Salmon Abundance Research Articles

Adapting management of Pacific salmon to a warming and more crowded ocean

Abstract The North Pacific Ocean is warming and overall Pacific salmon abundance is higher now than at any other time in the past century. This increase in abundance is in large part due to warming-related changes in marine ecosystems at northern latitudes that primarily benefit pink salmon, and industrial-scale hatchery production to support commercial fisheries. A large body of evidence indicates that increasing and more variable ocean temperatures, as well as competition among salmon at sea, are associated with shifts in salmon productivity, body size, and age at maturation. However, these relationships vary by species, location, and time, resulting in increased harvest opportunities in some regions and exacerbated conservation concerns in others. The weight-of-evidence suggests North Pacific salmon nations should, as a minimum, limit further increases in hatchery salmon production until there is a better scientific understanding of hatchery and wild salmon distribution at sea, how they interact, and how the consequences of these interactions are influenced by broader climate and ecosystem conditions. Coordinated research to overcome knowledge gaps and develop strategies to reduce unintended interactions between hatchery and wild salmon could be funded (in part) by a tax placed on industrial-scale hatchery salmon releases. A tax would formalize recognition that there are finite prey resources to support salmon in the ocean and that both prey and wild salmon represent a “common property” whose use should not be without cost to those that seek to benefit from them. We highlight additional approaches salmon nations can take to adapt to changing conditions and suggest that improved communication and collaboration among North Pacific salmon research and management agencies will be key to balancing the benefits and risks of a warming and more crowded ocean.

Continental-scale nutrient and contaminant delivery by Pacific salmon.

The movement of large amounts of nutrients by migrating animals has ecological benefits for recipient food webs1,2 that may be offset by co-transported contaminants3,4. Salmon spawning migrations are archetypal of this process, carrying marine-derived materials to inland ecosystems where they stimulate local productivity but also enhance contaminant exposure5-7. Pacific salmon abundance and biomass are higher now than in the last century, reflecting substantial shifts in community structure8 that probably altered nutrient versus contaminant delivery. Here we combined nutrient and contaminant concentrations with 40 years of annual Pacific salmon returns to quantify how changes in community structure influenced marine to freshwater inputs to western North America. Salmon transported tonnes of nutrients and kilograms of contaminants to freshwaters annually. Higher salmon returns (1976-2015) increased salmon-derived nutrient and contaminant inputs by 30% and 20%, respectively. These increases were dominated by pink salmon, which are short-lived, feed lower in marine food webs than other salmon species, and had the highest nutrient-to-contaminant ratios. As a result, the delivery of nutrients increased at a greater rate than the delivery of contaminants, and salmon inputs became more ecologically beneficial over time. Even still, contaminant loadings may represent exposure concerns for some salmon predators. The Pacific salmon example demonstrates how long-term environmental changes interact with nutrient and contaminant movement across large spatial scales and provides a model for exploring similar patterns with other migratory species9.

Description of regression models for predicting the dynamics of pink salmon returns in the Kamchatka region based on climate-oceanological and population-genetic data

Several regression models for predicting returns of pink salmon in the Kamchatka region are presented. The data for 1990–2023 were analyzed. Among available climatic and oceanological indices, the most suitable for using as predictors for forecasting of pink salmon returns were the Pacific Decadal Oscillation (PDO) index, Western Pacific Cyclonic Index (WP), Arctic Oscillation (AO) index, and the sea surface temperature anomaly in the North Pacific. Multi-dimensional models of the «stock–recruitment» type were built on identified statistical patterns, which allowed to estimate potential abundance of the pink salmon returns to northeastern and western Kamchatka. Besides, methods for predicting the abundance of pink salmon returns on the data of fish counting in the sea are considered, using the materials of TINRO trawl surveys conducted in the Bering and Okhotsk Seas in the fall seasons of 2012–2023. To determine the abundance of pink salmon originated from West Kamchatka, genetic identification of regional composition of juveniles in mixed trawl catches was used. All tested methods have a high level of determination, but simpler regressive models are more prospective for practical forecasting of general trend in dynamics of pink salmon stocks in the Kamchatka region due to very weak generalization ability of more complicated models.

