteomic changes at the portals of entry in rainbow trout after infection with the myxozoan parasites, Myxobolus cerebralis, and Tetracapsuloides bryosalmonae. Whirling disease (WD) is a severe disease of salmonids, caused by the myxosporean M. cerebralis, while, proliferative kidney disease (PKD) is caused by T. bryosalmonae, which instead belongs to the class Malacosporea. Climate change is providing more suitable conditions for myxozoan parasites lifecycle, posing a high risk to salmonid aquaculture and contributing to the decline of wild trout populations in North America and Europe. Therefore, the aim of this study was to provide the first proteomic profiles of the host in the search for evasion strategies during single and coinfection with M. cerebralis and T. bryosalmonae. One group of fish was initially infected with M. cerebralis and another group with T. bryosalmonae. After 30 days, half of the fish in each group were co-infected with the other parasite. Using a quantitative proteomic approach, we investigated proteomic changes in the caudal fins and gills of rainbow trout before and after co-infection. In the caudal fins, 16 proteins were differentially regulated post exposure to M. cerebralis, whereas 27 proteins were differentially modulated in the gills of the infected rainbow trout post exposure to T. bryosalmonae. After co-infection, 4 proteins involved in parasite recognition and the regulation of host immune responses were differentially modulated between the groups in the caudal fin. In the gills, 11 proteins involved in parasite recognition and host immunity, including 4 myxozoan proteins predicted to be virulence factors, were differentially modulated. The results of this study increase our knowledge on rainbow trout co-infections by myxozoan parasites and rainbow trout immune responses against myxozoans at the portals of entry, supporting a better understanding of these host-parasite interactions.

Effects of handling and surgical implant on juvenile wild trout: Physiological and behavioral impact.

Fish involved in telemetry studies are usually handled, anaesthetized, and subjected to internal tag implantation, all of which have the potential to disrupt the fish's physiology, migratory patterns, food-seeking behavior, growth, and survival. As fish tagging is a widespread standard method in aquaculture, it is crucial to gain a better understanding of the short- and long-term effects on various aspects of fish welfare. The experimental outcomes of each phase of the surgical process and how it affects the fish's ability to recover during the post-tagging period, particularly in small salmonids, are poorly understood. Thus, it is essential to measure the extent of these adverse effects to precisely extrapolate differences in surgical intervention for tagged fish compared to their unmanipulated counterparts. In our study, we explored the post-tagging impacts on wild-captured brown trout (Salmo trutta) (1+) juveniles. We examined the time effect on fish recovery after each phase of the surgical procedure, which included anaesthetized fish, followed by surgery with/without internal tagging, within the first 2 and 25 h post-surgery. Common blood parameters (glucose levels and haematocrit) and endpoints related to swimming behavior, including fish movement, direction, and social distancing aspects, were studied. The findings of the study indicated no significant changes in glucose and haematocrit levels over the observational period. Fish subjected to anesthesia and tagging procedures exhibited a significant decrease in swimming activity, recovering to baseline levels 1 h after anesthesia and 24 h after surgical (tagging) procedures in recirculating aquaculture systems. Tagged fish showed less social proximity than non-tagged fish in the first 2 h post-surgery. A significant effect size was observed between nonsurgical (anesthesia) and surgical (non-tagged/tagged) groups, highlighting a more pronounced impact associated with surgical tag interventions. Our data indicate that the magnitude of behavioral response was significantly influenced by tag weight (~1.4% body/tag weight ratio in water) among manipulated fish groups. Consequently, our study reveals that wild-captured S. trutta juveniles, tagged with commercially available low-weight acoustic tags (V6, 69 kHz), experienced negative effects on swimming performance. In surgical studies, acknowledging potential influences is crucial for accurately inferring fish physiological and behavioral status. Emphasizing fish recovery potential in both short- and long-term periods is essential for quantifying tagging effects. Future research should prioritize exploring alternative tagging technologies and refining methodologies, with a particular focus on assessing telemetry's impact on socioeconomically relevant small salmonids.

