Mortality Of Salmon Research Articles

Sex-specific differences in physiological recovery and short-term behaviour following fisheries capture in adult sockeye salmon (Oncorhynchus nerka)

Numerous laboratory and field studies have found that female Pacific salmon have higher mortality than males during their once-in-a-lifetime upriver spawning migration. However, the proximate cause(s) of this increased mortality are poorly understood. This study exposed sockeye salmon (Oncorhynchus nerka) to a mild capture and tagging stressor and evaluated physiological recovery and movement behaviour at 1 and 4 h postrelease. Female sockeye salmon did not expend more anaerobic energy in response to the stressor but did have higher plasma lactate levels 4 h after the stressor, indicating that females took longer to physiologically recover compared with males. In addition, female salmon had lower plasma glucose but higher plasma cortisol, plasma K+, and cardiac lactate levels compared with males. Male and female salmon had markedly different postrelease behaviours within the first hour of release; males were more likely to hold position within the staging area. Two potential mechanisms leading to increased mortality in female salmon were identified in this study: (a) prolonged recovery duration (possibly mediated by elevated plasma cortisol levels) and (b) insufficient oxygen delivery to the heart.

Development of a Piscirickettsia salmonis immersion challenge model to investigate the comparative susceptibility of three salmon species.

Piscirickettsia salmonis, the aetiological agent of salmonid rickettsial septicaemia (SRS), is a global pathogen of wild and cultured marine salmonids. Here, we describe the development and application of a reproducible, standardized immersion challenge model to induce clinical SRS in juvenile pink (Oncorhynchus gorbuscha), Atlantic (Salmo salar) and sockeye salmon (O. nerka). Following a 1‐hr immersion in 105 colony‐forming units/ml, cumulative mortality in Atlantic salmon was 63.2% while mortality in sockeye salmon was 10%. Prevalence and levels of the bacterium in kidney prior to onset of mortality were lower in sockeye compared with Atlantic or pink salmon. The timing and magnitude of bacterial shedding were estimated from water samples collected during the exposure trials. Shedding was estimated to be 82‐fold higher in Atlantic salmon as compared to sockeye salmon and peaked in the Atlantic salmon trial at 36 d post‐immersion. These data suggest sockeye salmon are less susceptible to P. salmonis than Atlantic or pink salmon. Finally, skin lesions were observed on infected fish during all trials, often in the absence of detectable infection in kidney. As a result, we hypothesize that skin is the primary point of entry for P. salmonis during the immersion challenge.

Reconsidering the Estimation of Salmon Mortality Caused by the State and Federal Water Export Facilities in the Sacramento–San Joaquin Delta, San Francisco Estuary

Combined water exports from Old River in the south end of California’s San Francisco Estuary (estuary) by state and federal pumping facilities entrain small fishes, including out-migrating juvenile salmon. Both export projects have fish salvage facilities that use behavioral barriers (louvers) in combination with screens to guide fish into collection areas from which they are trucked to release points in the western Delta. Sacramento River-origin Chinook Salmon are regularly taken in the projects’ fish salvage operations. Survival has been estimated within the boundaries of both intake structures, but not in Old River. Prevailing methods for estimating fish losses are based on studies of louver efficiency, near-field survival at the state facility, and assumed survival at the federal facility. The efficiency of the fish salvage operations is affected by several factors, including intake velocity, debris build-up on the louvers and trash racks, and by the omnipresence of predators in front of and within the fish guidance structures. Analysis of existing data suggests that under average conditions, juvenile salmon survive entrainment into the forebay of the state facility at a rate of less than 10%. There is no evidence for better survival at the federal facility. We found no data on predation outside of either the state’s forebay or the federal trash boom, structures which are separated by an approximately 2-km reach of Old River where predation on small fish is thought to be intense. We suggest an improvement to the existing loss estimation, and discuss some features of the studies needed to increase its accuracy and precision.

Documentation of en route mortality of summer chum salmon in the Koyukuk River, Alaska and its potential linkage to the heatwave of 2019.

