Native Salmonids Research Articles

Integrated field, laboratory, and theoretical study of PKD spread in a Swiss prealpine river

Proliferative kidney disease (PKD) is a major threat to wild and farmed salmonid populations because of its lethal effect at high water temperatures. Its causative agent, the myxozoan Tetracapsuloides bryosalmonae, has a complex lifecycle exploiting freshwater bryozoans as primary hosts and salmonids as secondary hosts. We carried out an integrated study of PKD in a prealpine Swiss river (the Wigger). During a 3-year period, data on fish abundance, disease prevalence, concentration of primary hosts' DNA in environmental samples [environmental DNA (eDNA)], hydrological variables, and water temperatures gathered at various locations within the catchment were integrated into a newly developed metacommunity model, which includes ecological and epidemiological dynamics of fish and bryozoans, connectivity effects, and hydrothermal drivers. Infection dynamics were captured well by the epidemiological model, especially with regard to the spatial prevalence patterns. PKD prevalence in the sampled sites for both young-of-the-year (YOY) and adult brown trout attained 100% at the end of summer, while seasonal population decay was higher in YOY than in adults. We introduce a method based on decay distance of eDNA signal predicting local species' density, accounting for variation in environmental drivers (such as morphology and geology). The model provides a whole-network overview of the disease prevalence. In this study, we show how spatial and environmental characteristics of river networks can be used to study epidemiology and disease dynamics of waterborne diseases.

Transmission routes maintaining a viral pathogen of steelhead trout within a complex multi-host assemblage.

This is the first comprehensive region wide, spatially explicit epidemiologic analysis of surveillance data of the aquatic viral pathogen infectious hematopoietic necrosis virus (IHNV) infecting native salmonid fish. The pathogen has been documented in the freshwater ecosystem of the Pacific Northwest of North America since the 1950s, and the current report describes the disease ecology of IHNV during 2000–2012. Prevalence of IHNV infection in monitored salmonid host cohorts ranged from 8% to 30%, with the highest levels observed in juvenile steelhead trout. The spatial distribution of all IHNV‐infected cohorts was concentrated in two sub‐regions of the study area, where historic burden of the viral disease has been high. During the study period, prevalence levels fluctuated with a temporal peak in 2002. Virologic and genetic surveillance data were analyzed for evidence of three separate but not mutually exclusive transmission routes hypothesized to be maintaining IHNV in the freshwater ecosystem. Transmission between year classes of juvenile fish at individual sites (route 1) was supported at varying levels of certainty in 10%–55% of candidate cases, transmission between neighboring juvenile cohorts (route 2) was supported in 31%–78% of candidate cases, and transmission from adult fish returning to the same site as an infected juvenile cohort was supported in 26%–74% of candidate cases. The results of this study indicate that multiple specific transmission routes are acting to maintain IHNV in juvenile fish, providing concrete evidence that can be used to improve resource management. Furthermore, these results demonstrate that more sophisticated analysis of available spatio‐temporal and genetic data is likely to yield greater insight in future studies.

Evaluation of Bayesian Networks for Predicting Spawning Habitat Quality of Chinook Salmon in Data‐Poor Watersheds

AbstractCalifornia's native salmonid populations are in decline, as evidenced by the 2008 fishing closures on one historically abundant species, Chinook Salmon Oncorhynchus tshawytscha. One major impact on spring‐run Chinook Salmon within the Central Valley has been the modification of natal rivers. Bayesian networks are one modeling method that could help researchers to understand these systems and direct restoration efforts. We constructed a Bayesian network for Deer Creek, in Tehama County, to assess its utility as a tool for guiding restoration of spawning habitats for spring‐run Chinook Salmon. We applied this network on a riffle–pool subreach scale to determine the suitability of each reach for spawning, indicated by the probability of redd presence. To evaluate the network we conducted sensitivity analyses and thereby determined the influence of each variable and the degree to which each variable informed the probability of redd presence. Sensitivity analyses were run for networks trained with two different stream alignments, one derived from the National Hydrography Dataset from the U.S. Geological Survey and one derived from tracing aerial imagery. We also conducted a Mann–Whitney test comparing redd densities from subreaches predicted to be good with those predicted to be poor for four fish passage condition scenarios. Of the four scenarios we modeled with the network, three exhibited significantly higher redd densities in subreaches designated as good by the network. Our results indicate that Bayesian networks can be used to predict habitat use and prioritize restoration in a data‐poor northern California watershed.Received November 30, 2015; accepted October 20, 2016Published online January 9, 2017

