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Predation mortality can influence the distribution and abundance of fish populations. While predation is often assessed using direct observations of prey consumption, potential predation can be predicted from co-occurring predator and prey densities under varying environmental conditions. Juvenile Pacific salmon Oncorhynchus spp. (i.e., smolts) from the Columbia River Basin experience elevated mortality during the transition from estuarine to ocean habitat, but a thorough understanding of the role of predation remains incomplete. We used a Holling type II functional response to estimate smolt predation risk based on observations of piscivorous seabirds (sooty shearwater [ Ardenna griseus ] and common murre [ Uria aalge ]) and local densities of alternative prey fish including northern anchovy ( Engraulis mordax ) in Oregon and Washington coastal waters during May and June 2010–2012. We evaluated predation risk relative to the availability of alternative prey and physical factors including turbidity and Columbia River plume area, and compared risk to returns of adult salmon. Seabirds and smolts consistently co-occurred at sampling stations throughout most of the study area (mean = 0.79 ± 0.41, SD), indicating that juvenile salmon are regularly exposed to avian predators during early marine residence. Predation risk for juvenile coho ( Oncorhynchus kisutch ), yearling Chinook salmon ( O . tshawytscha ), and subyearling Chinook salmon was on average 70% lower when alternative prey were present. Predation risk was greater in turbid waters, and decreased as water clarity increased. Juvenile coho and yearling Chinook salmon predation risk was lower when river plume surface areas were greater than 15,000 km 2 , while the opposite was estimated for subyearling Chinook salmon. These results suggest that plume area, turbidity, and forage fish abundance near the mouth of the Columbia River, all of which are influenced by river discharge, are useful indicators of potential juvenile salmon mortality that could inform salmonid management.

Advances in telemetry and modeling of physical processes expand opportunities to assess relationships between marine predators and their dynamic habitat. The Columbia River plume (CRP) attracts sooty shearwaters Ardenna grisea and common murres Uria aalge, but how seabirds respond to variability in plume waters is unknown. We characterized seabird distributions in relation to hourly, daily, monthly, and seasonal variation in CRP location and surface area by attaching satellite telemetry tags to shearwaters in 2008 and 2009, and to murres in 2012 and 2013. We matched seabird locations to surface salinity from a high-resolution hydrodynamic model of the CRP and adjacent waters. Utilization distributions indicated high-use areas north of the Columbia River mouth and in continental shelf waters. Shearwater and murre occupancy of tidal (< 21 psu), recirculating (21–26 psu), and boundary (26–31 psu) plume waters was on average 31% greater than expected and positively correlated with CRP surface area. Seabird latitude was positively correlated with latitude of the geographic center of the CRP, indicating that birds move in phase with the plume. We detected a threshold response of seabirds to plume size, and birds were closer to the convergent CRP boundary (28 psu isohaline) after a surface area threshold between 1500 and 4000 km² was exceeded. We conclude that shearwaters and murres selectively occupy and track plume waters, particularly dynamic boundary waters where foraging opportunities may be enhanced by increases in surface area and associated biophysical coupling that aggregates zooplankton and attracts prey fishes.