Comparing the migration behavior and survival of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) smolts.

Many organisms rely on migrations between habitats to maximize lifetime fitness, but these migrations can be risky due to a suite of factors. In anadromous salmonids, the smolt migration from fresh water to sea is a critical life stage, during which smolts can experience high mortality from multiple sources. This study investigated the migratory behavior and survival of Atlantic salmon (Salmo salar) and anadromous brown trout (Salmo trutta) smolts during their seaward migration using acoustic telemetry between March and May 2021. Due to the extinction of wild salmon in the River Gudenaa after the construction of the Tange hydropower plant, this study used hatchery-reared salmon originating from a nearby Danish river. A total of 75 hatchery-reared salmon smolts, 75 hatchery-reared trout smolts, and 75 wild trout smolts were tagged with acoustic transmitters and released into River Gudenaa, Denmark. The downstream movements of tagged fish were monitored using acoustic receivers deployed in the river and fjord. Hatchery-reared trout initiated migration first, followed by hatchery-reared salmon, with wild trout being the last to migrate. There was no difference in riverine progression rates among the three smolt groups, but noticeable differences emerged once in the fjord: trout (wild and hatchery) slowed down, whereas hatchery-reared salmon maintained their speed. Riverine migration was predominantly nocturnal for all smolts; however, daytime migration increased at the fjord arrays. Day-of-year significantly influenced diurnal patterns in the river and fjord, where daytime migration increased later in the year. Hatchery-reared salmon and wild trout had reasonably good overall survival from river to sea entry (≥66%), whereas hatchery-reared trout had poor survival (c.26%). The fjord was the major bottleneck for survival of hatchery-reared trout. We found no strong evidence for differences in progression rate or diurnal patterns between wild and hatchery-reared trout to explain the lower survival. This study demonstrates that salmon and trout differ in their life-history strategy already in the post-smolt phase, and that stocking is a sub-optimal strategy to aid wild populations.

Association of ectoparasite Lepeophtheirus salmonis counts on farmed Atlantic salmon and wild sea trout in Scotland.

Parasitic sea lice (Copepoda: Caligidae) colonising marine salmonid (Salmoniformes: Salmonidae) aquaculture production facilities have been implicated as a possible pressure on wild salmon and sea trout populations. This investigation uses monitoring data from the mainland west coast and Western Isles of Scotland to estimate the association of the abundance of adult female Lepeophtheirus salmonis (Krøyer) colonising farmed Atlantic salmon Salmo salar L. with the occurrence of juvenile and mobile L. salmonis on wild sea trout, anadromous S. trutta L. The associations were evaluated using generalised linear mixed models incorporating farmed adult female salmon louse abundances which are temporally lagged relative to dependent wild trout values. The pattern of lags, which is consistent with time for L. salmonis development between egg and infective stage, was evaluated using model deviances. A significant positive association is identified between adult female L. salmonis abundance on farms and juvenile L. salmonis on wild trout. This association is consistent with a causal relationship in which increases in the number of L. salmonis copepodids originating from lice colonising farmed Atlantic salmon cause an increase of L. salmonis abundance on wild sea trout.

Local environments, not invasive hybridization, influence cardiac performance of native trout under acute thermal stress.

