Introduction Of Trout Research Articles

‘A Most Excellent Thing: Ecological Imperialism and the Introduction of Trout to Canterbury, New Zealand’

History CompassVolume 13, Issue 4 p. 159-170 Article ‘A Most Excellent Thing: Ecological Imperialism and the Introduction of Trout to Canterbury, New Zealand’ Jack Kós, Corresponding Author Jack Kós History and Art History, University of OtagoCorrespondence: History and Art History, University of Otago. Email: [email protected].Search for more papers by this author Jack Kós, Corresponding Author Jack Kós History and Art History, University of OtagoCorrespondence: History and Art History, University of Otago. Email: [email protected].Search for more papers by this author First published: 22 April 2015 https://doi.org/10.1111/hic3.12230Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Volume13, Issue4April 2015Pages 159-170 RelatedInformation

Population genetic SNP analysis of marble and brown trout in a hybridization zone of the Adriatic watershed in Slovenia

Marble trout (Salmo marmoratus) is an extensively managed salmonid taxon threatened by historical introductions of non-native brown trout (Salmo trutta) of different origins into habitats in the northern part of its distribution, namely the Po river system and the Slovenian part of the Adriatic watershed. Hybridization between marble trout and brown trout results in fertile offspring, creating an extensive zone of genetic mixing, with varying amount of introgression. A rehabilitation programme for marble trout has been undertaken in Slovenia for two decades, dependent upon eight non-introgressed marble trout populations inhabiting isolated streams in the upper Soča river valley. MtDNA and a new marker system based on nuclear DNA, designed for identification of trouts and their hybrids in the genus Salmo, were used to determine the extent of introgressive hybridization between marble and brown trout across a large area of the Adriatic watershed in Slovenia. Individual admixture coefficients, the posterior probability of each individual belonging to one or other of the parental species and user-defined categories of admixture were determined along with basic population genetic parameters, revealing variable intensity and patterns of individual introgression throughout the zone of hybridization. In most of the populations analysed, hybridization of native marble trout with the introduced Atlantic lineage of brown trout was observed. In the upper Soča river system a high proportion of genetically non-introgressed (pure) marble trout was found, enabling selection of these fish for supplementary material in the reintroduction programme. Conclusions regarding management of marble trout in Slovenia can be inferred from the patterns observed in the present study: (1) transfer of pure populations to fishless or fished-out streams is recommended, (2) to sustain genetic diversity of the species, non-introgressed individuals sorted out from the zone of hybridization should be used to produce juveniles for supplementation stocking in fish farms, and (3) extension of the marble trout repopulation programme to highly introgressed populations in small unmanaged streams, which represent a potential source for downstream genetic ‘contamination’.

The Political Economy of South African Trout Fisheries

:I analyze the evolution of trout recreational fisheries with the objective of identifying possible factors that might be driving current controversies in biodiversity policy reforms on the governance of alien and invasive species. Findings suggest that trout species have attained a cultural status, which makes it difficult to have them eradicated without facing stiff resistance from the trout-dependent sector. Results suggested that a process of leisure augmentation through environmental greed underpinned the introduction and continuance of spreading trout. With this process came also the development of a complex set of institutions that protected trout since they served an honorific role. My findings also suggest that the Ayresian thesis that all ceremonial systems are past-binding could be relaxed because ceremonial interests also envision alternative futures that can entrench and further protect ceremonial systems. To that end, these ceremonial interests facilitate path-breaking, but harmful institutional change. I extend Paul Dale Bush's concept of institutional spaces by assigning faces to the concept, resulting in expansion of possible institutional adjustment configurations. I find that this reorganization of Bush's concept tells the historical evolution of institutions of the leisure class in South Africa in a more dynamic way.

Potential population and assemblage influences of non‐native trout on native nongame fish in Nebraska headwater streams

