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.     

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

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Salmon species, density and watershed size predict magnitude of marine enrichment in riparian food webs

Resource subsidies across habitat boundaries can structure recipient communities and food webs. In the northern Pacific region, bears Ursus spp. foraging on anadromous salmon Oncorhynchus spp. provide a key link between marine and terrestrial ecosystems, with salmon density, fish size and watershed size as potential predictors of the magnitude of marine subsidy to terrestrial habitats. We use nitrogen and carbon stable isotopes to provide an assessment of the patterns of marine‐enrichment in riparian plants (11 species, 4 guilds) and litter invertebrates (4 guilds) sampled from 27 watersheds in coastal British Columbia, Canada. Watersheds occurred in three geographical regions (Vancouver Island, mainland midcoast and Haida Gwaii) and varied in size, and in biomass (kg m^?1 of spawning length) and species of salmon (chum O. keta, pink O. gorbuscha and coho O. kisutch). δ¹⁵N values in all plant species and invertebrate guilds were positively predicted by total salmon biomass (kg m^?1) and negatively predicted by watershed size. We observed replicated parallel slopes among plant species and invertebrate guilds across the gradient in salmon biomass, with differences in means hypothesized to be due to plant fractionation and animal trophic position. As such, we derived a watershed δ¹⁵N‐index averaged across guilds, and using an information theoretic approach we find that the biomass of chum salmon is a much stronger predictor of the δ¹⁵N‐index than either pink or coho salmon, or the sum biomass of all species. The top linear model contained chum biomass and watershed size. Chum salmon biomass independently predicted δ¹⁵N‐index variation in all three regions of British Columbia. Chum salmon are larger than pink or coho and provide an energetic reward for bears that facilitates carcass transfer, tissue selective foraging, and nutrient distribution by insect scavengers. Analyses of biodiversity and habitat data across many watersheds moves towards a long‐term goal in fisheries ecology to better integrate ecosystem values in salmon conservation.     

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.     

Straying of hatchery salmon in Prince William Sound,Alaska

The straying of hatchery salmon may harm wild salmon populations through a variety of ecological and genetic mechanisms. Surveys of pink (Oncorhynchus gorbuscha), chum (O. keta) and sockeye (O. nerka) salmon in wild salmon spawning locations in Prince William Sound (PWS), Alaska since 1997 show a wide range of hatchery straying. The analysis of thermally marked otoliths collected from carcasses indicate that 0–98% of pink salmon, 0–63% of chum salmon and 0–93% of sockeye salmon in spawning areas are hatchery fish, producing an unknown number of hatchery-wild hybrids. Most spawning locations sampled (77%) had hatchery pink salmon from three or more hatcheries, and 51% had annual escapements consisting of more than 10% hatchery pink salmon during at least one of the years surveyed. An exponential decay model of the percentage of hatchery pink salmon strays with distance from hatcheries indicated that streams throughout PWS contain more than 10% hatchery pink salmon. The prevalence of hatchery pink salmon strays in streams increased throughout the spawning season, while the prevalence of hatchery chum salmon decreased. The level of hatchery salmon strays in many areas of PWS are beyond all proposed thresholds (2–10%), which confounds wild salmon escapement goals and may harm the productivity, genetic diversity and fitness of wild salmon in this region     

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 spawning Pacific salmon on the isotopic composition of biota differ among southeast Alaska streams

