Alaskan brown bears (Ursus arctos) aggregate and display fidelity to foraging neighborhoods while preying on Pacific salmon along small streams

The interaction between brown bears (Ursus arctos) and Pacific salmon (Oncorhynchus spp.) is important to the population dynamics of both species and a celebrated example of consumer‐mediated nutrient transport. Yet, much of the site‐specific information we have about the bears in this relationship comes from observations at a few highly visible but unrepresentative locations and a small number of radio‐telemetry studies. Consequently, our understanding of brown bear abundance and behavior at more cryptic locations where they commonly feed on salmon, including small spawning streams, remains limited. We employed a noninvasive genetic approach (barbed wire hair snares) over four summers (2012–2015) to document patterns of brown bear abundance and movement among six spawning streams for sockeye salmon, O. nerka, in southwestern Alaska. The streams were grouped into two trios on opposite sides of Lake Aleknagik. Thus, we predicted that most bears would forage within only one trio during the spawning season because of the energetic costs associated with swimming between them or traveling around the lake and show fidelity to particular trios across years because of the benefits of familiarity with local salmon dynamics and stream characteristics. Huggins closed‐capture models based on encounter histories from genotyped hair samples revealed that as many as 41 individuals visited single streams during the annual 6‐week sampling season. Bears also moved freely among trios of streams but rarely moved between these putative foraging neighborhoods, either during or between years. By implication, even small salmon spawning streams can serve as important resources for brown bears, and consistent use of stream neighborhoods by certain bears may play an important role in spatially structuring coastal bear populations. Our findings also underscore the efficacy of noninvasive hair snagging and genetic analysis for examining bear abundance and movements at relatively fine spatial and temporal scales.     

Effects of habitat features on size-biased predation on salmon by bears

Predators can drive trait divergence among populations of prey by imposing differential selection on prey traits. Habitat characteristics can mediate predator selectivity by providing refuge for prey. We quantified the effects of stream characteristics on biases in the sizes of spawning salmon caught by bears (Ursus arctos and U. americanus) on the central coast of British Columbia, Canada by measuring size-biased predation on spawning chum (Oncorhynchus keta) and pink (O. gorbuscha) salmon in 12 streams with varying habitat characteristics. We tested the hypotheses that bears would catch larger than average salmon (size-biased predation) and that this bias toward larger fish would be higher in streams that provide less protection to spawning salmon from predation (e.g., less pools, wood, undercut banks). We then we tested for how such size biases in turn translate into differences among populations in the sizes of the fish. Bears caught larger-than-average salmon as the spawning season progressed and as predicted, this was most pronounced in streams with fewer refugia for the fish (i.e., wood and undercut banks). Salmon were marginally smaller in streams with more pronounced size-biased predation but this predictor was less reliable than physical characteristics of streams, with larger fish in wider, deeper streams. These results support the hypothesis that selective forces imposed by predators can be mediated by habitat characteristics, with potential consequences for physical traits of prey.     

Salmon-derived nitrogen delivery and storage within a gravel bed: Sediment and water interactions

Post-spawning salmon carcasses are broadly recognized as a source of organic matter- and marine-derived nutrients (MDN) in Pacific salmon streams, but MDN delivery and retention processes are not well understood. Recent studies emphasize the interaction of inorganic particulate matter and salmon organic matter, through flocculation, as a delivery mechanism for MDN to the streambed. This study builds upon previous flocculation studies to look at nitrogen delivery and storage within the gravel bed of a re-circulating flume. Findings indicate that nitrogen storage in surface and interstitial water is lower than sediment-associated nitrogen. Flocculation of salmon organic matter and inorganic sediment is presented as a delivery mechanism in spawning and post-spawning periods that helps to maintain ecological productivity within Pacific salmon streams. Based on these findings it is recommended that salmon enhancement activities should include leaving post-spawn carcasses in-stream and that fertilization programs should consider flocculation processes to increase nutrient delivery to the streambed.     

Can intense predation by bears exert a depensatory effect on recruitment in a Pacific salmon population?