AbstractNon‐native trout are currently stocked to support recreational fisheries in headwater streams throughout Nebraska. The influence of non‐native trout introductions on native fish populations and their role in structuring fish assemblages in these systems is unknown. The objectives of this study were to determine (i) if the size structure or relative abundance of native fish differs in the presence and absence of non‐native trout, (ii) if native fish‐assemblage structure differs in the presence and absence of non‐native trout and (iii) if native fish‐assemblage structure differs across a gradient in abundances of non‐native trout. Longnose dace Rhinichthys cataractae were larger in the presence of brown trout Salmo trutta and smaller in the presence of rainbow trout Oncorhynchus mykiss compared to sites without trout. There was also a greater proportion of larger white suckers Catostomus commersonii in the presence of brown trout. Creek chub Semotilus atromaculatus and fathead minnow Pimephales promelas size structures were similar in the presence and absence of trout. Relative abundances of longnose dace, white sucker, creek chub and fathead minnow were similar in the presence and absence of trout, but there was greater distinction in native fish‐assemblage structure between sites with trout compared to sites without trout as trout abundances increased. These results suggest increased risk to native fish assemblages in sites with high abundances of trout. However, more research is needed to determine the role of non‐native trout in structuring native fish assemblages in streams, and the mechanisms through which introduced trout may influence native fish populations.

Revised Predictive Model for Successful Introduction of Native Ohio Brook Trout (Salvelinus fontinalis) in Select Streams in Geauga and Lake Counties, Ohio a

Eight cold-water streams in Lake and Geauga Counties, Ohio were evaluated to develop a revised predictive model to assess future streams for the introduction of threatened native Ohio Brook Trout (Salvelinus fontinalis). A 15-month study was conducted, ending May 2010, in streams where Brook Trout were previously introduced during the years 1997-2002. Fifty percent of the sixteen original reintroduction streams failed to support the establishment of self-sustaining populations of Brook Trout, indicating there were additional factors contributing to their success and failure. Of the 8 streams included in this study, 4 streams were designated successful, 2 variable, and 2 failed, in terms of the Brook Trout’s ability to establish self-sustaining populations by the Ohio Department of Natural Resources (ODNR). Multivariate statistical methods including Principal Component Analysis (PCA) and Agglomerative Hierarchical Cluster Analysis (AHCA) were implemented to determine the most important characteristics in Brook Trout stream selection. Factors found to be significant for Brook Trout success were stream velocities within the range of 1.4-4.7 cm/s, high hydraulic conductivity of the headwater bedrock aquifer (K>4.7 x 103 cm/s), lighter average δ18O ‰ (-9.8 to -10.4), either high percent canopy cover (40%-55%) or high percent instream cover (18%-37%), a high number of cold-water adapted benthic macroinvertebrate cold-water taxa (10-16 species), yearly average hyporheic water temperature of 4.6°C-17.2°C, average surface water turbidity of 7 to 31 NTU and long-term surface water temperature and dissolved oxygen monitoring over both winter and summer seasons is recommended prior to Brook Trout introduction to ensure their sustainability.

Evidence of exotic trout mediated minnow invasion in Pyrenean high mountain lakes

Although high mountain lakes are naturally fishless, there have been numerous trout introductions to such ecosystems in many areas of the world with negative ecological consequences. In recent decades other fishes, such as minnows, have been introduced to some mountain areas, including the Pyrenees. These introductions may cause further ecological problems, since minnows also occupy the top of the food chain, and are difficult to manage since such introductions occur without permission from the authorities. In this study we have analyzed the process of minnow introductions in all high mountain lakes of the southern slope of the Pyrenees to find out which particular factors best explained their present distribution and to evaluate which management measures have been most effective for stopping introductions. We found 27 % of the lakes had minnows (Phoxinus sp.) present, 52 % had trout and 47 % were fishless. Trout presence was the most significant variable explaining 27 % of deviance of minnow presence data in a generalized additive model. Recreational fishing using minnows as live bait is likely responsible for these introductions. Minnow introductions are therefore mediated by a preceding invasive species and facilitated by human activity. We also compared the number of minnow introductions in non-fishing areas of National Parks with other areas where managed fishing takes place. We found that the number of lakes with minnow introductions was increasing in all areas except those where fishing was prohibited, indicating that prohibiting fishing is an effective management practice for stopping minnow introductions.