1. Adult Pacific salmon (Oncorhynchus spp.) transport marine nutrients to fresh waters and disturb sediments during spawning. The relative importance of nutrient fertilisation and benthic disturbance by salmon spawners can be modulated by environmental conditions (e.g. biological, chemical and physical conditions in the catchment, including human land use). 2. To determine the importance of the environmental context in modifying the uptake and incorporation of salmon‐derived material into stream biota, we measured the nitrogen (δ¹⁵N) and carbon (δ¹³C) isotopic composition of benthic algae (i.e. epilithon) and juvenile coho salmon (Oncorhynchus kisutch) in seven streams across a timber‐harvest gradient (8–69% catchment area harvested), both before and during the salmon run. Conditional bootstrap modelling simulations were used to assess variability in the response of epilithon and juvenile coho salmon to spawning salmon. 3. In response to spawning salmon, epilithon exhibited enrichment in both δ¹⁵N (mean: 1.5‰) and δ¹³C (2.3‰). Juvenile coho were also enriched in both δ¹⁵N (0.7‰) and δ¹³C (1.4‰). Conditional bootstrap models indicate decreased variation in data as spatial replication increases, suggesting that the number of study sites can influence the results of Pacific salmon isotope studies. 4. Epilithon isotopic enrichment was predicted by environmental conditions, with δ¹⁵N enrichment predicted by stream temperature and timber harvest (R² = 0.87) and δ¹³C enrichment by discharge, sediment size, timber harvest and spawner density (R² = 0.96). Furthermore, we found evidence for a legacy effect of salmon spawners, with pre‐spawner δ¹⁵N and δ¹³C of both epilithon and juvenile coho predicted by salmon run size in the previous year. 5. Our results show that the degree of incorporation of salmon‐derived nitrogen and carbon differs among streams. Furthermore, the environmental context, including putative legacy effects of spawning salmon, can influence background isotopic concentrations and utilisation of salmon‐derived materials in southeast Alaska salmon streams. Future studies should consider the variation in isotopic composition of stream biota when deciding on the number of study sites and samples needed to generate meaningful results.     

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.     

Adaptive strategies and life history characteristics in a warming climate: Salmon in the Arctic?

In the warming Arctic, aquatic habitats are in flux and salmon are exploring their options. Adult Pacific salmon, including sockeye (Oncorhynchus nerka), coho (O. kisutch), Chinook (O. tshawytscha), pink (O. gorbuscha) and chum (O. keta) have been captured throughout the Arctic. Pink and chum salmon are the most common species found in the Arctic today. These species are less dependent on freshwater habitats as juveniles and grow quickly in marine habitats. Putative spawning populations are rare in the North American Arctic and limited to pink salmon in drainages north of Point Hope, Alaska, chum salmon spawning rivers draining to the northwestern Beaufort Sea, and small populations of chum and pink salmon in Canada’s Mackenzie River. Pacific salmon have colonized several large river basins draining to the Kara, Laptev and East Siberian seas in the Russian Arctic. These populations probably developed from hatchery supplementation efforts in the 1960’s. Hundreds of populations of Arctic Atlantic salmon (Salmo salar) are found in Russia, Norway and Finland. Atlantic salmon have extended their range eastward as far as the Kara Sea in central Russian. A small native population of Atlantic salmon is found in Canada’s Ungava Bay. The northern tip of Quebec seems to be an Atlantic salmon migration barrier for other North American stocks. Compatibility between life history requirements and ecological conditions are prerequisite for salmon colonizing Arctic habitats. Broad-scale predictive models of climate change in the Arctic give little information about feedback processes contributing to local conditions, especially in freshwater systems. This paper reviews the recent history of salmon in the Arctic and explores various patterns of climate change that may influence range expansions and future sustainability of salmon in Arctic habitats. A summary of the research needs that will allow informed expectation of further Arctic colonization by salmon is given.     

Linking climate change projections for an Alaskan watershed to future coho salmon production

Climate change is predicted to dramatically change hydrologic processes across Alaska, but estimates of how these impacts will influence specific watersheds and aquatic species are lacking. Here, we linked climate, hydrology, and habitat models within a coho salmon (Oncorhynchus kisutch) population model to assess how projected climate change could affect survival at each freshwater life stage and, in turn, production of coho salmon smolts in three subwatersheds of the Chuitna (Chuit) River watershed, Alaska. Based on future climate scenarios and projections from a three‐dimensional hydrology model, we simulated coho smolt production over a 20‐year span at the end of the century (2080–2100). The direction (i.e., positive vs. negative) and magnitude of changes in smolt production varied substantially by climate scenario and subwatershed. Projected smolt production decreased in all three subwatersheds under the minimum air temperature and maximum precipitation scenario due to elevated peak flows and a resulting 98% reduction in egg‐to‐fry survival. In contrast, the maximum air temperature and minimum precipitation scenario led to an increase in smolt production in all three subwatersheds through an increase in fry survival. Other climate change scenarios led to mixed responses, with projected smolt production increasing and decreasing in different subwatersheds. Our analysis highlights the complexity inherent in predicting climate‐change‐related impacts to salmon populations and demonstrates that population effects may depend on interactions between the relative magnitude of hydrologic and thermal changes and their interactions with features of the local habitat.     