This paper documents a mass en route mortality event of adult summer chum salmon (Oncorhynchus keta) returning to the Koyukuk River, Alaska in the Yukon River basin. In response to reports from local communities, a small team of researchers (including the author) surveyed ca. 275 km of river on July 26 and 27, 2019 and counted 1,364 dead salmon. Although the total magnitude of mortality is unknown, counts from the survey certainly represent only a small fraction of the true number of fish that died. We sampled 73 carcasses to confirm death occurred prematurely prior to complete maturation and spawning, and to quantify sex and length. Visual inspection revealed a substantial fraction exhibited patterns of fungal growth consistent with secondary infections of skin lesions caused by the ubiquitous natural bacterial pathogen Flavobacterium columnare. Water temperatures during the survey averaged 17.1°C and the water was approximately 85% saturated with oxygen (ca. 8.5 mg/L), which likely contributed to the stress for upstream migrants. Evidence suggests size‐selective en route mortality as female migrants that died were 2% and male migrants 5% shorter than individuals that survived to their spawning grounds on Henshaw Creek. This translates to very strong estimates of natural selection using standardized selection differentials, yet it is unclear whether selection acts on body size directly or indirectly through correlated phenotypic traits such as run timing. The mortality event likely underpins the below average returns of summer chum salmon to the Koyukuk River in 2019, suggesting an impact on spawner abundance. The future consequences of this, or potentially increasingly frequent, en route mortality events for population productivity and the extent to which genetic adaptation or adaptive phenotypic plasticity of migration behavior may facilitate persistence of these populations is unknown.

Mortality of Atlantic salmon after catch and release angling: assessment of a recreational Atlantic salmon fishery in a changing climate

Human activities have the potential to accelerate population-level decline by contributing to climate warming and decreasing the capacity of species to survive warming temperatures. Here we build a predictive model to test interactions between river warming and catch and release mortality in recreational fisheries for Atlantic salmon (Salmo salar) by compiling and analyzing published data. We then test whether warming has occurred in rivers where angling occurs and whether angling opportunities have been restricted through increased river closures due to high water temperatures. We find that catch and release mortalities are low (<0.05) at cool river temperatures (<12 °C). At river temperatures often leading to fishery closures (between 18 and 20 °C), mortalities range from 0.07 to 0.33 (mean = 0.16). River temperatures on the east and southeast coasts of Newfoundland have warmed, leading to an increase in fishery closures in recent years. By contrast, river temperatures in southern Labrador have warmed slightly, with only one documented river closure. Accordingly, increasing temperatures will increase the frequency of river closures and likely result in higher mortality in caught and released Atlantic salmon in rivers that remain open to catch and release angling at warm water temperatures.

Detection of Piscine orthoreoviruses (PRV-1b AND PRV-3a) in farmed Coho salmon with jaundice syndrome from Chile

Piscine orthoreoviruses (PRVs) represent a group of marine viruses belonging to Reoviridae family, which are associated to emerging diseases in farmed Atlantic salmon, Rainbow trout and Coho salmon around the world. In Chile, the prevalence and deleterious effects of PRV infection in farmed salmonids had been studied recently, showing the occurrence of PRV-1 (formerly PRV-Ss) in Atlantic salmon with Heart and Skeletal Muscle Inflammation (HSMI), PRV-3 (formerly PRV-Om) in Rainbow trout with HSMI-like disease, and both viruses (PRV-1 and PRV-3) in Coho salmon with HSMI-like disease. In 1997, Jaundice Syndrome in Coho salmon was first described in Chile associated to outbreaks with high mortality, which occur during autumn season. Then, it was reported in 2006 that the jaundice condition in Coho salmon is consistent with an infectious form of hemolytic anemia and probably a virus was the causative agent, however the identification of the putative virus was not possible. Currently, the etiology of this syndrome is unknown, but several of the clinical signs exhibited by diseased Coho salmon are comparable to the signs showed by salmonids affected with some disease which are caused by PRVs. Therefore, it was of interest to determine if some PRVs could be related to Jaundice Syndrome in Coho salmon. In this manner, heart and head kidney tissue samples obtained from Coho salmon mortalities were analyzed in search of viral pathogens, where the presence of both PRV-1 and PRV-3 was confirmed by RT-qPCR and Sanger sequencing. Moreover, full-length genomic segment S1 sequences were obtained to determine the phylogenetic relationship with others PRV isolates previously described in Europe, America and Japan. One group of sequences found in Coho salmon with Jaundice Syndrome clustered into Sub-genotype Ib together with PRV-1 isolate described in Coho salmon with HSMI-like condition from Chile, and other group of sequences clustered into Genotype II with the first PRV-3 detected in Rainbow trout from Norway into previously described PRV-3a clade. Our results also suggest that PRVs has developed an infection cycle, which involves to Atlantic salmon, Rainbow trout and Coho salmon.