Climate-induced expansion of invasive hybridization (breeding between invasive and native species) poses a significant threat to the persistence of many native species worldwide. In the northern U.S. Rocky Mountains, hybridization between native cutthroat trout and non-native rainbow trout has increased in recent decades due, in part, to climate-driven increases in water temperature. It has been postulated that invasive hybridization may enhance physiological tolerance to climate-induced thermal stress because laboratory studies indicate that rainbow trout have a higher thermal tolerance than cutthroat trout. Here, we assessed whether invasive hybridization improves cardiac performance response to acute water temperature stress of native wild trout populations. We collected trout from four streams with a wide range of non-native admixture among individuals and with different temperature and streamflow regimes in the upper Flathead River drainage, USA. We measured individual cardiac performance (maximum heart rate, "MaxHR", and temperature at arrhythmia, "ArrTemp") during laboratory trials with increasing water temperatures (10-28°C). Across the study populations, we observed substantial variation in cardiac performance of individual trout when exposed to thermal stress. Notably, we found significant differences in the cardiac response to thermal regimes among native cutthroat trout populations, suggesting the importance of genotype-by-environment interactions in shaping the physiological performance of native cutthroat trout. However, rainbow trout admixture had no significant effect on cardiac performance (MaxHR and ArrTemp) within any of the three populations. Our results indicate that invasive hybridization with a warmer-adapted species does not enhance the cardiac performance of native trout under warming conditions. Maintaining numerous populations across thermally and hydrologically diverse stream environments will be crucial for native trout to adapt and persist in a warming climate.

Species‐ and origin‐specific susceptibility to bird predation among juvenile salmonids

AbstractJuvenile salmonids often experience high mortality rates during migration and bird predation is a common source of mortality. Research suggests that hatchery‐reared salmonids are more prone to predation than wild salmonids, and that Atlantic salmon (Salmo salar) experience lower predation than Sea trout (Salmo trutta), yet telemetry studies have displayed equivocal results. Here, using a large data set on passive integrated transponder (PIT) tagged hatchery‐reared and wild juveniles of Atlantic salmon and Sea trout (25,769 individuals) we investigate predation probability by piscivorous birds (mainly Great Cormorants Phalarocorax carbo) on salmonids originating from River Dalälven in Sweden. Bird colonies and roosting sites were scanned annually (2019–2021), and the temporal dynamics of bird predation on salmonids released in 2017–2021 was assessed. Hatchery‐reared trout was clearly most susceptible to cormorant predation (0.31, 90% credibility interval [CRI] = 0.14–0.53), followed by wild trout (0.19, 90% CRI = 0.08–0.37), hatchery‐reared salmon (0.13, 90% CRI = 0.07–0.23), and wild salmon (0.08, 90% CRI = 0.04–0.14), in subsequent order. This order in predation probability was consistent across all studied tag‐ and release‐years, suggesting that the opportunistic foraging of cormorants affects the overall survival of juvenile salmonids, but that the inherent predation risk between different salmonid types differs systematically.

Changes in the spatio-temporal genetic structure of Baltic sea trout (Salmo trutta L.) over two decades: direct and indirect effects of stocking

Several countries have implemented stocking programmes to enhance abundance and fish production by releases of hatchery-reared fish. However, due to fluctuations in population size, stocking history, and potential indirect effects of straying of hatchery-reared fish, it is often difficult to predict how these factors will affect genetic diversity and differentiation patterns among wild populations. This study characterized the population genetic structure and temporal variability of four Estonian sea trout populations by evaluating the degrees of direct and indirect genetic impacts of stocking over two decades using 14 microsatellite loci. Our results demonstrate considerable temporal change combined with weak genetic structuring among studied sea trout populations. We found a reduction of the overall level of genetic differentiation combined with the tendency for increased genetic diversity, and an effective number of breeders (Nb) over the study period. Furthermore, we found that immigration rates (m) from hatchery stocks were highest in the population subjected to direct stocking and in non-stocked populations that were located geographically closer to the stocked rivers. This work suggests that hatchery releases have influenced the genetic diversity and structuring of studied sea trout populations. However, the impact of hatchery releases on the adaptive variation and fitness-related traits in wild trout populations remains to be revealed by more informative genetic markers. This study illustrates the dynamic nature of the population genetic structure of sea trout and the value of long-term genetic monitoring for management and conservation.

