Juvenile Pacific Salmon Oncorhynchus Research Articles

Microhabitat conditions and habitat configuration influence juvenile salmonid use of off-channel features in the Cedar River, Washington

Abstract Objective Off-channel habitats are crucial for freshwater rearing of juvenile Pacific salmon Oncorhynchus spp., but many of these river floodplain habitats have been lost to human development. While there is a clear need to restore and reconnect off-channel habitats to help recover imperiled salmonid populations, uncertainty exists regarding which characteristics to prioritize when constructing off-channel habitats to maximize the salmonid rearing capacity of these features. To address this uncertainty, we quantified the relative influence of microhabitat conditions and habitat configuration on juvenile salmonid densities within off-channel features of the Cedar River (Washington State). Methods We measured microhabitat conditions (water velocity, water depth, cover, and large wood density), habitat configuration (habitat type, distance from the main-stem river, and feature width), and salmonid densities at randomly selected sample units within side channels and backwaters. Result Water velocity and depth were strongly associated with subyearling Chinook Salmon O. tshawytscha, subyearling Coho Salmon O. kisutch, and yearling Coho Salmon densities, although the relationships varied among the species–age cohorts and some relationships changed during the spring sampling season. Cover and wood density were also associated with salmonid densities, but the relationships were weaker than for velocity and depth. Habitat configuration was associated with salmonid densities as well, although the relationships varied among the species–age cohorts and between habitat types. For example, within backwaters, subyearling Chinook Salmon densities were highest close to the main-stem river but yearling Coho Salmon densities were highest far from the main stem. Additionally, subyearling Chinook Salmon and Coho Salmon densities were highest within intermediate-width side channels, while yearling Coho Salmon densities exhibited a more complex response to side channel width. Conclusion Our results indicate that both microhabitat conditions and spatial configuration influence juvenile salmonid densities within off-channel habitats. These results also suggest that restoration practitioners should carefully weigh trade-offs among design alternatives because different habitat configurations will differentially benefit certain species or life stages.

Implications of increased spatial and trophic overlap between juvenile Pacific salmon and Sablefish in the northern California Current

AbstractObjectiveThe study was designed to assess long‐term variability in the distribution of juvenile Pacific salmon Oncorhynchus spp. and Sablefish Anoplopoma fimbria. The study also evaluated whether Sablefish and Pacific salmon shared food resources and looked to characterize Sablefish during an understudied period of their life cycle.MethodsTo meet the objectives, the study used data from 26 years of surface trawls conducted in Oregon and Washington coastal waters (1998–2023). Spatial–temporal models were used to measure changes in abundance and distribution of Pacific salmon and Sablefish along with covariates of ocean temperature. The study evaluated trophic characteristics of Pacific salmon and Sablefish from 2020 for differences. The temporal variation in size and diets of Sablefish were also analyzed, along with energy density of fish caught in 2020.ResultThe spatial–temporal model demonstrated that there has been a nearshore expansion of juvenile Sablefish over the past 26 years that was correlated with increased ocean temperature. The nearshore expansion of Sablefish resulted in increased spatial and trophic overlap with juvenile Pacific salmon. While feeding in nearshore waters, juvenile Sablefish demonstrated competitive feeding advantages over juvenile Pacific salmon during a critical phase of salmonid early marine life history. Juvenile Sablefish exhibited significant ontogenetic diet and energetic shifts, and even the smallest (68–80 mm fork length) were piscivorous.ConclusionsIf juvenile Sablefish numbers continue to increase relative to Pacific salmon, they could exert more competitive pressure, especially if food resources become limited. Pacific salmon may experience adverse effects from competition, regardless of whether or not juvenile Sablefish, which have recently expanded into nearshore waters, successfully recruit to the adult population.

Variation in estuary use patterns of juvenile Chinook salmon in the Fraser River, BC

Juvenile Pacific salmon ( Oncorhynchus spp.) use estuary habitats to varying degrees with some species and populations thought to rely heavily on these areas for early growth. In the Fraser River, British Columbia, there are 18 distinct conservation units of Chinook salmon ( O. tshawytscha), and all but one is of conservation concern. Our study compares the outmigration timing, size, and habitat use of juvenile Chinook salmon in the Fraser River estuary. Over 5 years (2016–2020), we captured 6493 juvenile Chinook salmon, with 3318 sampled for stock identification. Fraser River Chinook salmon extensively used estuary habitats, but patterns varied considerably by population. Juvenile Chinook salmon from the Lower Fraser River were most abundant and present the longest, arriving the smallest in late March and early April, and captured until July. South Thompson ocean-type Chinook salmon entered the estuary later, starting to arrive in late May or early June and remaining present until mid-August. Overall, juvenile Chinook salmon varied considerably in their estuary use across populations. Understanding this variation can inform differences in productivity and guide recovery actions.

