Marine-derived Nutrient Research Articles

Conservation of the surface-nesting Kermadec Petrel Pterodroma neglecta neglecta in the South Pacific: clarifying breeding ecology and the threat of avian ground predators

Summary Understanding population dynamics and impacts on island ecology remain top priorities for the conservation management of seabirds, particularly when attempting species-recovery for island restoration. Identifying suitable nesting habitat is a keystone detail in seabird restoration which can be complicated when predator pressures impact colony productivity and population viability. The surface-nesting Kermadec Petrel Pterodroma neglecta neglecta is dispersed on remote islands through the tropics and subtropics. We examined their breeding ecology on Phillip Island, Norfolk Group in the South Pacific and considered the limitations to re-establishment at their previous breeding location on Lord Howe Island (900 km south-east). On Phillip Island, the petrels were mostly monogamous with nesting locations generally within the vicinity of the previous season’s attempts. Breeding sites were limited to sloping terrain 182–228 m above the shoreline and up to 85 m from the coast in small sub-colonies under low scrubby woodland. Based on observations of 56 pairs and data gleaned from Global Light Sensing devices, we determined that breeding (incubation and provisioning) occurs in all calendar months of the year, with a laying peak in the late austral spring. Breeding success in the first year of study was 25% but improved to 56% once management of Purple Swamphens Porphyrio melanotus was implemented. Successful nests were located at sites with naturally limited egress from cleared or lightly vegetated areas. Prudent management of the petrel population on Phillip Island could aid in the regional recovery of the species and, in the process, assist the island’s restoration by significantly increasing transfers of marine-derived nutrient. Their reintroduction to Lord Howe Island is also possible but will likely rely upon consistent productivity of the Phillip Island population for founding immigrants. Moreover, successful establishment will require adaptive management of selected sites to ensure native avian predators do not overly impact breeding.

The diadromous watersheds-ocean continuum: Managing diadromous fish as a community for ecosystem resilience

Diadromous fishes play important ecological roles by delivering ecosystem services and making crucial connections along the watersheds-ocean continuum. However, it is difficult to fully understand the community-level impacts and cumulative benefits of diadromous fish migrations, as these species are most often considered individually or in small groups. Their interactions at a community level (e.g., interdependencies such as predation, co-migration, and habitat conditioning) and the connections between their ecosystem roles and functions (e.g., cumulative marine-derived nutrient contributions, impacts on stream geomorphology) are yet to be fully understood. Similarly, freshwater, estuarine, and marine ecosystems are often considered as independent parts, limiting understanding of the importance of connections across systems. We argue that not considering the ecosystem interdependence and importance of diadromous fish as a community currently hinders the implementation of the large-scale management required to increase ecosystem resilience and fish productivity across the full range of these species. We developed a conceptual model, the Diadromous Watersheds-Ocean Continuum (DWOC), that uses ecosystem services to promote a more holistic approach to the management of the diadromous community and encourages an integrated understanding of the ecosystem connections made by these species. DWOC provides a framework for discussions that can help identify research and management needs, discuss the trade-offs of different management options, and analyze what pressing questions impede the implementation of large-scale management solutions toward a more ecosystem-based management approach.

Climate and human exploitation have regulated Atlantic salmon populations in the River Spey, Scotland, over the last 2000 years

Historical salmon catch records suggest that climatic variability, and more recently human exploitation, control patterns of abundance in Atlantic salmon populations. We present the first long-term (2000-year) reconstruction of Atlantic Salmon population variations based on a Marine Derived Nutrient (MDN) lake sediment record. Our record is constructed from nitrogen isotopes (δ15N) measured from a lake sediment core, which we compare with an escapement index (EI) derived from historic net catch data on major Scottish salmon rivers. We used an isotope mixing model to demonstrate that the N isotope values are likely enriched with MDN and demonstrate that Loch Insh sediments are enriched compared with a control site (Loch Vaa) that has never had exposure to salmon. We demonstrate that current adult spawner returns are around half that of historic values prior to major human exploitation. Before the onset of widespread human exploitation and habitat degradation, large fluctuations in salmon abundance are attributed to variations in North Atlantic sea surface temperature. While our data support published reconstructions of declining Atlantic salmon stocks in Northwest European rivers over the last 1000 years, rather than point to a solely human cause, the human impact appears to be overprinted on larger-scale changes in marine habitat occurring at the transition from the warmer Medieval Climatic Anomaly (MCA) to the cooler Little Ice Age (LIA).

