Native Salmonids Research Articles

Food habits of nonnative Walleyes in Lake Pend Oreille, Idaho

AbstractObjectiveWalleye Sander vitreus is an important species that has been widely introduced outside of its native distribution. The goal of this study was to assess the effects of an established Walleye population in the Lake Pend Oreille (LPO) system, Idaho.MethodsFood habits of Walleyes were described using stomach contents and stable isotopes (δ15N, δ13C). Trophic structure of the LPO system's food web was identified using stable isotopes. Annual consumption by Walleyes of important prey items was estimated using a bioenergetics model.ResultWalleyes consumed a diversity of prey items, including macroinvertebrates and fishes. Kokanee Oncorhynchus nerka, the most frequently consumed prey item, occurred in 23% of all Walleye diets. Combined, native cyprinids and catostomids occurred in 31% of all Walleye stomachs. Select taxa (e.g., native cyprinids, kokanee) were consistently consumed by Walleyes across seasons, regions, and cohorts, whereas other taxa (e.g., Westslope Cutthroat Trout O. lewisi, Smallmouth Bass Micropterus dolomieu) were consumed inconsistently. Stable isotope analysis suggested that Walleyes occupied similar trophic positions as other top‐level piscivores in the system. As Walleye age increased, δ15N increased and δ13C decreased, indicating increased consumption of pelagic prey resources and prey at higher trophic positions. The estimated biomass of kokanee consumed annually by Walleyes was 27,121 kg (95% confidence interval = 9178–61,603). Comparatively, native cyprinids represented about 46% of the total biomass of kokanee consumed by Walleyes, whereas native catostomids represented about 11% and native salmonids represented about 15% of the total biomass of kokanee consumed by Walleyes.ConclusionThis study revealed that Walleyes consumed various fishes across the LPO system. Although kokanee were the most frequently consumed prey item, native cyprinids and catostomids (combined) occurred at similar proportions. This study contributes to our growing knowledge of the effects of nonnative Walleyes on important salmonids and native fishes in western systems.

Health Monitoring of Invasive Pink Salmon Oncorhynchus gorbuscha in Norway in 2023

ABSTRACTSince 2019, the Norwegian Veterinary Institute (NVI) has conducted health monitoring of pink salmon in Norway. The primary purpose of health monitoring has been to examine the presence of selected notifiable infections in pink salmon, but also to gain general knowledge regarding the health of pink salmon to inform assessments of risks to wild and farmed native salmonids. In 2023, post‐mortem examinations were performed on 184 pink salmon captured in rivers Lakselv, Tana, Neiden, Komagelva, Kongsfjordelva and Karpelv in the County of Finnmark, Norway. Tissue samples from 161 of these were analysed with specific PCR assays aimed at detecting infectious salmon anaemia virus (ISAV), viral haemorrhagic septicaemia virus (VHSV), infectious hematopoietic necrosis virus (IHNV), piscine orthoreovirus genotype 1 (PRV‐1) and Renibacterium salmoninarum. In addition, cultivation methods were applied to detect the presence of cultivable virus and bacteria. ISAV, VHSV, IHNV PRV‐1 and R. salmoninarum were not detected by PCR. Furthermore, R. salmoninarum, Aeromonas salmonicida subsp. salmonicida and cultivable virus were not detected by cultivation. With the exception of the absence of PRV‐1, the results correspond to previous years' health‐monitoring projects.

Evaluating the success of eradicating nonnative Brook Trout using Trojan Y Chromosome fish in southern Rocky Mountain streams

AbstractObjectiveBrook Trout Salvelinus fontinalis often outcompete and displace native salmonids in western North America. Due to the limitations of traditional removal methods, managers are implementing the Trojan Y Chromosome approach to extirpate nonnative Brook Trout populations. However, there are uncertainties of the comparative performance (i.e., survival, reproductive success) of YY‐male (MYY) fish and their wild counterparts. In addition, it is unclear how effective YY‐female (FYY) fish compare with MYY fish for extirpation of wild Brook Trout. To address these uncertainties, we modeled the time to extirpation of wild Brook Trout using 5 years of empirical data, assuming introductions of either MYY or FYY fish.MethodsOur simulations assumed varying stocking (0% to 100%) and removal (0% to 90%) scenarios. In addition, we evaluated model sensitivity to changes in the reproductive success and survival of MYY fish.ResultThe simulations suggest that the average time to extirpation would be 14 years using MYY fish and 8 years for FYY fish, assuming 50% annual removal and 50% initial stocking of the wild Brook Trout abundance. Increasing removal and/or stocking levels resulted in reductions in extirpation times using either MYY or FYY fish. Sensitivity analyses indicated that the model was relatively insensitive to changes in the reproductive success and survival of MYY Brook Trout.ConclusionOur results suggest that either MYY or FYY fish can be used to eradicate wild Brook Trout populations, but FYY fish are likely the most efficient means of eradicating nonnative Brook Trout in streams typical of western North America.

