Salmon Carcasses Research Articles

Salmon Carcass Movements in Forest Streams

AbstractThe movements of salmon carcasses over time were studied in two forest streams in the context of a large‐scale salmon carcass supplementation program. The objectives were to assess both the level of treatment after stream flows had displaced carcasses and to evaluate whether the magnitude of carcass movements outside of a given reach could be predicted. The movements of hand‐placed, radio‐tagged salmon carcasses were studied and compared with those of flagged carcasses dropped by helicopter. Repeated surveys showed that, in both streams, radio‐tagged carcasses moved, on average, only a short distance, even after high‐flow events, the maximum observed movement being 1.14 km after 77 d. In‐channel wood and slow‐water habitats contributed most to the retention of carcasses. The amount of wood that was incorporated into jams of medium and high complexity (including accumulated pieces of small wood) was the best predictor of the proportion of carcasses that would be retained within a given length of stream, whereas the amount of pool habitat contributed to a lesser degree. A particular high‐complexity debris jam, however, confounded the use of hand‐placed carcasses to predict the distribution of movements of helicopter‐dropped carcasses by retaining almost all carcasses that encountered it. As in other studies, our results demonstrate the importance of wood in retaining salmon carcasses and show that complexity in the form of branches and accumulated small wood makes wood jams effective carcass collectors.

Uptake of Allochthonous Dissolved Organic Matter from Soil and Salmon in Coastal Temperate Rainforest Streams

Dissolved organic matter (DOM) is an important component of aquatic food webs. We compare the uptake kinetics for NH4–N and different fractions of DOM during soil and salmon leachate additions by evaluating the uptake of organic forms of carbon (DOC) and nitrogen (DON), and proteinaceous DOM, as measured by parallel factor (PARAFAC) modeling of DOM fluorescence. Seasonal DOM slug additions were conducted in three headwater streams draining a bog, forested wetland, and upland forest using DOM collected by leaching watershed soils. We also used DOM collected from bog soil and salmon carcasses to perform additions in the upland forest stream. DOC uptake velocity ranged from 0.010 to 0.063 mm s−1 and DON uptake velocity ranged from 0.015 to 0.086 mm s−1, which provides evidence for the whole-stream uptake of allochthonous DOM. These findings imply that wetlands could potentially be an important source of DOM to support stream heterotrophic production. There was no significant difference in the uptake of DOC and DON across the soil leachate additions (P > 0.05), although differential uptake of DOM fractions was observed as protein-like fluorescence was removed from the water column more efficiently than bulk DOC and DON (P < 0.05). Moreover, PARAFAC analysis of DOM fluorescence showed that protein-like fluorescence decreased downstream during all DOM additions, whereas humic-like fluorescence did not change. This differential processing in added DOM suggests slow and fast turnover pools exist for aquatic DOM. Taken together, our findings argue that DON could potentially fill a larger role in satisfying biotic N demand in oligotrophic headwater streams than previously thought.

Modeling Stream Network‐Scale Variation in Coho Salmon Overwinter Survival and Smolt Size

AbstractWe used multiple regression and hierarchical mixed‐effects models to examine spatial patterns of overwinter survival and size at smolting in juvenile coho salmon Oncorhynchus kisutch in relation to habitat attributes across an extensive stream network in southwestern Oregon over 3 years. Contributing basin area explained the majority of spatial variation (R2 = 0.57‐0.63) in coho salmon overwinter survival (range = 0.02‐0.63), with highest survival rates observed in smaller headwater and intermittent streams. Other habitat attributes, including proportional pool area, percent exposed bedrock substrate, percent broadleaf canopy cover, and adult salmon carcass density, were relatively poor predictors of survival. Indices of individual fish condition, including fall parr fork length, condition factor, and parasite infestation rates, were also relatively uninformative in coho salmon overwinter survival models. Coho salmon smolt length was primarily a function of length at the time of fall tagging, but stream type, contributing basin area (positive effect), thermal history (positive effect), and black spot infestation (i.e., trematode metacercariae; negative effect) were also important. The consistent, broad spatial gradients in overwinter survival observed in this study can help guide efforts designed to enhance coho salmon production in coastal streams and suggest that habitat protection, restoration, and enhancement strategies will be best guided by a whole‐basin context.