Comparison of triploid and diploid rainbow trout (Oncorhynchus mykiss) fine-scale movement, migration and catchability in lowland lakes of western Washington

Fisheries managers stock triploid (i.e., infertile, artificially produced) rainbow trout Oncorhynchus mykiss in North American lakes to support sport fisheries while minimizing the risk of genetic introgression between hatchery and wild trout. In Washington State, the Washington Department of Fish and Wildlife (WDFW) allocates approximately US $3 million annually to stock hatchery-origin rainbow trout in > 600 lakes, yet only about 10% of them are triploids. Many lakes in Washington State drain into waters that support wild anadromous steelhead O. mykiss that are listed as threatened under the U.S. Endangered Species Act. As a result, there is a strong interest in understanding the costs and benefits associated with stocking sterile, triploid rainbow trout as an alternative to traditional diploids. The objectives of this study were to compare triploid and diploid rainbow trout in terms of: (1) contribution to the sport fishery catch, (2) fine-scale movements within the study lakes, (3) rate of emigration from the lake, and (4) natural mortality. Our results demonstrated that triploid and diploid trout had similar day-night distribution patterns, but triploid trout exhibited a lower emigration rate from the lake and lower catch rates in some lakes. Overall, triploid rainbow trout represent a viable alternative to stocking of diploids, especially in lakes draining to rivers, because they are sterile, have comparable home ranges, and less often migrate.

A rapid and reliable detection procedure of Atlantic trout introgression at the diagnostic lactate dehydrogenase chain‐1 gene

AbstractThe Italian‐native Mediterranean brown trout (Salmo ghigii) is a seriously threatened freshwater fish, especially by anthropogenic hybridisation with the domestic strains of Atlantic origin that have been repeatedly released into the wild for angling. A PCR‐restriction fragment length polymorphism (RFLP) assay of the diagnostic lactate dehydrogenase chain‐1 (LDH‐C1) gene sequences has been routinely applied to distinguish exotic from native brown trout lineages and detect Atlantic introgression signals in the Mediterranean wild populations. Here, we used dermal swab DNA obtained from 28 wild trout to improve laboratory procedures to genetically characterise trout samples at the LDH‐C1gene through (1) a capillary electrophoresis analysis of the RFLP fragments and (2) the optimisation of a diagnostic single nucleotide polymorphism analysable through mini‐sequencing approaches. The developed methods were fully consistent with those obtained through the traditional approach, but their analytical process is almost entirely automated and digitalised, thus improving result readability and accuracy in the detection of alien introgressed traces in wild Mediterranean brown trout populations.

A comparative analysis of multi-biomarker responses to environmental stress: Evaluating differences in landfill leachate and pathogenic oomycete effects between wild and captive Salmo trutta

Phenotypic plasticity is one of the major means by which organisms can manage with environmental factor changes. Captivity-related stress and artificial rearing settings have been shown to dramatically alter fish response plasticity in terms of physiology, behavior, and health, potentially reducing overall fitness and fish survival. Understanding the variations in plasticity between captive-bred (kept in a homogenous environment) and wild fish populations in response to varied environmental pressures is becoming increasingly important, particularly in risk assessment research. In this study, we investigated whether captive-bred trout (Salmo trutta) are more susceptible to stress stimuli than their wild counterparts. In both wild and captive-bred trout, we investigated a battery of biomarkers that depicts the effects at various levels of biological organization in response to landfill leachate as a chemical pollutant, and after exposure to pathogenic oomycetes Saprolegnia parasitica. According to the findings, wild trout were more susceptible to chemical stimuli based on cytogenetic damage and catalase activity changes, whereas captive-bred trout were more sensitive to biological stress as evidenced by changes in overall fish activity and increasing cytogenetic damage in gills erythrocytes. Our findings emphasize the significance of exercising caution when conducting risk assessments of environmental pollutants using captive-bred animals, especially when seeking to extrapolate hazards and better understand the consequences of environmental contamination on wild fish populations. Additional comparative studies are required to investigate the impact of environmental stressors on multi-biomarker responses in both wild and captive fish populations in order to uncover changes in the plasticity of various traits that can result in adaptation or maladaptation to environmental stimuli within these fish populations, affecting data comparability and transferability to wildlife.

A travel cost recreation demand model examining the economic benefits of acid mine drainage remediation to trout anglers.