Marine Protected Areas (MPAs) provide an important tool for conservation of marine ecosystems. To be most effective, these areas should be strategically located in a manner that supports ecosystem function. To inform marine spatial planning and support strategic establishment of MPAs within the California Current System, we identified areas predicted to support multispecies aggregations of seabirds (\"hotspots\"). We developed habitat-association models for 16 species using information from at-sea observations collected over an 11-year period (1997-2008), bathymetric data, and remotely sensed oceanographic data for an area from north of Vancouver Island, Canada, to the USA/Mexico border and seaward 600 km from the coast. This approach enabled us to predict distribution and abundance of seabirds even in areas of few or no surveys. We developed single-species predictive models using a machine-learning algorithm: bagged decision trees. Single-species predictions were then combined to identify potential hotspots of seabird aggregation, using three criteria: (1) overall abundance among species, (2) importance of specific areas (\"core areas\") to individual species, and (3) predicted persistence of hotspots across years. Model predictions were applied to the entire California Current for four seasons (represented by February, May, July, and October) in each of 11 years. Overall, bathymetric variables were often important predictive variables, whereas oceanographic variables derived from remotely sensed data were generally less important. Predicted hotspots often aligned with currently protected areas (e.g., National Marine Sanctuaries), but we also identified potential hotspots in Northern California/Southern Oregon (from Cape Mendocino to Heceta Bank), Southern California (adjacent to the Channel Islands), and adjacent to Vancouver Island, British Columbia, that are not currently included in protected areas. Prioritization and identification of multispecies hotspots will depend on which group of species is of highest management priority. Modeling hotspots at a broad spatial scale can contribute to MPA site selection, particularly if complemented by fine-scale information for focal areas.

Studies estimating species' distributions require information about animal locations in space and time. Location data can be collected using surveys within a predetermined frame of reference (i.e., Eulerian sampling) or from animal‐borne tracking devices (i.e., Lagrangian sampling). Integration of observations obtained from Eulerian and Lagrangian perspectives can provide insights into animal movement and habitat use. However, contemporaneous data from both perspectives are rarely available, making examination of biases associated with each sampling approach difficult. We compared distributions of a mobile seabird observed concurrently from ship, aerial, and satellite tag surveys during May, June, and July 2012 in the northern California Current. We calculated utilization distributions to quantify and compare variability in common murre (Uria aalge) space use and examine how sampling perspective and platform influence observed patterns. Spatial distributions of murres were similar in May, regardless of sampling perspective. Greatest densities occurred in coastal waters off southern Washington and northern Oregon, near large murre colonies and the mouth of the Columbia River. Density distributions of murres estimated from ship and aerial surveys in June and July were similar to those observed in May, whereas distributions of satellite‐tagged murres in June and July indicated northward movement into British Columbia, Canada, resulting in different patterns observed from Eulerian and Lagrangian perspectives. These results suggest that the population of murres observed in the northern California Current during spring and summer includes relatively stationary individuals attending breeding colonies and nonstationary, vagile adults and subadults. Given the expected growth of telemetry studies and advances in survey technology (e.g., unmanned aerial systems), these results highlight the importance of considering methodological approaches, spatial extent, and synopticity of distribution data sets prior to integrating data from different sampling perspectives. We compared distributions of a mobile seabird observed concurrently from ship, aerial, and satellite tag surveys during May, June, and July 2012 in the northern California Current. Spatial distributions of common murres (Uria aalge) were similar early in the study period, but sampling perspective influenced observed patterns later in the season. These results highlight the importance of considering methodological approaches, spatial extent, and synopticity of distribution data sets prior to integrating data from different sampling perspectives into habitat models.

Abstract Objective Because predation is thought to be the primary source of natural mortality for juvenile salmon first entering the ocean, we sought to identify regions where, on average, stock‐specific spatial overlap between the distribution of threatened and endangered juvenile Chinook Salmon Oncorhynchus tshawytscha and abundant fish‐eating seabirds (common murres Uria aalge and sooty shearwaters Ardenna grisea) suggests the greatest potential for ocean predation risk to juvenile Chinook Salmon. Methods The relative abundance and spatial distribution of seabird predators and juvenile Chinook Salmon were quantified as part of long‐term ecosystem surveys during May 2003–2012 and June 2003–2022. Genetic stock identification methods were used to assign individual fish to their respective stock groups. Stock‐specific species distribution models then generated maps and indices of average annual spatial overlap between predators and prey within the survey area. Result There is unequivocal evidence for spatial overlap between common murres, sooty shearwaters, and five genetic groups of interior and lower Columbia River juvenile Chinook Salmon. We found strongly positive (≥0.70) spatial correlations between predator and prey densities in both May and June, although spatial overlap was, in general, greater during May. The region of highest spatial overlap occurred on the inner continental shelf between the Columbia River mouth (46.2°N) and Grays Harbor (47.0°N), a region at the beginning of the juvenile salmon migratory pathway that is strongly affected by freshwater outflow from the river. Conclusion Our findings support the idea that ocean avian predation during early marine residence has the potential to affect marine survival of juvenile Chinook Salmon and should be further investigated to better inform and implement ecological models and possible recovery actions for Chinook Salmon populations of the Columbia River basin. Impact statement Our findings identify the geographical region where high numbers of fish‐eating seabirds occur along the migration pathway of juvenile salmon that recently entered the ocean. This information advances work to understand the role ocean predators play in affecting salmon survival.