Elodea mediates juvenile salmon growth by altering physical structure in freshwater habitats

Invasive species introductions in high latitudes are accelerating and elevating the need to address questions of their effects on Subarctic and Arctic ecosystems. As a driver of ecosystem function, submerged aquatic vegetation is one of the most deleterious biological invasions to aquatic food webs. The aquatic plant Elodea spp. has potential to be a widespread invader to Arctic and Subarctic ecosystems and is already established in 19 waterbodies in Alaska, USA. Elodea spp. has been found to alter ecosystem processes through multiple pathways; yet little is known about the impact of Elodea spp. on fish life history. A primary concern is the effect of Elodea spp. on juvenile Pacific salmon (Oncorhynchus spp.), because this invading plant can form dense stands in littoral zones, potentially impacting important freshwater rearing habitats used by juvenile fish for foraging and refuge from predators. We used a field experiment to test the effect of Elodea spp. on juvenile coho salmon (O. kisutch) growth in an infested lake near Cordova, Alaska, USA. We found that Elodea spp. stands result in reduced growth and a lower trophic position for juvenile coho salmon over the summer compared to habitats dominated by a native assemblage of aquatic plants. While infested sites were not associated with significant changes in water condition or primary productivity compared to sites dominated by native vegetation, zooplankton densities were reduced, and Elodea spp. height and vegetation richness increased macroinvertebrate densities. Combined, these results indicate that Elodea spp. may alter the flow of energy to juvenile salmon by restructuring space and affecting prey resources for rearing fish. Furthermore, these results suggest that widespread establishment of Elodea spp. may alter the quality of habitat for juvenile salmon and, by affecting juvenile fish growth, could lead to population-level impacts on salmon returns.

Migration timing affects the foraging ecology of Fraser River sockeye salmon stocks in coastal waters of British Columbia, Canada

Coastal migrations of juvenile Pacific salmon Oncorhynchus spp. have evolved to take advantage of optimal ocean foraging conditions and maximize early marine growth and survival. The growth and survival of salmon during the early marine period is affected by both the diversity of encountered coastal habitats, with varying productivity and plankton phenology, and stock-specific migration timings that determine the match-mismatch with their prey. In 2015 and 2016, we investigated temporal and spatial patterns in environmental conditions and zooplankton prey as well as diets and stock composition of juvenile Fraser River sockeye O. nerka during their outmigration through the tidally mixed Discovery Islands and Johnstone Strait in British Columbia (Canada). Three groups of sockeye diet profiles reflected variations in environmental conditions and prey communities. First, in the Discovery Islands, earlier migrating stocks primarily encountered and foraged on small, energy-poor zooplankton prey (barnacles and cladocerans). Second, later migrating stocks foraged mainly on larger, more energy-rich copepod and larvacean prey. And third, in the highly mixed waters of Johnstone Strait, large energy-rich calanoid copepods dominated diets irrespective of migration timing and year. Foraging success was typically low throughout the areas sampled and across the migration period, which may amplify the importance of prey nutritional quality. Our findings highlight the importance of accounting for spatial and temporal differences in foraging environments for migrating species such as juvenile salmon. Furthermore, we demonstrate that the timing of stock migration affects the foraging conditions experienced.