Do Dead Fish Tell Tales? DNA Degradation in Chinook Salmon (Oncorhynchus tshawytscha) Carcasses

Carcasses are potentially a valuable source of DNA for genetic studies of Pacific salmon (Oncorhynchus sp.). They can be collected at spawning grounds across a wide geographic area and include populations in streams in which it is logistically difficult or too intrusive to sample spawning fish. However, the quality and quantity of DNA from salmon carcasses is highly variable. Using a standardized set of microsatellite loci, we investigated the relationship of amplification success and genotyping errors to time since death and locus size in Chinook salmon (O. tshawytscha) carcasses. Amplification success declined rapidly from death, smaller loci had greater amplification success than larger loci, and genotyping errors were present in 5% of scored samples. Salmon carcasses can be a valuable source of genetic information; however, the level of effort needed to produce accurate and reliable data with microsatellite loci is considerable. Ideally, field collections should be frequent and focus on fresh carcasses. Data replication should also be incorporated into analyses to reduce amplification failures and genotyping errors. New techniques and markers should improve future cost-effectiveness when analyzing degraded tissue samples. However, we recommend that an evaluation of cost and time be performed at the onset of any population genetic study using salmon carcass tissues, regardless of the technique used.

Transportation of Pacific salmon carcasses from streams to riparian forests by bears

Predation on Pacific salmon by bears (genus Ursus L., 1758) can be an important ecosystem process because the spatial distribution of carcasses largely determines whether marine-derived nutrients cycle through aquatic or terrestrial pathways. Direct observations on three streams in southeastern Alaska indicated that 49% of the pink ( Oncorhynchus gorbuscha (Walbaum, 1792)) and chum ( Oncorhynchus keta (Walbaum in Artedi, 1792)) salmon killed by bears were carried into the forest. The tendency of bears to transport carcasses was independent of the sex and species of salmon, but unspawned fish were more often transported than fish that had completed spawning. Data on tagged sockeye salmon ( Oncorhynchus nerka (Walbaum in Artedi, 1792)) in one southwestern Alaska stream indicated that 42.6% of the killed salmon were transported, and that higher percentages were transported in years when salmon densities were greater. At six other streams, on average, 68.1% of the sockeye salmon killed were apparently transported away from the stream into the forest. Combining the data from all sites, the proportion of carcasses transported increased with water depth at the site. These results emphasize the role that bears play in mediating the interactions between nutrients from salmon and the terrestrial and aquatic ecosystems, and the variation in carcass distribution among streams and among years.

The ecology of terrestrial invertebrates on Pacific salmon carcasses

AbstractIn coastal streams throughout the north Pacific region, spawning salmon (Oncorhynchus spp.) subsidize terrestrial communities with their nutrients and carcasses. We document the previously unreported composition and ecology of terrestrial invertebrates using salmon carcasses in forest habitats from two high salmon density watersheds in coastal British Columbia. From experimental placement of 186 carcasses, terrestrial Diptera‐dominated salmon carcass decay (85.5% of carcasses). Overall, we recorded over 60 species from salmon carcasses, including saprophagous Diptera and Coleoptera (15 spp.), dipteran predators (eight spp.) and parasitoids (four spp.), and opportunistic predators, scavengers, and detritivores (24 spp.). Using stable isotopes of nitrogen and carbon, we reconstruct the dietary niches of select species relative to salmon muscle tissue and previously sampled non‐salmon feeding invertebrate species. From comparisons across seasons, sampling locations and larval and adult life stages, we find evidence for a diet of salmon tissue in flies (Calliphora terraenovae and Dryomyza anilis), and beetles (Nicrophorus investigator and Anthobium fimetarium). The parasitic wasps Alysia alticola and Atractodes sp. had the highest levels of enrichment of all species, representing their larval diet of fly larvae that have fed on salmon carcasses 1 year prior to adult wasp sampling. Temporal and spatial isotopic variation in insect indicator species varies by species mobility and the pathway of salmon nutrient uptake. Cataloguing these associations may be useful for developing indices of intact salmon runs, bear foraging, and subsequent nutrient transfer in coastal watersheds.