Enhanced growth and decreased mortality in Atlantic salmon (Salmo salar) after probiotic bath.

Disease in farmed Atlantic salmon occurs in all its life stages. Salmon are particularly vulnerable to infectious diseases at transition from the freshwater stage to the saltwater stage. Our aim in these studies reported was to investigate the possibility that waterborne delivery of a probiotic comprised of naturally occurring marine bacterial species would reduce the mortality and improve the health and growthof farmed Atlantic salmon. In three trials at two aquaculture production sites in Norway, isolates of Aliivibrio bacteria were added to the rearing water of Atlantic salmon. The fish were followed in 4-6months after one single bath with observations and samplings. Growth, ulcers and survival were recorded. At the end of the studies growth was up to 31% larger in the probiotic enhanced groups and in trial 1 both mortality and prevalence of ulcer were significantly lower in the probiotic enhanced group compared to the control. Feed conversion rates were recorded in trial 1 and 2 and were from 9 to 28 % better for the probiotic enhanced groups compared to the control groups. Bathing of Atlantic salmon with probiotic Aliivibrio strains increased growth, reduced mortality and improved FCR in the postsmolt period. The study demonstrates the potential to enhance growth, prevent ulcers and decrease mortality in Atlantic salmon after adding probiotic strains of Aliivibrio spp. into the rearing water. The study can have impact on animal welfare, economy and sustainability in the aquaculture industry.

Increased prespawning mortality threatens an integrated natural- and hatchery-origin sockeye salmon population in the Lake Washington Basin

The life cycle of diadromous fishes such as salmonids involves natural mortality in a series of distinct life history stages, occurring sequentially in different habitats. Decades of research have emphasized mortality at the embryo, juvenile, and sub-adult stages but it is increasingly clear that some adults that survive and return to freshwater habitats die during the final homeward migration or after they reach the spawning grounds, prior to breeding. These are termed “en route” and “prespawning” mortality, respectively, and can threaten populations depleted by mortality at previous stages. In this study, we present evidence that the sockeye salmon, Oncorhynchus nerka, population that returns to the Lake Washington Basin, in Washington State, USA, is experiencing both forms of adult mortality. Counts of the salmon entering the basin on their return migration in June and July were compared to counts in the major spawning grounds in September through November for 1995–2018. The disparity has increased markedly in recent years. The counts on the spawning grounds have decreased as a proportion of the number entering the system with an average 49 % of sockeye unaccounted for, consistent with increased en route mortality. In addition, prespawning mortality rates have increased in salmon that reach the Cedar River, the main spawning tributary, both at a hatchery holding adult fish in 1995–2018, and in the naturally spawning populations when monitored in the last five years. Hatchery records indicated <10 % prespawning mortality for 1995–2010, increasing to an average 43 % for 2014 – 2018. Recent carcass surveys in the Cedar River documented that 33.6% (2014), 22.3% (2015), 30.3% (2016) and 50.0% (2018) of female sockeye died before completing spawning. These recent increases in prespawning mortality have been associated with warm water during entry to freshwater, but comparably warm water in past decades had no such effect. Steady warming of river temperatures around the median run completion date from < 8.0 °C to > 13.0 °C was correlated with increased prespawning mortality rates at the hatchery from 1995–2018. We conclude that warming conditions during migration and spawning, in concert with other factors such as infections with pathogens, are responsible for the increased prespawning mortality of adult sockeye salmon that are high enough to threaten the population’s viability.

Transcriptomic response to elevated water temperatures in adult migrating Yukon River Chinook salmon (Oncorhynchus tshawytscha).