It has long been recognized that, as populations increase in density, ecological processes affecting growth and survival reduce per capita recruitment in the next generation. In contrast to the evidence for such “compensatory” density dependence, the alternative “depensatory” process (reduced per capita recruitment at low density) has proven more difficult to demonstrate in the field. To test for such depensation, we measured the spawner–recruit relationship over five decades for a sockeye salmon (Oncorhynchus nerka) population in Alaska breeding in high-quality, unaltered habitat. Twenty-five years of detailed estimates of predation by brown bears, Ursus arctos, revealed strong density dependence in predation rate; the bears killed ca. 80 % of the salmon in years of low salmon spawning abundance. Nevertheless, the reconstructed spawner–recruit relationship, adjusted to include salmon intercepted in the commercial fishery, provided no evidence of demographic depensation. That is, in years when few salmon returned and the great majority were killed by bears, the few that spawned were successful enough that the population remained highly productive, even when those killed by bears were included as potential spawners. We conclude that the high quality of breeding habitat at this site and the productive nature of semelparous Pacific salmon allowed this population to avoid the hypothesized depressed recruitment from depensatory processes expected at low density. The observed lack of demographic depensation is encouraging from a conservation standpoint because it implies that depleted populations may have the potential to rebound successfully given suitable spawning and rearing habitat, even in the presence of strong predation pressure.     

Diverse foraging opportunities drive the functional response of local and landscape-scale bear predation on Pacific salmon

The relationship between prey abundance and predation is often examined in single habitat units or populations, but predators may occupy landscapes with diverse habitats and foraging opportunities. The vulnerability of prey within populations may depend on habitat features that hinder predation, and increased density of conspecifics in both the immediate vicinity and the broader landscape. We evaluated the relative effects of physical habitat, local, and neighborhood prey density on predation by brown bears on sockeye salmon in a suite of 27 streams using hierarchical Bayesian functional response models. Stream depth and width were inversely related to the maximum proportion of salmon killed, but not the asymptotic limit on total number killed. Interannual variation in predation was density dependent; the number of salmon killed increased with fish density in each stream towards an asymptote. Seven streams in two geographical groups with ≥23 years of data in common were then analyzed for neighborhood density effects. In most (12 of 18) cases predation in a stream was reduced by increasing salmon abundance in neighboring streams. The uncertainty in the estimates for these neighborhood effects may have resulted from interactions between salmon abundance and habitat that influenced foraging by bears, and from bear behavior (e.g., competitive exclusion) and abundance. Taken together, the results indicated that predator–prey interactions depend on density at multiple spatial scales, and on habitat features of the surrounding landscape. Explicit consideration of this context dependency should lead to improved understanding of the ecological impacts of predation across ecosystems and taxa.     

Spatial and temporal variability of macroinvertebrates in spawning and non-spawning habitats during a salmon run in Southeast Alaska

Spawning salmon create patches of disturbance through redd digging which can reduce macroinvertebrate abundance and biomass in spawning habitat. We asked whether displaced invertebrates use non-spawning habitats as refugia in streams. Our study explored how the spatial and temporal distribution of macroinvertebrates changed during a pink salmon (Oncorhynchus gorbuscha) spawning run and compared macroinvertebrates in spawning (riffle) and non-spawning (refugia) habitats in an Alaskan stream. Potential refugia included: pools, stream margins and the hyporheic zone, and we also sampled invertebrate drift. We predicted that macroinvertebrates would decline in riffles and increase in drift and refugia habitats during salmon spawning. We observed a reduction in the density, biomass and taxonomic richness of macroinvertebrates in riffles during spawning. There was no change in pool and margin invertebrate communities, except insect biomass declined in pools during the spawning period. Macroinvertebrate density was greater in the hyporheic zone and macroinvertebrate density and richness increased in the drift during spawning. We observed significant invertebrate declines within spawning habitat; however in non-spawning habitat, there were less pronounced changes in invertebrate density and richness. The results observed may be due to spawning-related disturbances, insect phenology, or other variables. We propose that certain in-stream habitats could be important for the persistence of macroinvertebrates during salmon spawning in a Southeast Alaskan stream.     

Cortisol and Pacific Salmon: A New Look at the Role of Stress Hormones in Olfaction and Home-stream Migration