Effectiveness of Salmon Carcass Tissue for Use in DNA Extraction and Amplification in Conservation Genetic Studies

AbstractA key concern in conservation genetic studies is obtaining viable DNA for analysis. In Pacific salmon Oncorhynchus spp., carcasses represent a feasible alternative for obtaining this tissue. However, the relative speed with which a salmon carcass decomposes can affect the quality of the extracted DNA. We extracted DNA from three different tissues (anal fin, operculum, and scales) obtained from carcasses of Chinook salmon O. tshawytscha at three different levels of decomposition (slight, moderate, and extensive). Freshly euthanized fish were included as a control. Extraction of DNA was accomplished using two common methods: the Chelex method and the Qiagen DNeasy spin column method. For each extracted sample, polymerase chain reaction (PCR) was used to amplify one small microsatellite locus (170–375 base pairs [bp]), one large nuclear locus (971 bp), and one large mitochondrial locus (1,300 bp). Results suggest that DNA suitable for PCR amplification at all three loci can be readily obtained from carcasses with slight decay. Qiagen spin columns appeared to provide the best extraction method, and fin clips were the optimal tissue type. Qiagen spin columns were the only extraction method that yielded DNA suitable for PCR from carcasses with moderate to extreme decay, although positive results were less reliable. Chelex extractions proved effective for amplifying small microsatellite loci from fin and scale samples but only those from carcasses with slight decay. With further optimization of techniques, we believe salmon carcasses can be used as a suitable source of tissue for DNA extraction and subsequent study in a conservation genetics framework.

Differential Incorporation of Natural Spawners vs. Artificially Planted Salmon Carcasses in a Stream Food Web: Evidence from δ15N of Juvenile Coho Salmon

AbstractPlacement of salmon carcasses is a common restoration technique in Oregon and Washington streams, with the goal of improving food resources and productivity of juvenile salmon. To explore the effectiveness of this restoration technique, we measured the Δ15N of juvenile coho salmon (Oncorhynchus kisutch) as an index of the incorporation of salmon‐derived nutrients (SDN) in streams with varying amounts of natural spawners and artificially placed carcasses. Although Δ15N of juvenile coho salmon increased with salmon carcass mass in the stream channel, this relationship was only significant for natural spawners. Artificially placed carcasses did not appear to increase SDN utilization by juvenile coho salmon. Possible explanatory factors include the restricted spatial and temporal distribution of carcasses, general absence of eggs, and lack of substrate bioturbation associated with artificially placed carcasses. Our study suggests that artificially placed carcasses at the levels added in the Oregon Coast Range may not directly mimic the role of natural spawners in stream foodwebs.

Tracing salmon nutrients in riparian food webs: isotopic evidence in a ground-foraging passerine

The predictable annual spawning of anadromous salmon (genus Oncorhynchus Suckley, 1861) provides an important marine resource subsidy to terrestrial species throughout the North Pacific. Using isotopic ratios of nitrogen (δ15N) and carbon (δ13C), we examine pathways of salmon nutrient uptake by a ground-foraging passerine, the winter wren ( Troglodytes troglodytes (L., 1758)), captured above and below waterfall barriers to salmon migration from two rivers on the central coast of British Columbia. Wren feathers exhibited substantial variation in δ15N (range = 18.6‰) and δ13C (range =7.9‰), approximating the isotopic range of invertebrates sampled at our study sites. Mean δ15N values of feathers and feces representing the fall diet of winter wrens were enriched by 4.8‰ and 8.0‰, respectively, below versus above the waterfall barriers. We observed positive relationships between δ15N and δ13C in fall-grown feathers and fecal samples below the falls but not above the falls, and large positive shifts (as high as 14.3‰) in15N from summer to fall within individual birds. The large isotopic variation observed in this system likely can be explained by the mobility of wrens, isotopic variation in their prey, and individual variation in the consumption of fly larvae from salmon carcasses. Our results identify both direct and indirect pathways by which salmon-derived nutrients are cycled into top-level consumers.

Colonization and development of an Alaskan stream community over 28 years

The application of general successional theory to stream ecosystems has not been widely addressed due to a lack of long-term studies on stream channels at sufficiently large spatial scales. Wolf Point Creek in Glacier Bay, Alaska, a lake-fed stream that began to emerge from under glacial ice in the mid-1940s, offers an opportunity to address this imbalance. We examine the stream's development from 1977 to 2005, with reference to concepts of succession and community assembly. Dispersal constraints have influenced the succession, as non-insect taxa required at least 20 years to colonize. We suggest that tolerance is a major mechanism of macroinvertebrate community assembly. Most taxa, with the exception of the cold-tolerant first colonizers, have persisted within the community following colonization, although relative abundance has changed markedly with time. However, biotic processes do influence colonization and succession. Redd (nest) digging by spawning salmon creates disturbed patches that facilitate the persistence of some early colonizers, and riparian vegetation facilitates colonization by caddisflies and chironomids. We further suggest that both deterministic and stochastic elements influence succession and community assembly in streams. Our study highlights the importance of reestablishing riparian vegetation during stream restoration programs and of increased in-stream habitat complexity from inputs of coarse woody debris to improve nutrient retention, particularly of salmon carcasses.