Acoustic Imaging Observes Predator–Prey Interactions between Bull Trout and Migrating Sockeye Salmon Smolts

AbstractAnnual migrations by juvenile Pacific salmon Oncorhynchus spp. smolts are predictable, presenting opportunities for predators to exploit these seasonal prey pulses. Directly observing predator–prey interactions to understand factors affecting predation may be possible via dual‐frequency identification sonar (DIDSON) acoustic imaging. Within Chilko Lake, British Columbia, prior telemetry and stomach content analyses suggested that the out‐migration of Sockeye Salmon Oncorhynchus nerka smolts influences the movements and aggregations of Bull Trout Salvelinus confluentus that feed extensively on smolts during their out‐migration. Bull Trout captured at a government‐installed counting fence exhibited high consumption of smolts, but it is only assumed that feeding occurred directly at the fence. We used DIDSON to assess fine‐scale predator–prey interactions between Sockeye Salmon smolts and Bull Trout over 10 d during the 2016 smolt out‐migration. We found that smolt–Bull Trout interactions were correlated with smolt densities at the counting fence, consistent with the prior diet studies in the system. Predator–prey interactions were also coupled with nocturnal migratory behaviors of Sockeye Salmon smolts, presumably to minimize predation risk. These results demonstrate that DIDSON technology can record interactions between predators and migrating prey at a resolution that can identify variability in space and time and provide insight on the role of anthropogenic structures (e.g., counting fences) in mediating such interactions.

Life in the Fast Lane: Feeding and Growth of Juvenile Steelhead and Chinook Salmon in Main‐Stem Habitats of the Columbia River Estuary

AbstractVery little is known about the ecology of juvenile Pacific salmon Oncorhynchus spp. that rapidly traverse estuaries. For these species and life history types, main‐stem habitats are thought to largely function as migratory corridors rather than as productive habitats that support feeding or growth, although little research has focused on this issue. This is especially true in the highly modified Columbia River estuary, where it is unclear whether fast‐migrating salmon benefit from extensive tidal marsh restoration. To address this deficit, we sampled migratory juvenile steelhead O. mykiss and Chinook Salmon O. tshawytscha at locations spread across nearly 200 km of the Columbia River estuary. Our results demonstrated that these juvenile salmon were actively feeding and growing as they moved downstream; dominant prey included chironomids, other insects, and corophiid and gammarid amphipods. We also observed variation in diet composition and quantity between years, which was likely associated with the highly contrasting river conditions. Insulin‐like growth factor 1 hormone levels and the size of juvenile salmon increased as the fish moved downstream, suggesting that prey quality and quantity were sufficient to fuel rapid growth, which may increase survival in marine waters. Our results have direct management implications for habitat restoration and suggest the potential for competition between hatchery and wild salmon. Overall, our results support a fundamental shift in the view of main‐channel estuarine habitats from serving primarily as a migration corridor to serving as productive habitat where rapidly moving salmon actively feed and grow.

Acoustic Classification of Juvenile Pacific Salmon (Oncorhynchus spp) and Pacific Herring (Clupea pallasii) Schools Using Random Forests

Acoustic surveys are the standard approach for evaluating many fish stocks around the world. The analysis of such survey data requires the accurate echo-classification of target species. This classification is often challenging as many organisms exhibit overlapping characteristics in terms of shape, acoustic amplitude, and behavior. In this study, a random forest approach was used to distinguish juvenile Pacific salmon (Oncorhynchus spp) from Pacific herring (Clupea pallasii) aggregations using the acoustic and morphological characteristics of their echo traces. The acoustic data was collected with an autonomous, multi-frequency echosounder deployed on the seafloor in the Discovery Islands, British Columbia from May to September 2015. The model was able to differentiate juvenile Pacific salmon from Pacific herring with a 98% accuracy. School depth and school mean volume backscattering strength were the most important predictors in determining the school classification. This study supports other publications suggesting that random forests represent a promising approach to acoustic target classification in fisheries science.

Environmental factors important to high-latitude nearshore estuarine fish community structure