Experimental evaluation of rainbow trout Oncorhynchus mykiss predation on longnose dace Rhinichthys cataractae

AbstractLaboratory and in‐stream enclosure experiments were used to determine whether rainbow trout Oncorhynchus mykiss influence survival of longnose dace Rhinichthys cataractae. In the laboratory, adult rainbow trout preyed on longnose dace in 42% of trials and juvenile rainbow trout did not prey on longnose dace during the first 6 h after rainbow trout introduction. Survival of longnose dace did not differ in the presence of adult rainbow trout previously exposed to active prey and those not previously exposed to active prey ( = 0.28, P = 0.60). In field enclosures, the number of longnose dace decreased at a faster rate in the presence of rainbow trout relative to controls within the first 72 h, but did not differ between moderate and high densities of rainbow trout (F2,258.9 = 3.73, P = 0.03). Additionally, longnose dace were found in 7% of rainbow trout stomachs after 72 h in enclosures. Rainbow trout acclimated to the stream for longer periods had a greater initial influence on the number of longnose dace remaining in enclosures relative to those acclimated for shorter periods regardless of rainbow trout density treatment (F4,148.5 = 2.50, P = 0.04). More research is needed to determine how predation rates will change in natural environments, under differing amounts of habitat and food resources and in the context of whole assemblages. However, if rainbow trout are introduced into the habitat of longnose dace, some predation on longnose dace is expected, even when rainbow trout have no previous experience with active prey.

Predatory impact of non-native rainbow trout on endemic fish populations in headwater streams in the Cape Floristic Region of South Africa

Non-native rainbow trout Oncorhynchus mykiss have been widely introduced in the Cape Floristic Region (CFR) at the south-western tip of Africa and may pose a serious threat to endemic freshwater fishes in the region. Quantitative information about trout impacts in the CFR is scarce but is urgently needed to guide legislation and management efforts. We used a combination of comparative and experimental approaches to evaluate impacts of introduced trout on native fish populations in headwater streams draining the upper Breede River catchment in the CFR. Fish populations were surveyed, and 19 environmental variables measured, in 24 minimally-disturbed streams, half of which had been invaded by trout. The mean densities of native Breede River redfin Pseudobarbus burchelli, Cape kurper Sandelia capensis and Cape galaxias Galaxias zebratus, were 89–97 % lower in invaded streams than in streams without trout. Furthermore, while native fish were present at all 12 sites without trout, they were not recorded at seven of the 12 invaded sites. None of the measured environmental variables differed significantly between sites with and without trout, and distance-based linear models identified trout density as the best predictor of redfin and kurper density, while galaxias density was best predicted by other environmental variables (riparian vegetation, canopy cover, substrate length, site slope). Native fish ≤40 mm in length were largely absent from invaded streams, but generally abundant in streams without trout, and a field experiment confirmed that trout selectively consume small redfin. Taken together, these findings constitute evidence that trout have depleted the abundance of CFR-endemic fishes through size-selective predation. It is recommended that managers aim to prevent new trout introductions and consider eradicating trout populations where they pose a threat to highly threatened native species.

Genetic introgression of cultured rainbow trout in the Mexican native trout complex

The Mexican native trout complex is the group of salmonids that naturally has the southernmost distribution in the world. Despite its unique status and evidence of more than 13 distinct lineages, there are only two described species (Oncorhynchus mykiss nelsoni and O. chrysogaster). These fishes are threatened by environmental and anthropogenic factors, most notably the introduction of the exotic species O. mykiss (rainbow trout) for aquaculture. Here we applied population genetics analysis in 1,017 wild and cultured trout to understand the extent of genetic introgression of rainbow trout in Mexican native trout. Present results indicate a high degree of introgression and genetic admixture among introduced rainbow trout and some populations of Mexican native trout, exposing them to loss of genetic diversity. Thus, introduction of exotic trout for aquaculture purposes must be strictly regulated or avoided and we advise the use of native trout for aquaculture.

Implications of Cutthroat Trout Declines for Breeding Ospreys and Bald Eagles at Yellowstone Lake