Effects of spawning Pacific salmon on terrestrial invertebrates: Insects near spawning habitat are isotopically enriched with nitrogen-15 but display no differences in body size.

When Pacific salmon (Oncorhynchus spp.) spawn and die, they deliver marine‐derived nutrient subsidies to freshwater and riparian ecosystems. These subsidies can alter the behavior, productivity, and abundance of recipient species and their habitats. Isotopes, such as nitrogen‐15 (15N), are often used to trace the destination of marine‐derived nutrients in riparian habitats. However, few studies have tested for correlations between stable isotopes and physiological responses of riparian organisms. We examined whether increases in δ 15N in terrestrial insect bodies adjacent to salmon spawning habitat translate to changes in percent nitrogen content and body size. This involved comparisons between distance from a salmon‐bearing river, marine‐derived nutrients in soils and insects, soil moisture content, and body size and nitrogen content in two common beetle families (Coleoptera: Curculionidae, Carabidae). As predicted, δ15N in riparian soils attenuated with distance from the river but was unaffected by soil moisture. This gradient was mirrored by δ15N in the herbivorous curculionid beetles, whereas carabid beetles, which feed at a higher trophic level and are more mobile, did not show discernable patterns in their δ15N content. Additionally, neither distance from the river nor body δ15N content was related to beetle body size. We also found that nitrogen‐15 was not correlated with total percent nitrogen in insect bodies, meaning that the presence of spawning salmon did not increase the percent nitrogen content of these insects. We conclude that while salmon‐derived nutrients had entered terrestrial food webs, the presence of δ15N alone did not indicate meaningful physiological changes in these insects in terms of percent nitrogen nor body size. While stable isotopes may be useful tracers of marine‐derived nutrients, they cannot necessarily be used as a proxy for physiologically important response variables.

Carcass Additions Influence Food Webs Through Bottom-Up and Direct Consumption Pathways Along a Fish Species Assemblage Gradient

The loss of subsidies delivered by anadromous fish to inland stream ecosystems may have profound influences on stream food webs. However, studies have focused on food web responses in ecosystems where the fish assemblage is dominated by salmonids. We evaluated food web responses to carcass additions in three locations of an interior Columbia River Basin stream with varying native fish assemblages. Periphyton biomass responses were mixed with increases in treatment compared to control reaches of the middle and downstream pairs, but not in the upstream pair where scavenging by bears removed the majority of carcasses. Stable isotope analysis revealed marine-derived nutrient (MDN) enrichment of periphyton and invertebrate functional feeding groups in the middle and downstream pairs (up to 12% MDN derived), but not in the upstream pair. Non-salmonid fish exhibited limited MDN assimilation (~ 5 and 10% MDN derived), with isotopic enrichment patterns similar to lower trophic levels and little evidence of eggs and carcass material consumption, suggesting bottom-up transfer of MDN to non-salmonid fishes. In contrast, across the three study pairs, juvenile Chinook Salmon (Oncorhynchus tshawytscha) and steelhead trout (O. mykiss) assimilated MDN rapidly, obtaining up to 25% and 57% of their nitrogen from carcasses, respectively. Diet analysis and isotopic enrichment patterns indicated that juvenile salmonid assimilation occurred primarily through direct consumption of eggs and carcass tissue. Our findings suggest that salmonids in this region may experience more benefit from carcass additions containing eggs, while non-salmonids are likely to experience minimal MDN incorporation whether or not eggs are present.