Physiological and morphological investigation of Arctic grayling (Thymallus arcticus) gill filaments with high salinity exposure and recovery.

Freshwater environments are at risk of increasing salinity due to multiple anthropogenic forces including current oil and gas extraction practices that result in large volumes of hypersaline water. Unintentional releases of hypersaline water into freshwater environments act as an osmoregulatory stressor to many aquatic organisms including native salmonids like the Arctic grayling (Thymallus arcticus). Compared to more euryhaline salmonids, Arctic grayling have a reduced salinity tolerance and develop an elevated interlamellar cell mass (ILCM) in response to salinity exposure (17 ppt). In this study, we described the gill morphology and cell types characterizing the ICLM. Further, we investigated whether Arctic grayling could recover in freshwater following a short-term (<48 h) salinity exposure. Arctic grayling were exposed to 17 ppt saline water for 12, 24 and 48 h. Following the 24 and 48 h salinity exposure, Arctic grayling were returned to freshwater for 24 h to assess their ability to recover from, and reverse, the osmotic disturbances. Physiological serum [Na+], [Cl-] and total osmolality were significantly elevated and progressively increased at 12, 24 and 48 h salinity exposures. The 24 h post-exposure recovery period resulted in Arctic grayling serum ion concentrations and total osmolality returning to near normal levels. Similar recovery patterns were observed in the salinity-induced ILCM, which developed as early as 12 h of exposure to 17 ppt, and then reverted to control levels following 24 h in freshwater. Gill histology indicates an increased number of apically located mucous cells in the interlamellar space following salinity exposure of Arctic grayling. The scanning electron microscopy and transmission electron microscopy data show the presence of granule containing eosinophil-like cells infiltrating the ILCM suggesting a salinity-induced immune response by the Arctic grayling.

Predicting shifts in the climate space of freshwater fishes in Great Britain due to climate change

The implications of climate change for terrestrial and aquatic taxa are for their dispersal pole-wards and/ or to higher altitudes as they track their climate niches. Here, bioclimatic models are developed to predict how projected climate change scenarios for a northern temperate region (Great Britain) shift the climate spaces (i.e. areas of suitable thermal habitat) for 12 freshwater fishes of the Salmonidae, Percidae, Esocidae and Cyprinidae families. Climate envelope models developed in Biomod2 used the current species' distributions and their relationships with current climatic variables, and projected these onto the BCC-CSM1-1 and HadGEM2-AO climate change scenarios (low and high emissions, 2050 and 2070) in full and no dispersal scenarios. Substantial contractions in climate spaces were predicted for native salmonid fishes, with decreases of up to 78% for Atlantic salmon Salmo salar, with these largely unchanged between the dispersal scenarios. Conversely, for the majority of cyprinid fishes, expansions were predicted, including into northern regions where they are current not present biogeographically. Only under the no dispersal scenarios did their predicted distributions remain the same as their current distributions. For all non-salmonid species, the most important climate variables in the model predictions related to temperature; for salmonids, they were a combination of temperature and shifts in annual mean precipitation. As these predictions suggest that there is potential for considerable alterations to the climate spaces of freshwater fishes in Great Britain during this century then regulatory and mitigation conservation actions should be undertaken to minimise these.

Islands in the ice stream: were spawning habitats for native salmonids in the Great Lakes created by paleo‐ice streams?