Resident Harbor Seals (&lt;I&gt;Phoca vitulina&lt;/I&gt;) in Iliamna Lake, Alaska: Summer Diet and Partial Consumption of Adult Sockeye Salmon (&lt;I&gt;Oncorhynchus nerka&lt;/I&gt;)

This study assessed the summer diet and consumption patterns of harbor seals (Phoca vitulina) resident in Iliamna Lake, Alaska. The authors predicted that adult sockeye salmon (Oncorhynchus nerka), a seasonally abundant and nutrient-rich prey source, would dominate diets when available and that seals would preferentially consume the most energetically profitable portion of salmon carcasses. Diet was examined by identifying hard parts of prey found in harbor seal scats, and consumption patterns were measured by collecting carcasses of harbor seal-killed sockeye salmon along island spawning grounds. Salmonids were present in 98% of scats that contained identifiable prey, followed by petromyzontids, osmerids, cottids, coregonids, and gasterosterids. The carcass surveys provided evidence of selective consumption patterns of sockeye salmon body parts. Harbor seals consumed the bodies of nearly all (96.6%) male salmon collected, leaving little but the head. In contrast, the belly and eggs were consumed in 63.6% of the female samples, and the entire body was eaten in only 31.3% of females. The harbor seals in Iliamna Lake thus took advantage of the seasonally abundant adult sockeye salmon by consuming them selectively and as a high proportion of their diet, but they also consumed smaller resident fishes, which presumably sustain them during the rest of the year.

Semipermeable membrane devices link site-specific contaminants to effects: Part 1 – Induction of CYP1A in rainbow trout from contaminants in Prince William Sound, Alaska

Extracts from semi-permeable membrane devices (SPMDs) deployed on beaches in Prince William Sound (PWS), Alaska, were used to evaluate if complex contaminant mixtures from different sources can be distinguished by the resulting cytochrome P450 1A (CYP1A) activity in exposed test animals. Deployment sites included canneries, salmon hatcheries, and beaches where lingering oil remains from discharges during the 1964 earthquake or the 1989 Exxon Valdez oil spill. Other sites were selected at random to evaluate region-wide contaminant inputs or were located in salmon streams to evaluate contaminants carried and released by migrating salmon carcasses following reproduction. Following standard deployments of approximately 28 d, an aliquot of the accumulated contaminants was intraperitoneally injected without cleanup into juvenile rainbow trout ( Oncorhynchus mykiss). After 2 d and 7 d, the activity of CYP1A was measured by the ethoxyresorufin- o-deethylase (EROD) assay. Exposure to extracts from the oiled sites and one hatchery site with numerous creosote pilings elicited strong EROD responses, whereas fish exposed to salmon stream extracts elicited weak but significant responses during late summer compared to late spring. Responses from the other sites were not significant, indicating contaminants from these sources are unlikely to cause CYP1A induction in resident biota. Rather than simply assessing extant contaminants, this method evaluates the potency of the different sites for bringing about aryl hydrocarbon receptor responses in resident biota.

Salmon carcasses as a marine-derived resource for benthic macroinvertebrates in a developing postglacial stream, Alaska

Streams formed in Glacier Bay, southeast Alaska, following glacial recession rapidly support populations of spawning salmon. Using both observational and experimental approaches, we examined the importance of pink salmon ( Oncorhynchus gorbuscha ) carcasses for benthic macroinvertebrate communities, approximately 10 years after salmon first colonized Wolf Point Creek, a first-order, lake-fed stream. Macroinvertebrate abundance on carcasses naturally accumulating in the stream channel was low, dominated by chironomids ( Eukiefferiella spp.). Macroinvertebrate drift was significantly lower from artificial experimental channels containing salmon flesh (P &lt; 0.001) compared with channels without. However, the density and community structure of macroinvertebrates colonizing rock baskets in channels was not significantly different between treatments. In a separate experiment, colonization of substrate trays with salmon and control (inert) fillets were associated with differences in community structure; simuliids dominated control fillets, and chironomids dominated fish fillets, while underlying stone substrate was colonized by a more diverse community including Ephemeroptera, Plecoptera, and Trichoptera taxa (EPT). Excluding simuliids, abundance was significantly higher on fish compared with control fillets (P &lt; 0.001). These data suggest that scales and mucus prevent the direct consumption of flesh from whole carcasses, highlighting the role predators–scavengers in facilitating flesh consumption by benthic macroinvertebrates.

