Northern California Current Research Articles

Edge computing at sea: high-throughput classification of in-situ plankton imagery for adaptive sampling

The small sizes of most marine plankton necessitate that plankton sampling occur on fine spatial scales, yet our questions often span large spatial areas. Underwater imaging can provide a solution to this sampling conundrum but collects large quantities of data that require an automated approach to image analysis. Machine learning for plankton classification, and high-performance computing (HPC) infrastructure, are critical to rapid image processing; however, these assets, especially HPC infrastructure, are only available post-cruise leading to an ‘after-the-fact’ view of plankton community structure. To be responsive to the often-ephemeral nature of oceanographic features and species assemblages in highly dynamic current systems, real-time data are key for adaptive oceanographic sampling. Here we used the new In-situ Ichthyoplankton Imaging System-3 (ISIIS-3) in the Northern California Current (NCC) in conjunction with an edge server to classify imaged plankton in real-time into 170 classes. This capability together with data visualization in a heavy.ai dashboard makes adaptive real-time decision-making and sampling at sea possible. Dual ISIIS-Deep-focus Particle Imager (DPI) cameras sample 180 L s-1, leading to >10 GB of video per min. Imaged organisms are in the size range of 250 µm to 15 cm and include abundant crustaceans, fragile taxa (e.g., hydromedusae, salps), faster swimmers (e.g., krill), and rarer taxa (e.g., larval fishes). A deep learning pipeline deployed on the edge server used multithreaded CPU-based segmentation and GPU-based classification to process the imagery. AVI videos contain 50 sec of data and can contain between 23,000 - 225,000 particle and plankton segments. Processing one AVI through segmentation and classification takes on average 3.75 mins, depending on biological productivity. A heavyDB database monitors for newly processed data and is linked to a heavy.ai dashboard for interactive data visualization. We describe several examples where imaging, AI, and data visualization enable adaptive sampling that can have a transformative effect on oceanography. We envision AI-enabled adaptive sampling to have a high impact on our ability to resolve biological responses to important oceanographic features in the NCC, such as oxygen minimum zones, or harmful algal bloom thin layers, which affect the health of the ecosystem, fisheries, and local communities.

Growth and diet of a larval myctophid across distinct upwelling regimes in the California Current

Abstract Eastern boundary systems support major fisheries of species whose early stages depend on upwelling production. However, upwelling can be highly variable at the regional scale, leading to complex patterns of feeding, growth, and survival for taxa that are broadly distributed in space and time. The northern California Current (NCC) is characterized by latitudinal variability in the seasonality and intensity of coastal upwelling. We examined the diet and larval growth of a dominant myctophid (Stenobrachius leucopsarus) in the context of their prey and predators in distinct NCC upwelling regimes. Larvae exhibited significant differences in diet and growth, with greater seasonal than latitudinal variability. In winter, during reduced upwelling, growth was substantially slower, guts less full, and diets dominated by copepod nauplii. During summer upwelling, faster-growing larvae had guts that were more full from feeding on calanoid copepods and relying less heavily on lower trophic level prey. Yet, our findings revealed a dome-shaped relationship with the fastest growth occurring at moderate upwelling intensity. High zooplanktivorous predation pressure led to above average growth, which may indicate the selective loss of slower-growing larvae. Our results suggest that species whose spatio-temporal distributions encompass multiple regional upwelling regimes experience unique feeding and predation environments throughout their range with implications for larval survivorship.

Planktonic foraminiferal assemblages reflect warming during two recent mid-latitude marine heatwaves