Pacific salmon (genus Oncorhynchus) exhibit an interesting anduncommon life-history pattern that combines semelparity, anadromy,and navigation (homing). During smoltification, young salmonimprint on the chemical composition of their natal stream water(the home-stream olfactory bouquet or "HSOB"); they then migrateto the ocean where they spend a few years feeding prior to migratingback to their natal freshwater stream to spawn. Upstream migrationis guided by the amazing ability to discriminate between thechemical compositions of different stream waters and thus identifyand travel to their home-stream. Pacific salmon demonstratemarked somatic and neural degeneration changes during home-streammigration and at the spawning grounds. The appearance of thesepathologies is correlated with a marked elevation in plasmacortisol levels. While the mechanisms of salmonid homing arenot completely understood, it is known that adult salmon continuouslyutilize two of their primary sensory systems, olfaction andvision, during homing. Olfaction is the primary sensory systeminvolved in freshwater homing and "HSOB" recognition, and willbe emphasized here. Previously, we proposed that the increasein plasma cortisol during Pacific salmon home-stream migrationis adaptive because it enhances the salmon's ability to recallthe imprinted memory of the "HSOB" (Carruth, 1998; Carruth etal., 2000b). Elevated plasma concentrations of cortisol couldprime the hippocampus or other olfactory regions of the brainto recall this memory and, therefore, aid in directing the fishto their natal stream. Thus, specific responses of salmon tostressors could enhance reproductive success.     

A challenge to dam improvement for the protection of both salmon and human livelihood in Shiretoko,Japan’s third Natural Heritage Site

The Shiretoko Peninsula was placed on the World Heritage List in July 2005 as Japan’s third Natural Heritage Site, being valued for its unique ecosystems formed by the interaction between marine and terrestrial environments. Prior to its listing, however, the International Union for Conservation of Nature and Natural Resources requested the development of measures to allow for the free movement of salmonids through the nominated area via artificial in-stream structures. This report introduces a pioneering effort to modify check dams in order to improve ecosystem linkages. The River Construction Working Group has intensively discussed and implemented restoration projects for 3 years, and the results have been monitored after implementation. Of 44 streams within the Shiretoko World Natural Heritage Site, 14 streams have had one or more in-stream structure(s) built. A total of 123 structures, mainly soil conservation dams, are distributed sporadically in these streams. Following assessment, the Working Group concluded that it would be reasonable to modify 31 structures in five streams, including 18 structures that were to be modified before listing. The most suitable design that met local stream conditions was selected, after accounting for salmon passage, adverse impacts on stream environments above and below the construction sites and on fishing grounds, efficiency of construction, and ease of postmodification maintenance. Working in cooperation with the group members and parties concerned, we succeeded in restoring salmonids’ upstream runs and extending their spawning habitat in streams above some of the dams, while keeping the disaster prevention function of the dams intact.     

Quantitative Links Between Pacific Salmon and Stream Periphyton

Species’ impacts on primary production can have strong ecological consequences. In freshwater ecosystems, Pacific salmon (Oncorhynchus spp.) may influence stream periphyton through substrate disturbance during spawning and nutrient subsidies from senescent adults. The shape of relationships between the abundance of spawning salmon and stream periphyton, as well as interactions with environmental variables, are incompletely understood and may differ across the geographic range of salmon. We examined these relationships across 24 sockeye salmon (Oncorhynchus nerka) spawning streams in north-central British Columbia, Canada. The influence of salmon abundance and environmental variables (temperature, light, dissolved nutrients, water velocity, watershed size, and invertebrate grazer abundance) on post-spawning periphyton abundance and nitrogen stable isotope signatures, which can indicate the uptake of salmon nitrogen, was evaluated using linear regression models and Akaike Information Criterion. Periphyton nitrogen stable isotope signatures were best described by a positive log-linear relationship with an upstream salmon abundance metric that includes salmon from earlier years. This suggests the presence of a nutrient legacy. In contrast, periphyton abundance was negatively related to the spawning-year salmon density, which likely results from substrate disturbance during spawning, and positively related to dissolved soluble reactive phosphorus prior to spawning, which may indicate phosphorus limitation in the streams. These results suggest that enrichment from salmon nutrients does not always translate into elevated periphyton abundance. This underscores the need to directly assess the outcome of salmon impacts on streams rather than extrapolating from stable isotope evidence for the incorporation of salmon nutrients into food webs.     

Stream network geomorphology mediates predicted vulnerability of anadromous fish habitat to hydrologic change in southeast Alaska