Comparison of Juvenile Pacific Salmon abundance, distribution, and body condition between Western and Eastern Bering Sea using spatiotemporal models

Recent changes in climate have had generally negative impacts on salmon stocks in the eastern and positive effects in the western Bering Sea (WBS). Eastern Bering Sea (EBS) salmon stocks have periodically experienced reduced productivity, size, and survival, whereas salmon stocks in the western Bering Sea (WBS) have tended to be more stable, with some stocks increasing in abundance. This difference could be partially attributed to the different habitats that juvenile salmon occupy during their first marine year, which often determines recruitment. Here, we used salmon surveys in the EBS and WBS to compare the status of juveniles in these two large marine ecosystems. We applied spatio-temporal vector autoregressive spatio-temporal (VAST) models to adjust for differences in timing, vessels and trawl gear used, and to compare the relative abundance and body condition indices (length, weight, and Fulton's condition factor) of three juvenile salmon species pink, chum and sockeye salmon (Oncorhynchus gorbuscha, O. keta, and O.nerka respectively) between the WBS and EBS from 2002 to 2022. Juvenile salmon in WBS exhibited a consistent trend of larger size and higher Fulton's condition factor, with greater abundance compared to EBS in even years. A clear even-odd year pattern, which is believed to be driven by pink salmon, occurred in all species’ relative abundance and body condition in the WBS, with a limited impact of interannual temperature changes. Conversely, temperature had a significant impact on EBS salmon, with much of the variation in relative abundance and body condition occurring between warm and cold periods, and even/odd patterns were found only in pink salmon relative abundance and body condition. We detected different WBS-EBS migration patterns, with EBS juveniles dispersing from nearshore habitats earlier than WBS juveniles but migrating offshore later that is connected to bathymetry and the broad shelf in the EBS.

Ecological implications of the pink salmon invasion in northern Norway-Aggregative responses and terrestrial transfer by white-tailed eagles.

Over the last 10 years, the spawning population of invasive pink salmon (Oncorhynchus gorbuscha) has increased in the river systems in northern Norway to a level that is causing concern about their impact on endemic fauna and ecosystem processes. The scale of transfer of pink salmon carcasses into the terrestrial ecosystem is likely to be a key determinant of terrestrial impact. Bears (Ursus sp.) are responsible for most such transfers in North America but are rare in Norway. The white-tailed eagle (Haliaeetus albicilla) is common however, and a candidate to be a main cause of such transfers. To evaluate this hypothesis, data on the abundance of white-tailed eagles and pink salmon were collected along the river Skallelv in northern Norway in 2021, a year the pink salmon spawned in the river, and in 2022, a year no pink salmon spawned in the river. The abundance of white-tailed eagles along the river was much higher the year pink salmon spawned in the river. Furthermore, white-tailed eagles were observed aggregating and catching pink salmon where and when pink salmon were present at the spawning and post-spawning stages. Based on our observations, we suggest that the white-tailed eagle is the main species involved in the transport of pink salmon from the river into the riparian zone in northern Norway and that other scavengers, in particular the red fox (Vulpes vulpes) and common raven (Corvus corax), play an important role in transporting pink salmon carcasses from the riparian zone to the wider terrestrial ecosystem.

Back from the brink: Estimating daily and annual abundance of natural-origin salmon smolts from 30-years of mixed-origin capture-recapture data

Evaluating the status and trends of natural-origin anadromous fish populations over time requires robust estimates of out-migrating juvenile abundance. Information on abundance is typically acquired by capturing actively migrating fish as they pass stationary monitoring platforms. Challenges to estimation include protracted migration timing, temporally varying capture probabilities and the contemporaneous presence of unmarked hatchery-origin fish. The confounding effects of unmarked hatchery fish are especially pernicious in systems hosting multiple hatchery programs with variable mark-rates among releases. Here, we address this problem for a regionally and culturally important population of Chinook salmon (Oncorhynchus tshawytscha) supported by a hatchery-supplementation program implemented in response to the listing of this population under the U.S. Endangered Species Act. We developed a model to estimate daily and annual abundance of naturally produced age-0 fall Chinook salmon passing Lower Granite Dam (Snake River, USA) for each of the last 30 years. We accounted for variable hatchery marking rates by integrating two related data sources: 1) release-recapture data of fish with individually identifiable tags and 2) counts of marked and unmarked sample of fish captured each day. We fit joint parameters for daily fish arrival and capture probabilities to these data to estimate the daily abundance of hatchery- and natural-origin fish. Our results show that from 1992 to 2021, the annual abundance of juvenile natural-origin Snake River fall Chinook salmon increased by two orders of magnitude. These results are the first comprehensive evaluation of multi-decadal trends in abundance and run-timing for this population. Our approach can be adapted to other runs and locations within the Columbia River basin or similar systems where out-migrating fish are monitored at fixed locations.