Under future climate scenarios, ocean temperatures that are presently extreme and qualify as marine heatwaves (MHW) are forecasted to increase in frequency and intensity, but little is known about the impact of these events on one of the most common paleoproxies, planktonic foraminifera. Planktonic foraminifera are globally ubiquitous, shelled marine protists. Their abundances and geochemistry vary with ocean conditions and fossil specimens are commonly used to reconstruct ancient ocean conditions. Planktonic foraminiferal assemblages are known to vary globally with sea surface temperature, primary productivity, and other hydrographic conditions, but have not been studied in the context of mid-latitude MHWs. For this study, the community composition and abundance of planktonic foraminifera were quantified for 2010-2019 along the Newport Hydrographic Line, a long-term monitoring transect at 44.6°N in the Northern California Current (NCC). Samples were obtained from archived plankton tows spanning 46 to 370 km offshore during annual autumn (August – October) cruises. Two MHWs impacted the region during this timeframe: the first during 2014-2016 and a second, shorter duration MHW in 2019. During the 2014-2016 MHW, warm water subtropical and tropical foraminifera species were more prevalent than the typical polar, subpolar, and transitional species common to this region. Cold water species were abundant again after the first MHW dissipated in late 2016. During the second, shorter-duration MHW in 2019, the assemblage consisted of a warm water assemblage but did not include tropical species. The foraminiferal assemblage variability correlated with changes in temperature and salinity in the upper 100 meters and was not correlated with distance offshore or upwelling. These results suggest that fossil foraminiferal assemblages from deep sea sediment cores may provide insight into the magnitude and frequency of past MHWs.

Fine-scale vertical distribution and diel migrations ofPyrosoma atlanticumin the northern California Current

AbstractBlooms of the colonial pelagic tunicate Pyrosoma atlanticum in 2014–2018 followed a marine heatwave in the eastern Pacific Ocean. Pyrosome blooms could alter pelagic food webs of the northern California Current (NCC) by accelerating the biological pump via active transport, fecal pellet production and mortality events. Although aggregations of P. atlanticum have the potential to shape marine trophic dynamics via carbon export, little is known about pyrosome vertical distribution patterns. In this study, we estimated the distribution of P. atlanticum in the NCC along transects off of Oregon (45°N and 124°W) and northern California (41°N and 124°W), USA during February and July 2018. Depth-stratified plankton tows provided volume-normalized pyrosome abundance and biovolume estimates that complemented fine-scale counts by a vertically deployed camera system. Pyrosomes were numerous offshore during February, especially off Oregon. Colonies were distributed non-uniformly in the water column with peak numbers associated with vertical gradients in environmental parameters, notably density and chl-a. Vertical distributions shifted over the 24-h period, indicative of diel vertical migration. Understanding the vertical distribution of these gelatinous grazers in the NCC gives insight to their behavior and ecological role in biologically productive temperate ecosystems as conditions become more favorable for recurring blooms.

Seasonal hydromedusan feeding patterns in an Eastern Boundary Current show consistent predation on primary consumers

Abstract Cnidarian jellyfish can be dominant players in the food webs of highly productive Eastern Boundary Currents (EBC). However, the trophic role of inconspicuous hydromedusae in EBCs has traditionally been overlooked. We collected mesozooplankton from five stations along two cross-shelf transects in the Northern California Current (NCC) during winter and summer of 2018–2019. We analyzed gut contents of 11 hydromedusan species and the prey community to (i) determine prey resource use by hydromedusae and (ii) determine temporal shifts in the trophic niche of hydromedusae, focusing on the two most collected species (Clytia gregaria and Eutonina indicans). Hydromedusae in the NCC fed mostly on copepods, appendicularians and invertebrate larvae. Nonmetric multidimensional scaling of hydromedusan diets showed seasonal shifts in prey resource driven by the abundant C. gregaria, which fed mostly on copepod eggs during winter and fed mostly on appendicularians and copepods during summer. Prey selectivity for copepod eggs increased during winter for C. gregaria and E. indicans. Intriguingly, theoretical ingestion rates show that both species acquire similar amounts of carbon during upwelling and nonupwelling conditions. Hydromedusae’s consistent presence and predation impact across seasons may lead to significant effects in carbon and energy transfer through the NCC food web.