In rivers supporting Pacific salmon in southeast Alaska, USA, regional trends toward a warmer, wetter climate are predicted to increase mid‐ and late‐21st‐century mean annual flood size by 17% and 28%, respectively. Increased flood size could alter stream habitats used by Pacific salmon for reproduction, with negative consequences for the substantial economic, cultural, and ecosystem services these fish provide. We combined field measurements and model simulations to estimate the potential influence of future flood disturbance on geomorphic processes controlling the quality and extent of coho, chum, and pink salmon spawning habitat in over 800 southeast Alaska watersheds. Spawning habitat responses varied widely across watersheds and among salmon species. Little variation among watersheds in potential spawning habitat change was explained by predicted increases in mean annual flood size. Watershed response diversity was mediated primarily by topographic controls on stream channel confinement, reach‐scale geomorphic associations with spawning habitat preferences, and complexity in the pace and mode of geomorphic channel responses to altered flood size. Potential spawning habitat loss was highest for coho salmon, which spawn over a wide range of geomorphic settings, including steeper, confined stream reaches that are more susceptible to streambed scour during high flows. We estimated that 9–10% and 13–16% of the spawning habitat for coho salmon could be lost by the 2040s and 2080s, respectively, with losses occurring primarily in confined, higher‐gradient streams that provide only moderate‐quality habitat. Estimated effects were lower for pink and chum salmon, which primarily spawn in unconfined floodplain streams. Our results illustrate the importance of accounting for valley and reach‐scale geomorphic features in watershed assessments of climate vulnerability, especially in topographically complex regions. Failure to consider the geomorphic context of stream networks will hamper efforts to understand and mitigate the vulnerability of anadromous fish habitat to climate‐induced hydrologic change.     

Movers and shakers: nutrient subsidies and benthic disturbance predict biofilm biomass and stable isotope signatures in coastal streams

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Watershed‐scale climate influences productivity of Chinook salmon populations across southcentral Alaska

The ecosystems supporting Pacific salmon (Oncorhynchus spp.) are changing rapidly as a result of climate change and habitat alteration. Understanding how—and how consistently—salmon populations respond to changes at regional and watershed scales has major implications for fisheries management and habitat conservation. Chinook salmon (O. tshawytscha) populations across Alaska have declined over the past decade, resulting in fisheries closures and prolonged impacts to local communities. These declines are associated with large‐scale climate drivers, but uncertainty remains about the role of local conditions (e.g., precipitation, streamflow, and stream temperature) that vary among the watersheds where salmon spawn and rear. We estimated the effects of these and other environmental indicators on the productivity of 15 Chinook salmon populations in the Cook Inlet basin, southcentral Alaska, using a hierarchical Bayesian stock‐recruitment model. Salmon spawning during 2003–2007 produced 57% fewer recruits than the previous long‐term average, leading to declines in adult returns beginning in 2008. These declines were explained in part by density dependence, with reduced population productivity following years of high spawning abundance. Across all populations, productivity declined with increased precipitation during the fall spawning and early incubation period and increased with above‐average precipitation during juvenile rearing. Above‐average stream temperatures during spawning and rearing had variable effects, with negative relationships in many warmer streams and positive relationships in some colder streams. Productivity was also associated with regional indices of streamflow and ocean conditions, with high variability among populations. The cumulative effects of adverse conditions in freshwater, including high spawning abundance, heavy fall rains, and hot, dry summers may have contributed to the recent population declines across the region. Identifying both coherent and differential responses to environmental change underscores the importance of targeted, watershed‐specific monitoring and conservation efforts for maintaining resilient salmon runs in a warming world.     

Density-dependence in the survival and reproduction of Bald Eagles: Linkages to Chum Salmon

During the late 20th Century, due to decreases in both contamination and persecution, bald eagle (Haliaeetus leucocephalus) populations increased dramatically. Currently, mechanisms regulating eagle populations are not well understood. To examine potential regulating processes in the Pacific Northwest, where eagles are no longer primarily regulated by contaminants or direct persecution, we examined bald eagle reproductive success, breeding populations, winter populations, mortality, and salmon stream use. Wintering and breeding eagle populations in south-coastal British Columbia (BC) quadrupled between the early 1980s and the late 1990s, and have since stabilized. Density-dependent declines in reproduction occurred during 1986–2009, but not through changes in site quality. Mid-winter survival was crucial as most mortality occurred then, and models showed that density-dependent reductions in population growth rates were partially due to reduced survival. Wintering eagles in British Columbia fed heavily on chum salmon (Oncorhynchus keta) runs, and then switched to birds in late winter, when mortality was highest. Eagles tended to arrive after the peak in salmon availability at streams in BC as part of a migration associated with salmon streams from Alaska to northern Washington. Eagles were most abundant in southern BC during cold Alaskan winters and in years of high chum salmon availability. We suggest that eagle populations in the Pacific Northwest are currently partially limited by density on the breeding grounds and partially by adult mortality in late winter, likely due to reduced late winter salmon stocks forcing eagles to exploit more marginal prey supplies. Larger eagle populations have affected some local prey populations. © 2011 The Wildlife Society.     

