All species inevitably leave genetic traces in their environments, and the resulting environmental DNA (eDNA) reflects the species present in a given habitat. It remains unclear whether eDNA signals ...can provide quantitative metrics of abundance on which human livelihoods or conservation successes depend. Here, we report the results of a large eDNA ocean survey (spanning 86 000 km
to depths of 500 m) to understand the abundance and distribution of Pacific hake (
), the target of the largest finfish fishery along the west coast of the USA. We sampled eDNA in parallel with a traditional acoustic-trawl survey to assess the value of eDNA surveys at a scale relevant to fisheries management. Despite local differences, the two methods yield comparable information about the broad-scale spatial distribution and abundance. Furthermore, we find depth and spatial patterns of eDNA closely correspond to acoustic-trawl estimates for hake. We demonstrate the power and efficacy of eDNA sampling for estimating abundance and distribution and move the analysis eDNA data beyond sample-to-sample comparisons to management relevant scales. We posit that eDNA methods are capable of providing general quantitative applications that will prove especially valuable in data- or resource-limited contexts.
Environmental conditions can have spatially complex effects on the dynamics of marine fish stocks that change across life-history stages. Yet the potential for non-stationary environmental effects ...across multiple dimensions, e.g. space and ontogeny, are rarely considered. In this study, we examined the evidence for spatial and ontogenetic non-stationary temperature effects on Pacific hake Merluccius productus biomass along the west coast of North America. Specifically, we used Bayesian additive models to estimate the effects of temperature on Pacific hake biomass distribution and whether the effects change across space or life-history stage. We found latitudinal differences in the effects of temperature on mature Pacific hake distribution (i.e. age 3 and older); warmer than average subsurface temperatures were associated with higher biomass north of Vancouver Island, but lower biomass offshore of Washington and southern Vancouver Island. In contrast, immature Pacific hake distribution (i.e. age 2) was better explained by a nonlinear temperature effect; cooler than average temperatures were associated with higher biomass coastwide. Together, our results suggest that Pacific hake distribution is driven by interactions between age composition and environmental conditions and highlight the importance of accounting for varying environmental effects across multiple dimensions.
In coastal marine ecosystems, the depletion of dissolved oxygen can cause behavioral and distributional shifts of organisms and thereby alter ecological processes. We used the spatiotemporal ...variation in the onset and intensity of low dissolved oxygen in Hood Canal, Washington, USA, to investigate consequences of seasonally reduced oxygen on fish–zooplankton predator–prey interactions. By simultaneously monitoring densities of zooplankton (primarily the euphausiid; Euphausia pacifica) and zooplanktivorous fish (Pacific herring, Clupea pallasii, and Pacific hake, Mercluccius productus), and the feeding of zooplanktivorous fish, we could separate the effects of dissolved oxygen on fish–zooplankton interactions from other seasonal changes. We expected that fish predators (especially Pacific herring) would be less abundant and have lower feeding rates when oxygen levels declined below biological thresholds, and that this would result in increased zooplankton abundance in areas with lowest dissolved oxygen. However, these expectations were not borne out. Overall, there was mixed evidence for an effect of dissolved oxygen on many of our response variables, and when effects were detected, they were frequently in the opposite direction of our expectations. Specifically, the pelagic fish community became more abundant (as measured by increasing acoustic backscatter), which was particularly pronounced for Pacific herring. Zooplankton had weak evidence for a response to dissolved oxygen, but the direction was negative instead of positive. Although predator feeding composition was unrelated to dissolved oxygen, stomach fullness (an index of feeding intensity) of Pacific herring declined, as per our expectations. These unexpected findings highlight the importance of in situ measurements of multiple aspects of predator–prey linkages in response to environmental stress to enhance our ability to predict ecological consequences of declining oxygen.