Acid mine drainage (AMD) is a global environmental problem which has well-documented effects on the physical, chemical, biological, and ecological characteristics of waterways. Less well examined are the economic benefits generated by costly AMD remediation efforts. This research examines trout angler willingness-to-pay (WTP) to maintain fisheries quality benefits resulting from AMD remediation on the North Branch of the Potomac River (NBPR), Maryland. The NBPR has a history of severe AMD-related degradation previously rendering the waterbody unsuitable for many outdoor recreational activities such as trout fishing. However, extensive NBPR AMD remediation activities including nine automated limestone dosers that neutralize acidic water and improve water quality have been critical to developing and maintaining a high quality recreational trout fishery with hatchery-raised stocked trout and naturally reproducing populations of wild trout. To examine trout angler WTP to maintain current NBPR trout fishing quality and hence avoid plausible fishing quality reductions associated with the elimination of nine NBPR limestone dosers, a travel cost recreation demand model of trout angler fishing site choice was developed using data from a survey of Maryland non-tidal anglers. Model results reveal that annual Maryland trout angler WTP to maintain current NBPR trout stocking levels and wild trout abundance, relative to the elimination of the NBPR as a trout fishing site, is about $850,000. These economic benefits to a single stakeholder group - Maryland trout anglers - exceeds the total estimated $340,000 in annual operations and maintenance costs associated with all Maryland limestone dosers. While the positive net economic benefits produced by limestone dosers provide a justification for continuing annual limestone doser expenditures, it's important to note that total economic benefits are likely much higher, as many non-trout fishing benefits remain unaccounted for in this analysis. The approach in this manuscript can serve as a model for practitioners regarding the use of fisheries data and recreational fisher site choice to estimate fishing-related economic benefits from past or for future AMD remediation activities. Given anticipated substantial investments in AMD remediation, additional research in this area is much needed to evaluate past and guide future remediation activities.

Linking omega-3 polyunsaturated fatty acids in natural diet with brain size of wild consumers.

Omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA) are key structural lipids and their dietary intake is essential for brain development of virtually all vertebrates. The importance of n-3 LC-PUFA has been demonstrated in clinical and laboratory studies, but little is known about how differences in the availability of n-3 LC-PUFA in natural prey influence brain development of wild consumers. Consumers foraging at the interface of aquatic and terrestrial food webs can differ substantially in their intake of n-3 LC-PUFA, which may lead to differences in brain development, yet this hypothesis remains to be tested. Here we use the previously demonstrated shift towards higher reliance on n-3 LC-PUFA deprived terrestrial prey of native brown trout Salmo trutta living in sympatry with invasive brook trout Salvelinus fontinalis to explore this hypothesis. We found that the content of n-3 LC-PUFA in muscle tissues of brown trout decreased with increasing consumption of n-3 LC-PUFA deprived terrestrial prey. Brain volume was positively related to the content of the n-3 LC-PUFA, docosahexaenoic acid, in muscle tissues of brown trout. Our study thus suggests that increased reliance on diets low in n-3 LC-PUFA, such as terrestrial subsidies, can have a significant negative impact on brain development of wild trout. Our findings provide the first evidence of how brains of wild vertebrate consumers response to scarcity of n-3 LC-PUFA content in natural prey.

Attraction, Entrance, and Passage Efficiency of Arctic Grayling, Trout, and Suckers at Denil Fishways in the Big Hole River Basin, Montana