Salmon-mediated nutrient flux in Snake River sockeye salmon nursery lakes: the influence of depressed population size and hatchery supplementation

Evans ML, Kohler AE, Griswold RG, Tardy KA, Eaton KR, Ebel JD. Salmon-mediated nutrient flux in Snake River sockeye salmon nursery lakes: the influence of depressed population size and hatchery supplementation. Lake Reserv Manage. 36:75–86.Since the 1970s, Snake River sockeye salmon (Oncorhynchus nerka) have returned in low numbers to nursery lakes in central Idaho, consequently diminishing marine-derived nutrient subsidies to freshwater spawning and rearing environments. In healthy Pacific salmon populations, returning adults generally import more nutrients than juveniles export, resulting in net positive salmon-mediated nutrient fluxes to freshwater ecosystems. To mitigate for declining sockeye salmon returns, Snake River nursery lakes have been stocked with embryos, with captive-reared juveniles and adults, and fertilized with inorganic nutrients. Here, we examine sockeye salmon-mediated nutrient flux in 3 nursery lakes, Alturas, Pettit, and Redfish, across 20 yr of study (1998–2017). In Redfish Lake, sockeye salmon-mediated nutrient flux was positive in all years when imports from the captive-rearing program were considered in our estimates. Without captive-rearing program inputs, Redfish Lake nutrient flux was positive in only 40% and 45% of years for phosphorus and nitrogen, respectively. In Alturas and Pettit lakes, nutrient export by juveniles exceeded nutrient import by adults in 67% and 56% of years for phosphorus and nitrogen, respectively. Overall, our findings suggest that a sufficient number of adult recruits relative to juvenile migrants, corresponding to a smolt-to-adult return rate > 0.78%, is needed to achieve positive salmon-mediated nutrient flux within lakes and that this will depend on both adequate population productivity and enhanced marine and freshwater habitat connectivity.

Effects of sea-level rise and freshwater management on long-term water levels and water quality in the Florida Coastal Everglades

Since the 1880s, hydrological modification of the Greater Florida Everglades has reduced water levels and flows in Everglades National Park (ENP). The Comprehensive Everglades Restoration Program (CERP) began in 2000 to restore pre-drainage flows and preserve the natural landscape of the Everglades. However, sea-level rise (SLR) was not considered in the development of CERP. We used long-term data (2001–2016) from the Florida Coastal Everglades-Long Term Ecological Research Program to quantify and model the spatial dynamics of water levels, salinity, and nutrients in response to changes in climate, freshwater management and SLR in the Shark River Slough (SRS), ENP. Results indicate that fresh-to-marine head difference (FMHD) was the single most important factor affecting marine-to-freshwater hydrologic connectivity and transport of salinity and phosphorous upstream from the Gulf of Mexico. Sea-level has increasingly exceeded ground surface elevation at the most downstream freshwater site in SRS, thereby reducing the FMHD. We showed a higher impact of SLR in the dry season when there was practically no freshwater inflow to raise FMHD. We also demonstrated effectiveness of inflow depends more on the monthly distribution than the total annual volume. Hence, the impact per unit volume of inflow is significantly higher in the dry season in preventing high salinity and marine-derived nutrient levels. We advocate that FMHD needs to be factored into water management decisions to reduce adverse and likely irreversible effects of SLR throughout the Everglades landscape.