AbstractLake trout Salvelinus namaycush, lake whitefish Coregonus clupeaformis and cisco Coregonus artedi are salmonid fishes native to the Laurentian Great Lakes that spawn on rocky substrates in the fall and early winter. After comparing the locations of spawning habitat for these species in the main basin of Lake Huron with surficial substrates and the hypothesized locations of fast‐flowing Late Wisconsinan paleo‐ice streams, we hypothesize that much of the spawning habitat for these species in Lake Huron is the result of deposition and erosion by paleo‐ice streams. This hypothesis may represent a new framework for the identification and protection of spawning habitat for these native species, some of which are currently rare or extirpated in some of the Great Lakes. We further suggest that paleo‐ice streams may have been responsible for the creation of native salmonid spawning habitat elsewhere in the Great Lakes and in other glaciated landscapes.

Trophic ecology of introduced populations of Alaska blackfish (Dallia pectoralis) in the Cook Inlet Basin, Alaska

Introduced non-native fishes have the potential to substantially alter aquatic ecology in the introduced range through competition and predation. The Alaska blackfish (Dallia pectoralis) is a freshwater fish endemic to Chukotka and Alaska north of the Alaska Range (Beringia); the species was introduced outside of its native range to the Cook Inlet Basin of Alaska in the 1950s, where it has since become widespread. Here we characterize the diet of Alaska blackfish at three Cook Inlet Basin sites, including a lake, a stream, and a wetland. We analyze stomach plus esophageal contents to assess potential impacts on native species via competition or predation. Alaska blackfish in the Cook Inlet Basin consume a wide range of prey, with major prey consisting of epiphytic/benthic dipteran larvae, gastropods, and ostracods. Diets of the introduced populations of Alaska blackfish are similar in composition to those of native juvenile salmonids and stickleback. Thus, Alaska blackfish may affect native fish populations via competition. Fish ranked third in prey importance for both lake and stream blackfish diets but were of minor importance for wetland blackfish.

Potential threat of introduced rainbow trout Oncorhynchus mykiss to native salmonids in the western part of Hokkaido, Japan

We obtained the following five results in 10 rivers while elucidating the potential threat of introduced rainbow trout to native salmonids. First, native salmonid density decreased with increasing trout density. Second, four environmental variables, namely altitude, distance from the sea, river width, and river depth, did not significantly affect trout density. Third, the distribution of trout expanded, and trout were the dominant species in an unconfined river. Fourth, the trout invasion has changed salmonid faunas over the last 20 years. Lastly, smolt trout were collected from a river. Therefore, the potential threat to native salmonids due to trout introduction is continuously increasing.

Genetic Diversity and Hybridisation between Native and Introduced Salmonidae Fishes in a Swedish Alpine Lake

Understanding the processes underlying diversification can aid in formulating appropriate conservation management plans that help maintain the evolutionary potential of taxa, particularly under human-induced activities and climate change. Here we assessed the microsatellite genetic diversity and structure of three salmonid species, two native (Arctic charr, Salvelinus alpinus and brown trout, Salmo trutta) and one introduced (brook charr, Salvelinus fontinalis), from an alpine lake in sub-arctic Sweden, Lake Ånn. The genetic diversity of the three species was similar and sufficiently high from a conservation genetics perspective: corrected total heterozygosity, H’T = 0.54, 0.66, 0.60 and allelic richness, AR = 4.93, 5.53 and 5.26 for Arctic charr, brown trout and brook charr, respectively. There were indications of elevated inbreeding coefficients in brown trout (GIS = 0.144) and brook charr (GIS = 0.129) although sibling relationships were likely a confounding factor, as a high proportion of siblings were observed in all species within and among sampling locations. Overall genetic structure differed between species, Fst = 0.01, 0.02 and 0.04 in Arctic charr, brown trout and brook charr respectively, and there was differentiation at only a few specific locations. There was clear evidence of hybridisation between the native Arctic charr and the introduced brook charr, with 6% of individuals being hybrids, all of which were sampled in tributary streams. The ecological and evolutionary consequences of the observed hybridisation are priorities for further research and the conservation of the evolutionary potential of native salmonid species.