In Yellowstone National Park (YNP), Ospreys (Pandion haliaetus) feed primarily on cutthroat trout (Oncorhynchus clarki bouvieri) and cutthroat trout represent approximately 23% of prey consumed by Bald Eagles (Haliaeetus leucocephalus) during the breeding season (Swenson 1978, Journal of Wildlife Management 42:87–90; Swenson et al. 1986, Wildlife Monographs 95:3–46). The introduction of exotic lake trout (Salvelinus namaycush) to Yellowstone Lake during the late 1980s caused substantial declines in populations of cutthroat trout. Historically, more than half of all breeding pairs of Ospreys and Bald Eagles in YNP have nested near and foraged at Yellowstone Lake and the decline in cutthroat trout numbers may affect rates of reproduction for these two species. We studied the relationship between an index of cutthroat trout abundance and spring weather on Osprey (1987–2009) and Bald Eagle (1987–2007) reproduction. We documented steep declines in an index of cutthroat trout abundance, Osprey productivity and nesting success, and a dramatic decline in the number of Osprey breeding pairs. Bald Eagle productivity and nesting success also declined, but at a slightly slower rate than that of Ospreys, and the number of breeding pairs of Bald Eagles increased over the study period. Osprey reproduction was positively correlated with an index of cutthroat trout abundance and spring temperatures. However, the relationship between Bald Eagle reproduction and the index of cutthroat trout abundance was unclear. Our study suggested that the recovery of cutthroat trout is important to maintaining a breeding population of Ospreys at Yellowstone Lake, but may be less important for the Yellowstone Lake Bald Eagle population.En el Parque Nacional Yellowstone, Pandion haliaetus se alimenta principalmente de truchas de la especie Oncorhynchus clarki bouvieri y estas truchas representan aproximadamente el 23% de las presas consumidas por individuos de Haliaeetus leucocephalus durante la época reproductiva (Swenson 1978, Journal of Wildlife Management 42:87–90; Swenson et al. 1986, Wildlife Monographs 95:3–46). La introducción de una especie de trucha exótica (Salvelinus namaycush) en el Lago Yellowstone durante la última parte de la década de 1980 causó disminuciones substanciales en las poblaciones de O. c. bouvieri. Históricamente, más de la mitad de todas las parejas reproductivas de P. haliaetus y H. leucocephalus han anidado y forrajeado en el Lago Yellowstone y la disminución en los números de O. c. bouvieri podría afectar las tasas reproductivas de estas dos especies. Estudiamos la relación entre el índice de abundancia de O. c. bouvieri y el clima de primavera sobre la reproducción de P. haliaetus (1987–2009) y H. leucocephalus (1987–2007). Documentamos marcadas disminuciones en el índice de abundancia de O. c. bouvieri, en la productividad y en el éxito de anidación de P. haliaetus, y una dramática disminución en el número de parejas reproductivas de P. haliaetus. La productividad y el éxito de anidación de H. leucocephalus también disminuyeron, pero a una tasa un poco menor que la de P. haliaetus, y el número de parejas reproductivas de H. leucocephalus aumentó durante el periodo de estudio. La reproducción de P. haliaetus estuvo positivamente correlacionada con el índice de abundancia de O. c. bouvieri y con las temperaturas primaverales. Sin embargo, la relación entre la reproducción de H. leucocephalus y el índice de abundancia de O. c. bouvieri no fue clara. Nuestro estudio sugirió que la recuperación de O. c. bouvieri es importante para mantener una población reproductiva de P. haliaetus en el Lago Yellowstone, pero puede ser menos importante para las poblaciones de H. leucocephalus del Lago Yellowstone.

Invasion Dynamics of a Fish-Free Landscape by Brown Trout (Salmo trutta)

Metapopulation dynamics over the course of an invasion are usually difficult to grasp because they require large and reliable data collection, often unavailable. The invasion of the fish-free freshwater ecosystems of the remote sub-Antarctic Kerguelen Islands following man-made introductions of brown trout (Salmo trutta) in the 1950's is an exception to this rule. Benefiting from a full long term environmental research monitoring of the invasion, we built a Bayesian dynamic metapopulation model to analyze the invasion dynamics of 85 river systems over 51 years. The model accounted for patch size (river length and connections to lakes), alternative dispersal pathways between rivers, temporal trends in dynamics, and uncertainty in colonization date. The results show that the model correctly represents the observed pattern of invasion, especially if we assume a coastal dispersal pathway between patches. Landscape attributes such as patch size influenced the colonization function, but had no effect on propagule pressure. Independently from patch size and distance between patches, propagule pressure and colonization function were not constant through time. Propagule pressure increased over the course of colonization, whereas the colonization function decreased, conditional on propagule pressure. The resulting pattern of this antagonistic interplay is an initial rapid invasion phase followed by a strong decrease in the invasion rate. These temporal trends may be due to either adaptive processes or environmental gradients encountered along the colonization front. It was not possible to distinguish these two hypotheses. Because invasibility of Kerguelen Is. freshwater ecosystems is very high due to the lack of a pre-existing fish fauna and minimal human interference, our estimates of invasion dynamics represent a blueprint for the potential of brown trout invasiveness in pristine environments. Our conclusions shed light on the future of polar regions where, because of climate change, fish-free ecosystems become increasingly accessible to invasion by fish species.