Model-based indices of juvenile Pacific salmon abundance highlight species-specific seasonal distributions and impacts of changes to survey design

Changes in survey implementation can bias traditional design-based indices of abundance, particularly for migratory species where local abundance can vary at short temporal scales. Spatiotemporal model-based estimators provide a flexible alternative that can better account for changes in the timing and location of surveys. Here we develop a geostatistical model, a generalized linear mixed effects model with Gaussian Markov random fields, that is sufficiently flexible to account for changes in survey design, as well as seasonal and interannual variability in abundance and spatial distribution. We then apply this model to a suite of migratory species—juvenile Pacific salmon—with survey data collected from southern British Columbia’s continental shelf over the previous 25 years. Juvenile Pacific salmon showed species-specific spatial distributions with considerable seasonal variability. While the location of species-specific hot spots varied among years, seasonal variation in distributions were greater than interannual variation. We found that a shift from standard transects and opportunistic sampling to a random stratified survey design resulted in lower estimates of abundance for four of five juvenile Pacific salmon species; however, these differences were not significant after accounting for other sampling attributes. Trends in abundance for several species shifted when models included survey and diel effects. Our approach provides a means of deriving indices of abundance for migratory species from surveys with variable effort in time and space. Ultimately such indices can be used to better understand the ecological mechanisms regulating productivity by linking distribution and abundance to environmental drivers.

Among‐river pattern in relative abundance of two salmonid fishes reflects temperature‐dependent competition

Abstract Temperature is a key driver in determining species distribution and abundance across climatic and geographical gradients. This occurs through direct physiological effects of temperature on performance, but may also result from temperature effects on species interactions. In the current study we experimentally test for temperature‐dependent competition between juvenile anadromous Atlantic salmon and brown trout and evaluate the role of temperature in shaping the relative abundance of these species across 88 Norwegian rivers. When the two species were reared in allopatry their somatic growth rates were similar across a range of temperatures (4–16°C). When reared in sympatry, the growth of salmon was greatly reduced at the coldest temperature relative to when reared in allopatry, whereas this was not the case for trout. The effect of interspecific competition on growth was more similar for the two species at warmer temperatures. Thus, interspecific competition effects were strongly asymmetric only at the lowest temperature, with trout outcompeting salmon. The results from the experiment were reflected in qualitative patterns of relative abundance of salmon and trout in natural populations, which shifted from being trout dominated in rivers with low summer temperatures to being salmon dominated in warmer rivers. These results provide an empirical example of how temperature effects on the relative abundance of species can only be understood in light of information about temperature‐dependent species interactions.

Pacific salmon in the Canadian Arctic highlight a range-expansion pathway for sub-Arctic fishes.

Rapid climate change is altering Arctic ecosystems at unprecedented rates. These changes in the physical environment may open new corridors for species range expansions, with substantial implications for subsistence-dependent communities and sensitive ecosystems. Over the past 20 years, rising incidental harvest of Pacific salmon by subsistence fishers has been monitored across a widening range spanning multiple land claim jurisdictions in Arctic Canada. In this study, we connect Indigenous and scientific knowledges to explore potential oceanographic mechanisms facilitating this ongoing northward expansion of Pacific salmon into the western Canadian Arctic. A regression analysis was used to reveal and characterize a two-part mechanism related to thermal and sea-ice conditions in the Chukchi and Beaufort seas that explains nearly all of the variation in the relative abundance of salmon observed within this region. The results indicate that warmer late-spring temperatures in a Chukchi Sea watch-zone and persistent, suitable summer thermal conditions in a Beaufort Sea watch-zone together create a range-expansion corridor and are associated with higher salmon occurrences in subsistence harvests. Furthermore, there is a body of knowledge to suggest that these conditions, and consequently the presence and abundance of Pacific salmon, will become more persistent in the coming decades. Our collaborative approach positions us to document, explore, and explain mechanisms driving changes in fish biodiversity that have the potential to, or are already affecting, Indigenous rights-holders in a rapidly warming Arctic.