An alien fish threatens an endangered parasitic bivalve: the relationship between brook trout (Salvelinus fontinalis) and freshwater pearl mussel (Margaritifera margaritifera) in northern Europe

AbstractHost–parasite interactions may play a significant role in biological invasions: for example, an invader may benefit from lower infectivity by native parasites in competition against the native hosts (‘enemy release hypothesis’).The invasive North American brook trout (Salvelinus fontinalis) is replacing the native salmonid hosts of the freshwater pearl mussel (FPM,Margaritifera margaritifera) in northern Europe, but the suitability of brook trout as a host for FPM is poorly known. In this study its suitability was investigated using an array of laboratory and field experiments, and several FPM populations from a catchment in northern Finland. The occurrence of brook trout in FPM rivers in the area was also studied.There were differences with respect to numbers and size of FPM larvae among the fish exposed to FPM, but generally brook trout was an unsuitable host or was clearly a poorer host for FPM than the native European salmonids, brown trout (Salmo trutta) or Atlantic salmon (Salmo salar). However, in one river a few FPM glochidia remained encysted in wild brook trout for at least 9 months, suggesting successful development – the first observation of its kind in Europe.A tendency was found for higher occurrence of brook trout in brown trout rivers with FPM than in brown trout rivers without FPM, being consistent with the enemy release hypothesis: as FPM infection is costly to the host, a higher glochidial load in brown trout may offer a competitive advantage to brook trout in FPM rivers.Generally, these results indicate that northern European FPM populations cannot typically use brook trout as their host fish. Therefore, the invasion and dispersal of brook trout, potentially leading to the disappearance of native salmonids, should also be considered as a potential threat to the endangered FPM populations.Copyright © 2016 John Wiley &amp; Sons, Ltd.

The Land Ethic and conservation of native salmonids

The <scp>L</scp>and <scp>E</scp>thic and conservation of native salmonids

Evaluation of strategies to conserve and restore intraspecific biodiversity of brown trout: outcomes from genetic monitoring in the French Alps

Introduction of nonnative cultured fish is one of the most important threats to native salmonid populations. In brown trout, more than a century of stocking practices has led to a large hybridization between initially geographically isolated lineages, threatening native populations and thereby intraspecific diversity. In the French region of Haute-Savoie, managers and scientists implemented together three management strategies (genetic refuge, direct translocation of wild spawners and stocking with native fry) on 19 test sites for more than 15 years, in the aim to recover pure or nearly pure native populations. Here we propose an assessment of the different management strategies based on a synthetic analysis of the evolution of the introgression rate. While none of the implemented strategies completely achieves the initial objective to restore pure native populations, they differ in their efficiency: introgression rates tend to decrease quickly when direct translocation of native spawners of stocking with native fry strategies are used. The genetic refuge strategy shows slower and more heterogeneous changes of introgression rates. In general, pure nonnative fish are efficiently removed but at the cost of an increased presence of hybrids. Our results imply that intraspecific dynamics react quickly to management practices and that these changes are probably fueled by evolutionary feedbacks that are not yet well understood.

Genetic Structure of Pacific Trout at the Extreme Southern End of Their Native Range.

Salmonid fishes are cold water piscivores with a native distribution spanning nearly the entire temperate and subarctic northern hemisphere. Trout in the genus Oncorhynchus are the most widespread salmonid fishes and are among the most important fish species in the world, due to their extensive use in aquaculture and valuable fisheries. Trout that inhabit northwestern Mexico are the southernmost native salmonid populations in the world, and the least studied in North America. They are unfortunately also facing threats to their continued existence. Previous work has described one endemic species, the Mexican golden trout (O. chrysogaster), and one endemic subspecies, Nelson’s trout (O. mykiss nelsoni), in Mexico, but previous work indicated that there is vastly more biodiversity in this group than formally described. Here we conducted a comprehensive genetic analysis of this important group of fishes using novel genetic markers and techniques to elucidate the biodiversity of trout inhabiting northwestern Mexico, examine genetic population structure of Mexican trout and their relationships to other species of Pacific trout, and measure introgression from non-native hatchery rainbow trout. We confirmed substantial genetic diversity and extremely strong genetic differentiation present in the Mexican trout complex, not only between basins but also between some locations within basins, with at least four species-level taxa present. We also revealed significant divergence between Mexican trout and other trout species and found that introgression from non-native rainbow trout is present but limited, and that the genetic integrity of native trout is still maintained in most locations. This information will help to guide effective conservation strategies for this important group of fishes.