Understanding the influence of ocean conditions on predator–prey relationships can provide insight for ecosystem‐based fisheries management. Pacific hake (Merluccius productus) are abundant and ...commercially important groundfish in the California Current Ecosystem (CCE) that consume euphausiids (krill) as a major prey item. We used data from the biennial joint U.S.‐Canada Integrated Ecosystem & Acoustic Trawl Survey for Pacific hake (2007–2019, n = 8 surveys) to quantify co‐occurrence of age 2+ hake with krill in relation to bottom depth, continental shelf break location, surface chlorophyll‐a, and 100‐m temperature. Vertical distributions of hake varied among years and were not correlated to krill depth. Hake hotspots occurred primarily off the Oregon coast and near Cape Mendocino, while most krill hotspots occurred in the northern CCE. The probability of co‐occurrence was greatest during cool ocean conditions (100‐m temperature 1°C below average), averaging 41.0% and extending throughout most of the CCE. During warm ocean conditions (100‐m temperature 1°C above average), predicted co‐occurrence averaged 17.0% and was concentrated near Cape Mendocino. These results indicate that hake‐krill co‐occurrence is a function of predator and prey spatial distributions and overall krill abundance. Furthermore, temperature influences hake‐krill co‐occurrence and may explain some of the variation in hake growth and recruitment to the fishery.
Abstract
Euphausiids, or krill, are important energy links between primary producers and higher trophic levels in the California Current Ecosystem (CCE), but a thorough understanding of their ...variability at the coast-wide scale is limited. Using fisheries acoustics data collected during biennial joint US–Canada Integrated Ecosystem and Acoustic Trawl Surveys for Pacific hake (Merluccius productus), we developed a time series (n = 8 years; 2007–2019 odd years inclusive, and 2012) of krill abundance and examined relationships with environmental factors. Krill were located in waters off the west coasts of the United States and Canada, primarily in shallow basins and on the continental shelf, with greatest kernel density estimates near Cape Mendocino and the Juan de Fuca eddy system. Coast-wide krill abundance was variable, and lowest in 2015 during an extended marine heat wave, when 91% were located in British Columbia. Using hierarchical generalized additive models, we predicted greatest krill abundance in cooler waters (0.2°C below the time series average), within 10–20 km of the shelf break, and in bottom depths between 200 and 400 m. This newly developed coast-wide time series of krill abundance and distribution will inform ecosystem-based fisheries management efforts, and offers additional opportunities for studies of krill-dependent fish, seabirds, and marine mammals.
In the California Current Ecosystem, the California Undercurrent (CU) is the predominate subsurface current that transports nutrient-rich water from southern California poleward. In this study, we ...used a large dataset of spatially explicit in situ observations of Pacific hake ( Merluccius productus) and the CU (36.5–48.3°N) to estimate relationships between northward undercurrent velocity and hake distribution and determine whether these relationships vary across space or life-history stage. We found that both hake occurrence and density had strong spatially complex relationships with the CU. In areas north of 44°N (central Oregon), the CU effect was spatially consistent and opposite for occurrence (negative) and density (positive), indicating that hake may aggregate in areas of high northward velocity in this region. In areas south of 44°N, the CU effect showed a cross-shelf gradient for both occurrence and density, indicating a more nearshore hake distribution when northward velocity is higher in this region. Together, our results suggest that future changes in the CU due to climate change are likely to impact hake differently in northern and southern areas.
•Coexisting dominant taxa in a coastal ecosystem were classified using a combination of multifrequency acoustics and net samplings.•DMVBS and school/single target detections allowed separation of ...zooplankton, Pacific Herring, and Pacific Hake.•Misclassification rates of fish or zooplankton were 0–5%.•At the species level, misclassifications of Pacific Herring and Pacific Hake were both ∼13%.
Classifying coexisting taxa in a coastal ecosystem remains an analytic challenge due to the difficulty in verifying species compositions within backscatter data. Multifrequency measurements (38, 70, 120, 200kHz) were combined with midwater trawls and zooplankton MultiNet tows in Hood Canal, WA, to classify backscatter dominated by single fish species (Pacific Herring, Pacific Hake) or major zooplankton taxa (euphausiids, copepods). Backscatter was categorized into aggregations, single targets, and layers based on morphology. Aggregations and single targets were identified in raw volume backscattering strength (Sv), while layers were classified using differences in mean volume backscattering strength (ΔMVBSi-j=MVBSi – MVBSj, where i and j denote frequency in kHz). Based on a subset of trawl-validated in situ acoustic measurements, backscatter with −16dB < ΔMVBS200-38≤2dB were classified as fish, and 2dB<ΔMVBS200-38<30dB as zooplankton. Backscatter identified as fish were further classified to hake when ΔMVBS120-38<−4.8dB, and herring when ΔMVBS120-38≥−4.8dB. The classification method was evaluated using a second set of trawl-validated acoustic data, resulting in classification accuracy of fish or zooplankton ranging from 95% to 100%. At the species level, misclassifications of herring and hake were both ∼13%. Removal of aggregations and single targets before calculating ΔMVBS values minimized the possibility of mixed species backscatter within layers. This classification technique provides an approach to separate coexisting aggregations of dominant taxa which are common in mid- and low-latitude coastal ecosystems.