Forest stream biota carbon : nitrogen ratios are unaffected by nutrient subsidies from breeding Westland petrels (Procellaria westlandica)

Stable isotope studies have repeatedly shown marine nutrient incorporation from seabirds, anadromous fish and tidal wrack into terrestrial and freshwater ecosystems. However, little is known about the physiological consequences of marine-derived nutrient subsidies. Protein content and lipid storage are important physiologically, and the C:N ratio is a widely used proxy that reflects changes in these quantities. In this study we tested the response of C:N ratios in stream biota to the presence of marine-derived nutrients from Westland petrels, a forest breeding seabird. Samples of different stream invertebrate functional feeding groups, predatory freshwater fish, coarse particulate organic matter and riparian soil and vegetation came from four reference streams and four streams with a wide range of seabird densities. Samples were analysed for percentage C, percentage N and δ15N using isotope ratio mass spectrometry. The data were tested against petrel colony size, stream size and distance from colony to sampling site. Despite increased δ15N accompanying petrel presence (as reported previously), Westland petrels had no effect on stream biota C:N ratios, regardless of colony presence or absence or colony size. Despite the nutrients provided by petrels, we conclude that petrel N replaced rather than enhanced non-marine N in these stream ecosystems.

Nutrient loading by anadromous fishes: species-specific contributions and the effects of diversity

Anadromous fishes exhibit diverse life history and functional traits. Spawning anadromous fishes bring nutrients to fresh waters, and their life history and functional traits can influence nutrient loading patterns. We asked how nutrient inputs varied across 12 species of North American anadromous fishes and how these patterns affected river-wide trends in nutrient loading. We used portfolio effect analyses to assess whether diversity stabilized or destabilized nutrient inputs to freshwater ecosystems over time. Recent decreases in body size reduced per individual nutrient loading for several key species, which in turn decreased cumulative loading. Invasion also altered nutrient loading; non-native American shad (Alosa sapidissima) now provide nearly half of the annual marine-derived nutrient inputs to the Columbia River. Counter to expectations, species diversity (richness and evenness) did not significantly increase the stability of nutrient inputs to fresh waters. Species loss in the Connecticut River increased stability, while species gain in the Columbia River decreased stability.

Sea lamprey carcasses exert local and variable food web effects in a nutrient-limited Atlantic coastal stream

Resource flows from adjacent ecosystems are critical in maintaining structure and function of freshwater food webs. Migrating sea lamprey (Petromyzon marinus) deliver a pulsed marine-derived nutrient subsidy to rivers in spring when the metabolic demand of producers and consumers are increasing. However, the spatial and temporal dynamics of these nutrient subsidies are not well characterized. We used sea lamprey carcass additions in a small stream to examine changes in nutrients, primary productivity, and nutrient assimilation among consumers. Algal biomass increased 57%–71% immediately adjacent to carcasses; however, broader spatial changes from multiple-site carcass addition may have been influenced by canopy cover. We detected assimilation of nutrients (via δ13C and δ15N) among several macroinvertebrate families including Heptageniidae, Hydropsychidae, and Perlidae. Our research suggests that subsidies may evoke localized patch-scale effects on food webs, and the pathways of assimilation in streams are likely coupled to adjacent terrestrial systems. This research underscores the importance of connectivity in streams, which may influence sea lamprey spawning and elicit varying food web responses from carcass subsidies due to fine-scale habitat variables.

Spatiotemporal responses of algal, fungal, and bacterial biofilm communities in Atlantic rivers receiving marine-derived nutrient inputs

AbstractSpawning anadromous fishes can enhance primary and secondary production in streams by providing marine-derived nutrients (MDN), but little is known of their effects on individual biofilm constituents (algae, fungi, and bacteria). To address this issue, we measured spatial and temporal changes in the abundance and composition of biofilm algae, fungi, and bacteria in 6 Atlantic rivers with contrasting influx of MDN. Each river had a natural barrier to anadromous fish migration, which established an upstream control and downstream treatment area, with Atlantic Salmon (Salmo salar), Alewife (Alosa pseudoharengus), Rainbow Smelt (Osmerus mordax), or Sea Lamprey (Petromyzon marinus) as the primary anadromous species. In areas receiving MDN, increases in algal and fungal biomass and bacterial density were maintained throughout the season. Algal communities showed the largest (2–10×) increase in abundance over control sites, and Rainbow Smelt spawning streams had the greatest accrual. The river with Alewi...