AbstractThe Big Hole River basin in southwestern Montana supports the only indigenous, self‐sustaining fluvial population of Arctic Grayling Thymallus arcticus in the conterminous United States, but the basin is fragmented by numerous low‐head irrigation diversion dams. Denil fishways at 63 diversion dams provide Arctic Grayling and other fishes opportunities for year‐round access to critical habitats; however, their efficiency has not been evaluated. We quantified attraction, entrance, and passage for hatchery‐reared Arctic Grayling, wild trout (Brook Trout Salvelinus fontinalis and Brown Trout Salmo trutta), and wild suckers (White Sucker Catostomus commersonii and Longnose Sucker C. catostomus) during 14 field trials conducted at six Denil fishways over a representative range of fishway slopes and hydraulic conditions using passive integrated transponder telemetry. Attraction (60.4–84.3%) and entrance (44.3–78.6%) efficiencies were variable across test conditions and reduced overall fishway efficiencies (19.1–55.8%). In contrast, upon entry, passage efficiencies were high (96.2–97.0%) for all taxa across all test conditions. Attraction of hatchery‐reared Arctic Grayling increased with upstream depth (a surrogate for fishway discharge) and attraction flow, but attraction of wild fish was less affected by these conditions. Entrance of Arctic Grayling, Brook Trout, and Brown Trout decreased with upstream depth and fishway slope, especially when plunging entrance conditions associated with shallow downstream depths were present. However, entrance of Arctic Grayling and both trout species increased with downstream depth, and submerged fishway entrances demonstrated promise for increasing entrance efficiency at fishways with high discharges and steep slopes. We demonstrate that comprehensive evaluations of fishway efficiency components can identify specific solutions that improve fishway efficiency; application of these engineering solutions at individual fishways (as needed) could improve their efficiency and further enhance aquatic connectivity for fishes in the Big Hole River basin and elsewhere.

Wild trout and hydrologic response to restoration of incised streams with improved water use for ranching and fisheries

AbstractIn the face of climate change and dwindling water supplies, this long‐term monitoring study examines the effectiveness of restoring two incised and degraded streams (Crystal Creek and Black Slough) to benefit livestock ranching and wild trout populations in a spring creek and wetland complex, central Idaho, USA. By raising the incised streambeds and connecting to historical floodplains, the restoration aimed to raise groundwater levels to create subsurface saturation conditions to sustain grazing forage and hay production without flood irrigation; diverted flows from the Big Wood River previously used for flood irrigation were instead routed through the restored systems. The restoration used natural channel design methods to reconstruct 5.2 km of Black Slough and 8.4 km of Crystal Creek to emulate a downstream reference reach (Willow Creek); within the relic incised channels, off‐channel oxbow ponds and wetlands were created while filling other segments. Additionally, screened gravels were placed in the reconstructed channels. We monitored before–after restoration changes in channel morphology, groundwater levels, wetlands, instream flows, spawning substrates and redds, fish populations, and invertebrates for the restored creeks and reference reach over 10+ years. Following restoration, average groundwater levels increased by 0.35 m, mean annual flows at Willow Creek (receiving stream) increased from 0.64 to 1.11 m3/s, and average days per year that baseflows were below 0.28 m3/s decreased from 44.5 to 5.4 despite a decrease in average annual precipitation (16.97 to 11.53 cm). Moreover, residual pool depths increased (<0.18 to >0.90 m), spawning substrate sizes increased (0.1 to 19.0 mm), and egg‐to‐fry trout survival rates increased from <20% to >90%. Six years after restoration, fish diversity increased from three to seven species, matching reference conditions. We also found dramatic increases in Brown Trout and Rainbow Trout redd counts (7 to 161 and 17 to 143, respectively) and relative abundance (0.013 to 1.166 trout/m and 0.029 to 0.222 trout/m, respectively). Invertebrate indices of restoration effectiveness were positive for both restored streams but negative for the reference reach. Overall, this study exemplifies how restoration reduced consumptive water use, created sustainable streams with hydrological connections and ponds and wetlands, and improved trout populations.