Chinook salmon (Oncorhynchus tshawytscha) declines are widespread and may be attributed, at least in part, to warming river temperatures. Water temperatures in the Yukon River and tributaries often exceed 18°C, a threshold commonly associated with heat stress and elevated mortality in Pacific salmon. Untangling the complex web of direct and indirect physiological effects of heat stress on salmon is difficult in a natural setting with innumerable system challenges but is necessary to increase our understanding of both lethal and sublethal impacts of heat stress on populations. The goal of this study was to characterize the cellular stress response in multiple Chinook salmon tissues after acute elevated temperature challenges. We conducted a controlled 4-hour temperature exposure (control, 18°C and 21°C) experiment on the bank of the Yukon River followed by gene expression (GE) profiling using a 3'-Tag-RNA-Seq protocol. The full transcriptome was analysed for 22 Chinook salmon in muscle, gill and liver tissue. Both the 21°C and 18°C treatments induced greater activity in genes associated with protein folding (e.g. HSP70, HSP90 mRNA) processes in all tissues. Global GE patterns indicate that transcriptomic responses to heat stress were highly tissue-specific, underscoring the importance of analyzing multiple tissues for determination of physiological effect. Primary superclusters (i.e. groupings of loosely related terms) of altered biological processes were identified in each tissue type, including regulation of DNA damage response (gill), regulation by host of viral transcription (liver) and regulation of the force of heart contraction (muscle) in the 21°C treatment. This study provides insight into mechanisms potentially affecting adult Chinook salmon as they encounter warm water during their spawning migration in the Yukon River and suggests that both basic and more specialized cellular functions may be disrupted.

Effect of simulated catch-and-release angling on postrelease mortality and egg viability in sockeye salmon (Oncorhynchus nerka)

Catch-and-release angling of salmonids is a widely implemented fisheries management strategy intended to provide recreational fishing opportunities while limiting harvest. Previous research has indicated mortality may be low, but the extent of sublethal impacts, including inability to reach spawning grounds or decreased spawning effectiveness, is relatively under-studied. Sockeye salmon (Oncorhynchus nerka) were subjected to simulated catch-and-release events, with a subset also enduring air exposure, to determine how physiological stress influences the ability of sockeye salmon to reach spawning grounds and the viability of eggs produced. Short-term postrelease mortality was highest for fish that were subjected to air exposure, but once recovered, neither simulated catch-and-release nor air exposure had a major impact on their ability to reach spawning grounds. Egg viability was similar between treatment groups, indicating physiological stress associated with the trials did not have a negative impact on reproduction. These results suggest that handling, especially air exposure, is the primary concern and substantially impacts the short-term postrelease mortality of sockeye salmon, which has important implications for management.

Spatial variation of juvenile Chinook Salmon (Oncorhynchus tshawytscha) survival in a modified California river

Spatial variation in habitat structure is a well-recognized characteristic of lotic ecosystems. The resulting patchwork mosaic provides opportunities for growth and survival as well as mortality risks for fish migrating through these systems. Dam construction within a watershed has numerous downstream tail water effects that alter creation and evolution of habitat features used by anadromous fishes that spawn and rear in these reaches. California Central Valley Pacific salmon (Oncorhynchus spp) are largely restricted to areas below impassable dams and these populations are undergoing widespread declines. High mortality of juvenile salmon in these reaches is thought to be a limitation to recovery. We conducted a radio telemetry study to evaluate spatial variation in juvenile Chinook Salmon (O. tschawytscha) survival and survival-environment relationships in three reaches of a Central Valley tail water. Additionally, we used mobile surveys to identify the location of last known detections to elucidate areas of disproportionate mortality. Our results indicated variation in flow and salmon size had greater explanatory power than year effects. However, relationships between flow, salmon size and survival were reach-specific indicating drivers of survival are spatially heterogeneous. Significant clumping of last known detections in the two lower reaches of the river suggests that mortality may be more frequent in these locations. Our results suggest that there is substantial spatial variation in migration mortality and both environmental conditions (flow) and salmon characteristics (size) have a strong influence on observed mortality. This information should be used to target actions that can support Chinook Salmon recovery.

Review of infectious agent occurrence in wild salmonids in British Columbia, Canada.