Monitoring and predicting the presence and abundance of juvenile Atlantic salmon in tributaries according to habitat characteristics using environmental DNA

AbstractConservation of the Atlantic salmon Salmo salar requires to monitor the spatial distribution and abundance of juveniles at a local scale in tributaries. However, tributaries are rarely accounted for in monitoring programs despite their importance for juvenile life stages. This is mainly because inventories of young salmon populations in tributaries can be technically challenging with traditional methods, as the number of tributaries in a watershed can be important and their access limited compared to the main stem. In this study, we tested the use of environmental DNA (eDNA) to quantify the abundance of juvenile Atlantic salmon in tributaries. We successfully detected eDNA of juvenile Atlantic salmon in 19 tributaries of three main rivers of the Gaspé Peninsula (Québec, Canada) using quantitative real‐time PCR analyses. By comparing the eDNA approach with electrofishing surveys conducted in parallel to water sampling, we found that eDNA concentrations positively correlated with juvenile abundance, total biomass, and body surface area. The use of the allometrically scaled mass (ASM) instead of abundance improved the correlation. Furthermore, we demonstrated that the levels of eDNA molecules detected for juvenile Atlantic salmon were also correlated with water temperature and canopy cover measured in each tributary. Finally, we tested if eDNA concentrations measured in a tributary could be used as a reliable indicator of juvenile abundance or biomass in that tributary. We found that our models slightly better predicted juvenile biomass than juvenile abundance. The use of ASM did not improve model prediction, suggesting that further refinement would be required in the future. Our method will facilitate the implementation of conservation practices appropriate to the ecology of juvenile Atlantic salmon in tributaries.

A Review of Pacific Salmon Hatcheries in British Columbia, Canada, and Interactions With Natural Populations

Pacific salmon Oncorhynchus spp. hatcheries in British Columbia (BC) have been the major activity of the Salmonid Enhancement Program since 1977. However, the present diminished abundance of Pacific salmon and loss of fisheries in BC called for a review of hatchery effectiveness. The Pacific Salmon Foundation conducted a review resulting in 15 reports publicly available on the Pacific Salmon Foundation marine science website. The review includes evaluations of effectiveness of hatchery production, evidence of interactions with natural salmon populations (including rebuilding natural populations with hatchery‐produced salmon), and future opportunities via genomics. Beyond the estimation of juvenile production and contribution to catches and spawning escapements, the ability to assess effectiveness was limited. The assessments of interactions provided a few examples of positive outcomes, but most were negative. The lack of comparative assessments in purely wild populations seriously limits interpretations of hatchery–wild interactions. Future evaluations of the hatchery effectiveness require more quantitative and integrated assessments of hatchery and wild salmon, open data sharing for public review or research, and related studies could benefit from application of new genomic technologies. This paper presents recommendations to address these needs and calls for regularized reviews of the Salmonid Enhancement Program and its net value to sustainability of BC's Pacific salmon.

Influence of ecotourism on grizzly bear activity depends on salmon abundance in the Atnarko River corridor, Nuxalk Territory

AbstractEcotourism management can draw on theory and data related to non‐consumptive effects of risk on wildlife. The asset protection principle (APP) predicts that variable food supply and its associated risks will affect antipredator behavior; responses to predation risk should dominate when food reserves are high, while nutritional risk becomes more important when food reserves are limited. Additionally, the human shield hypothesis (HSH) describes how some individuals might seek human presence if it repels potential sources of risk. Using camera traps, we used generalized linear mixed effects and multinomial regression models to test components of the APP and HSH where ecotourism co‐occurs with grizzly bear (Ursus arctos) foraging during hyperphagia. When salmon abundance was high (+1 SD), bear activity (weekly detections) decreased by 13% with every 100 visitors/week. Under low salmon conditions, bear activity increased with visitor numbers, creating ‘high bear‐high visitor’ conditions. Consistent with HSH, detection data revealed an increased likelihood of detecting subordinate age‐sex classes compared with adult males when visitor numbers were high. Our findings suggest that when salmon are low, managers might consider limiting visitors to mitigate disturbance. More broadly, understanding how wildlife allocate anti‐predator behavior as a function of risk and food can inform conservation science and practice.