Biogeomorphic impacts of migration and disturbance: Implications of salmon spawning and decay

Geomorphologic processes often involve a biotic element that acts to regulate landform development. This biotic element can be plant or animal-based with a feedback that ultimately benefits the ecology of the organism. Pacific salmon (Oncorhynchus sp.) are an example of an animal biogeomorphic agent exhibiting such feedbacks and, because of long migrations from the sea to freshwater spawning grounds, are a species of interest that act on both local and regional scales. Upon returning to their natal streams, salmon generate a dual disturbance, resuspending large amounts of sediment as they construct nests while at the same time generating a substantial nutrient pulse through post-spawn die-off and decay. The retention and export of these nutrients are of importance to any hypothesized productivity boost driven by the marine derived nutrients (MDNs). Using experimental enclosures in the Horsefly River spawning channel in north-central British Columbia, our objectives for this study were to i) quantify the magnitude of organic and inorganic sediment export and retention from an active-spawning area and ii) determine the contribution of fine sediment MDN storage. Using a suspended sediment mass balance model, marine isotope enrichment and a time series of gravel bed sediment infiltration, we found strongly linear relationships between sediment infiltration and marine-derived nutrient enrichment. Elevated suspended sediment produced by salmon redd (nest) construction acted as an effective vector for MDN infiltration into the gravel bed. This study demonstrated that localized patterns of sediment deposition are regulated by salmon activity which in turn act to control MDN storage within, and release from, the gravel bed. Furthermore, this study demonstrates the ability of a biogeomorphic agent like salmon to establish a feedback mechanism that creates favorable conditions which ultimately benefit the organism.

Carcass analog addition enhances juvenile Atlantic salmon (Salmo salar) growth and condition

Our study used historic marine-derived nutrient (MDN) delivery timing to simulate potential effects of restored connectivity on juvenile Atlantic salmon (ATS; Salmo salar) growth and condition. Four headwater streams were stocked with ATS young of the year (YOY) and received carcass analog additions (0.10 kg·m–2wetted area) in treatment reaches to match the timing of sea lamprey (Petromyzon marinus) spawning. Individual ATS mass was 33%–48% greater and standard length was 9%–15% greater in treatment reaches relative to control reaches for 4 months following nutrient additions. Percent total lipids in YOY ATS were twice as great in treatment reaches 1 month following carcass analog additions and remained elevated in treatment fish for 2 more months. Absolute growth rates, based on otolith microstructure analysis, correlated with water temperature fluctuations in all reaches and were elevated by an average of 0.07 mm·day–1in treatment reaches for 1 month following carcass analog additions. Simulated sea lamprey MDNs increased juvenile ATS growth, which, via potential increases in overwinter survival and decreases in smolt age, may contribute to population persistence and ecosystem productivity.

Nutrient Enrichment with Salmon Carcass Analogs in the Columbia River Basin, USA: A Stream Food Web Analysis

AbstractAnadromous fishes represent an important ecosystem linkage between marine and inland aquatic and terrestrial habitats. These fishes carry organic matter and marine‐derived nutrient (MDN) subsidies across a vast landscape, often with profound influences on recipient ecosystem food web structure and function. In the Columbia River basin, century‐long declines in the abundance of anadromous fish populations have focused attention on potential mitigation efforts to address MDN deficits. In this study, we evaluate components of the stream food web response (periphyton, macroinvertebrate, and fish) to pasteurized salmon carcass analog (SCA) treatments in 15 streams across the Columbia River basin. Periphyton standing crop, macroinvertebrate density, and salmonid fish growth rates and stomach fullness measures increased following the addition of SCA. We found no significant change in dissolved nutrient concentrations after treatment, suggesting that biological demand exceeded supply. Nitrogen stable isotope signatures confirmed trophic transfer from SCA to lower trophic levels but were noticeably weak in fish tissue samples despite our marked growth and stomach fullness measures. These data indicate that SCA has the potential to increase the productivity of nutrient‐limited freshwater ecosystems and may provide a nutrient mitigation tool in ecosystems where MDNs are severely limited or unavailable.