Survey of the invasive alien pink salmon (Oncorhynchus gorbuscha) for infective agents in the Fennoscandian Rivers Tana and Neiden

Pink salmon (Oncorhynchus gorbuscha, Walbaum 1792) is an invasive alien species in the Fennoscandian rivers, and its growing populations cause concern for their impact on the native salmonids and river ecosystems. One of the major concerns is fish diseases and the potential ability of pink salmon to transmit pathogens to the native salmonid species. In this study, pink salmon were sampled during their spawning migration in the Rivers Tana and Neiden in northern Norway and Finland in 2021. The fish were surveyed for viral hemorrhagic septicemia virus (VHSV), infectious pancreatic necrosis virus (IPNV), infectious salmon anemia virus (ISAV), salmonid alphavirus (SAV), piscine orthoreovirus (PRV), Renibacterium salmoninarum, and Gyrodactylus salaris. No viral diseases or infections with R. salmoninarum or G. salaris were detected. However, 23 % of the fish were found infected with a parasitic roundworm, Hysterothylacium aduncum. Additionally, external anomalies, such as hemorrhages or lesions in the skin, were observed in 35 % of the fish studied, and some of the fish exhibited degenerative myopathy. Based on this study, pink salmon appears not to pose a high infection risk to the native salmonid fish populations in the Rivers Tana and Neiden. Nevertheless, the results demonstrate that pink salmon have the potential to carry salmonid pathogens. More information on the susceptibility of pink salmon to different pathogens and on the overall effect this invasive species has on native fish species is needed. As the number of migrating pink salmon in the Fennoscandian rivers appears to increase, the potential risk of parasites and other pathogens spreading between fish species will also rise.

A review of the hybridisation potential of brown trout with particular reference to invaded environments, and a case study from Newfoundland

AbstractHybridisation is a complex process that begins with mating. Females have more to lose with each hybrid fertilisation than males, so they should avoid it. Even if females choose con‐specific males as preferred mates, they often cannot control which additional males release sperm during spawning. Polyandry is ubiquitous and may result in hetero‐specific sperm competition between males of different species. In such cases, cryptic female choice (the ability to bias paternity towards certain males under sperm competition) is the last line of defence to prevent hybridisation of her eggs and is highly adaptive if it enables con‐specific sperm preference. Such seems to be the case with the hybridisation of Atlantic salmon (Salmo salar) and brown trout (S. trutta) in their native Europe. Under hetero‐specific sperm competition, hybrid fertilisations in these fish are reported to be reduced via ovarian fluid‐mediated cryptic female choice. It is not known, however, whether the strength of this mechanism is dependent on reinforcement and thus, the historical sympatry/allopatry of hybridising populations. Brown trout are one of the world's worst invasive species. Ecological impacts arise through competition with other species (e.g. Galaxids in the southern hemisphere, Oncorhynchus in western North America). Eastern North America contains native salmonids that evolved in the absence of brown trout but have gametes that are compatible. The 140 ‐year‐old brown trout invasion of Newfoundland is ground zero to study these potential interactions. Their relatively low spread rate across the island may be the result of inherent poor productivity, but data suggest it could also be a function of hybridisation with native Atlantic salmon and brook char (Salvelinus fontinalis).