Self-sustaining populations, population sinks or aggregates of strays: chum (Oncorhynchus keta) and Chinook salmon (Oncorhynchus tshawytscha) in the Wood River system, Alaska

Small populations can provide insights into ecological and evolutionary aspects of species distributions over space and time. In the Wood River system in Alaska, USA, small aggregates of Chinook (Oncorhynchus tshawytscha) and chum salmon (O. keta) spawn in an area dominated by sockeye salmon (O. nerka). Our objective was to determine whether these Chinook and chum salmon are reproductively isolated, self-sustaining populations, population sinks that produce returning adults but receive immigration, or strays from other systems that do not produce returning adults. DNA samples collected from adult chum salmon from 16 streams and Chinook salmon from four streams in the Wood River system over 3 years were compared to samples from large populations in the nearby Nushagak River system, a likely source of strays. For both species, microsatellite markers indicated no significant genetic differentiation between the two systems. Simulations of microsatellite data in a large source and a smaller sink population suggested that considerable immigration would be required to counteract the diverging effects of genetic drift and produce genetic distances as small as those observed, considering the small census sizes of the two species in the Wood River system. Thus, the Wood River system likely receives substantial immigration from neighbouring watersheds, such as the Nushagak River system, which supports highly productive runs. Although no data on population productivity in the Wood River system exist, our results suggest source-sink dynamics for the two species, a finding relevant to other systems where salmonid population sizes are limited by habitat factors.     

Temporal patterns in adult salmon migration timing across southeast Alaska

Pacific salmon migration timing can drive population productivity, ecosystem dynamics, and human harvest. Nevertheless, little is known about long‐term variation in salmon migration timing for multiple species across broad regions. We used long‐term data for five Pacific salmon species throughout rapidly warming southeast Alaska to describe long‐term changes in salmon migration timing, interannual phenological synchrony, relationships between climatic variation and migratory timing, and to test whether long‐term changes in migration timing are related to glaciation in headwater streams. Temporal changes in the median date of salmon migration timing varied widely across species. Most sockeye populations are migrating later over time (11 of 14), but pink, chum, and especially coho populations are migrating earlier than they did historically (16 of 19 combined). Temporal trends in duration and interannual variation in migration timing were highly variable across species and populations. The greatest temporal shifts in the median date of migration timing were correlated with decreases in the duration of migration timing, suggestive of a loss of phenotypic variation due to natural selection. Pairwise interannual correlations in migration timing varied widely but were generally positive, providing evidence for weak region‐wide phenological synchrony. This synchrony is likely a function of climatic variation, as interannual variation in migration timing was related to climatic phenomenon operating at large‐ (Pacific decadal oscillation), moderate‐ (sea surface temperature), and local‐scales (precipitation). Surprisingly, the presence or the absence of glaciers within a watershed was unrelated to long‐term shifts in phenology. Overall, there was extensive heterogeneity in long‐term patterns of migration timing throughout this climatically and geographically complex region, highlighting that future climatic change will likely have widely divergent impacts on salmon migration timing. Although salmon phenological diversity will complicate future predictions of migration timing, this variation likely acts as a major contributor to population and ecosystem resiliency in southeast Alaska.     