The evolution of antipredator behaviour following relaxed and reversed selection in Alaskan threespine stickleback fish

Changing environments, whether through natural or anthropogenic causes, can lead to the loss of some selective pressures ('relaxed selection') and possibly even the reinstatement of selective agents not encountered for many generations ('reversed selection'). We examined the outcome of relaxed and reversed selection in the adaptive radiation of the threespine stickleback fish, Gasterostues aculeatus L., in which isolated populations encounter a variety of predation regimes. Oceanic stickleback, which represent the ancestral founders of the freshwater radiation, encounter many piscivorous fish. Derived, freshwater populations, on the other hand, vary with respect to the presence of predators. Some populations encounter native salmonids, whereas others have not experienced predation by large fish in thousands of generations (relax-selected populations). Some relax-selected populations have had sport fish, including rainbow trout, Oncorhynchus mykiss, introduced within the past several decades (reverse-selected). We examined the behavioural responses of stickleback from three populations of each type to simulated attacks by trout and birds to determine whether relaxed and reversed selection has led to divergence in behaviour, and whether this divergence was predator specific. Fish from trout-free populations showed weak responses to trout, as predicted, but these responses were similar to those of oceanic (ancestral) populations. Fish from populations that co-occur with trout, whether native or introduced, showed elevated antipredator responses, indicating that in freshwater, trout predation selects for enhanced antipredator responses, which can evolve extremely rapidly. Comparison of laboratory-reared and wild-caught individuals suggests a combination of learned and genetic components to this variation. Responses to a model bird flyover were weakly linked to predation environment, indicating that the loss of predation by trout may partially influence the evolution of responses to birds. Our results reject the hypothesis that the consistent presence of predatory birds has been sufficient to maintain responses to piscivorous fish under periods of relaxed selection.

Seasonal Variation in Estuarine Habitat Use by Native Atlantic Salmon (Salmo salar) and Invasive Brown Trout (Salmo trutta) in Southeast Newfoundland

In North America, Salmo salar (Atlantic Salmon) populations evolved in the absence of Salmo trutta (Brown Trout) and use estuaries more extensively than in Europe. European Brown Trout were introduced to Newfoundland in the 1880s and are spreading along its coast. Most of the colonized watersheds include distinct estuaries. Unlike for river habitats, knowledge of estuary use by Brown Trout outside of their native range is very limited. We investigated seasonal estuary use by Atlantic Salmon and Brown Trout of different sizes in eastern Newfoundland. We observed parr-sized Brown Trout in June and July, whereas Atlantic Salmon parr were present from April to August. Smolts of both spe- cies were most prevalent in spring, but we found them throughout the year. Large Brown Trout were present in the estuary over much of the year. Brown Trout are a freshwater invasive species in much of their introduced range and their year-round estuarine presence in our system raises concerns for native salmonids, particularly Atlantic Salmon on New- foundland's south coast, which have been identified as at-risk and evolved in the absence of these invaders.