Change in growth and prey utilization for a native salmonid following invasion by an omnivorous minnow in an oligotrophic reservoir

Change in growth and prey utilization for a native salmonid following invasion by an omnivorous minnow in an oligotrophic reservoir

Landscape influences on thermal sensitivity and predicted spatial variability among brook trout streams in the southeastern USA

AbstractWarming water temperatures as a result of climate change pose a major threat to coldwater organisms. However, the rate of warming is not spatially uniform due to surface‐ground‐water interactions and stream and watershed characteristics. Coldwater habitats that are most resistant to warming serve as thermal refugia and identifying their locations is critical to regional aquatic conservation planning. We quantified the thermal sensitivity of 203 streams providing current and potential habitat for brook trout (Salvelinus fontinalis) across nearly 1000 linear km of their native range in the southern and central Appalachian Mountains region, USA, and characterized their spatial variability with landscape variables available in the National Hydrography Dataset. Using the Bayesian framework, we calculated the maximum slope of the logistic function relating paired weekly mean air temperature and stream temperature as an index of stream thermal sensitivity. Streams differed greatly in thermal sensitivity and those with more resistant water temperature regimes (i.e., thermal refugia) were consistently characterized by southerly latitudes and groundwater input. Landscape variables derived from a principal component analysis explained 16% of the variation in thermal sensitivity, indicating that the existing landscape variables were modestly successful in explaining spatial thermal heterogeneity. Using our model and spatial interpolation, we predicted thermal sensitivity at 8695 stream segments potentially suitable for brook trout in the study region. Thermal refugia were more common southward presumably due to higher elevations, but elsewhere they were also clustered at finer spatial scales. Our analysis informs prioritizing habitat conservation and restoration of this native salmonid and other aquatic organisms that depend on coldwater habitats in a warming world.

Population assignment of migratory Westslope Cutthroat Trout (WCT) in the Clark Fork–Pend Oreille River basin

AbstractObjectiveThe Clark Fork–Pend Oreille River basin of northeastern Washington and the Idaho Panhandle historically supported a robust metapopulation of the Westslope Cutthroat Trout (WCT) Oncorhynchus lewisi, a western native salmonid of high cultural and economic value. The construction of impassible hydroelectric dams and smaller instream barriers has prevented the return of migratory WCT to spawning tributaries, leading to the fragmentation of this metapopulation over the past 100 years. One such impassible barrier is Albeni Falls Dam (AFD) near Newport, Washington, which was completed without fish passage capabilities in 1955. We sought to examine large‐scale genetic patterns in the study area and determine the most likely spawning tributary of origin for migratory WCT captured below AFD.MethodsWe created a genetic baseline representative of populations within the Clark Fork–Pend Oreille River basin from upstream and downstream of the dam using 191 biallelic single‐nucleotide polymorphism genetic markers. Our data set included 124 collections, which allowed for an examination of population structure and hatchery influence across the study area and provided a robust tool for population assignment. Population assignment tests were conducted using the program RUBIAS.ResultPopulation assignment tests were successful for all pure WCT of unknown origin despite potential influence from hatchery lineages across the study area. Of 83 migratory WCT captured below AFD, approximately 80% were assigned to tributaries upstream of AFD with a posterior assignment probability of at least 90%. Only one fish was assigned to a tributary downstream of AFD.ConclusionOur results indicate that AFD disrupts the natural metapopulation dynamics of WCT populations in the basin. Passage for WCT at this barrier would reestablish metapopulation connectivity within the basin by allowing migratory individuals to make genetic contributions to populations upstream of the dam.

Indications of reproductive disturbance by the invasive round goby on a native salmonid

Invasive species have a wide array of effects on species in their introduced ranges, including sub-lethal effects such as disruption of courtship or parental care, with potential negative influence on fitness of the native species. The invasive round goby (Neogobius melanostomus) is a door-knocker species to freshwater rivers and streams connected to the Baltic Sea. In the case of establishment of round goby in freshwater it may overlap spatially with native salmonids like Baltic Sea Atlantic salmon and sea trout, and we therefore experimentally studied interactions between round goby and salmon during salmon spawning. We found that salmon spawning behaviour was unaffected by the presence of round goby. However, salmon displayed aggression towards the round goby and the salmon male and female resided closer to each other in the presence of round goby at higher round goby densities, which we interpret to be a protective behaviour. In addition, salmon spawning was delayed in the presence of round goby. Altogether, our results imply that the reproductive success of vulnerable Baltic salmonids may be impaired under a scenario where round goby migrates upstream and establishes in Baltic rivers and streams. Consequently, we see the need for management actions to hinder spread and subsequent establishment of round goby in freshwaters along the Baltic coastline.