Estuarine habitats exhibit spatial and temporal variability due to fluctuating hydrological inputs and heterogenous coastal formations. In high-latitude regions, such as the Gulf of Alaska, shifts in coastal hydrology may be further exacerbated by climate warming leading to structural responses in nearshore estuarine fish communities. To better understand relationships between physicochemical factors and fish community structure within high-latitude estuarine systems, we conducted systematic beach seine collections at multiple river mouth sites in a macro-tidal estuary located in the northern Gulf of Alaska (Kachemak Bay) during summer and fall of 2018. The effect of local physicochemical conditions (temperature, salinity, dissolved oxygen, turbidity), spatial factors (site, bay location, current type), and temporal factors (sampling date, month) were assessed in relation to community structure (taxonomic abundances). Juvenile fishes made up a vast majority of the community (99% of total catch) confirming the importance of estuaries as nursery habitats. Variability in fish community was best explained by the combined effects of site and month. Bay location (inner versus outer bay) and current type (low versus high water speeds) represented intermediate spatial factors (broader than site-level) that also had significant effects on community structure. Month and bay location were characterized by spatiotemporal gradients in dissolved oxygen and turbidity, respectively. Juvenile Pacific herring (Clupea pallasii) were associated with more turbid, less saline conditions found in the inner bay, while saffron cod (Eleginus gracilis) were associated with less turbid, more saline conditions of the outer bay. Juvenile Pacific salmon (Oncorhynchus spp.), starry flounder (Platichthys stellatus), and crescent gunnel (Pholis laeta) were associated with low current sites that were not well-defined by any measured physicochemical factor. Adult and juvenile Pacific sand lance (Ammodytes personatus) were good indicators of both the inner bay and high current sites. Temporal patterns in fish abundances confirmed the findings of prior studies in the area - juvenile salmonids appeared early, followed by juvenile gadids, and then sand lance later in the fall. Overall, variability in fish community structure was better explained by overarching spatial and temporal factors rather than by the local physicochemical conditions. These results point to broader scale coastal shifts as potential mechanisms driving changes in community structure within similar systems in this region.

Environmental Drivers of Nearshore Fish Community Composition and Size Structure in Glacially Influenced Gulf of Alaska Estuaries

Coastal ecosystems in Alaska are undergoing rapid change due to warming and glacier recession. We used a natural gradient of glacierized to non-glacierized watersheds (0–60% glacier coverage) in two regions along the Gulf of Alaska—Kachemak Bay and Lynn Canal—to evaluate relationships between local environmental conditions and estuarine fish communities. Multivariate analyses of fish community data collected from five sites per region in 2019 showed that region accounted for the most variation in community composition, suggesting that local effects of watershed type were masked by regional-scale variables. Seasonal shifts in community composition were driven largely by the influx of juvenile Pacific salmon (Oncorhynchus spp.) in late spring. Spatiotemporal differences among fish communities were partly explained by salinity and temperature, which accounted for 19.5% of the variation in community composition. We used a multi-year dataset from Lynn Canal (2014–2019) to examine patterns of mean length for two dominant species. Generalized additive mixed models indicated that Pacific staghorn sculpin (Leptocottus armatus) mean length varied along site-specific seasonal gradients, increasing gradually through the summer in the least glacially influenced sites and increasing rapidly to an asymptote of ~ 150 mm in the most glacially influenced sites. Starry flounder (Platichthys stellatus) mean length was more strongly related to environmental conditions, increasing with temperature and turbidity. Together, our findings suggest that community compositions of estuarine fishes show greater variation at the regional scale than the watershed scale, but species-specific variation in size distributions may indicate differences in habitat quality across watershed types within regions.

Nocturnal distributions of juvenile Pacific salmon along an eco-engineered marine shoreline

Nocturnal distributions and habitat preferences of juvenile fish along urban shorelines are understudied relative to daytime investigations. As a case study, nocturnal distributions of juvenile Pacific salmon (Oncorhynchus spp.) among ecological engineered and conventional seawall and pier habitats were characterized from May through August 2019 along the Seattle, WA, USA, waterfront. A multibeam sonar mounted beneath a kayak enabled day-night fish density comparisons to identify distribution and habitat use differences. Ecological engineering included mattresses (mesh bags filled with rocks) to create intertidal benches, a textured seawall to increase invertebrate colonization, and embedded glass blocks in an overhanging sidewalk to increase ambient light. Overall juvenile salmon night presence was twice that of daytime, and counts were 1.5 times higher during peak salmon densities. Juvenile salmon presence in eco-engineered and reference habitats was more similar at night compared to day. At night, juvenile salmon avoided traditional under-pier habitats and were more likely to navigate around piers that lack nearshore eco-engineered habitats. Increased use of eco-engineered habitats by juvenile salmon at night highlights the need to incorporate diel fish distribution differences in resource abundance estimates, and in the design and construction of coastline modifications.