Location Is Everything: Evaluating the Effects of Terrestrial and Marine Resource Subsidies on an Estuarine Bivalve

Estuaries are amongst the world’s most productive ecosystems, lying at the intersection between terrestrial and marine environments. They receive substantial inputs from adjacent landscapes but the importance of resource subsidies is not well understood. Here, we test hypotheses for the effects of both terrestrial- and salmon-derived resource subsidies on the diet (inferred from stable isotopes of muscle tissue), size and percent nitrogen of the soft-shell clam (Mya arenaria), a sedentary estuarine consumer. We examine how these relationships shift across natural gradients among 14 estuaries that vary in upstream watershed size and salmon density on the central coast of British Columbia, Canada. We also test how assimilation and response to subsidies vary at smaller spatial scales within estuaries. The depletion and enrichment of stable isotope ratios in soft-shell clam muscle tissue correlated with increasing upstream watershed size and salmon density, respectively. The effects of terrestrial- and salmon-derived subsidies were also strongest at locations near stream outlets. When we controlled for age of individual clams, there were larger individuals with higher percent nitrogen content in estuaries below larger watersheds, though this effect was limited to the depositional zones below river mouths. Pink salmon exhibited a stronger effect on isotope ratios of clams than chum salmon, which could reflect increased habitat overlap as spawning pink salmon concentrate in lower stream reaches, closer to intertidal clam beds. However, there were smaller clams in estuaries that had higher upstream pink salmon densities, possibly due to differences in habitat requirements. Our study highlights the importance of upstream resource subsidies to this bivalve species, but that individual responses to subsidies can vary at smaller scales within estuaries.     

Fecundity and egg size variation in North American Pacific salmon (Oncorhynchus)

We examined regional and latitudinal variation in fecundity and egg weight for five species of Pacific salmon ( Oncorhynchus ) along the Pacific coast of North America. Data were examined for 24 chum salmon, 15 pink salmon, 34 sockeye salmon, 44 chinook salmon, and 40 coho salmon populations from published sources, unpublished Canadian hatchery records, our own laboratory investigations, and other unpublished sources. Substantial regional variation in fecundity and egg weight was observed, with salmon on the Queen Charlotte Islands and Vancouver Island in British Columbia generally having lower fecundity and larger egg size than nearby mainland populations. The relative distance of freshwater migration to the spawning grounds generally had a marked effect on both fecundity and egg size, with populations spawning in the upper portions in the drainages of large rivers like the Fraser River in British Columbia having reduced fecundity and egg size compared with coastal spawning populations. Fecundity was generally higher and egg size generally lower in more northern populations of sockeye, chinook, and coho salmon compared with southern ones. We suggest that egg size tends to be lower in northern populations of some species as a result of increased fecundity due to their older ages at maturity and a limited amount of energy that can be expended on egg production.     

Wild and hatchery reproduction of pink and chum salmon and their catches in the Sakhalin-Kuril region,Russia

In the Sakhalin-Kuril region hatchery culture of pink and chum salmon is of great importance compared to other regions of the Russian Far East. During the last 30 years the number of hatcheries increased two-fold, and significant advances were made in hatchery technologies. As a result, chum salmon capture in regions where hatcheries operate (southwestern and eastern Sakhalin coasts, and Iturup Island) was 9 times as high during 2006–2010 than during 1986–1990, whereas wild chum salmon harvest markedly declined. Recent dynamics in pink salmon catch appear to track trends in natural spawning in monitored index rivers, suggesting natural-origin pink salmon play a dominant role in supporting the commercial fishery. It remains uncertain as to whether hatcheries have substantially supplemented commercial catch of pink salmon in this region, and I recommend continued research (including implementing mass marking and recovery programs) before decisions are made regarding increasing pink salmon hatchery production. Location of hatcheries in spawning river basins poses problems for structuring a management system that treats hatchery and wild populations separately. Debate continues regarding the existence and importance of density-dependent processes operating in the ocean environment and the role hatcheries play in these processes. Loss of critical spawning habitat for chum salmon in the Sakhalin-Kuril region has lead to significant declines in their abundance. I conclude by recommending increases in releases of hatchery chum salmon numbers in the region to help recover depressed wild populations and provide greater commercial fishing benefits in the region.     