Differential tree and shrub production in response to fertilization and disturbance by coastal river otters in Alaska

We explored the interacting effects of marine-derived nutrient fertilization and physical disturbance introduced by coastal river otters (Lontra canadensis) on the production and nutrient status of pristine shrub and tree communities in Prince William Sound, Alaska, USA. We compared production of trees and shrubs between latrines and non-latrines, while accounting for otter site selection, by sampling areas on and off sites. Nitrogen stable isotope analysis (delta15N) indicated that dominant tree and shrub species assimilated the marine-derived N excreted by otters. In association with this uptake, tree production increased, but shrub density and nonwoody aboveground shrub production decreased. The reduced shrub production was caused by destruction of ramets, especially blueberry (Vaccinium spp.), through physical disturbance by river otters. False azalea (Menziesia ferruginea) ramets were less sensitive to otter disturbance. Although surviving individual blueberry ramets showed a tendency for increased production per plant, false azalea allocated excess N to storage in leaves rather than growth. We found that plant responses to animal activity vary among species and levels of biological organization (leaf, plant, ecosystem). Such differences should be accounted for when assessing the influence of river otters on the carbon budget of Alaskan coastal forests at the landscape scale.

Differential production of trees and shrubs in response to fertilization and disturbance by coastal river otters in Alaska

We explored the interacting effects of marine-derived nutrient fertilization and physical disturbance introduced by coastal river otters (Lontra canadensis) on the production and nutrient status of pristine shrub and tree communities in Prince William Sound, Alaska, USA. We compared production of trees and shrubs between latrines and non-latrines, while accounting for otter site selection, by sampling areas on and off sites. Nitrogen stable isotope analysis (δ15N) indicated that dominant tree and shrub species assimilated the marine-derived N excreted by otters. In association with this uptake, tree production increased, but shrub density and nonwoody aboveground shrub production decreased. The reduced shrub production was caused by destruction of ramets, especially blueberry (Vaccinium spp.), through physical disturbance by river otters. False azalea (Menziesia ferruginea) ramets were less sensitive to otter disturbance. Although surviving individual blueberry ramets showed a tendency for increased production per plant, false azalea allocated excess N to storage in leaves rather than growth. We found that plant responses to animal activity vary among species and levels of biological organization (leaf, plant, ecosystem). Such differences should be accounted for when assessing the influence of river otters on the carbon budget of Alaskan coastal forests at the landscape scale.

Investigating the effect of marine-derived nutrients from spawning salmon on macroinvertebrate secondary production in southeast Alaskan streams

It is generally accepted that juvenile salmonid production is linked, via bottom-up pathways, to marine-derived nutrient (MDN) inputs from spawning salmon. Many studies have used standing stock biomass estimates of aquatic macroinvertebrates to infer relationships between MDN and secondary production in streams that receive spawners. However, no study has measured aquatic macroinvertebrate secondary production in relation to MDN. To assess the relationship between MDN and aquatic insect production, we measured secondary production of the 5 dominant mayfly genera (Baetis spp., Drunella spp., Cinygmula spp., Epeorus spp., and Rhithrogena spp.) and chironomids throughout the primary growing season in 2 streams in southeastern Alaska. Both streams had upstream reaches blocked from spawning salmonids by a waterfall barrier and downstream reaches that received large spawning runs of pink and chum salmon. Four of the mayfly genera studied (Drunella spp., Cinygmula spp., Epeorus spp., and Rhithrogena spp.) had significantly greater production in upstream than in spawning reaches. Secondary production of Baetis spp. was similar between upstream and spawning reaches. Chironomid production was significantly greater in spawning than in upstream reaches. However, biomass of each taxon was maximized in the spring and summer before the primary period of MDN input. These patterns indicate that another factor, primarily spawning disturbance, is an important driver of benthic insect secondary production in these streams and might provide the community structure within which MDN subsidies occur. If these patterns are common in streams that receive salmon runs, then secondary production-mediated links between MDN and juvenile salmonid production might be mostly the result of responses of chironomids and other benthic organisms with similar life histories.