Capturing copepod dynamics in the Northern California Current using sentinel stations

Ecosystem indicators track information about states and trends in ocean systems and are a valuable tool for ecosystem assessment and management. To ensure that indicators are used appropriately in both science and management contexts, it is important to understand the extent to which they represent broader spatial patterns. In the Northern California Current (NCC) off the Oregon and Washington state coasts in the USA, copepod metrics derived from data collected at a well-sampled station on the Newport Hydrographic (NH) Line (‘NH05’, five nmi offshore from Newport, OR, USA) are commonly used as indicators of the region’s general ocean conditions. Using correlation analyses, we examined the utility of NH05 as a sentinel station (i.e. representative of a broader region) with respect to the abundance and biomass of warm-water and cold-water copepods in the NCC. Copepod correlations between NH05 and other locations in the NCC were higher for the warm-water copepods than the cold-water group, with correlations being slightly higher for abundance than biomass. Paracalanus parvus had the highest NH05 correlations among the warm-water copepod species, and Acartia longiremis had the highest NH05 correlations among the cold-water copepod species. We also broadened our analysis to evaluate other sampling sites as sentinel stations and found that some sites were equally or more representative of copepod dynamics in the NCC than NH05, though none of these sites tend to be sampled as often as NH05. This analysis emphasized that the warm-water copepods tend to be more similarly distributed in the NCC than the cold-water copepods, and thus their changes through time are better captured by most stations. In contrast, correlations among the cold-water copepods in the NCC vary by location and appear to be strongly related to depth, with the highest correlations at mid-shelf stations.

Abundance, distribution, and feeding ecology of Pyrosoma atlanticum in the Northern California Current

During 2016-2018, unprecedented aggregations of the colonial pelagic tunicate Pyrosoma atlanticum were observed in the Northern California Current (NCC). Pyrosomes are common in tropical and sub-tropical ocean waters, but little is known about their abundance, distribution, and trophic ecology in mid-latitude systems. To assess these factors, pyrosomes were collected during cruises in the NCC in May and August 2017. A generalized additive model (GAM) was used to identify relationships between in situ environmental variables (temperature, salinity, fluorescence) and distribution and abundance patterns of pyrosomes in May 2017. Fatty acid (FA) profiles were then characterized as diet indicators, and bulk stable isotope analysis of carbon and nitrogen was used to examine spatial variations in potential food sources and trophic level. The GAM identified sea surface temperature and surface salinity as significant variables related to pyrosome densities. The most abundant FA in the pyrosomes was docosahexanoic acid (22:6ω3), which serves in pelagic systems as a biomarker for dinoflagellates. Common FA biomarkers for bacteria, carnivory, and dinoflagellates differed by latitude, suggesting that pyrosomes have different diets over a broad latitudinal range. The δ15N values of P. atlanticum indicate that pyrosomes may be feeding at a relatively low trophic level compared to other zooplankton groups in this region. Offshore pyrosomes had lower δ13C values than those collected on the shelf, suggesting incorporation of nearshore carbon in pyrosome tissues. Previously documented rapid reproduction and growth of pyrosomes coupled with efficient feeding behavior for common NCC plankters may support their continued presence in this mid-latitude region.

Three-dimensional cross-shelf zooplankton distributions off the Central Oregon Coast during anomalous oceanographic conditions

The Northern California Current (NCC) is a complex, dynamic system experiencing distinctly different levels of upwelling and downwelling, ranging from intermittent upwelling in summer to downwelling in winter. In recent years, warm water anomalies along the Oregon coast have had significant effects on coastal plankton assemblages. To resolve some of the fine-scale responses to these conditions, we used a towed, undulating underwater imaging system to investigate fine-scale (1 m vertical) zoo- and ichthyoplankton distributions along a 57-km section parallel to the Newport Hydrographic Line encompassing the shelf, shelf break, and slope off the central coast of Oregon. A sparse Convolutional Neural Network was used to automate the identification of 52 million plankton images of 64 plankton taxa, ranging from protists to copepods, larval fishes, and gelatinous organisms. Taxa distributions were interpolated over the whole transect, providing unprecedented insight into their horizontal and vertical distributions and revealing seven broad patterns of distribution. Additional fine-scale distribution data enable examination of some of the physical and biological processes underlying these fine-scale distribution patterns, building upon the historical time series data that exists for this region and advancing our knowledge of planktonic processes in this productive region of the NCC.