Wild Pacific salmonids (WPS) are economically and culturally important to the Pacific North region. Most recently, some populations of WPS have been in decline. Of hypothesized factors contributing to the decline, infectious agents have been postulated to increase the risk of mortality in Pacific salmon. We present a literature review of both published journal and unpublished data to describe the distribution of infectious agents reported in wild Pacific salmonid populations in British Columbia (BC), Canada. We targeted 10 infectious agents, considered to potentially cause severe economic losses in Atlantic salmon or be of conservation concern for wild salmon in BC. The findings indicated a low frequency of infectious hematopoietic necrosis virus, piscine orthoreovirus, viral haemorrhagic septicaemia virus, Aeromonas salmonicida, Renibacterium salmoninarum, Piscirickettsia salmonis and other Rickettsia-like organisms, Yersinia ruckeri, Tenacibaculum maritimum and Moritella viscosa. No positive results were reported for infestations with Paramoeba perurans in peer-reviewed papers and the DFO Fish Pathology Program database. This review synthesizes existing information, as well as gaps therein, that can support the design and implementation of a long-term surveillance programme of infectious agents in wild salmonids in BC.

A 2-stage hierarchical interrupted time-series analysis to quantify the long-term effect of subclinical bacterial kidney disease on performance of farmed Atlantic salmon (Salmo salar L.)

Bacterial Kidney Disease (BKD) is an economically significant disease in salmonid aquaculture and commonly requires antibiotic treatments to reduce its impact. Once a pen of fish is diagnosed with BKD, fish are considered chronically infected, potentially until harvest. Although there appears to be little or no evidence to support it, it is often assumed that subclinical infections affect productivity over the long term. We used a 2-stage hierarchical interrupted time series (ITS) analysis in an attempt to quantify the effect of subclinical BKD on mortality, growth, and food conversion ratio (FCR) of Atlantic salmon cultured in marine farms in Atlantic Canada. For all three outcomes, BKD had for some site cycles a positive effect, and for others a negative effect. Overall, the effect of BKD on mortality and growth could not be detected (effect -0.08 ((95% ci: -0.51, 0.35) and 0.00 (-0.02, 0.02)), while a very small effect showing an increase in FCR was detected (0.07 (-0.01, 0.15)). We hypothesized that minimal interference with fish performance may be compatible with the ecology of Renibacterium salmoninarum, the causative agent of BKD. For this organism, vertical transmission is a primary mode of propagation in low-density host populations as found in the wild. Since farms are always adapting and optimizing their farm management of BKD, these constant adjustments may also have negated our ability to detect the effect of many factors contributing to BKD productivity impacts. Hierarchical ITS analysis is considered an appropriate methodology to investigate the complex relationships with productivity measures over time under farming conditions. In the highly innovative salmon aquaculture industry, health records generating data available for time-series analysis is expected to become more accurate and abundant in the future, providing more opportunities for time-series regression studies.

Cumulative Effects of Thermal and Fisheries Stressors Reveal Sex-Specific Effects on Infection Development and Early Mortality of Adult Coho Salmon (Oncorhynchus kisutch).

Multiple stressors are commonly encountered by wild animals, but their cumulative effects are poorly understood, especially regarding infection development. We conducted a holding study with repeated gill and blood sampling to characterize the effects of cumulative stressors on infection development in adult coho salmon. Treatments included chronic thermal stress (15°C vs. 10°C) and acute gill net entanglement with an air exposure (simulating fisheries bycatch release). The potential loadings of 35 infectious agents and the expression of 17 host immune genes were quantified using high-throughput quantitative polymerase chain reaction, while host physiology was characterized with chemical analysis of blood. Temporal increases in infectious agent richness and loads were concurrent with decreased expression of immune genes in fish sampled in the river. In the laboratory, mortality was minimal in cool water regardless of fishery treatment (<15%). Elevated water temperature under laboratory conditions increased mortality of males and females (8% and 28% mortality, respectively, delayed by >1 wk) and enhanced mortality associated with handling and biopsy (∼40% both sexes). Experimental gillnetting at high temperature further enhanced female mortality (73%). Fish held at high temperature demonstrated heavier infectious agent loads, osmoregulatory impairment, suppressed female maturation, and upregulation of inflammatory and extracellular immune genes. At high temperature, heavy Parvicapsula minibicornis loads were associated with premature mortality. Females exhibited physiological impairment from both stressors after 1 wk, and infection burdens correlated poorly with immune gene regulation compared with males. Cumulative effects of multiple stressors on female mortality are likely a function of physiological impairment and enhanced infections at high temperature.