Influence of marine-derived nutrients from spawning salmon on aquatic insect communities in southeast Alaskan streams

The objective of this study was to evaluate the influence of the nutrient transfer system between anadromous salmon and aquatic insect communities across multiple, natural stream systems. Between 2000 and 2002, we sampled seven streams in southeast Alaska, seasonally. Of the seven study streams, four received large annual salmon runs (high-run streams), and three were no-run streams. All the streams selected had a natural waterfall barrier to salmon, providing an upstream control reach for each study stream. Insect density, biomass, richness, diversity and functional feeding groups were analyzed before, during and after the fall salmon run in each stream section (i.e. above and below the barrier) of the seven study streams between 2001 and 2002. Results showed that diversity and richness were similar across stream sections and run size within each period, except for during the run when both were significantly lower in downstream sections of high-run streams. Functional feeding group patterns showed higher abundance and biomass of collector–gatherers and shredders during the post spawning, carcass decomposition period. High-run streams had upstream sections with greater abundance and biomass of mayflies (dominated by Baetidae, Heptageniidae and Ephemerellidae) during the run, and downstream sections with greater abundance and biomass of dipterans (dominated by Chironomidae). This study suggests that the often published positive relationship between MDN and stream insect abundance and biomass may only exist for certain taxa, primarily chironomid midges.     

Going with the Flow: Spatial Distributions of Juvenile Coho Salmon Track an Annually Shifting Mosaic of Water Temperature

A critical challenge for ecologists is to understand the functional significance of habitat heterogeneity and connectivity for mobile animals. Here, we explore how a thermo-regulating fish responds to annual variation in the spatial patterning of thermal and trophic resources. In a third-order stream in coastal Alaska, juvenile coho salmon forage on sockeye salmon eggs at night in cold water and then move to warmer water to increase their digestive capacity. We mapped the spatial distributions of water temperature, juvenile coho salmon, and spawning sockeye salmon across a 5-year period during which summer discharge varied by greater than fivefold. In low flow years, warm water (9–12°C) was only available in thalweg (that is, main-channel) habitat at least approximately 400 m upstream of the cooler habitat (3–7°C) where sockeye salmon spawned. In high flow years, the entire stream thalweg was isothermal at 7–8°C, but inundated off-channel areas generated warm habitats (9–12°C) laterally adjacent to the downstream regions where sockeye salmon spawned. The daytime spatial distribution of juvenile coho salmon shifted from headwater thalweg habitats in low flow years, to downstream off-channel habitats in high flow years. In all years, the majority of juvenile coho salmon sampled during the daytime were found in warm habitat units without sockeye salmon present, yet they exhibited diet contents comprised virtually entirely of sockeye salmon eggs. Thus, thermoregulatory movements by coho salmon were able to track an annually shifting mosaic of water temperature. Our results demonstrate how the spatial habitat heterogeneity and connectivity of intact floodplains can in turn buffer aquatic organisms from high levels of temporal variation in habitat conditions and resource abundance.     

Climate Change Sensitivity Index for Pacific Salmon Habitat in Southeast Alaska

Global climate change may become one of the most pressing challenges to Pacific Salmon conservation and management for southeast Alaska in the 21^st Century. Predicted hydrologic change associated with climate change will likely challenge the ability of specific stocks to adapt to new flow regimes and resulting shifts in spawning and rearing habitats. Current research suggests egg-to-fry survival may be one of the most important freshwater limiting factors in Pacific Salmon''s northern range due to more frequent flooding events predicted to scour eggs from mobile spawning substrates. A watershed-scale hydroclimatic sensitivity index was developed to map this hypothesis with an historical stream gauge station dataset and monthly multiple regression-based discharge models. The relative change from present to future watershed conditions predicted for the spawning and incubation period (September to March) was quantified using an ensemble global climate model average (ECHAM5, HadCM3, and CGCM3.1) and three global greenhouse gas emission scenarios (B1, A1B, and A2) projected to the year 2080. The models showed the region''s diverse physiography and climatology resulted in a relatively predictable pattern of change: northern mainland and steeper, snow-fed mountainous watersheds exhibited the greatest increases in discharge, an earlier spring melt, and a transition into rain-fed hydrologic patterns. Predicted streamflow increases for all watersheds ranged from approximately 1-fold to 3-fold for the spawning and incubation period, with increased peak flows in the spring and fall. The hydroclimatic sensitivity index was then combined with an index of currently mapped salmon habitat and species diversity to develop a research and conservation priority matrix, highlighting potentially vulnerable to resilient high-value watersheds. The resulting matrix and observed trends are put forth as a framework to prioritize long-term monitoring plans, mitigation experiments, and finer-scale climate impact and adaptation studies.     