Acoustic characterization of juvenile Pacific salmon distributions along an eco-engineered seawall

A mobile dual-frequency identification sonar (DIDSON) was used to characterize juvenile Pacific salmon (Oncorhynchus spp.) daytime use of armored and eco-engineered seawall habitats along an urbanized shoreline in Puget Sound, Washington, USA. Eco-engineering included intertidal benches to elevate the seafloor, a textured seawall to provide refuge and encourage invertebrate colonization, and glass blocks in an overhanging sidewalk to increase ambient light. A DIDSON multibeam sonar gave salmon counts twice that of visual surveys, and was thus deemed effective as a mobile sampling tool for small fish (~40-90 mm) and can be advantageous relative to visual methods depending on research goals, habitats, and ambient light levels. Increased salmon presence in the eco-engineered intertidal corridor relative to traditional seawall and pier habitats showed that the combination of increased light, reduced infrastructure (e.g. pier pilings), increased texture, and a shallower seafloor improves habitat function for juvenile salmon. High densities of juvenile salmon along pier ends show that salmon also use alternative migration pathways, with unknown energetic costs and predations risks.

Density‐dependent effects of eastern Kamchatka pink salmon (Oncorhynchus gorbuscha) and Japanese chum salmon (O. keta) on age‐specific growth of western Alaska chum salmon

AbstractHatcheries release >4.5 billion juvenile Pacific salmon (Oncorhynchus spp.) into the North Pacific Ocean annually, raising concerns about competition with wild salmon populations. We used retrospective scale analysis to investigate how the growth of chum salmon (O. keta) from western Alaska is affected by the abundance of chum salmon from Japanese hatcheries and wild pink salmon (O. gorbuscha) from the Russian Far East. Over nearly five decades, the growth of Kuskokwim River chum salmon was negatively correlated with the abundance of Japanese hatchery chum salmon after accounting for the effects of sex and spring/summer sea‐surface temperature in the Bering Sea. An effect of wild eastern Kamchatka pink salmon abundance on the growth of Kuskokwim River salmon was detectable but modest compared to the intraspecific competitive effect. A decrease in Japanese hatchery chum salmon releases in 2011–2013 was not associated with increased growth of Bering Sea chum salmon. However, the abundance of wild chum salmon from the Russian Far East increased during that time, possibly obscuring reduced competition with hatchery chum salmon. Our results support previous evidence that chum salmon are affected by intraspecific competition, and to a lesser extent interspecific competition, in the North Pacific, underscoring that the effects of salmon hatchery production transcend national boundaries.

Trophic responses of juvenile Pacific salmon to warm and cool periods within inside marine waters of Southeast Alaska

Marine growth and survival of juvenile Pacific salmon (Oncorhynchus spp.) have been linked to marine temperatures and feeding conditions during their first few months at sea. Therefore, understanding what salmon consume under varying environmental conditions is important to understanding how their growth and survival are affected by climate change. Here, we examined how warm/cool-phase variation in water temperature, salinity, wind speed, and pycnocline depth influenced the diet composition and quality of four species of juvenile Pacific salmon in Icy Strait (Southeast Alaska) from 2013 to 2017. During the five-year period, water temperatures shifted from a cool phase in 2013 to warm in 2014–2016, then back to cool in 2017. Overall, the diet composition and prey diversity varied among zooplanktivorous species (pink salmon O. gorbuscha, chum salmon O. keta, and sockeye salmon O. nerka) and piscivorous (coho salmon O. kisutch) species, with the exception of 2015, when euphausiids were the dominant prey for all four species. The summer of 2015 was notable for its deep pycnocline although it was not the warmest year in the study. Zooplankton nutritional quality was below average in 2015, but lipid intake by juvenile salmon appeared to be supplemented by the availability of larger euphausiid prey. Across years for all species, diet composition was weakly correlated with a combination of water temperature, salinity, and wind (Pearson correlation = 0.216). We conclude that while the marine heat wave altered the Gulf of Alaska ecosystem, within the range of variability observed during the study period, juvenile salmon were able to meet their energetic demands by switching to alternative prey.