Biological and environmental covariates of juvenile sockeye salmon distribution and abundance in the southeastern Bering Sea, 2002-2018.

Climate change is altering the distribution and abundance of marine species, especially in Arctic and sub-Arctic regions. In the eastern Bering Sea, home of the world's largest run of sockeye salmon (Oncorhynchus nerka), juvenile sockeye salmon abundance has increased and their migration path shifted north with warming, 2002-2018. The reasons for these changes are poorly understood. For these sockeye salmon, we quantify environmental and biological covariate effects within spatio-temporal species distribution models. Spatio-temporally, with respect to juvenile sockeye salmon densities: (1) sea surface temperature had a nonlinear effect, (2) large copepod, Calanus, a minor prey item, had no effect, (3) age-0 pollock (Gadus chalcogrammus), a major prey item during warm years, had a positive linear effect, and (4) juvenile pink salmon (O. gorbuscha) had a positive linear effect. Temporally, annual biomass of juvenile sockeye salmon was nonlinearly related to sea temperature and positively related to age-0 pollock and juvenile pink salmon abundance. Results indicate that sockeye salmon distributed with and increased in abundance with increases in prey, and reached a threshold for optimal temperatures in the eastern Bering Sea. Changes in population dynamics and distribution of sockeye salmon in response to environmental variability have potential implications for projecting specific future food securities and management of fisheries in Arctic waters.

Human disturbance in riparian areas disrupts predator-prey interactions between grizzly bears and salmon.

Wildlife must increasingly balance trade-offs between the need to access important foods and the mortality risks associated with human-dominated landscapes. Human disturbance can profoundly influence wildlife behavior, but managers know little about the relationship between disturbance-behavior dynamics and associated consequences for foraging. We address this gap by empirically investigating the consequences of human activity on a keystone predator-prey interaction in a region with limited but varied industrial disturbance. Using stable isotope data from 226 hair samples of grizzly bears (Ursus arctos horribilis) collected from 1995 to 2014 across 22 salmon-bearing watersheds (88,000 km2) in British Columbia, Canada, we examined how human activity influenced their consumption of spawning salmon (Oncorhynchus spp.), a fitness-related food. Accounting for the abundance of salmon and other foods, salmon consumption strongly decreased (up to 59% for females) with increasing human disturbance (as measured by the human footprint index) in riparian zones of salmon-bearing rivers. Declines in salmon consumption occurred with disturbance even in watersheds with low footprints. In a region currently among the least influenced by industrial activity, intensification of disturbance in river valleys is predicted to increasingly decouple bears from salmon, possibly driving associated reductions in population productivity and provisioning of salmon nutrients to terrestrial ecosystems. Accordingly, we draw on our results to make landscape-scale and access-related management recommendations beyond current streamside protection buffers. This work illustrates the interaction between habitat modification and food security for wildlife, highlighting the potential for unacknowledged interactions and cumulative effects in increasingly modified landscapes.

Visual and genetic stock identification of a test fishery to forecast Columbia River spring Chinook salmon stocks 2 weeks into the future.