Separating physical disturbance and nutrient enrichment caused by Pacific salmon in stream ecosystems

1. Pacific salmon (Oncorhynchus spp.) deliver marine‐derived nutrients to the streams in which they spawn and die, and these resource subsidies can increase the abundance of stream biota. In strong contrast, physical disturbance from salmon spawning activity can reduce the abundance of benthic organisms. Previous experimental designs have not established the relative effects of these two contrasting processes on stream organisms during a salmon run. 2. We combined manipulative and observational field studies to assess the degree of nutrient enrichment, physical disturbance, and the net effect of salmon on the abundance of benthic periphyton. Related salmon‐mediated processes were also evaluated for benthic macroinvertebrates. Mesh exclosures (2 × 2 m plots) prevented salmon from disturbing areas of the stream channel, which were compared with areas to which salmon had access. Sampling was conducted both before and during the late‐summer spawning run of pink (O. gorbushca) and chum (O. keta) salmon. 3. Streamwater nitrogen and phosphorus concentrations increased sharply with the onset of the salmon run, and highly significant positive relationships were observed between the numbers of salmon present in the stream and these dissolved nutrients. Before the salmon run, periphyton biomass (as chlorophyll a) and total macroinvertebrate abundance were very similar between control and exclosure plots. During the salmon run, exclosures departed substantially from controls, suggesting significant disturbance imparted on benthic biota. 4. Comparing exclosures before and during the salmon run enabled us to estimate the effects of salmon in the absence of direct salmon disturbance. This ‘nutrient enrichment potential’ was significant for periphyton biomass, as was a related index for macroinvertebrate abundance (although enhanced invertebrate drift into exclosures during the salmon run could also have been important). Interestingly, however, the net effect of salmon, evaluated by comparing control plots before and during the salmon run, was relatively modest for both periphyton and macroinvertebrates, suggesting that nutrient enrichment effects were largely offset by disturbance. 5. Our results illustrate the importance of isolating the specific mechanisms via which organisms affect ecosystems, and indicate that the relative magnitude of salmon nutrient enrichment and benthic disturbance determines the net effect that these ecologically important fish have on stream ecosystems.     

Geomorphology and fish assemblages in a Piedmont river basin, U.S.A.

1. We investigated linkages between fishes and fluvial geomorphology in 31 wadeable streams in the Etowah River basin in northern Georgia, U.S.A. Streams were stratified into three catchment sizes of approximately 15, 50 and 100 km², and fishes and geomorphology were sampled at the reach scale (i.e. 20–40 times stream width). 2. Non‐metric multidimensional scaling (NMDS) identified 85% of the among‐site variation in fish assemblage structure and identified strong patterns in species composition across sites. Assemblages shifted from domination by centrarchids, and other pool species that spawn in fine sediments and have generalised food preferences, to darter‐cyprinid‐redhorse sucker complexes that inhabit riffles and runs, feed primarily on invertebrates, and spawn on coarser stream beds. 3. Richness and density were correlated with basin area, a measure of stream size, but species composition was best predicted (i.e. |r| between 0.60–0.82) by reach‐level geomorphic variables (stream slope, bed texture, bed mobility and tractive force) that were unrelated to stream size. Stream slope was the dominant factor controlling stream habitat. Low slope streams had smaller bed particles, more fines in riffles, lower tractive force and greater bed mobility compared with high slope streams. 4. Our results contrast with the ‘River Continuum Concept’ which argues that stream assemblages vary predictably along stream size gradients. Our findings support the ‘Process Domains Concept’, which argues that local‐scale geomorphic processes determine the stream habitat and disturbance regimes that influence stream communities.