Shift in piscivory by salmonids following invasion of a minnow in an oligotrophic reservoir

AbstractPredation can play an important role in structuring ecological communities, and predator–prey dynamics can be altered following the introduction of new species. An unauthorized introduction of redside shiner (Richardsonius balteatus) into reservoirs in the Upper Skagit River, Washington, USA created concern that a consequent shift in predator–prey dynamics in the reservoirs could reduce recruitment and production of native salmonids in the basin. We estimated predation mortality in Ross Lake on nonnative redside shiner and juvenile native salmonids to evaluate the potential role of predation in regulating these populations and limiting survival of native species of concern. We used bioenergetics modelling and stable isotope analysis combined with directed field measurements of growth, seasonal diet and thermal experience of piscivorous salmonids to quantify their consumption demand on prey fishes to evaluate the relative magnitude of predation mortality on invasive redside shiners and native salmonids. While redside shiner are the dominant prey fish species in Ross Lake, the modest biomass of native salmonids consumed could translate into substantial mortality, the magnitude of which depended on the timing and size at which prey fishes were eaten. This information provides important context for how nonnative species may indirectly impact native species through shared predation (apparent competition) and can inform conservation decisions surrounding nonnative species control, sustainability of native salmonids and introductions of anadromous fishes.

Population genetics, demography and conservation of Mediterranean brown trout from Sardinia

Abstract Brown trout is a species complex (Salmo trutta complex, L., 1758) including both widespread invasive (non‐native hatchery strains) lineages and endangered local‐endemic lineages, among which is the Sardinian trout, the only native salmonid present in Sardinia. Multiple stressors (e.g. the spread of stocked brown trout of Atlantic origin, habitat alteration and climate change) combine to seriously threaten the persistence of wild native populations. In this study, the origin, population genetics and demography of wild Sardinian brown trout populations were extensively investigated. A total of 274 trout individuals collected from 12 hydro‐geographical basins were analysed using both mitochondrial (control region) and nuclear (LDH‐C1* locus and 10 microsatellites) markers. Although stocking activities have altered the native genetic makeup of some populations in the study area, several (almost) uncontaminated populations showing strong genetic structure were detected. Eroded intra‐population diversity, as well as small effective population size, sometimes associated with a bottleneck signal was also found. The genetic characteristics of Sardinian trout populations described in this study are probably due, at least partly, to the peculiarity of local environmental conditions at the margin of the ecological niche for salmonids. Based on the results of this study, the need for urgent measures of conservation aimed to ensure the near future viability of the last wild Sardinian trout populations was discussed.

Active feeding of downstream migrating juvenile pink salmon (Oncorhynchus gorbuscha) revealed in a large Barents Sea river using diet and stable isotope analysis.

The recent, rapid spreading of non-native pink salmon Oncorhynchus gorbuscha in the North Atlantic area has raised concerns about their possible negative impacts on native salmonid species. Potential interactions include competition for food resources during the short freshwater phase of juvenile O. gorbuscha, but little is known about their feeding behavior in the newly occupied North Atlantic rivers. Using stable isotope and stomach content analyses, patterns of freshwater feeding of non-native O. gorbuscha fry were studied in a large Fennoscandian river, the Teno, that discharges to the Barents Sea. Changes in stable isotope values (δ13 C, δ15 N, δ34 S) and stomach contents from the period of emergence (April to mid-May) to estuarine entry (late May/June) were examined and provided both temporally integrated and short-term indicators of freshwater feeding dependency. In addition, the occurrence of juvenile O. gorbuscha and changes in their length and weight during their emergence/migration period were investigated. Juvenile O. gorbuscha were at the spawning grounds from April through to mid-May with abundance peaking in mid-May. Fish moved to the estuary by late May and their abundance decreased toward June, and their body size increased concurrently. Stomach analyses indicated no feeding activity in April-early May in the spawning areas, but the stomach fullness indices increased markedly in fish sampled in the estuary in May and June. The most important prey items in stomachs were Chironomidae and Ephemeroptera larvae. Significant changes in all analysed stable isotopes were detected among sample periods, with a peak in mid-May and June showing significantly lower values than other sample periods. A change from the higher values reflective of parental marine feeding to the lower values reflective of freshwater feeding indicated active in-river feeding by juveniles during the study period. The documented active freshwater feeding of non-native juvenile O. gorbuscha suggests potential resource competition with native fluvial fishes, particularly salmonids.