Evaluating the effects of an electric barrier on fish entrainment in an irrigation canal in Colorado

AbstractEntrainment of fish in irrigation canals is a source of mortality for sport and native fishes and can affect populations and species diversity. To reduce entrainment of wild trout, an electric barrier was installed on the South Canal, an irrigation ditch on the Gunnison River in western Colorado, USA. The objective of this study was to evaluate the effectiveness of the barrier by marking fish upstream of it and estimating fish populations in the canal downstream before and after the barrier was operational. Three groups of fish were tagged and released upstream of the barrier: fish from the canal, wild fish from the Gunnison River, and hatchery‐reared Rainbow Trout. Boat electrofishing was completed in the canal reach below the barrier, and population estimates were made with the Huggins Closed Capture Model. The estimated Brown Trout population of the canal declined following the installation of the barrier, but Rainbow Trout remained stable because of the entrainment of small fish and their growth and survival in the canal. A total of 288 tagged fish less than 300 mm and 4 fish greater than 300 mm were recovered below the barrier, representing 1.3% of all tagged fish. The electric barrier appears to successfully exclude a portion of adult‐sized trout from the irrigation canal, but smaller adult and sub‐adult trout can pass the barrier. The entrainment, growth, and survival of smaller fish maintain a reduced but stable population of fish in the canal, but fewer entrained mature fish is likely a benefit to the populations of the Gunnison River.

Prevalence and distribution of Renibacterium salmoninarum, causative agent of bacterial kidney disease, in wild trout fisheries in Colorado.

Detections of Renibacterium salmoninarum in Colorado USA fish hatcheries have become more frequent in recent years, including one disease outbreak that originated with a wild broodstock. Our objectives were to document the prevalence and distribution of R. salmoninarum in Colorado's wild trout fisheries, investigate variables that influence that distribution, and evaluate the effectiveness of common testing methods on non-anadromous trout. We sampled wild trout across Colorado and tested kidney tissue with enzyme-linked immunosorbent assay (ELISA), quantitative polymerase chain reaction (qPCR), nested polymerase chain reaction (nPCR), and direct fluorescent antibody test (DFAT). Screening with ELISA showed high prevalence of R. salmoninarum among fish populations, but antigen levels were low. No clinical disease was observed in any of the fish sampled despite the antigen of R. salmoninarum being common. Antigen levels measured by ELISA increased in smaller streams with lower historic fish stocking rates. Brook trout Salvelinus fontinalis had the highest prevalence of the bacterium among fish species and highest ELISA antigen levels. The distribution of brook trout in the smallest streams may help explain the patterns of R. salmoninarum across the landscape. The most effective assays for screening wild trout were qPCR and ELISA; DFAT was inconsistent for bacterial levels encountered in wild trout and generally uninformative. Additionally, qPCR and ELISA can provide quantitative information about bacteria levels. The bacterium R. salmoninarum is ubiquitous in Colorado trout fisheries but is generally found at low levels. Active infections are rare and overt bacterial kidney disease appears more common in Colorado hatcheries than in wild fish.

Genetic Divergence and Diversity Reflect a Predominant Freshwater Resident Life History in Rainbow Trout from Southwestern Alaska

AbstractRainbow Trout Oncorhynchus mykiss in southwestern Alaska occupy coastal watersheds near the northern boundary of the species’ native range and support a world‐class wild trout sport fishery. Although low freshwater temperatures and a short growing season in this region may favor anadromy, these populations appear to exhibit a freshwater resident life history strategy. We used genetic data to evaluate two hypotheses regarding the influence of the presumed migratory behavior of these Rainbow Trout on reproductive isolation among and within watersheds. The results were largely consistent with the predictions, but there were exceptions. The data supported the hypothesis that the freshwater resident behavior precludes marine‐mediated gene flow, resulting in large genetic divergence and low admixture among watersheds. The estimate of among‐watershed differentiation (FCT = 0.350) was large and reflected over 96% of the variation among all sampled aggregations (FST = 0.363). However, evidence of admixed individuals in two adjacent watersheds and five first‐generation migrants among five watersheds suggests that the potential for coastal migration with gene flow exists in these populations. The data also supported the hypothesis that aggregations formed within watersheds during the spawning period (May–June) represent reproductively isolated populations. Pairwise estimates of FST and the G‐test results revealed population structure in four of the six watersheds tested. However, not all aggregation pairs were found to be genetically distinct and there was notable variation in the pairwise FST estimates (0.000–0.067). In summary, the data reflected the predicted results for each hypothesis but also revealed exceptions that, consistent with tagging studies, demonstrate the complexity of migratory behavior in southwestern Alaska Rainbow Trout. We discuss the implications of these results for fishery management and conservation.