IntroductionEastern Boundary Upwelling Systems are some of the most productive marine ecosystems in the world. Little is known about habitat associations and spatial distributions of marine predators during seasonal periods of low productivity because there are few at-sea surveys during this period. During low productivity or prey scarcity, predators consuming similar prey in the same time and space may compete for limited resources, or they may avoid competition by exploiting different habitats or occupying separate spaces (i.e. niche partitioning). In this study, we examined habitat associations and niche partitioning of marine predators during the low-productivity winter downwelling season of the northern California Current Ecosystem (CCE). MethodsSeabird and marine mammal counts were continuously collected during systematic at-sea surveys during February–March/April in the northern California Current across four years (2006, 2008, 2009, and 2012). We examined seabird and marine mammal distributions in relation to seven habitat characteristics [i.e., sea surface temperature (°C), salinity, depth (m), seafloor slope (%), distance from shore (km), and distance from the 100 m and 200 m isobaths (km)]. We used a non-parametric multivariate analysis [i.e. canonical correspondence analysis (CCA)] to quantify species’ habitat associations and directional distribution ellipses to explore overlap in species core winter habitat.ResultsResults show 49 seabird and ten marine mammal species inhabit the CCE during this low productivity period, including endangered southern resident killer whales (Orcinus orca). Seabirds and marine mammals exhibited significant but low overlap in habitat associations (i.e. weak niche partitioning) and similar habitat associations to summer studies. DiscussionWe also found that some species with similar foraging strategies showed asymmetrical spatial range overlap (i.e. common murre (Uria aalge) and parakeet auklet (Aethia psittacula)), which may mean that expected increased competition due to climate change can negatively affect some species more than others. Given that climate change is leading to increased frequencies, intensities, and durations of marine heat waves during winter months, addressing the winter ecology knowledge gap will be important to understanding how climate change is going to affect species that reside in or migrate through the northern California Current during the low productivity downwelling season.

Rare species are difficult to observe in the wild, particularly in the ocean where large spatial scales and accessibility hinder effective sampling. Environmental DNA (eDNA) is a non-destructive, scalable sampling method with the potential to inform the distribution of rare species in marine ecosystems. We sample eDNA within the California Current ecosystem to estimate the distribution of eulachon (Thaleichthys pacificus), a threatened anadromous smelt ranging along the coastal Northeast Pacific. We amplify eulachon DNA from thousands of water samples collected at night across two years and more than 200,000 square kilometers along the U.S. west coast. We then use spatiotemporal models to derive quantitative estimates of eulachon DNA across space, depth, and time relative to environmental covariates. We find that eulachon DNA has a distribution weighted towards the ocean surface, spatially associated with major river mouths and productive offshore banks. Temperature and prey density are key covariates, with eulachon more likely to be found in warmer waters with higher prey concentrations. We discuss how our results can augment the information currently used in eulachon recovery planning, and describe the wide applicability of our statistical models for estimating distribution and abundance for other species of conservation concern.