Historical use, fishing management and lake characteristics explain the presence of non-native trout in Pyrenean lakes: Implications for conservation

High mountain lakes are naturally fishless, although many have had introductions of non-native fish species, predominantly trout. Predation on native fauna by introduced trout involves profound ecological changes. The objective of this study was to reconstruct the historical process of trout introduction in 520 high mountain lakes >0.5ha of the southern Pyrenees and quantify which particular factors either environmental or anthropogenic, best explained their present distribution and lake conservation status. The first written evidence of trout introductions dated back to 1371 AD. By 1900, trout had been introduced to 26.5% of lakes during preceding centuries. A subsequent wave of introductions began in 1960 when stocking led to trout becoming established in 52.5% of lakes. From 1900 to 1950, walking distance from nearby urban centres was the dominant factor explaining 29–60% of the variation in trout distribution, indicating that trout introductions were in lakes closer to human settlements. In contrast, with the onset of modern fish management that took place during the period 1960–2000, the most significant factors were both the management practices and lake characteristics which are likely related with the probability of survival of the fish populations. After 2000, the remaining fishless lakes were those of highest altitudes, and the shallowest and those with the lowest surface area. The recent fishing ban in protected areas of National Parks has resulted in a stabilization of the number of lakes with fish, suggesting that this is the only reliable management policy to avoid new introductions and preserve the conservation status of high mountain lakes.

Chromosome rearrangements, recombination suppression, and limited segregation distortion in hybrids between Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri) and rainbow trout (O. mykiss)

BackgroundIntrogressive hybridization is an important evolutionary process that can lead to the creation of novel genome structures and thus potentially new genetic variation for selection to act upon. On the other hand, hybridization with introduced species can threaten native species, such as cutthroat trout (Oncorhynchus clarkii) following the introduction of rainbow trout (O. mykiss). Neither the evolutionary consequences nor conservation implications of rainbow trout introgression in cutthroat trout is well understood. Therefore, we generated a genetic linkage map for rainbow-Yellowstone cutthroat trout (O. clarkii bouvieri) hybrids to evaluate genome processes that may help explain how introgression affects hybrid genome evolution.ResultsThe hybrid map closely aligned with the rainbow trout map (a cutthroat trout map does not exist), sharing all but one linkage group. This linkage group (RYHyb20) represented a fusion between an acrocentric (Omy28) and a metacentric chromosome (Omy20) in rainbow trout. Additional mapping in Yellowstone cutthroat trout indicated the two rainbow trout homologues were fused in the Yellowstone genome. Variation in the number of hybrid linkage groups (28 or 29) likely depended on a Robertsonian rearrangement polymorphism within the rainbow trout stock. Comparison between the female-merged F1 map and a female consensus rainbow trout map revealed that introgression suppressed recombination across large genomic regions in 5 hybrid linkage groups. Two of these linkage groups (RYHyb20 and RYHyb25_29) contained confirmed chromosome rearrangements between rainbow and Yellowstone cutthroat trout indicating that rearrangements may suppress recombination. The frequency of allelic and genotypic segregation distortion varied among parents and families, suggesting few incompatibilities exist between rainbow and Yellowstone cutthroat trout genomes.ConclusionsChromosome rearrangements suppressed recombination in the hybrids. This result supports several previous findings demonstrating that recombination suppression restricts gene flow between chromosomes that differ by arrangement. Conservation of synteny and map order between the hybrid and rainbow trout maps and minimal segregation distortion in the hybrids suggest rainbow and Yellowstone cutthroat trout genomes freely introgress across chromosomes with similar arrangement. Taken together, these results suggest that rearrangements impede introgression. Recombination suppression across rearrangements could enable large portions of non-recombined chromosomes to persist within admixed populations.