Implications of Pyrosoma atlanticum range expansion on phytoplankton standing stocks in the Northern California Current

Coinciding with the marine heat wave in 2014 and a strong El Niño in 2016, blooms of the tropical-temperate pelagic colonial tunicate, Pyrosoma atlanticum, expanded into the Northern California Current (NCC). Given the high densities of P. atlanticum colonies observed in the NCC, and their potential for high grazing rates, we hypothesized that the species could have a substantial negative effect on the phytoplankton standing stock in the Northeastern Pacific Ocean. In the late winter, spring and early autumn of 2018, we measured abundances and length frequencies of P. atlanticum colonies off the coast of Oregon and estimated their spring grazing impact on the local phytoplankton standing stock. Abundances of P. atlanticum peaked in late winter (0.7 colonies m−3), dropped off in spring (0.42 colonies m−3), and declined to near-absent in early autumn (0.07 colonies m−3). We observed a doubling in the median size of P. atlanticum colonies from late winter to spring, followed by a decline in size in early autumn. With their high individual colony gut pigment contents and bloom densities, the overall spring grazing impact of P. atlanticum was substantial, reaching a maximum of 22% of the available phytoplankton standing stock.

Comparison of condition metrics and lipid content between Euphausia pacifica and Thysanoessa spinifera in the northern California Current, USA

Krill are a key component of pelagic food webs where they are vital to the transfer of energy from phytoplankton to higher trophic levels. Krill have a high lipid content compared to other zooplankton and form dense aggregations, making them an important prey source for higher trophic level fish, seabirds, and marine mammals. The two dominant euphausiid species in the northern California Current (NCC) are Euphausia pacifica and Thysanoessa spinifera. E. pacifica is the most abundant species of euphausiid in the NCC, but T. spinifera has a higher potential energetic content due to its larger body size and higher lipid density. Most studies have inferred differences in lipid content and body condition between the two species, but few studies have quantified these differences in the NCC. Here, we report on the body condition, carbon and nitrogen content, as well as lipid and fatty acid composition of these two krill species, and the extent to which these metrics vary across season, year, and reproductive status. Body condition, elemental composition, and total lipids strongly differed between the species. T. spinifera had higher length-weight, Fulton’s K, hepato-somatic index, carbon to nitrogen ratio, total lipid per wet weight, and storage lipid compared to E. pacifica, indicating that T. spinifera has a higher energetic value for predators. However, there were strong seasonal differences in the energetics of T. spinifera. Carbon and lipids were highest in non-reproductive T. spinifera from August through October. Although there were strong ontogenetic and inter-specific differences, the lipid and fatty acid compositions of both species followed a seasonal progression characterized by low lipids during the pre-upwelling period, an increase in lipids, triacylglycerols, and diatom markers during upwelling, and increased proportions of dinoflagellate and bacterial diet markers during the fall post-upwelling period.

Examining the ecological role of jellyfish in the Eastern Bering Sea

AbstractWithin the Eastern Bering Sea, the jellyfish Chrysaora melanaster has fluctuated widely over recent decades. We examined the role of C. melanaster as an ecosystem-structuring agent via application of ecosystem models representing inner-, mid-, and outer-shelf regions of comparable areal coverage. Chrysaora melanaster utilize 1% of total mid-shelf consumer production, or 1/4th the energy required by forage fish (capelin Mallotus villosus, Pacific herring Clupea pallasii, age-0 Pacific cod Gadus macrocephalus, age-0 walleye pollock Gadus chalcogrammus). Model simulations show the impacts of C. melanaster are broadly distributed across consumer groups with increasingly negative impacts with higher jellyfish biomass. Age-0 pollock represent the greater part of the forage fish biomass, and observed pollock biomass during low jellyfish years (2004–2007) was significantly greater than during high jellyfish years (2009–2014). However, sensitivity among consumer groups to observed jellyfish variability is small, within 5% of baseline (2004–2015) conditions. Estimates using similar models for the Coastal Gulf of Alaska (CGoA) and Northern California Current (NCC) suggest large differences in the role of scyphozoans among northern Pacific shelf ecosystems. Only 0.1% of total summer consumer production is required to support CGoA Chrysaora, while the coastal NCC population uses 19%.