Fish disease dynamics in changing rivers: Salmonid Ceratomyxosis in the Klamath River

The myxozoan parasite Ceratomyxa shasta has been identified as the main contributor of mortality in salmon in the Klamath River, California. The life cycle of the parasite is complex, involving a polychaete and a salmonid host. Infection dynamics are greatly influenced by environmental factors, such as temperature and water velocity. If we are to control the disease it is important to predict the impact of environmental scenarios on spore concentration and infection prevalence. Here, we introduce a model based on partial differential equations to study the spore concentration in the river and the infection prevalence of returning salmon. The analysis of the model shows that for current climate conditions, additional dam release can reduce the actinospore concentration up to 48% and the prevalence up to 40% thus providing a potential disease management option. However, the infection risk is likely to increase for future climate conditions by 10–54% and this will lead to an infection level comparable to that of a recent high-disease year. Our simulations show that dam removal cannot be assumed to mitigate the effect of climate change or to have influence on infection prevalence. We show that our detailed model system can be reduced to a simpler exponential dose-response function, which predicts infection levels based on transmission rates, travel time and mean spore concentration, quantities that can be measured in-situ or given empirically. The dose-response function may therefore be a useful tool in disease management.

Survival under the Gravel: the Effects of Egg Size and Parental Identity on the In Situ Hatching Success of Chinook Salmon

AbstractEarly‐life mortality in salmon is influenced by both extrinsic (i.e., environmental) and intrinsic (i.e., parental) factors; however, the intrinsic factors have rarely been studied in the wild and relatively little is known about how egg mass and parental effects mediate in situ survival. Thus, we addressed four research objectives related to intrinsic factors and the early‐life survival of Chinook Salmon Oncorhynchus tshawytscha: (1) assess the effect of egg mass and parental effects on in situ hatching success, (2) assess the effect of redd gravel composition and its interaction with egg mass on in situ hatching success, (3) determine whether in situ egg mass–survival relationships have population‐level consequences, and (4) determine whether results from hatchery studies translate to the wild. Embryos from three Chinook Salmon populations located in the Laurentian Great Lakes were reared from fertilization to 15 d posthatch, both in the hatchery and in the wild. We found that egg mass influenced in situ hatching success in the Credit (concave down) and Sydenham rivers (positive‐linear), whereas there was no relationship between the two variables in the Pine River. Maternal identity explained a significant amount of the variation in hatching success for the Pine (20%) and Sydenham (12%) rivers. Within populations, no interaction was found between egg mass and gravel composition; however, hatching success was negatively related to the sand content of a redd when the data were pooled among the populations. Importantly, egg mass and quantitative genetic parameter estimates were different between samples from the hatchery and those from the wild, indicating that hatchery‐based observations cannot be transferred to the wild. Finally, we used both the in situ survival data and previously published data in a simulation analysis, which showed that both linear and nonlinear egg mass–survival relationships can greatly influence the number of alevins produced by a population, depending on where the population egg mass distribution falls along an egg mass selection gradient.

Salmonid alphavirus subtype I isolated from clinically-diseased Atlantic salmon, Salmo salar, in freshwater culture

Pancreas disease (PD) is a disease of cultivated salmonids caused by salmonid alphavirus (SAV) and it is responsible for significant losses within the aquaculture industry, at the marine stage, in Ireland, Norway and Scotland.In May 2018, an increased mortality of Atlantic salmon at a freshwater hatchery on the west coast of Scotland was reported to Marine Scotland Science, Fish Health Inspectorate. Site staff suspected an infectious pancreatic necrosis (IPN) outbreak, a condition frequently diagnosed at salmonid freshwater sites, but the RT-qPCR results were negative for IPNV and positive for salmonid alphavirus. This detection was subsequently confirmed as SAV subtype I by partial sequence analysis of the E2 gene. Histological examination revealed lesions in the pancreas, heart and skeletal muscle consistent with PD pathology.This is the first published description of a confirmed clinical field case of SAV with pathology consistent with PD in freshwater Atlantic salmon.