Seawall as salmon habitat: Eco-engineering improves the distribution and foraging of juvenile Pacific salmon

Urban nearshore ecosystems are built environments that differ structurally and functionally from the natural ecosystems they replace. Eco-engineering offers the ability to enhance these ecosystems by reducing the impacts of shoreline modification. Recent studies have linked shoreline armoring and pier shade—common features of modified shorelines—to lower foraging rates, altered distribution patterns, and increased predation risk for juvenile salmon. The 2017 replacement of the Elliott Bay seawall in Seattle, WA, USA incorporated novel eco-engineering design elements, including glass light penetrating surfaces (LPS) in overwater structures, an elevated seafloor, and textured substrates in the seawall face to enhance nearshore habitat for juvenile Pacific salmon (Oncorhynchus spp.) migrating from natal streams to the ocean. To examine the effectiveness of seawall eco-engineering, we used snorkel surveys to assess changes in juvenile salmon spatial distribution and foraging patterns during peak outmigration (March–August), and measured ambient light penetration before and after seawall replacement. Overall, we found that juvenile salmon were distributed more evenly across a spatial mosaic of habitats following eco-engineering. LPS enhanced ambient light penetration in nearshore under-pier habitats, and juvenile salmon use of these habitats increased concurrently. Salmon densities under piers tended to be greatest at low tides, indicating that light availability may mediate selective habitat use. Feeding rates at sites immediately adjacent to the seawall increased both between and under piers following habitat enhancement. Our findings suggest that the elevated light levels facilitated by LPS, in concert with physical enhancements that provided more complex shallow water habitat, temper the negative effects of pier shade and shoreline armoring on juvenile salmon.

Spatiotemporal Variation and Size-Selective Predation on Hatchery- and Wild-Born Juvenile Chum Salmon at Marine Entry by Nearshore Fishes in Southeast Alaska

Abstract Juvenile Pacific salmon Oncorhynchus spp. experience high mortality at marine entry, yet our quantitative understanding of predation during this critical period is limited. We evaluated spatial, temporal, and size-based patterns of predation on hatchery- and wild-born juvenile Chum Salmon O. keta by two abundant predators in Southeast Alaska estuaries: the Pacific Staghorn Sculpin Leptocottus armatus and Dolly Varden Salvelinus malma. For the predators we sampled, Chum Salmon comprised 4.5% of the diet by weight for Pacific Staghorn Sculpin (n =937) and 19.6% of the diet by weight for Dolly Varden (n =448), with 88% of the individual Chum Salmon consumed originating from hatcheries. Variation in occurrence of Chum Salmon in diets was driven by date, site, and local Chum Salmon density. The quantity of Chum Salmon consumed by Pacific Staghorn Sculpin varied with predator length, Chum Salmon density, and the proportion of hatchery fish present; however, date was the only important predictor for Dolly Varden. The mean length of Chum Salmon in the diets of both predators was significantly shorter than that of concurrent hatchery releases or seine catches, suggesting size-selective predation on smaller individuals. This pattern indicates that hatchery strategies of releasing larger individuals may reduce the initial vulnerability of hatchery salmon to estuarine predators.

Vertical Distribution of Juvenile Salmon in a Large Turbid River

Abstract We know very little about the vertical distribution of downstream-migrating juvenile Pacific salmon Oncorhynchus spp. in large rivers. It is important for project engineers and fisheries managers to understand the potential interactions of fishes with in-river hydrokinetic devices, which harness a river's energy by spinning a turbine to produce electrical current without damming or impounding water. Currently, several rural Alaskan communities are considering development projects for hydrokinetic devices, including projects in the Tanana River, near Nenana, Alaska. Therefore, the goal of this study was to determine the vertical distribution of juvenile Chinook Salmon Oncorhynchus tshawytscha, Coho Salmon Oncorhynchus kisutch, and Chum Salmon Oncorhynchus keta, in the Tanana River, at a site (bottom depth ∼ 8, channel width ∼ 150 m) where communities may deploy future hydrokinetic turbines. Using a suspended wingless fyke net system during diurnal periods (0800–1800 hours), we found juveniles of all three species at all depths of the water column and no significant differences in catch-per-unit-effort among four depth categories (surface, midwater, deep water, bottom water). The occurrence of juvenile salmon throughout the water column indicates that they may interact with hydrokinetic devices, regardless of their depth. Future research to more comprehensively characterize fish distribution patterns and describe the outcomes of fish–turbine interactions may inform practices aimed at reducing potentially deleterious impacts of hydrokinetic devices on juvenile Pacific salmon.