The Evolution of Trout Stream Management in the Black Hills, 1883–2023, as Evidenced Through Hatchery Activities and Stocking

In this paper, we review and synthesize trout management philosophy and policies for Black Hills streams in the states of South Dakota and Wyoming through the roles of hatcheries, fish stocking, and natural production from 1883 to 2023. We identified three specific fisheries management time periods (eras) in the Black Hills: (1) the early management–fry stocking era (pre‐1950), (2) the catchable‐trout stocking era (1950s to mid‐1990s), and (3) the integrated management era (mid‐1990s to present). The first era emphasized juvenile trout stocking, with little evaluation of effectiveness in terms of return to creel. The second era was characterized by greater emphasis on stocking catchable‐sized trout and a substantial increase in hatchery rearing capabilities. In the first two eras, instead of fish stocking being one of many potentially useful tools for managers of Black Hills streams, stocking was the dominant activity—leading and directing fisheries management rather than supporting it. By the third era, trout management began to embody a more encompassing and complex national ideology and philosophy that more fully considered ecology, including habitat improvement, natural reproduction of trout, defining the role of fish stocking, and understanding the relationship of trout to the depauperate native (nongame) fish fauna. We also discuss the three eras in the Black Hills in relation to national trends in trout management. In contrast to many other localities, the lack of any native salmonids and the lack of native game fish species have facilitated the development of valuable, self‐sustaining recreational fisheries in streams that support trout, with less concern for native species than in many areas with harvestable native fish species. The cultural ideology that brought western expansion of settlers to the Black Hills in search of gold, land, freedom, and prosperity was emboldened by a science‐based belief and confidence in technology and industry to rapidly shape lands and waters to improve the human condition. In part, the sculpting and shaping of Mount Rushmore symbolize that ideology. In the context of fisheries management in the Black Hills, the introduction of trout, development of stream fisheries, and investment in hatchery technology to produce catchable trout for an expanding tourist economy also exemplify that prevailing ideology, modulated by improved ecological understanding and an accompanying shift toward a more ecological management philosophy.

Smallmouth bass (Micropterus dolomieu) suppress Atlantic salmon (Salmo salar) feeding activity and increase aggressive behaviours at warmer temperatures

AbstractBy 2050, mean temperature in the state of Maine, located in the Northeastern USA, is expected to increase nearly 1°C, which could directly affect native coldwater salmonid behaviour and increase competition with warmwater smallmouth bass. We conducted a microcosm experiment to examine the feeding and agonistic behaviour of endangered juvenile Atlantic Salmon (Salmo salar) at two temperatures (18 and 21°C) in the presence and absence of non‐native Smallmouth Bass (Micropterus dolomieu). By visually reviewing footage of fish competition in our tanks, we quantified feeding and agonistic interactions. We predicted salmon would exhibit lower feeding activity than bass at 21°C and antagonistic interactions between the two species would increase with warming. We found salmon feeding activity was reduced by smallmouth bass presence and this effect was stronger at 21°C. We also found smallmouth bass aggression was strongest at 21°C when salmon were present. Lastly, feeding activity and aggression in both species changed with food availability. These findings illustrate the potential for invasive warmwater species to outcompete native salmonids for resources, especially under the warmer conditions predicted by climate change scenarios.

Colonization of a temperate river by mobile fish following habitat reconnection

AbstractMobile species are particularly affected by artificial barriers requiring large investments to restore connectivity. However, few large‐scale, long‐term studies have investigated the ecological outcomes of restoring connectivity for these species. Our study, spanning 15–20 years, quantified response trajectories, which represent temporal trends following disturbance, of three native salmonids colonizing 20 km of protected habitat following restoration of fish passage at Landsburg Dam, Cedar River, WA, in 2003. Built in 1901, the dam blocked the upriver movement of native anadromous coho and Chinook salmon and nonanadromous mountain whitefish for 102 years. Restoration effectiveness was also assessed by comparing temporal trends in freshwater productivity of juvenile coho and Chinook salmon in the Cedar River after restoration to a nearby undammed subbasin. We also compared summer densities of juvenile coho and Chinook salmon, and mountain whitefish above the dam measured a decade after restoration to undammed reference systems. Anadromous salmon and nonanadromous mountain whitefish populations increased linearly or nonlinearly following restoration. The positive, asymptotic response represented by adult Chinook salmon counts indicates a slowing in population recovery rate, plateauing a decade after restoration. In contrast, annual abundance of adult coho salmon increased at a constant rate, indicating additional capacity 15 years post‐restoration. Salmonid compositional diversity, driven largely by juvenile coho salmon, also increased nonlinearly, plateauing in a decade. We observed substantial spatial variation in the temporal response, as juvenile coho salmon and mountain whitefish population expansion slowed linearly with upstream distance from the restoration site. There was evidence that some of the annual variation in salmonid biomass in summer was a result of discharge variability in winter and spring, with biomass declining as flow variability increased. Species reintroduction and establishment had no discernible effect on stream‐rearing salmonids living above the dam before restoration, while increasing coho freshwater productivity at the subbasin scale. Results from our study showed recolonization by three mobile species, each with a unique life history, takes at least a decade or more and was dependent on species and life stage, size of the spawning population, distance from restoration site, and annual variability in streamflow.