Recent Ecosystem Disturbance in the Northern California Current

An extended marine heat wave occurred across the North Pacific during 2014–2016, including the formation of the warm “Blob” followed by a strong El Niño in 2016. Coincident with this marine heat wave, we documented unprecedented biological changes in plankton and nekton in the Northern California Current (NCC) within pelagic surveys conducted over 20 years (1998–2017). The recent warm period was dominated by warmwater gelatinous invertebrates and fishes, some of which were previously either extremely rare or absent. Mixing of organisms originating from more southern or western regions with those previously present in the NCC may have resulted in novel and unpredictable trophic interactions that produced some of the observed changes in relative abundance. Continued long‐term monitoring is needed to determine whether this is a temporary ecosystem disturbance or a fundamental change in the very productive NCC upwelling region.

Phenological and distributional shifts in ichthyoplankton associated with recent warming in the northeast Pacific Ocean.

Understanding changes in the migratory and reproductive phenology of fish stocks in relation to climate change is critical for accurate ecosystem-based fisheries management. Relocation and changes in timing of reproduction can have dramatic effects upon the success of fish populations and throughout the food web. During anomalously warm conditions (1-4°C above normal) in the northeast Pacific Ocean during 2015-2016, we documented shifts in timing and spawning location of several pelagic fish stocks based on larval fish samples. Total larval concentrations in the northern California Current (NCC) during winter (January-March) 2015 and 2016 were the highest observed since annual collections first occurred in 1998, primarily due to increased abundances of Engraulis mordax (northern anchovy) and Sardinops sagax (Pacific sardine) larvae, which are normally summer spawning species in this region. Sardinops sagax and Merluccius productus (Pacific hake) exhibited an unprecedented early and northward spawning expansion during 2015-16. In addition, spawning duration was greatly increased for E.mordax, as the presence of larvae was observed throughout the majority of 2015-16, indicating prolonged and nearly continuous spawning of adults throughout the warm period. Larvae from all three of these species have never before been collected in the NCC as early in the year. In addition, other southern species were collected in the NCC during this period. This suggests that the spawning phenology and distribution of several ecologically and commercially important fish species dramatically and rapidly changed in response to the warming conditions occurring in 2014-2016, and could be an indication of future conditions under projected climate change. Changes in spawning timing and poleward migration of fish populations due to warmer ocean conditions or global climate change will negatively impact areas that were historically dependent on these fish, and change the food web structure of the areas that the fish move into with unforeseen consequences.

A physically coupled end-to-end model platform for coastal ecosystems: Simulating the effects of climate change and changing upwelling characteristics on the Northern California Current ecosystem

We describe a spatially explicit, intermediate complexity end-to-end model platform that integrates physical, trophic, and nutrient cycling processes. A two-dimensional advection and mixing model drives nitrate input into the model continental shelf domain, the transport of nutrients and plankton between sub-regions, and the export of nutrients and plankton from the model domain. Trophic relationships are defined by classical mass-balanced food web model techniques (e.g., ECOPATH). Inclusion of nitrate and ammonium nutrient pools and bacterial recycling of detritus allows consideration of alternate “new” vs. “recycled” production regimes. The model platform was applied to the Northern California Current (NCC) shelf ecosystem. Seasonal upwelling of nutrients along the coast is the primary driver of NCC productivity, however the characteristics of upwelling vary considerably between years and are expected to change into the future as a result of global climate change. The model was run under alternate physical driver scenarios to examine the effects of changing upwelling characteristics on the production and spatial distribution of functional groups across all trophic levels. Productivity on the shelf had a dome-shaped relationship with upwelling strength. As the intensity of individual upwelling events increased, productivity increased throughout the food web. However, strong upwelling had a detrimental effect when the physical export of plankton exceeded the capacity of phytoplankton to exploit higher nutrient supply rates and the capacity of zooplankton to exploit higher phytoplankton production. As the duration of individual upwelling events increased (an implication of some climate change scenarios) model simulations predicted an overall reduction of productivity at all trophic levels and a shift in the size composition of the phytoplankton community, especially within the nearshore region.