American mink (Neovison vison) preying on hatchery and wild brown trout (Salmo trutta) juveniles in semi‐natural streams

AbstractPredator–prey interactions are one of the main ecological factors influencing the structure of fish communities. The impact of wading and diving semi‐aquatic predators on riverine fish populations is poorly known. We studied the effect of feral American mink predation on brown trout juveniles during winter in two experiments conducted in semi‐natural streams (length 26 m, width 1.5 m).In the first experiment, we compared the vulnerability of age‐1+ hatchery (length 142 ± 16 mm, average ±SD) and wild (112 ± 8 mm) brown trout of similar genetic origin in sympatry and allopatry. In the second experiment we used age‐0+ brown trout (79 ± 5 mm), increased habitat heterogeneity by addition of fine wood, and compared those to treatments without fine wood addition.Hatchery fish were more vulnerable to mink predation than their wild counterpart, and the predation rate increased with increasing body size among the hatchery trout. Predation by mink on wild trout was higher in sympatric than in allopatric treatments suggesting that stocking of hatchery fish may increase predation on wild conspecifics. Increased habitat heterogeneity resulted in reduced predation rate.The results show that a large size of hatchery fish in small streams was a negative trait, which was opposite to the mainstream observations of salmonid stockings made directly of feeding areas in lakes and oceans. Adding habitat heterogeneity was found important for habitat enhancements in streams with mammalian predators.We highlight the importance of taking all the habitats during the life cycle of migratory fish into account in management decisions and carefully considering whether using hatchery fish to support wild populations in streams.

Different antibiotic profiles in wild and farmed Chilean salmonids. Which is the main source for antibiotic in fish?

Fish from both aquaculture and wild capture are exposed to veterinary and medicinal antibiotics (ABs). This study explored the occurrence and probable source of 46 antibiotic residues in muscle of farmed salmon and wild trout from Chile. Results showed that at least one AB was detected in all studied samples. Diverse patterns were observed between farmed and wild specimens, with higher ABs concentrations in wild fish. Considering antimicrobial resistance, detected ABs corresponded to the categories B (Restrict), C (Caution) and D (Prudence) established by Antimicrobial Advice Ad Hoc Expert Group (European Medicines Agency). Multivariate statistic was used to verify differences between farmed and wild populations, looking for the probable source of ABs as well. Principal components analysis (PCA) revealed that ciprofloxacin, moxifloxacin, enrofloxacin, amoxicillin, penicillin G, oxolinic acid, sulfamethoxazole, trimethoprim and clarithromycin were associated with wild samples, collected during the cold season. Conversely, norfloxacin, sulfaquinoxaline, sulfadimethoxine, nitrofurantoin, nalidixic acid, penicillin V, doxycycline, flumequine, oxacillin, pipemidic acid and sulfamethizole were associated with wild samples collected during the warm season. All farmed salmon samples were associated with ofloxacin, tetracycline, cephalexin, erythromycin, azithromycin, roxithromycin, sulfabenzamide, sulfamethazine, sulfapyridine, sulfisomidin, and sulfaguanidine. In addition, linear discriminant analysis showed that the AB profile in wild fish differ from farmed ones. Most samples showed ABs levels below the EU regulatory limit for edible fish, except for sulfaquinoxaline in one sample. Additionally, nitrofurantoin (banned in EU) was detected in one aquaculture sample. The differences observed between farmed and wild fish raise questions on the probable source of ABs, either aquaculture or urban anthropic activities. Further research is necessary for linking the ABs profile in wild fish with the anthropic source. However, to our knowledge, this is the first report showing differences in the ABs profile between wild and aquaculture salmonids, which could have both environmental and health consequences.