The role of semen cryobanks for protecting endangered native salmonids: Advantages and perspectives as outlined by the LIFE Nat.Sal.Mo. project on Mediterranean brown trout (Molise region – Italy)

The Mediterranean brown trout is one of the most endangered freshwater species. A complicated network of climate and human influences has severely harmed its biodiversity. The introduction of alien trout is one of the most serious threats to native populations’ intraspecific diversity. In Molise region (south-Italy) an important conservation program (LIFE Nat.Sal.Mo project) has recently been proposed to preserve the genetic integrity of native Mediterranean trout. This project, alongside safeguarding and re-establishing the habitats’ usefulness aims to restore the genetic integrity of the autochthonous population. This is one of the major goals, and it is accomplished by employing frozen wild breeder semen in conjunction with proper fertilization techniques to carry out artificial reproduction to enhance genetic diversity in the progeny and maintain fitness within self-sustaining populations. In this regard, the implementation of the first European semen cryobank has played a strategic role for conserving extant genomic diversity of native population. The goal of this review is to outline the procedures developed and guidelines established for the creation of a Mediterranean trout sperm cryobank. Here, we specifically provide an overview of some of the main challenges associated with the implementation of semen cryobank, the results achieved, the prospects for restoring genetic integrity in native populations, and lastly, future views for hatchery management to preserve the wild biodiversity of native salmonid species. During the project timeframe 1,683 semen doses, from 150 native breeders were stored inside the cryobank. Our results clearly showed the efficiency of the freezing procedure used, both in vitro and in vivo. In fact, we recorded satisfactory values of post-thaw sperm motility and viability that ranged from 40% to 80%, and excellent fertilization rate in vivo, which ranged from 64% to 81%.

Streamwide Evaluation of Survival and Reproduction of MYY and Wild Brook Trout Populations

AbstractBrook Trout Salvelinus fontinalis have been introduced across the western USA, where the species competes with and often replaces native salmonids. Nonnative Brook Trout are difficult to eradicate; thus, new removal strategies are needed. One novel methodology couples the partial suppression of wild Brook Trout with the replacement of MYY Brook Trout (males with two Y chromosomes). If MYY fish survive to reproduce with wild female Brook Trout, their progeny will be 100% male, which eventually shifts the sex ratio and theoretically extirpates the population. However, the effectiveness of this approach depends on survival and reproduction of MYY fish relative to the surviving wild conspecifics. From 2018 to 2020, we annually removed an estimated 45.7% of wild Brook Trout from three streams in New Mexico and stocked fingerling MYY Brook Trout (mean TL = 94 mm; range = 61–123 mm) targeting 50.0% of wild annual abundance estimates. Annual survival for MYY and wild Brook Trout was similar in Leandro Creek (MYY = 0.63 and wild = 0.63) and Rito de los Piños (MYY = 0.37 and wild = 0.46) but differed in Placer Creek (MYY = 0.28 and wild = 0.75). During spawning, we evaluated the reproductive potential of MYY Brook Trout by comparing the percentage of sexually mature male Brook Trout comprised of MYY fish to the percentage of hybrid (MYY × wild) F1 progeny. By the second spawning season (2019), MYY fish comprised 59.8, 50.4, and 34.5% of milt‐producing Brook Trout, which resulted in 55.1, 33.3, and 0% hybrid progeny in Leandro Creek, Rito de los Piños, and Placer Creek, respectively. We demonstrated that MYY fish exhibit similar vital rates compared with wild conspecifics in two of three streams; however, differences among streams highlights unforeseen variables that influence MYY survival and reproduction. The study offers promising results of the MYY approach for potentially eradicating unwanted Brook Trout populations.