Anomalous ichthyoplankton distributions and concentrations in the northern California Current during the 2010 El Niño and La Niña events

In late spring of 2010, the northern California Current (NCC) experienced a transition from El Niño to La Niña conditions resulting in anomalous distributions and concentrations within the ichthyoplankton community. We analyzed larval fish data collected during the four months before and after this transition and compared them to data from three previous studies conducted in the NCC. In one comparison, concentrations of larvae collected during winter from stations 2 to 46km offshore along the central Oregon coast were higher in 2010 than in any other year from 1998 to 2011. In a second comparison of nearshore larvae collected during six periods (1971–1972, 1978, 1983, 1998, 1999–2002, and 2003–2005) previous to 2010, concentrations of total larvae and most dominant larval taxa were higher during the winter/spring and lower during the summer/fall seasons in 2010 (corresponding to the shift from El Niño to La Niña conditions) than during similar seasons in any other annual period. In a third comparison, larvae collected from stations 21 to 102km offshore along the southern Washington to south-central Oregon coast in May 2010, at the end of the El Niño event, were found in higher concentrations than during any May from 2004 to 2009 and 2011. The high concentration of larvae in the winter and spring of 2010 was likely the direct result of El Niño and warm-ocean conditions (high values of the MEI, NOI, and PDO) along with strong downwelling and onshore transport that increased the abundance of offshore taxa over the shelf. Continued monitoring of the NCC is warranted as El Niño effects on larval fish observed in the past may not be indicative of future effects.

Large-scale forcing of environmental conditions on subarctic copepods in the northern California Current system

In the ocean, dominant physical processes often change at various spatial and temporal scales. Here, we examined associations between large-scale physical forcing indexed by the Pacific Decadal Oscillation (PDO), regional ocean conditions including alongshore currents in relation to the abundance of two subarctic oceanic copepods, Neocalanus plumchrus, and N. cristatus in the offshore portions of the northern California Current (NCC) system in spring of 1998–2008. We found significant relationships between the abundance of copepods, water temperature, and alongshore currents with a lag of two or four months in response to the PDO in the NCC system. During the growth season in March/April both subarctic copepod species displayed consistent cross-shelf patterns with shoreward decreasing gradient in abundance, and were negatively correlated with the PDO, sea water temperature, and alongshore currents. Our studies highlight the responses of regional ocean conditions to large-scale physical forcing and illustrate the potential for Neocalanus copepods as unique vectors for a new understanding of the ecological response in the offshore oceanic waters of the NCC system to climate variability.

Commercial fishers' perceptions of jellyfish interference in the Northern California Current

Abstract Jellyfish disruption of fisheries has been described in some coastal systems, but few thorough investigations have been conducted. To ascertain the economic impact and trend of jellyfish blooms in the Northern California Current (NCC), we mailed surveys to resident commercial shrimpers, salmon trollers, rockfish (blue, black), and groundfish fishers (n = 872). We asked fishers to estimate the damages caused by jellyfish—including costs of relocating to avoid blooms, lost fishing time, time lost to bycatch sorting, fish depreciation, and gear damage. Of the total respondents (n = 111), 67% reported that jellyfish reduce their seasonal revenue, but the degree of impact ranged considerably by fishery and location. Highest jellyfish nuisance corresponded to regions with the most salmon trolling effort. Using the mean revenue losses provided by respondents, we estimate that the combined economic impact of jellyfish on Oregon's salmon and pink shrimp fishers was over $650 000 in peak jellyfish season (June–September) in 2012. Fishers reported that jellyfish biomass varies annually, but most respondents (51%) reported observing no appreciable change in jellyfish populations in the last 5 years. Since economic impact analyses have been conducted primarily in areas with anomalous, high-density blooms, data from the NCC, which is not known to be experiencing increases in jellyfish abundance, provides baseline information on the socio-economic impact of jellyfish blooms in this region. In addition, the finding that jellyfish impact hook and line fisheries—not solely net fisheries—has implications for many other regions where fishers employ this gear type.