Hypoxia can cause significant disturbances in aquatic ecosystems, but the impacts of moderately low dissolved oxygen (DO) where physiological tolerance levels vary among organisms and likely have ...consequences for key food web linkages are not well understood. We hypothesized that the greater sensitivity of fish to DO, compared with their zooplankton prey, would reduce spatial overlap between fish and zooplankton at moderately low DO (2–4 mg l−1). We used a combination of multifrequency acoustics and net samples to characterize distributions and abundances of zooplankton and pelagic fish in Hood Canal, Washington, a seasonally hypoxic fjord. We employed a sampling design that included both high and moderately low DO sites sampled prior to, during, and after the onset of seasonally low DO over 2 yr. Contrary to our hypothesis, we found that fish and their zooplankton prey did not change their horizontal or vertical distributions during periods and in locations with moderately low DO levels. Consequently, the vertical overlap between fish and zooplankton did not change with DO concentration. The apparent lack of response of fish to moderately low DO in our system may result from decreased metabolic oxygen demand due to cool temperatures, availability of prey in moderately low DO waters, increased predation risk at shallower depths, and/or phenotypic adaptations to chronic exposure. Stability in distributions of pelagic communities suggests resilience of trophic coupling to moderately low DO in Hood Canal.
Changing ecosystem conditions present a challenge for the monitoring and management of living marine resources, where decisions often require lead-times of weeks to months. Consistent improvement in ...the skill of regional ocean models to predict physical ocean states at seasonal time scales provides opportunities to forecast biological responses to changing ecosystem conditions that impact fishery management practices. In this study, we used 8-month lead-time predictions of temperature at 250 m depth from the J-SCOPE regional ocean model, along with stationary habitat conditions (e.g., distance to shelf break), to forecast Pacific hake (Merluccius productus) distribution in the northern California Current Ecosystem. Using retrospective skill assessments, we found strong agreement between hake distribution forecasts and historical observations. The top performing models (based on out-of-sample skill assessments using the area-under-the-curve (AUC) skill metric) were a generalized additive model (GAM) that included shelf-break distance (i.e., distance to the 200 m isobath) (AUC = 0.813) and a boosted regression tree (BRT) that included temperature at 250 m depth and shelf-break distance (AUC = 0.830). An ensemble forecast of the top performing GAM and BRT models only improved out-of-sample forecast skill slightly (AUC = 0.838) due to strongly correlated forecast errors between models (r = 0.88). Collectively, our results demonstrate that seasonal lead-time ocean predictions have predictive skill for important ecological processes in the northern California Current Ecosystem and can be used to provide early detection of impending distribution shifts of ecologically and economically important marine species.
The Bering Sea is a highly productive ecosystem with abundant prey populations in the summer that support some of the largest seabird colonies in the Northern Hemisphere. In the fall, the Bering Sea ...is used by large numbers of migrants and post-breeding seabirds. We used over 22 000 km of vessel-based surveys carried out during summer (June to July) and fall (late August to October) from 2008 to 2010 over the southeast Bering Sea to examine annual and seasonal changes in seabird communities and spatial relationships with concurrently sampled prey. Deep-diving murres Uria spp., shallow-diving shearwaters Ardenna spp., and surface-foraging northern fulmars Fulmarus glacialis and kittiwakes Rissa spp. dominated summer and fall seabird communities. Seabird densities in summer were generally less than half of fall densities and species richness was lower in summer than in fall. Summer seabird densities had high interannual variation (highest in 2009), whereas fall densities varied little among years. Seabirds were more spatially clustered around breeding colonies and the outer continental shelf in the summer and then dispersed throughout the middle and inner shelf in fall. In summer, the abundance of age-1 walleye pollock Gadus chalcogrammus along with spatial (latitude and longitude) and temporal (year) variables best explained broad-scale seabird distribution. In contrast, seabirds in fall had weaker associations with spatial and temporal variables and stronger associations with different prey species or groups. Our results demonstrate seasonal shifts in the distribution and foraging patterns of seabirds in the southeastern Bering Sea with a greater dependence on prey occurring over the middle and inner shelf in fall.
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