Consequences of Actively Managing a Small Bull Trout Population in a Fragmented Landscape

AbstractHabitat fragmentation, which affects many native salmonid species, is one of the major factors contributing to the declines in distribution and abundance of Bull Trout Salvelinus confluentus. Increasingly, managers are considering options to maintain and enhance the persistence of isolated local populations through active management strategies. Understanding the ecological consequences of such actions is a necessary step in conservation planning. We used an individual‐based model to evaluate the consequences of an ongoing management program aimed at mitigating the anthropogenic fragmentation of the lower Clark Fork River in Montana. Under this program juvenile Bull Trout are trapped and transported from small, headwater source populations to Lake Pend Oreille, Idaho, for rearing, and adults are subsequently recaptured in their upstream migration and returned to the natal population for spawning. We examined one of these populations and integrated empirical estimates of demographic parameters to simulate different management scenarios where moderate (n = 4) and high (n = 8) numbers of age‐2, age‐3, or age‐4 Bull Trout were removed for transport with variable return rates under both demographic stochasticity and environmental perturbations. Our results indicated the risks from removal with no returns increased substantially when removal totals and age of Bull Trout removed from the simulated population increased. Specifically, removing eight age‐3 or age‐4 individuals resulted in 26% and 62% reductions in average adult population size, respectively, across simulations. We found the risks of transport were not likely alleviated with low (3%) or moderate (6%) return rates, and there were considerable risks of declines for the source population even when return rates were extremely high (&gt;12%). Our simulations indicated little risk of declines for the source population with removals of age‐2 Bull Trout, and any risks were alleviated with low return rates. However, we found higher return rates were particularly beneficial in the presence of large, density‐independent perturbations.

Fish Assemblage Structure and Habitat Associations in a Large Western River System

Longitudinal gradients of fish assemblage and habitat structure were investigated in the Kootenai River of northern Idaho. A total of 43 500-m river reaches was sampled repeatedly with several techniques (boat-mounted electrofishing, hoop nets and benthic trawls) in the summers of 2012 and 2013. Differences in habitat and fish assemblage structure were apparent along the longitudinal gradient of the Kootenai River. Habitat characteristics (e.g. depth, substrate composition and water velocity) were related to fish assemblage structure in three different geomorphic river sections. Upper river sections were characterized by native salmonids (e.g. mountain whitefish Prosopium williamsoni), whereas native cyprinids (peamouth Mylocheilus caurinus, northern pikeminnow Ptychocheilus oregonensis) and non-native fishes (pumpkinseed Lepomis gibbosus, yellow perch Perca flavescens) were common in the downstream section. Overall, a general pattern of species addition from upstream to downstream sections was discovered and is likely related to increased habitat complexity and additions of non-native species in downstream sections. Assemblage structure of the upper sections were similar, but were both dissimilar to the lower section of the Kootenai River. Species-specific hurdle regressions indicated the relationships among habitat characteristics and the predicted probability of occurrence and relative abundance varied by species. Understanding fish assemblage structure in relation to habitat could improve conservation efforts of rare fishes and improve management of coldwater river systems. Copyright © 2015 John Wiley & Sons, Ltd.

Status of Redband Trout in the Upper Snake River Basin of Idaho

AbstractRedband Trout Oncorhynchus mykiss gairdneri are likely the most abundant and most widely distributed native salmonid in the Columbia River basin, yet their current distribution and abundance across the landscape have not been well documented. We sampled 1,032 randomly distributed stream sites (usually 100 m in length) across more than 60,000 km of stream network to assess Redband Trout occupancy, abundance, and genetic purity in the upper Snake River basin of Idaho. Study locations were more often in dry desert subbasins (49% of sites) than in montane subbasins (20%), and 25% of the dry “stream sites” had no discernible stream channel whatsoever, indicating a lack of flowing water for perhaps millennia. Redband Trout were estimated to occupy 13,485 km of stream (22% of the total) and were captured more often (389 sites) than Brook Trout Salvelinus fontinalis (128 sites), Bull Trout Salvelinus confluentus (37 sites), or Brown Trout Salmo trutta (16 sites). Redband Trout were also the most abundant species of trout, with an approximate abundance of 3,449,000 ± 402,000 (90% confidence interval) of all sizes, followed by Brook Trout (1,501,000 ± 330,000), Bull Trout (159,000 ± 118,000), and Brown Trout (43,000 ± 25,000). Approximately 848,000 ± 128,000 Redband Trout were adults. From 1913 (the earliest year of record) to 2001, roughly 43 million hatchery Rainbow Trout were stocked in streams in the study area, 17.5 million of which were of catchable size (i.e., ≥200 mm total length); since 2001, all catchable trout have been sterilized prior to stocking. Genetic results from 61 study sites suggest that hybridization with hatchery Rainbow Trout is more likely to occur in streams that were directly stocked with catchable trout from 1913 to 2001. Applying these results across the landscape, we estimated that Redband Trout likely remain pure in about 68% of the streams occupied in the upper Snake River basin.Received November 13, 2012; accepted January 9, 2014