Population Characteristics of Brook Trout in Idaho Streams and Alpine Lakes

In western North America, nonnative brook trout (Salvelinus fontinalis) frequently threaten native salmonids via competition and hybridization, so fisheries managers often implement eradication programs for conservation purposes. In conjunction with such programs, managers often construct population models to evaluate the effects of different management strategies designed to control the undesirable population, but such models require demographic data (e.g., age, growth, sex ratios, and survival), which are lacking for western brook trout populations. Brook trout were sampled from 12 alpine lakes and two streams in Idaho, with total length varying from 80 to 380 mm and age varying from 1 to 11 yrs. Across all waters, the von Bertalanffy growth parameters L∞ varied from 231 to 490 mm (mean = 345 mm) and K varied from 0.15 to 0.76 (mean = 0.37). Survival estimates, constructed from age-length keys, were corrected for streams with mark-recapture data; for alpine lakes, corrections were made via gill net selectivity data. Survival varied from 0.30 to 0.56 (mean = 0.45), and except for one waterbody, estimates were minimally affected by correcting for capture efficiency. The proportion of the population that was male varied from 0.34 to 0.75 (mean = 0.53). Our results indicate that brook trout population vital rates in Idaho were similar to those observed in their native range, and were surprisingly similar between alpine lake and stream environments.

Loss of riparian forests from wildfire led to increased stream temperatures in summer, yet salmonid fish persisted

AbstractNative salmonid fishes—cutthroat trout (Oncorhynchus clarkii) and steelhead/rainbow trout (Oncorhynchus mykiss)—are ecologically, culturally, and economically important species distributed across western North America. These fish are generally considered “cold‐water” adapted species. As such, recent studies have speculated about the potential effects of climate change on these native salmonids if stream temperature thresholds exceed 16–20°C during the summer. However, the magnitude of stream thermal responses to the slow but steady increases in regional temperatures associated with climate change remains uncertain and hard to predict. Comparatively, abrupt disturbances, such as wildfire, may produce almost instantaneous and substantial increases in stream temperatures that may persist for multiple years until near‐stream vegetation becomes re‐established. In the first summer following a severe wildfire in western Oregon, we observed the initial persistence of populations of O. clarkii clarkii (coastal cutthroat trout) and O. mykiss (rainbow/steelhead trout). The fire burned the entire catchment, including the riparian area (~76% of the watershed area burned at moderate or high severity), resulting in stream temperature that regularly exceeded 20°C and represented increases of 6–7°C relative to prefire conditions. However, the mechanisms enabling the persistence of cold‐water fishes despite the dramatic increases in stream temperature remain unclear and require further investigation. Nevertheless, wildfires represent acute natural disturbances that can substantially alter stream thermal regimes and provide unique insights that allow us to better understand how native fishes in natural systems cope with projected increases in stream temperatures.

Increasing dominance of non-native fishes in the yield of central European streams and rivers

Introductions of non-native fishes are among the biggest threats to freshwater ecosystems globally. However, the effects of such introductions on fisheries management and fish yields are understudied. This study is trying to fill this knowledge gap by analyzing relationships between yields of native and non-native fishes with regards to important fisheries management factors, such as fish stocking, fishing effort, and the type or size of the fishery. I analyzed the changes in fish yields in the regions of Prague and central Bohemia from 1986 to 2017. Based on the 32 years of reported data that included 60 million harvested fish weighing 80 tons that were harvested by 250,000 anglers, I conclude that while predominately native fish species were harvested by anglers in the 1980, non-native fish species started dominating in the 21st century. This was true on both small trout streams (1–m wide) and medium-sized cyprinid rivers (5–250 m wide). This dominance is partially explained by the decreasing percentage of native salmonids and cyprinids and by the decreasing or stagnating percentage of native piscivores in the overall fish yield. Conversely, the percentage of non-native salmonids in the overall fish yield was increasing on both streams and rivers. Most importantly though, the percentage of non-native cyprinids – which made 60–80 % of all harvested fish – increased by 16 % over 32 years. Fish yield had a strong positive relationship with fish stocking intensity, a strong negative relationship with angling effort, and strong but mixed relationships with size of the fishery. In conclusion, native fishes are being slowly replaced by non-native fishes in the yield of recreational anglers in central Europe.