Tidal rips and jets are common features associated with archipelagos and complex coastlines. In habitats where rips and jets develop, energy flow to piscivorous predators is hypothesized to be strongly associated with tidal phase due to interactions between currents, plankton, and schooling planktivorous fishes (the ‘tidal coupling hypothesis’). This study tests 1 component of the tidal coupling hypothesis, that the feeding activity of piscivorous predators and the availability of planktivorous fishes are both strongly associated with the same tidal phase. During 1994 to 1997, I made visual counts of actively feeding, mixed-species seabird flocks and hydroacoustic measurements of the relative abundance and distribution of schooling fishes. Median feeding activity, median backscatter m–1transect, and median prey encounter probabilities were greatest during the tidal periods Slow flood 1 and Fast flood. Positive feeding anomalies and positive prey-encounter anomalies were significantly associated with tidal flood phases, but positive backscatter anomalies were not. The results support the tidal-coupling hypothesis, but suggest that changes in the distribution or behavior of schooling fish are as important as, if not more important than, changes in relative fish abundance when determining prey availability and predator foraging-success. The data show that tidal currents can play an important role in structuring nearshore predator–prey interactions.

The population of Eulachon (Thaleicthys pacificus) spawning in the Columbia River and its tributaries is thought to be the largest in the world. Eulachon historically supported indigenous, commercial, and sport harvests, but were listed as threatened under the US Endangered Species Act in 2010. This study tested the use of a small research trawl and 38-kHz echosounder to provide new, fishery-independent data for Columbia River Eulachon. During January–March 2013, we used a semi-balloon shrimp trawl and an uncalibrated 38-kHz downward-looking echosounder to sample estuarine and tidal freshwater habitats. Eulachon were present in the estuary on every sampling day. Direct mortality in the trawl was very low (<0.1%). We observed sex ratios closer to 1:1 than previously reported for the Columbia River, and trawl-caught Eulachon were longer and heavier than Eulachon caught with the same gear in the same season during 1980–1981. The largest catches occurred after 11 February 2013, when midwater estuary temperature warmed and remained above 5.5°C. Tributary spawning began in mid-March after estuary warming and continued for at least 2 wk. Observations suggested that Eulachon occurred in low densities and remained dispersed in deeper waters of the estuary for at least 2 mo before upstream migration. The estuary may therefore serve as an important staging area prior to upstream migration and subsequent spawning. Catch data and qualitative acoustic images suggest that a combination of trawl and acoustic surveys could provide direct estimates of adult biomass. If Eulachon populations are to recover from the threat of extinction, additional data will be needed to resolve uncertainties regarding spawner condition, adult spawning-stock biomass, and variation in run timing relative to river and estuary conditions. We recommend implementation of systematic surveys for adult Eulachon in the Columbia River to further understand how environmental factors drive variation in run size and run timing for this species.

Tidal currents interacting with complex topographies are common features of coastal environments. These interactions are hypothesized to have significant effects on local plankton distribution and abundance, and therefore on food availability to planktivorous fishes. The purpose of this study was to test the hypothesis that tidal currents interacting with an island archipelago create significant and predictable increases in copepod availability to planktivores. Copepod densities during flood and ebb tides were sampled weekly during July-October of 1995-1997. Copepods of the genera Pseudocalanus, Paracalanus, and Corycaeus were the numerically dominant zooplankton except during a bloom of dinoflagellates (Noctiluca spp.) in July 1996. At sampling locations within the main tidal current, median copepod densities were 47 to 252 copepods m super(-3) greater during flood tides. In contrast, median densities outside of the main current were not significantly different between tides. An unexpected finding was the presence of an abundant, large centric diatom (Coscinodiscus wailesii), which showed a prolonged bloom from early July through early October in all years. Changes in copepod abundance and maintenance of the C. wailesii bloom were most likely caused by the advection of copepod aggregations and nutrients from near or below the pycnocline in the Strait of Juan de Fuca. Conversion of copepod numerical abundance to biomass estimates suggests that tidal differences in copepod abundance could affect fish growth. The predictability of changes in copepod abundance may explain why tidal rips and jets are important feeding areas for planktivorous fishes, as well as for fish predators such as marine mammals seabirds.