Northeast US Shelf Research Articles

Disentangling bottom-up and top-down controls on fish consumption of key prey in the Northeast US Shelf ecosystem

Abstract Exploited forage fishes serve a dual role in marine ecosystems by supporting directed fisheries and predator productivity, and thus both harvest and predatory removals should be accounted for when developing stock assessments and evaluating management trade-offs. Predator catch and stomach content data collected on the Northeast US Shelf from 1978 to 2019 by two fisheries-independent surveys were combined within multivariate spatiotemporal models to estimate time-series of consumptive removals during spring and fall for four commercially exploited prey; Atlantic herring (Clupea harengus), silver hake (Merluccius bilinearis), butterfish (Peprilus triacanthus), and longfin squid (Doryteuthis pealeii). Seasonal consumption trends were mostly synchronous for Atlantic herring and silver hake, asynchronous for butterfish and longfin squid, and predatory removals were generally greater during fall. Consumption has increased since the 1990s for all prey except Atlantic herring and butterfish during fall, which coincides with the widespread implementation of harvest constraints meant to rebuild predator and prey populations. These time-series were linked to hypothesized drivers using state-space regression models; prey availability (bottom-up; positive relationships) and commercial catch (top-down; primarily negative relationships) were the strongest predictors of consumption. Although the mechanisms underlying these relationships remain unresolved, these linkages highlight connections among the systemic drivers of productivity on the Northeast Shelf.

Distinct responses to warming within picoplankton communities across an environmental gradient.

Picophytoplankton are a ubiquitous component of marine plankton communities and are expected to be favored by global increases in seawater temperature and stratification associated with climate change. Eukaryotic and prokaryotic picophytoplankton have distinct ecology, and global models predict that the two groups will respond differently to future climate scenarios. At a nearshore observatory on the Northeast US Shelf, however, decades of year-round monitoring have shown these two groups to be highly synchronized in their responses to environmental variability. To reconcile the differences between regional and global predictions for picophytoplankton dynamics, we here investigate the picophytoplankton community across the continental shelf gradient from the nearshore observatory to the continental slope. We analyze flow cytometry data from 22 research cruises, comparing the response of picoeukaryote and Synechococcus communities to environmental variability across time and space. We find that the mechanisms controlling picophytoplankton abundance differ across taxa, season, and distance from shore. Like the prokaryote, Synechococcus, picoeukaryote division rates are limited nearshore by low temperatures in winter and spring, and higher temperatures offshore lead to an earlier spring bloom. Unlike Synechococcus, picoeukaryote concentration in summer decreases dramatically in offshore surface waters and exhibits deeper subsurface maxima. The offshore picoeukaryote community appears to be nutrient limited in the summer and subject to much greater loss rates than Synechococcus. This work both produces and demonstrates the necessity of taxon- and site-specific knowledge for accurately predicting the responses of picophytoplankton to ongoing environmental change.

Assessing small pelagic fish trends in space and time using piscivore stomach contents

Changing distribution and abundance of small pelagic fishes may drive changes in predator distributions, affecting predator availability to fisheries and surveys. However, small pelagics are difficult to survey directly, so we developed a novel method of assessing the aggregate abundance of 21 small pelagic forage taxa via predator stomach contents. We used stomach contents collected from 22 piscivore species captured by multiple bottom trawl surveys within a vector autoregressive spatio-temporal model to assess trends of small pelagics on the Northeast US shelf. The goal was to develop a spatial “forage index” to inform survey and (or) fishery availability in the western North Atlantic bluefish ( Pomatomus saltatrix) stock assessment. This spatially resolved index compared favorably with more traditional design-based survey biomass indices for forage species well sampled by surveys. However, our stomach content-based index better represented smaller unmanaged forage species that surveys are not designed to capture. The stomach-based forage index helped explain bluefish availability to the recreational fishery for stock assessment and provided insight into pelagic forage trends throughout the regional ecosystem.

Oligotrophic waters of the Northwest Atlantic support taxonomically diverse diatom communities that are distinct from coastal waters.

Diatoms are important components of the marine food web and one of the most species-rich groups of phytoplankton. The diversity and composition of diatoms in eutrophic nearshore habitats have been well documented due to the outsized influence of diatoms on coastal ecosystem functioning. In contrast, patterns of both diatom diversity and community composition in offshore oligotrophic regions where diatom biomass is low have been poorly resolved. To compare the diatom diversity and community composition in oligotrophic and eutrophic waters, diatom communities were sampled along a 1,250 km transect from the oligotrophic Sargasso Sea to the coastal waters of the northeast US shelf. Diatom community composition was determined by amplifying and sequencing the 18S rDNA V4 region. Of the 301 amplicon sequence variants (ASVs) identified along the transect, the majority (70%) were sampled exclusively from oligotrophic waters of the Gulf Stream and Sargasso Sea and included the genera Bacteriastrum, Haslea, Hemiaulus, Pseudo-nitzschia, and Nitzschia. Diatom ASV richness did not vary along the transect, indicating that the oligotrophic Sargasso Sea and Gulf Stream are occupied by a diverse diatom community. Although ASV richness was similar between oligotrophic and coastal waters, diatom community composition in these regions differed significantly and was correlated with temperature and phosphate, two environmental variables known to influence diatom metabolism and geographic distribution. In sum, oligotrophic waters of the western North Atlantic harbor diverse diatom assemblages that are distinct from coastal regions, and these open ocean diatoms warrant additional study, as they may play critical roles in oligotrophic ecosystems.