Trophic interactions between introduced lake trout (Salvelinus namaycush) and native Arctic charr (S. alpinus) in a large Fennoscandian subarctic lake

AbstractIntroduced fishes may have major impacts on community structure and ecosystem function due to competitive and predatory interactions with native species. For example, introduced lake trout (Salvelinus namaycush) has been shown to replace native salmonids and induce major trophic cascades in some North American lakes, but few studies have investigated trophic interactions between lake trout and closely related native Arctic charr (S. alpinus) outside the natural distribution of the former species. We used stomach content and stable isotope analyses to investigate trophic interactions between introduced lake trout and native Arctic charr in large subarctic Lake Inarijärvi in northern Finland. Both salmonids had predominantly piscivorous diets at &gt;280 mm total length and were mainly caught from the deep profundal zone. However, lake trout had a more generalist diet and showed higher reliance on littoral prey fish than Arctic charr, whose diet consisted mainly of pelagic planktivorous coregonids. According to length at age and condition data, lake trout showed slightly faster growth but lower condition than Arctic charr. The results indicate that introduced lake trout may to some extent compete with and prey upon native Arctic charr, but currently have only a minor if any impact on native fishes and food web structure in Inarijärvi. Future monitoring is essential to observe potential changes in trophic interactions between lake trout and Arctic charr in Inarijärvi, as well as in other European lakes where the two salmonids currently coexist.

Occupancy dynamics of escaped farmed Atlantic salmon in Canadian Pacific coastal salmon streams: implications for sustained invasions

Farmed non-native Atlantic salmon (Salmo salar) is the largest agriculture export product of British Columbia, Canada. Chronic low-volume escapes of salmon from farms into Pacific waters (“leakage”) are typically undetectable (Britton et al. 2011). Analysis of escape-reporting from farmers indicates that reports greatly underestimate the true number of Atlantic salmon inadvertently released from open-net pen rearing sites (Morton and Volpe 2002). To quantify the spatial extent of escaped Atlantic salmon in Canadian Pacific rivers, we systematically snorkel-surveyed 41 known Pacific salmon (Oncorhynchus spp.)-supporting rivers and creeks on Vancouver Island over a span of 3 years. We estimated and accounted for imperfect detections using multi-season occupancy models. We detected Atlantic salmon in 36.6 % of surveyed rivers. After accounting for imperfect detection, occupancy models estimated that over half of surveyed streams across the study area contained Atlantic salmon, and that 97 % of streams with high native salmon diversity were occupied by Atlantic salmon. Even in intensive snorkel surveys, Atlantic salmon are detected in occupied streams only 2/3 the time, suggesting abundance and distribution of non-native salmon is greater than indicated by the only existing data. Further, Atlantic salmon are more likely to occupy streams with high native Pacific salmon diversity—and more likely to maintain occupancy across years—potentially increasing competitive pressure on native salmonids. Understanding local biotic and abiotic predictors of Atlantic salmon occupancy, stream colonization, and local extinction requires more data; the same is true for the effects of escaped Atlantic salmon on local salmon diversity and sustainability. These data for the first time show that Atlantic salmon occupy Pacific coastal rivers for multiple years. The impact of Atlantic salmon occupancy in British Columbia rivers must be factored into policy decisions regarding the future of salmon farming in the provincial waters.