Estimation of spatiotemporal transmission dynamics and analysis of management scenarios for sea lice of farmed and wild salmon

Parasite transmission between farmed and wild salmon affects the sustainability of salmon aquaculture in Pacific Canada. Understanding and managing parasites in aquaculture is challenged by spatial and temporal variation in transmission dynamics. We developed a mechanistic model that connects sea louse (Lepeoptheirus salmonis) outbreak and control on farmed salmon (Salmo salar) to spatiotemporal dynamics of sea lice on migrating wild juvenile salmon (Oncorhynchus keta and Oncorhynchus gorbuscha). We fitted the model to time series of sea lice on farmed salmon and spatial surveys of juvenile wild salmon in the Broughton Archipelago. We used the parameterized model to evaluate alternative management scenarios based on the resulting sea louse infestations and predicted mortality of wild salmon. Early and coordinated management of sea lice on salmon farms was most effective for controlling outbreaks in wild salmon, while uncoordinated treatments led to a resurgence of sea lice on salmon farms during the juvenile salmon migration. This study highlights the importance of incorporating spatiotemporal variability when considering infectious disease dynamics shared by farmed and wild hosts, particularly when migratory wildlife are involved.

Ocean predation and mortality of adult Atlantic salmon

Predation and mortality are often difficult to estimate in the ocean, which hampers the management and conservation of marine fishes. We used data from pop-up satellite archival tags to investigate the ocean predation and mortality of adult Atlantic salmon (Salmo salar) released from 12 rivers flowing into the North Atlantic Ocean. Data from 156 tagged fish revealed 22 definite predation events (14%) and 38 undetermined mortalities (24%). Endothermic fish were the most common predators (n = 13), with most of these predation events occurring in the Gulf of St. Lawrence and from the Bay of Biscay to the Irish Shelf. Predation by marine mammals, most likely large deep-diving toothed whales (n = 5), and large ectothermic fish (n = 4) were less frequent. Both the estimated predation rates (ZP) and total mortality rates (ZM) where higher for Atlantic salmon from Canada, Ireland, and Spain (ZP = 0.60–1.32 y−1, ZM = 1.73–3.08 y−1) than from Denmark and Norway (ZP = 0–0.13 y−1, ZM = 0.19–1.03 y−1). This geographical variation in ocean mortality correlates with ongoing population declines, which are more profound for southern populations, indicating that low ocean survival of adults may act as an additional stressor to already vulnerable populations.

Estimating the take of migrating adult chum salmon (Oncorhynchus keta) caused by confined underwater rock blasting

This paper describes a methodology for estimating the take of upstream migrating adult chum salmon (Oncorhynchus keta) caused by confined underwater rock blasting. Because these fish are listed under the Endangered Species Act, it is unlawful to take (i.e., harm, capture, collect, injure, kill, etc.) them without a federal permit. In the permit for an underwater blasting project to deepen a 2 km section of the Columbia River navigation channel linking Portland, Oregon, to the Pacific Ocean, regulators defined take as the mortality of adult chum salmon due to underwater blasting. They required monitoring to estimate take to track compliance with the permit. Conventional predictive models of fish mortality from underwater blasting depend on data about the explosive charges; however, such data for this project were not available for proprietary reasons. Therefore, an innovative approach had to be conceived. The dose-exposure-response methodology we developed provided an unobtrusive, science-based methodology for monitoring and near real-time reporting of adult chum salmon take. We applied the methodology for 99 blasting events from November 1, 2009, through February 5, 2010, in the lower Columbia River (rkm 139–141). The mean absolute peak pressure in underwater sound generated by blast events was 151,685 Pa (22 psi) at a range of 42.7 m. The estimated cumulative take for the project was 0.126 adult chum salmon, far below the 10-fish mortality limit regulators set for the project. We propose that this dose-exposure-response methodology be considered wherever underwater blasting has the potential to have an adverse effect on important fish species.