Spatial overlap between forage fishes and the large medusa Chrysaora fuscescens in the northern California Current region

As in many regions of the world, the shelf waters of the western United States have experienced large increases and high interannual variability in jellyfish populations in recent decades. The northern California Current (NCC) is a productive upwelling zone that is home to large populations of medusae, particularly during some years. Seasonal trawl surveys in the NCC over 13 yr have documented a substantial biomass of jellyfish consisting primarily of one species, the sea nettle Chrysaora fuscescens, with abundances generally peaking in late summer. Trophic overlap can be high in the NCC with planktivorous species such as Pacific sardines and herring that consume copepods and other zooplankton. In this study, we examine the spatial overlap and co-occurrence of C. fuscescens and Pacific herring Clupea pallasii, northern anchovy Engraulis mordax and Pacific sardine Sardinops sagax in the NCC using spatial analysis tools to determine the species that have the potential to be most affected by high jellyfish biomass and the geo- graphic areas in which these interactions are likely to occur. Significant spatial overlap of C. fuscescens with these pelagic fishes occurred during certain months and years, although the results were highly variable. There was an overall negative relationship between the abundance of C. fuscescens and the catch of the 3 forage fishes for both June and September. End-to-end food web models show that jellyfish have a greater potential to affect production of pelagic forage fishes than the reverse.

Diet variability of forage fishes in the Northern California Current System

As fisheries management shifts to an ecosystem-based approach, understanding energy pathways and trophic relationships in the Northern California Current (NCC) will become increasingly important for predictive modeling and understanding ecosystem response to changing ocean conditions. In the NCC, pelagic forage fishes are a critical link between seasonal and interannual variation in primary production and upper trophic groups. We compared diets among dominant forage fish (sardines, anchovies, herring, and smelts) in the NCC collected in May and June of 2011 and June 2012, and found high diet variability between and within species on seasonal and annual time scales, and also on decadal scales when compared to results of past studies conducted in the early 2000s. Copepoda were a large proportion by weight of several forage fish diets in 2011 and 2012, which differed from a preponderance of Euphausiidae found in previous studies, even though all years exhibited cool ocean conditions. We also examined diet overlap among these species and with co-occurring subyearling Chinook salmon and found that surf smelt diets overlapped more with subyearling Chinook diets than any other forage fish. Herring and sardine diets overlapped the most with each other in our interdecadal comparisons and some prey items were common to all forage fish diets. Forage fish that show plasticity in diet may be more adapted to ocean conditions of low productivity or anomalous prey fields. These findings highlight the variable and not well-understood connections between ocean conditions and energy pathways within the NCC.

A comparative analysis of coastal and shelf‐slope copepod communities in the northern California Current system: Synchronized response to large‐scale forcing?

The synchrony between coastal and shelf‐slope copepod communities was investigated in the northern California Current (NCC) system, a strong upwelling zone, using time series of zooplankton sampled from a nearshore station (9 km offshore, water depth 62 m) and a shelf‐slope station (46 km offshore, water depth 297 m). Long‐term trends and seasonal changes were constructed for the dissimilarity index (Euclidean distance) between the two stations and for the biomass of three different copepod assemblages at the two stations: cold neritic, southern, and warm neritic copepods. The dissimilarity between the community structures of the two stations showed little variation in the long‐term trend, but exhibited a clear seasonal pattern. All three copepod assemblages showed similar long‐term trends in relation to the large‐scale forcing as indexed by the Pacific Decadal Oscillation at both stations, but variations in the long‐term trend at the nearshore station were much higher than the offshore station. Most copepod groups exhibited regular seasonal patterns at both stations except southern copepods at the nearshore station. All three copepod assemblages exhibited more pronounced seasonal fluctuations at the nearshore station compared with the slope station, and this difference is likely driven by higher productivity nearshore fueled by nutrient‐enriched upwelled water. Copepods in the inshore and offshore waters in the NCC ecosystem showed synchronized response to the large‐scale variability in physical forcing and copepods in the coastal waters were more responsive to local perturbations than were those in the slope waters.