Going, Going, Gone: Is Animal Migration Disappearing

Going, Going, Gone: Is Animal Migration Disappearing

Dispersal mechanisms of macroinvertebrates colonizing salmon flesh in a developing Alaskan stream

Patterns of carcass colonization by macroinvertebrates suggest deterministic resource tracking. As salmon carcasses decay in riverine ecosystems, exudates and abraded particles enter the water column accompanied by qualitative changes in benthic biofilms. We used a combination of implanted substrates, controlled exposure experiments and descriptive surveys to examine dispersal and colonization behaviour of macroinvertebrates in a developing stream in the presence/absence of decaying salmon flesh. Macroinvertebrate colonization of implanted trays containing salmon flesh showed no relationship with their sequential downstream position and (potential) increasing gradient of decay products. Dispersal to trays was principally due to drift with colonization patterns indicative of resource evaluation by direct contact, as opposed to remote detection. Macroinvertebrates in enclosed exposure chambers switched between artificial channels with and without salmon flesh demonstrated no significant difference related to exposure history; chamber opening was followed by a net migration from both treatment and controls. In a companion experiment, macroinvertebrates freely colonizing rock baskets switched between channel treatments, demonstrated no significant difference related to exposure history. Although the distribution of macroinvertebrates in the stream corresponded to habitats where carcasses accumulated, macroinvertebrate density in respective habitats was unrelated to carcass densities. The absence of a behavioural dispersal response to the presence of decaying salmon flesh in this study might be explained by the spatio-temporal design of the experiments, the use of previously frozen flesh as opposed to fresh carcasses, and the relatively early developmental stage of this stream and its associated macroinvertebrate community.

Return of Salmon-Derived Nutrients from the Riparian Zone to the Stream during a Storm in Southeastern Alaska

Spawning salmon deliver nutrients (salmon-derived nutrients, SDN) to natal watersheds that can be incorporated into terrestrial and aquatic food webs, potentially increasing ecosystem productivity. Peterson Creek, a coastal watershed in southeast Alaska that supports several species of anadromous fish, was sampled over the course of a storm during September 2006 to test the hypothesis that stormflows re-introduce stored SDN into the stream. We used stable isotopes and PARAFAC modeling of fluorescence excitation–emission spectroscopy to detect flushing of DOM from salmon carcasses in the riparian zone back into a spawning stream. During the early storm hydrograph, streamwater concentrations of NH4–N and total dissolved phosphorus (TDP), the fluorescent protein tyrosine and the δ15N content of DOM peaked, followed by a rapid decrease during maximum stormflow. Although δ15N has previously been used to track SDN in riparian zones, the use of fluorescence spectroscopy provides an independent indicator that SDN are being returned from the riparian zone to the stream after a period of intermediate storage outside the stream channel. Our findings further demonstrate the utility of using both δ15N of streamwater DOM and fluorescence spectroscopy with PARAFAC modeling to monitor how the pool of streamwater DOM changes in spawning salmon streams.

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.

Salmon carcass retention in recently formed stream habitat

Habitat manipulation for migratory fish is typically aimed at fulfilling particular life-cycle stages and rarely acknowledges the wider role of these fish in lotic ecosystems. Recent research, indicating the importance of salmon carcasses in stream ecosystems of the Pacific Northwest has highlighted their potential contribution in restoring degraded rivers. Previous research has emphasised the importance of in-stream habitat features such as pools and debris dams for carcass retention in mature river systems but carcass retention in other river habitats has not been well studied. This study documents the spawning runs of pink salmon (Oncorhynchus gorbuscha) and the retention of their carcasses in the early successional habitat of a recently formed stream. Eight years after initial colonisation, the estimated number of pink salmon spawners exceeded 10,000. Overall carcass retention was low and strongly related to discharge, with most carcasses transported out of the freshwater system to the estuary within a few days. The majority of carcasses that were retained accumulated in the slower moving waters in the stream margins, whilst the spatially scarce habitats of boulders, shallow areas and coarse woody debris were associated with comparatively high carcass densities. Mark and recapture of carcasses indicated that, with the exception of coarse wood debris, the retention of carcasses was an ephemeral and highly dynamic process with the structurally simple retentive features of this stream associated with a high turnover of carcasses.