Complex littoral habitat influences the response of native minnows to stocked trout: evidence from whole-lake comparisons and experimental predator enclosures

Strong negative effects of introduced predatory fishes on native species are frequently reported but may not be universal. Recent research from productive lakes, for example, has documented few serious negative effects. Our objective was to determine how complex littoral habitat mediates the response of adult and young-of-year (YOY) native dace (Chrosomus spp.) and fathead minnow (Pimephales promelas) to the introduction of rainbow trout (Oncorhynchus mykiss) in productive lakes in Alberta, Canada. We first quantified inshore–offshore habitat use of native fish in stocked and unstocked lakes with heavily vegetated littoral zones. We then manipulated the presence or absence of trout and densities of macrophytes within enclosures in an unstocked lake and assessed the behavioral response of native fish. Our whole-lake comparisons revealed that adult and YOY fishes occurred in vegetated inshore areas to a greater extent in stocked relative to unstocked lakes. In the enclosure experiment, native fishes did not respond to the introduction of trout at natural macrophyte densities, but dace significantly reduced their occupation of enclosures with reduced macrophytes once trout were added. Our results suggest that complex littoral macrophyte beds provide important refuge habitat for native fishes, which can potentially mitigate negative effects associated with introductions of a piscivorous predator.

Present status of trout fisheries in jammu and kashmir

––The research paper provided detailed information about the introduction of Trout in Kashmir. In Kashmir Valley Trout was introduced in 1899 by F.J. Mitchell who bought a consignment of eyed-eggs of Brown Trout from England (Mitchell, 1918). This whole consignment however, perished in the way due to heat. In 1900 another consignment of eyed-eggs of same species was produced from Howeitonin in Scotland. The swim-up fry from these eggs were successfully reared up to the adulthood and the first spawning or eggtaking of the Brown-Trout (Salmo trutta fario), was done in December 1905 at Harwan near Srinagar (Kashmir). In the 1905-1906, Mitchell succeeded in establishing a regular Trout Hatchery at Harwan Srinagar Kashmir, having a capacity of 100,000 green eggs. Since then brown trout has established itself throughout Kashmir and parts of Jammu notably in Badarwah and Poonch providing fly and spoon fishing year after year to thousands of anglers visiting the state.It was from Kashmir that brown trout was spread to other sections of the Himalayas. In 1912, Mitchell Succeeded in hatching and rearing rainbow trout eggs from a consignment presented to him by the Bristol Water Works England. Today J&K Fisheries Department ha achieved remarkable success and at present 32 trout rearing units are functioning in various Districts of J&K state. The Fisheries Department collects huge revenue from sale outlets of trout fishes located at various

Status of brown trout (Salmo trutta fario L.) in Garhwal Himalaya with a note on it morphometric characteristics

The history of introduction of brown trout in Garhwal Himalaya is 100 years. However, the scientific information on brown trout is grossly lacking. The present study is a part of investigation on various aspects of brown trout inhabiting the River Asiganga in Uttarkashi district of Uttarakhand. The status of brown trout was ascertained in River Asiganga and other reports from elsewhere in the region. The morphometric study was based on 253 fish specimens collected from River Asiganga. In addition to the 12 body measurements of the fish, red/orange and brown spots on body were also studied.

Patterns of Hybridization of Nonnative Cutthroat Trout and Hatchery Rainbow Trout with Native Redband Trout in the Boise River, Idaho

AbstractHybridization is one of the greatest threats to native fishes. Threats from hybridization are particularly important for native trout species as stocking of nonnative trout has been widespread within the ranges of native species, thus increasing the potential for hybridization. While many studies have documented hybridization between native cutthroat trout Oncorhynchus clarkii and nonnative rainbow trout O. mykiss, fewer have focused on this issue in native rainbow trout despite widespread threats from introductions of both nonnative cutthroat trout and hatchery rainbow trout. Here, we describe the current genetic (i.e., hybridization) status of native redband trout O. mykiss gairdneri populations in the upper Boise River, Idaho. Interspecific hybridization was widespread (detected at 14 of the 41 sampled locations), but high levels of hybridization between nonnative cutthroat trout and redband trout were detected in only a few streams. Intraspecific hybridization was considerably more widespread (almost 40% of sampled locations), and several local populations of native redband trout have been almost completely replaced with hatchery coastal rainbow trout O. mykiss irideus; other populations exist as hybrid swarms, some are in the process of being actively invaded, and some are maintaining genetic characteristics of native populations. The persistence of some redband trout populations with high genetic integrity provides some opportunity to conserve native genomes, but our findings also highlight the complex decisions facing managers today. Effective management strategies in this system may include analysis of the specific attributes of each site and population to evaluate the relative risks posed by isolation versus maintaining connectivity, identifying potential sites for control or eradication of nonnative trout, and long‐term monitoring of the genetic integrity of remaining redband trout populations to track changes in their status.Received February 24, 2011; accepted August 4, 2011