Modern fisheries management strives to balance opposing goals of protection for weak stocks and opportunity for harvesting healthy stocks. Test fisheries can aid management of anadromous fishes if they can forecast the strength and timing of an annual run with adequate time to allow fisheries planning. Integration of genetic stock identification (GSI) can further maximize utility of test fisheries by resolving run forecasts into weak- and healthy-stock subcomponents. Using 5 years (2017-2022) of test fishery data, our study evaluated accuracy, resolution, and lead time of predictions for stock-specific run timing and abundance of Columbia River spring Chinook salmon (Oncorhynchus tshawytscha). We determined if this test fishery (1) could use visual stock identification (VSI) to forecast at the coarse stock resolution (i.e., classification of "lower" vs. "upriver" stocks) upon which current management is based and (2) could be enhanced with GSI to forecast at higher stock resolution. VSI accurately identified coarse stocks (83.3% GSI concordance), and estimated a proxy for abundance (catch per unit effort, CPUE) of the upriver stock in the test fishery that was correlated (R 2 = 0.90) with spring Chinook salmon abundance at Bonneville dam (Rkm 235). Salmon travel rates (~8.6 Rkm/day) provided predictions with 2-week lead time prior to dam passage. Importantly, GSI resolved this predictive ability as finely as the hatchery broodstock level. Lower river stock CPUE in the test fishery was correlated with abundance at Willamette Falls (Rkm 196, R 2 = 0.62), but could not be as finely resolved as achieved for upriver stocks. We described steps to combine VSI and GSI to provide timely in-season information and with prediction accuracy of ~12.4 mean absolute percentage error and high stock resolution to help plan Columbia River mainstem fisheries.

Biennial patterns in Alaskan sockeye salmon ocean growth are associated with pink salmon abundance in the Gulf of Alaska and the Bering Sea

Abstract In response to ocean heating and hatchery production, pink salmon (Oncorhynchus gorbuscha) returning from the North Pacific Ocean steadily increased after 1975, leading to concerns about their influence on food webs and competition with other species. Using measurements of distance between scale annuli of 24 584 individual sockeye salmon (O. nerka), we examined growth during their 2 or 3 years at sea from 1977 to 2015 for eight populations in Alaska. We found significant, negative autocorrelations at 1 lag year in annual growth of sockeye salmon, with a consistent pattern of lower growth in odd years, i.e. opposite to the biennial pattern of pink salmon abundance. Peak pink salmon abundances reduced growth of sockeye salmon from 7 to 14% during the second year in the ocean compared with growth when pink salmon abundance was low, while third-year growth was reduced up to 17%. The overall effect of pink salmon abundance on sockeye growth was over two times greater than the effect of sockeye salmon abundance. Production hatcheries and ocean heating contribute to the competitive dominance of pink salmon, underscoring the need to consider this unintended anthropogenic effect on the growth and productivity of sockeye salmon throughout the North Pacific.

Juvenile salmonid abundance in a diamictic semi‐fluvial stream in Norway—does stream bed shelter beat large woody debris?

AbstractThis study investigates the effect of large woody debris (LWD) on the abundance of juvenile Atlantic salmon (Salmo salar, L.) and anadromous brown trout (Salmo trutta, L.) in semi‐alluvial side channels of the river Aurlandselva (Norway) using point electrofishing and microhabitat mapping. Not the presence of LWD, but stream bed shelter availability and the distance to spawning grounds affected the fish abundance (fish/point), independent of other habitat components. LWD showed only an effect on fish abundance when in interaction with other habitat components. This discrepancy can be explained by the availability of cavities in the shelter‐rich coarse substrate which provide sufficient cover and territory for juvenile fish at the given carrying capacity of river Aurlandselva. Whilst LWD may be most effective to provide shelter in lowland streams (bed slope <0.005), maintaining or restoring shelter‐rich coarse substrates should be considered a key priority in steeper salmonid rivers and associated semi‐fluvial streams.

Identifying drivers of demographic rates in an at‐risk population of marine mammals using integrated population models

AbstractEffective conservation of threatened populations requires identification of the processes limiting recovery. When multiple population processes are potentially limiting, they are sometimes analyzed independently, often using different datasets. Analytically, this is suboptimal, as processes are often correlated, which can lead to biased estimates of parameters and quantities of interest. Integrated population models (IPMs) can synthesize several data streams in the same probabilistic framework to circumvent these issues. Lack of prey was identified as one of the primary threats to recovery of critically endangered southern resident killer whales (SRKWs), Orcinus orca. Previous studies have correlated SRKW demographic rates with indices of Chinook salmon Oncorhynchus tshawytscha abundance, but these approaches have modeled reproduction and survival independently. We developed an IPM for SRKWs that models both processes simultaneously, as a function of Chinook salmon abundance along the west coast of North America. We used this model to evaluate the relationship between Chinook salmon abundance and demographic rates of SRKWs, with the goal of updating our understanding of a potential causal relationship between prey availability and SRKW population dynamics, and how these relationships may have changed over time. Results suggest that SRKW mortality rates are more strongly associated with Chinook salmon abundance than birth rates. Our analysis also suggests northern resident killer whale abundance could also be affecting SRKW carrying capacity, possibly through competition over shared prey resources. The IPM fit observed total abundance data well and predicted temporal changes in population demographics with reasonable accuracy, but small sample size may have opacified model selection. Our approach offers a valuable tool for predicting the response of the SRKW population to alternative management strategies involving the recovery of Chinook salmon stocks in the eastern Pacific.