Asynchrony in population dynamics of sockeye salmon in southwest Alaska

Ecologists have examined the synchronization of population dynamics across space as a means to understand how populations respond to climate variation. However, response diversity may reflect important variation among local population dynamics driven by population‐specific responses to regional environmental change. We used long‐term data on sockeye salmon Oncorhynchus nerka from pristine watersheds of southwestern Alaska to show that populations spawning in close proximity (<40 km) to one another have a limited degree of synchrony in their dynamics, even after accounting for density‐dependent processes. In fact, the dynamics of local populations of stream‐spawning sockeye salmon were no more coherent than those of stocks at a much coarser resolution across this region of Alaska. We examined four hypotheses to explain the observed patterns of asynchrony among stream‐spawning populations, and found that populations spawning in dissimilar habitats, and using different nursery lakes were less synchronized in their productivity. Similarity in the age structure of spawning adults was less correlated with synchrony in productivity. These results emphasize the importance of maintaining diverse spawning and rearing habitat for the conservation of Pacific salmon, and should guide conservation planning for Pacific salmon populations in regions where natural dynamics have been altered by habitat loss, hatchery practices, and over‐fishing.     

Effects of Salmon-Derived Nutrients and Habitat Characteristics on Population Densities of Stream-Resident Sculpins

Movement of nutrients across ecosystem boundaries can have important effects on food webs and population dynamics. An example from the North Pacific Rim is the connection between productive marine ecosystems and freshwaters driven by annual spawning migrations of Pacific salmon (Oncorhynchus spp). While a growing body of research has highlighted the importance of both pulsed nutrient subsidies and disturbance by spawning salmon, their effects on population densities of vertebrate consumers have rarely been tested, especially across streams spanning a wide range of natural variation in salmon densities and habitat characteristics. We studied resident freshwater prickly (Cottus asper), and coastrange sculpins (C. aleuticus) in coastal salmon spawning streams to test whether their population densities are affected by spawning densities of pink and chum salmon (O. gorbuscha and O. keta), as well as habitat characteristics. Coastrange sculpins occurred in the highest densities in streams with high densities of spawning pink and chum salmon. They also were more dense in streams with high pH, large watersheds, less area covered by pools, and lower gradients. In contrast, prickly sculpin densities were higher in streams with more large wood and pools, and less canopy cover, but their densities were not correlated with salmon. These results for coastrange sculpins provide evidence of a numerical population response by freshwater fish to increased availability of salmon subsidies in streams. These results demonstrate complex and context-dependent relationships between spawning Pacific salmon and coastal ecosystems and can inform an ecosystem-based approach to their management and conservation.     

Size-selective and sex-selective predation by brown bears on sockeye salmon

Breeding activity increases the vulnerability of many animals to predation, and such predation can affect the subset of animals successfully reproducing. To study the ways in which predation might affect the evolution of Pacific salmon, we measured the intensity and selectivity of predation by bears (primarily brown bears, Ursus arctos) on mature sockeye salmon (Oncorhynchus nerka) breeding in a series of small, spring-fed ponds and creeks near Pedro Bay, Alaska, from 1994 to 1998. Bears killed male salmon more often than females; males constituted 60% of the kills but only 35% of the salmon that died of senescence. The bears also killed fish that were larger, on average, than those dying of senescence (males: 462 vs 452 mm; females: 453 vs 443 mm). The level of predation varied greatly, from 4% (females) and 10% (males) in 1994 to 100% of both sexes in 1996 and 1997. The rate of predation also varied among habitats, being lower in larger ponds than in smaller, shallower ponds and the very small interconnecting creeks. Despite the intense and size-selective predation, the salmon in safer habitats (large ponds) were not larger than those in riskier habitats, and salmon densities were only slightly higher in the safer areas. Compared to a nearby population that experiences no bear predation (Woody Island), the male sockeye salmon from the Pedro Pond system had shallower bodies (i.e., less exposure in shallow water) for a given length, consistent with the hypothesis that selective predation can affect the extent of sexual dimorphism among populations. However, the average length at age for both males and females was greater in the Pedro Pond fish, indicating that selective factors besides predation affect length. Overall, the results indicate that bears can be an agent of natural selection within (and perhaps between) sockeye salmon populations, and predation can greatly affect reproductive success among individuals and years for the population as a whole. Received: 6 April 1999 / Accepted: 1 June 1999     