Trophic relationships between a native and a nonnative predator in a system of natural lakes

Abstract – Bull trout, a species of char listed as threatened under the US Endangered Species Act, have been displaced from portions of their historic range following the introduction of nonnative lake trout. It has been suggested that competitive exclusion as a result of trophic overlap between bull trout and lake trout may be the causal mechanism associated with displacement of bull trout. This study used stable isotope data to evaluate trophic relationships among native bull trout, nonnative lake trout and other fishes in seven lakes in Glacier National Park (GNP), Montana. Bull trout and lake trout had greater δ15N values relative to other fishes among lakes (δ15N ≥ 3.0‰). Lake trout had greater δ15N values relative to bull trout (δ15N = +1.0‰). Bull trout had greater δ13C values relative to lake trout in six of the seven lakes examined. Although both bull trout and lake trout had greater δ15N values relative to other fishes within lakes in GNP, differences in δ15N and δ13C between bull trout and lake trout suggest that they are consuming different prey species or similar prey species in different proportions. Therefore, displacement of bull trout as a direct result of complete overlap in food resource use is not anticipated unless diet shifts occur or food resources become limiting. Additionally, future studies should evaluate food habits to identify important prey species and sources of partial dietary overlap between bull trout and lake trout.

Landscape factors that shape a slow and persistent aquatic invasion: brown trout in Newfoundland 1883–2010

AbstractAim We investigated watershed‐scale abiotic environmental factors associated with population establishment of one of the ‘world’s 100 worst alien invaders’ on a temperate Atlantic island. Within the context of the conservation implications, we aimed to quantify (1) the early history and demographics (numbers and origins) of human‐mediated brown trout (Salmo trutta) introductions, (2) the current distribution of established populations, and (3) the watershed‐scale environmental factors that may resist or facilitate trout establishment.Location Island of Newfoundland, Canada.Methods We combined field sampling with historical and contemporary records from literature to assemble a presence–absence and physical habitat database for 312 watersheds on Newfoundland. Probability of watershed establishment was modelled with general additive ANCOVA models to control for nonlinear effects of propagule pressure (i.e. the distance to and number of invasion foci within a biologically relevant range) and model performance based on AIC.Results Between 1883 and 1906, 16 watersheds were introduced with brown trout from the Howietoun Hatchery, near Stirling, Scotland. Since that time, populations have established in 51 additional watersheds at an estimated rate of spread of 4 km per year. We did not detect any obvious abiotic barriers to resist trout establishment, but showed that for a given amount of propagule pressure that relatively large and productive watersheds were most likely to be established.Main conclusions Brown trout have successfully invaded and established populations in watersheds of Newfoundland and are currently slowly expanding on the island. Populations are more likely to establish in relatively large and productive watersheds, thereby supporting predictions of island biogeography theory. However, we suggest that all watersheds in Newfoundland are potentially susceptible to successful brown trout invasion and that abiotic factors alone are unlikely to act sufficiently as barriers to population establishment.

Indirect effects of introduced trout on Cascades frogs (Rana cascadae) via shared aquatic prey

Summary 1. The introduction of trout to montane lakes has negatively affected amphibian populations across the western United States. In northern California’s Klamath–Siskiyou Mountains, introduced trout have diminished the distribution and abundance of a native ranid frog, Rana (=Lithobates) cascadae. This is primarily thought to be the result of predation on frog larvae. However, if trout feed on larval aquatic insects that are available to R. cascadae only after emergence, then resource competition may also affect this declining native amphibian. 2. Stomach contents of R. cascadae were compared between lakes that contained trout and those from which introduced trout were removed. Total prey mass in stomach contents relative to frog body mass was not significantly different between lakes with fish and fish-removal lakes, but in the former R. cascadae consumed a smaller proportion of adult aquatic insects. The stomach contents of fish included larvae of aquatic insects that are, as adults, eaten by R. cascadae. 3. Rana cascadae consumed fewer caddisflies (Trichoptera) and more grasshoppers (Orthoptera) at lakes with higher densities of fish. At lakes with greater aquatic habitat complexity, R. cascadae consumed more water striders (Hemiptera: Gerridae) and terrestrial spiders (Araneae). 4. We suggest that reductions in the availability of emerging aquatic insects cause R. cascadae to consume more terrestrial prey where trout are present. Thus, introduced trout may influence native amphibians directly through predation and, indirectly, through pre-emptive resource competition.