Seasonal variations in stable isotope ratios of juvenile coho salmon (Oncorhynchus kisutch) from western Washington rivers

Salmon carcasses provide a marine derived nutrient (MDN) subsidy to river systems, but the extent to which it affects juvenile salmon growth is unclear. To evaluate temporal and spatial nutrient contributions from watershed sources and MDNs using stable isotopes, Skagit River (Washington, USA) juvenile coho salmon (Oncorhynchus kisutch) were collected. Muscle samples were taken from fry through smolts to measure temporal changes in δ15N and δ13C. δ15N and δ13C levels declined from emergence until fall, when they approached values for resident cutthroat trout (Oncorhynchus clarkii) collected above anadromous barriers. Muscle δ13C was highly variable and did not increase subsequently. However, coho salmon δ15N increased during the winter. March coho salmon parr δ15N levels suggested high variability in carcass availability for consumption. During the next spring, δ15N levels again declined. In Griffin Creek, a Snoqualmie River tributary, a significant relationship between carcass density and δ15N and δ13C levels was found in March coho salmon parr. At high spawner densities, some parr δ15N exceeded carcass values; however, parr δ13C increased moderately. These findings show that stable isotope data provide insights on seasonal sources of nutrients. In addition, results indicate that March coho salmon parr δ15N levels would be a useful index of carcass availability for overwintering juvenile consumption.

Do bears facilitate transfer of salmon resources to aquatic macroinvertebrates?

In coastal areas of the Pacific Northwest bears (Ursus spp.) prey heavily on spawning Pacific salmon (Oncorhynchus spp.) and selectively kill energy-rich individuals that are the most recent arrivals on spawning grounds. Pacific salmon eventually die in spawning habitats anyway, albeit with considerably lower energetic content. We investigated whether foraging activities of bears facilitate growth of stream invertebrates by increasing the duration of salmon carcass availability and the nutritional value of carcasses for scavengers. Our survey in southwest Alaska showed that carcasses are highly colonized by caddisfly (Trichoptera) larvae. Caddisflies show a strong preference for bear-killed over senescent carcasses, which may be a result of extended temporal availability, improved accessibility of consumable tissue, and higher energetic content of bear-killed fish. Isotope analyses further indicate uptake of marine-derived nutrients in caddisflies during the salmon run, which, however, does not extend into subsequent generations. Thus, species with life histories linked to the annual marine derived nutrient pulse gain the biggest advantage from the salmon resource subsidy. A long-term survey in several creeks in this region showed that bear predation intensity varied greatly among creeks and years, therefore indirect effects of bear predation on aquatic scavengers are likely highly patchy in time and space.

Implications of Trends in Marine-derived Nutrient Influx to South Coastal British Columbia Salmonid Production

Marine nutrients and carbon transported by adult salmon are important to the productivity of the oligotrophic lakes and streams in which salmon spawn. Reduced carcass availability results in a decline in nutrient and carbon sources for stream-rearing salmonids. We examined 42 years of escapement records for five species of Pacific salmon for Georgia Strait, the west coast of Vancouver Island, and the mainland coast of British Columbia to estimate the status of this nutrient source. Salmon stocks from enhanced streams frequently dominated the total escapement of entire regions. As a result, most of the influx of marine nutrients is focused toward a few large stream systems already undergoing significant salmon enhancement, while nutrient influx to the more-numerous unenhanced streams is declining. In the large number of streams with smaller salmon escapements, stream-rearing species already in decline may decrease further from oligotrophication. Risk-averse escapement targets for wild salmon stocks need to include sufficient spawners to provide the nutrient influx linked to the maintenance of stream productivity. Declining trends in nutrient influx to wild salmon streams in most regions are a cause for concern and more intensive examination.