Stable isotope evidence indicates the incorporation into Japanese catchments of marine‐derived nutrients transported by spawning Pacific Salmon

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Stream geomorphology regulates the effects on periphyton of ecosystem engineering and nutrient enrichment by Pacific salmon

1. Pacific salmon (Oncorhynchus spp.) returning to streams deliver substantial quantities of nutrients (nitrogen and phosphorus) that may stimulate primary production. Salmon can also affect the phytobenthos negatively via physical disturbance during nest excavation, a process that may counteract the positive effects of salmon‐derived nutrients on benthic algae. The ability of salmon to disturb benthic habitats may be a function of substratum particle size, and therefore, the geomorphology of streams could determine the net effect of salmon on benthic communities. 2. Based on surveys of 17 streams in southwest Alaska before the salmon run and during peak salmon density, we identified size thresholds for the disturbance of substratum particles by salmon and classified particles as vulnerable (<60 mm B‐axis), invulnerable (>110 mm) or transitional (61–110 mm). At the scale of individual rocks, algal biomass on vulnerable substrata decreased at peak spawning (relative to values before the run) as a power function of salmon density; transitional and invulnerable substrata showed no quantifiable pattern. However, invulnerable substrata in streams with more than 0.11 salmon m^?2 showed net algal accrual, or relatively smaller declines in algal biomass, than vulnerable substrata, indicating that large rocks provide refuge for benthic algae from salmon disturbance. 3. We expected that streams with proportionally larger rocks would respond positively to salmon at the whole‐stream scale, after accounting for the relative abundance of rocks of different sizes within streams. Invulnerable rocks made up only 0–12% of the total substratum particle size distribution in salmon‐bearing streams, however, and algal accrual on invulnerable substrata did not outweigh the strong disturbance effects on the more spatially extensive vulnerable substrata. The change in whole‐stream benthic algal biomass among streams was negatively related to salmon density. 4. Stable isotopes of nitrogen (δ¹⁵N) were used to track nutrients from salmon into benthic biota. Periphyton δ¹⁵N on rocks of all size classes was higher at peak salmon spawning than before the salmon run, indicating the uptake of salmon‐derived nitrogen. Peak δ¹⁵N values were positively related to salmon abundance and followed a two‐isotope mixing relationship. The per cent of N from salmon in periphyton was also related to salmon density and was best explained by a saturating relationship. Spring δ¹⁵N was unrelated to salmon returns in the previous year, suggesting little annual carryover of salmon nutrients.     

Predation on adult Atlantic salmon, Salmo salar L., by otters, Lutra lutra (L.), within the River Dee system, Aberdeenshire, Scotland

Predation on adult salmon, Salmo salur L., by otters, Lurra lutra (L.), varied seasonally on the R. Dee, Aberdeenshire, Scotland, being highest during the spawning season in winter. Predation is described for some tributaries of the river. Male fish were caught by otters more often than females, and it is suggested that they were most vulnerable during their extensive excursions up and down stream, particularly as they negotiated shallow riffles. Otters appeared to prey upon healthy fish rather than those infested with Saprolegnia sp. but there was no difference in the size, freshwater- or sea-age offish killed by otterscompared with 'kelts' which had died non-violently. At least some of the otters obtained most of their daily food requirements by catching a single salmon per night.
Considerable numbers of adult salmon may be killed by otters during the spawning period but it is suggested that, because most are males, this is unlikely to affect the breeding success of the salmon population. Most predation occurs outside the fishing season and so is unlikely to reduce the numbers of salmon caught by anglers.     

Interaction between hatchery and wild Pacific salmon in the Far East of Russia: A review

We review studies of interactions between hatchery and wild Pacific salmon in the Russian Far East. This includes the role of hatchery practices that result in premature migration to the sea and increased mortality, and data on feeding and territorial competition between juveniles of hatchery and wild origin. In the course of downstream migration many juvenile hatchery salmon are eliminated by wild salmon predation. During the marine period, Japanese hatchery chum salmon (Oncorhynchus keta) distribution overlaps the distribution of Russian wild salmon. Consequently, replacement of wild populations by hatchery fishes, as a result of abundant juvenile hatchery releases combined with extensive poaching in spawning grounds, is apparent in some Russian rivers. Interactions between the populations occur in all habitats. The importance of conservation of wild salmon populations requires a more detailed study of the consequences of interactions between natural and artificially reared fishes.