Effect of a Mammal Excluder Device on Trawl Catches of Salmon and Other Pelagic Animals

AbstractConcern about bycatch of marine mammals by fishery research gear has led to the use of mammal excluder devices (MEDs) in some surveys. We measured the effect of a rigid‐grate MED on trawl fishing effectiveness for juvenile Pacific salmon Oncorhynchus spp. and other upper‐pelagic species in the northern California Current. Catches of 11 taxa were compared among sets of a pelagic rope trawl without an MED and with an MED in two orientations: angled upward or angled downward. We estimated differences in overall catch ratios and size selectivity for each MED orientation compared to the unmodified net. The MED orientation had a species‐specific influence on both catch ratio and size selectivity, with median ratios ranging from 0.0 to 1.8 for the upward orientation and from 0.4 to 2.7 for the downward orientation. Video observations helped to explain performance differences and may help to improve MED design. Use of an MED in trawl surveys may substantially increase or decrease the catch of various taxa, contributing further uncertainty to the analysis of survey time series for either abundance or catch composition.

Quantifying the Net Benefits of Suppression: Truncated Size Structure and Consumption of Native Salmonids by Invasive Northern Pike in an Alaska Lake

AbstractPredation by nonnative Northern Pike Esox lucius is implicated in the decline of native salmonid Oncorhynchus spp. populations in south‐central Alaska. In response to the ongoing invasion of Northern Pike, eradication and suppression programs are under way in several severely impacted watersheds throughout the region. As previous research indicates that owing to the particular composition of their diet small Northern Pike may have disproportionate impacts on juvenile Pacific salmon Oncorhynchus spp. (hereafter, “salmon”) populations, there is a potentially important trade‐off between overall predator reduction and truncation of the size structure of the population. We examined this trade‐off using bioenergetics models and field data collected during a 5‐year Northern Pike suppression program in Shell Lake, a heavily impacted Sockeye Salmon O. nerka nursery lake in the Susitna River drainage of south‐central Alaska. Our results confirm that Northern Pike of all sizes consumed salmon and that on average salmon made up a smaller proportion of the diets of large Northern Pike. However, large Northern Pike consumed more salmon biomass per capita than smaller Northern Pike, according to the bioenergetics model incorporating size‐dependent metabolic needs. Compared with the period prior to initiation of suppression efforts in 2013, Northern Pike consumed 68% less salmon in 2014, 87% less in 2015, and 88% less in 2016. Despite the fact that the catches are now dominated by smaller and younger Northern Pike, the suppression program has been highly effective at reducing the consumption of salmon in Shell Lake, with stark reductions in the catch of all size‐ and age‐classes. Importantly, the reduction in the abundance of the oldest (age 5) Northern Pike to near zero was an key element in this reduction in predator consumption. Therefore, we conclude that suppression strategies targeting a broad range of size‐classes of Northern Pike are more likely to be effective at reducing the consumption of salmon than strategies targeting only smaller size‐classes.

Microtrolling: an Economical Method to Nonlethally Sample and Tag Juvenile Pacific Salmon at Sea

AbstractMortality of juvenile Pacific salmon Oncorhynchus spp. in their first marine year is hypothesized to be a primary driver of variable recruitment in a changing ocean. Much contemporary research focuses on diet, distribution, growth, and survival during this period; however, existing methods of capturing juvenile salmon at sea are expensive and may be limited by topography and tidal currents. We assessed the feasibility of using a small vessel and modified recreational fishing gear (microtrolling) to nonlethally capture, sample, and tag juvenile Chinook Salmon O. tshawytscha during their first marine summer. Sampling was conducted in the Strait of Georgia, British Columbia, from August to October 2014. We captured 168 Chinook Salmon, 13 Coho Salmon O. kisutch, and 1 Chum Salmon O. keta in 72 h of active fishing; Chinook Salmon CPUE was greatest between 6 and 19 m and increased late in the afternoon and on the flood tide. To assess short‐term mortality related to capture, PIT tagging, and sampling, we maintained 66 microtroll‐captured Chinook Salmon (41 of which were PIT‐tagged) overnight in a net pen; only one mortality and one incidence of tag loss were observed. Microtrolling proved effective for systematically sampling juvenile Chinook Salmon across depths and habitats. Unlike alternative methods of sampling juvenile Pacific salmon (trawling and purse seining), the utility of microtrolling is likely limited to studies of Chinook Salmon and possibly Coho Salmon. The low cost of this method has potential to facilitate participation of frequently excluded stakeholders, including First Nations and community groups, in marine research on juvenile Pacific salmon.Received June 10, 2016; accepted November 1, 2016 Published online February 21, 2017