Validity of winter sampling for estimation of salmonid abundance by electrofishing

Electrofishing removal sampling is a common method for abundance estimation of juvenile salmonids in streams and is usually performed during autumn. European guidelines specify that sampling should be conducted at water temperatures ≥ 5 °C, but studies investigating the validity of sampling below this threshold are lacking. We conducted electrofishing experiments during autumn and winter in three Norwegian rivers, comparing abundance estimation accuracy, rate of fish emigration from electrofishing sites, and mortality for juvenile Atlantic salmon Salmo salar and brown trout Salmo trutta. Abundance estimates were significantly more accurate during autumn than during winter, less accurate for age-0 fish than for older parr, and more accurate when the number of electrofishing passes was increased. By digging through the substrate after electrofishing, we found that a higher proportion of fish were uncatchable during winter than during autumn, and that age-0 fish were more likely than older fish to be uncatchable. For salmon older than age-0, both autumn and winter sampling consistently yielded relatively accurate abundance estimates. However, the rate of fish emigration from electrofishing sites was significantly higher during autumn than during winter and if block nets are not used, emigration during autumn sampling could cause severe underestimation of Atlantic salmon abundance. Fish mortality did not differ between autumn and winter sampling and was low for both species and all age groups. We recommend the use of block nets and conducting five electrofishing passes to reduce the likelihood of severe estimation error.

Estimating multiple years, tributary-specific, and overall Atlantic salmon smolt abundance in a large Canadian catchment using capture-mark-recapture experiments.

Population monitoring of Atlantic salmon (Salmo salar L.) abundance is an essential element to understand annual stock variability and inform fisheries management processes. Smolts are the life stage marking the transition from the freshwater to the marine phase of anadromous Atlantic salmon. Estimating smolt abundance allows for subsequent inferences on freshwater and marine survival rates. Annual abundances of out-migrating Atlantic salmon smolts were estimated using Bayesian models and an 18-year capture-mark-recapture time series from two to five trapping locations within the Restigouche River (Canada) catchment. Some of the trapping locations were at the outlet of large upstream tributaries, and these sampled a portion of the total out-migrating population of smolts for the watershed, whereas others were located just above the head of tide of the Restigouche River and sampled the entire run of salmon smolts. Due to logistic and environmental conditions, not all trapping locations were operational each year. Additionally, recapture rates were relatively low (<5%), and the absolute number of recaptures was relatively few (most often a few dozen), leading to incoherent and highly uncertain estimates of tributary-specific and whole catchment abundance estimates when the data were modeled independently among trapping locations and years. Several models of increasing complexity were tested using simulated data, and the best-performing model in terms of bias and precision incorporated a hierarchical structure among years on the catchability parameters and included an explicit spatial structure to account for the annual variations in the number of sampled locations within the watershed. When the best model was applied to the Restigouche River catchment dataset, the annual smolt abundance estimates varied from 250,000 to 1 million smolts, and the subbasin estimates of abundance were consistent with the spatial structure of the monitoring programme. Ultimately, increasing the probabilities of capture and the absolute number of recaptures at the different traps will be required to improve the precision and reduce the bias of the estimates of smolt abundance for the entire basin and within subbasins of the watershed. The model and approach provide a significant improvement in the models used to date based on independent estimates of abundance by trapping location and year. Total abundance and relative production in discrete spawning, nesting, or rearing areas provide critical information to appropriately understand and manage the threats to species that can occur at subpopulation spatial scales.