Stream food web response to a salmon carcass analogue addition in two central Idaho, U.S.A. streams

Pacific salmon and steelhead once contributed large amounts of marine-derived carbon, nitrogen and phosphorus to freshwater ecosystems in the Pacific Northwest of the United States of America (California, Oregon, Washington and Idaho). Declines in historically abundant anadromous salmonid populations represent a significant loss of returning nutrients across a large spatial scale. Recently, a manufactured salmon carcass analogue was developed and tested as a safe and effective method of delivering nutrients to freshwater and linked riparian ecosystems where marine-derived nutrients have been reduced or eliminated.We compared four streams: two reference and two treatment streams using salmon carcass analogue(s) (SCA) as a treatment. Response variables measured included: surface streamwater chemistry; nutrient limitation status; carbon and nitrogen stable isotopes; periphyton chlorophyll a and ash-free dry mass (AFDM); macroinvertebrate density and biomass; and leaf litter decomposition rates. Within each stream, upstream reference and downstream treatment reaches were sampled 1 year before, during, and 1 year after the addition of SCA.Periphyton chlorophyll a and AFDM and macroinvertebrate biomass were significantly higher in stream reaches treated with SCA. Enriched stable isotope (δ15N) signatures were observed in periphyton and macroinvertebrate samples collected from treatment reaches in both treatment streams, indicating trophic transfer from SCA to consumers. Densities of Ephemerellidae, Elmidae and Brachycentridae were significantly higher in treatment reaches. Macroinvertebrate community composition and structure, as measured by taxonomic richness and diversity, did not appear to respond significantly to SCA treatment. Leaf breakdown rates were variable among treatment streams: significantly higher in one stream treatment reach but not the other. Salmon carcass analogue treatments had no detectable effect on measured water chemistry variables.Our results suggest that SCA addition successfully increased periphyton and macroinvertebrate biomass with no detectable response in streamwater nutrient concentrations. Correspondingly, no change in nutrient limitation status was detected based on dissolved inorganic nitrogen to soluble reactive phosphorus ratios (DIN/SRP) and nutrient-diffusing substrata experiments. Salmon carcass analogues appear to increase freshwater productivity.Salmon carcass analogues represent a pathogen-free nutrient enhancement tool that mimics natural trophic transfer pathways, can be manufactured using recycled fish products, and is easily transported; however, salmon carcass analogues should not be viewed as a replacement for naturally spawning salmon and the important ecological processes they provide.

The Influence of Water Temperature on Time to Surface for Adult Sockeye Salmon Carcasses and the Limitations in Estimating Salmon Carcasses in the Fraser River, British Columbia

AbstractWe determined the postmortem specific gravity and examined the relationship between water temperature and the time to surface for carcasses of adult sockeye salmon Oncorhynchus nerka. These results, in conjunction with other biophysical factors, were used to explain the difficulties associated with the detection of the carcasses of sockeye salmon that die during upstream migration in the Fraser River. Large negative differences have occurred between the sockeye salmon estimates at the river entry and spawning escapement (&gt;0.5 million in 8 of the past 15 years), but relatively few carcasses are observed. Without confirmation of mortality, managers are left with the dilemma of how to partition these differences among lower river escapement, spawning ground escapement, catch, en route mortality, and unreported catch. We investigated the reasons for the paucity of carcass observations by experimentally testing the influence of water temperature on carcass buoyancy and by examining hydrological factors that may affect carcass visibility. Because the Fraser River is turbid (Secchi depth, &lt;40 cm), only carcasses on the surface can be observed. Initially, all fish had specific gravity estimates of greater than 1 (mean ± SD, 1.057 ± 0.010) and were negatively buoyant when placed in tanks at different water temperatures. The time for a sockeye salmon carcass to surface was inversely related to water temperature and ranged from 1–3 d at 20°C to 12–18 d at 6°C. Annual counts of dead fish floating on the surface were positively correlated with river water temperature. Fisheries managers should not always expect to see large numbers of dead salmon in years of high estimated en route losses, given the factors that influence the visibility of salmon carcasses in a large river system. We recommend alternative methods for achieving more accurate and precise estimates of en route mortalities.