Spatio-temporal species distribution models reveal dynamic indicators for ecosystem-based fisheries management

Abstract Many economic sectors rely on marine ecosystem services, and holistic management is necessary to evaluate trade-offs between sectors and facilitate sustainable use. Integrated ecosystem assessments (IEA) integrate system components so that managers can evaluate pathways to achieve desired goals. Indicators are a central element of IEAs and capture the status and trend of individual components and should be sensitive to changes in the system; however, most indicators are aggregated over space and time as annual values, potentially leading to incomplete or inaccurate inferences about system change. Here, we demonstrate the utility of spatially and temporally explicit ecological indicators by fitting multivariate spatio-temporal models to survey data from the northeast US Shelf Ecosystem, encompassing three distinct ecoregions: Georges Bank, Gulf of Maine, and mid-Atlantic Bight. We evaluate three case studies to explore how these models can help assess ecosystem performance relative to management objectives, such as to: (1) identify dominant modes of variation in zooplankton communities; (2) quantify components of system stability; and (3) assess the density-dependent condition of groundfish over time. Collectively, these three examples demonstrate multiple interesting processes, but particularly highlight the rapid zooplankton changes and associated changes in benthivore condition and stability in the Gulf of Maine. Attributing changes in ecosystem indicators to localized processes is difficult using conventional “regionally aggregated” indicators, so this example highlights the benefits of spatio-temporal methods for integrated ecosystem analysis in this and other regions.

Temperature dependence of parasitoid infection and abundance of a diatom revealed by automated imaging and classification

Diatoms are a group of phytoplankton that contribute disproportionately to global primary production. Traditional paradigms that suggest diatoms are consumed primarily by larger zooplankton are challenged by sporadic parasitic "epidemics" within diatom populations. However, our understanding of diatom parasitism is limited by difficulties in quantifying these interactions. Here, we observe the dynamics of Cryothecomonas aestivalis (a protist) infection of an important diatom on the Northeast U.S. Shelf (NES), Guinardia delicatula, with a combination of automated imaging-in-flow cytometry and a convolutional neural network image classifier. Application of the classifier to >1 billion images from a nearshore time series and >20 survey cruises across the broader NES reveals the spatiotemporal gradients and temperature dependence of G. delicatula abundance and infection dynamics. Suppression of parasitoid infection at temperatures <4 °C drives annual cycles in both G. delicatula infection and abundance, with an annual maximum in infection observed in the fall-winter preceding an annual maximum in host abundance in the winter-spring. This annual cycle likely varies spatially across the NES in response to variable annual cycles in water temperature. We show that infection remains suppressed for ~2 mo following cold periods, possibly due to temperature-induced local extinctions of the C. aestivalis strain(s) that infect G. delicatula. These findings have implications for predicting impacts of a warming NES surface ocean on G. delicatula abundance and infection dynamics and demonstrate the potential of automated plankton imaging and classification to quantify phytoplankton parasitism in nature across unprecedented spatiotemporal scales.

Temperature regulates Synechococcus population dynamics seasonally and across the continental shelf

AbstractHourly, year‐round flow cytometry has made it possible to relate seasonal environmental variability to the population dynamics of the smallest, most abundant phytoplankton on the Northeast US Shelf. To evaluate whether the insights from these data extend to Synechococcus farther from shore, we analyze flow cytometry measurements made continuously from the underway systems on 21 cruises traveling between the Martha's Vineyard Coastal Observatory (MVCO) and the continental shelf break. We describe how seasonal patterns in Synechococcus, which have been documented in detail at MVCO, occur across the region with subtle variation. We find that the underlying relationship between temperature and division rate is consistent across the shelf and can explain much of the observed spatial variability in concentration. Connecting individual cell properties to annual and regional patterns in environmental conditions, these results demonstrate the value of autonomous monitoring and create an improved picture of picophytoplankton dynamics within an economically important ecosystem.

Increased gulf stream warm core ring formations contributes to an observed increase in salinity maximum intrusions on the Northeast Shelf

We present observational evidence of a significant increase in Salinity Maximum intrusions in the Northeast US Shelf waters in the years following 2000. This increase is subsequent to and influenced by a previously observed regime-shift in the annual formation rate for Gulf Stream Warm Core Rings, which are relatively more saline than the shelf waters. Specifically, mid-depth salinity maximum intrusions, a cross-shelf exchange process, has shown a quadrupling in frequency on the shelf after the year 2000. This increase in intrusion frequency can be linked to a similar increase in Warm Core Ring occupancy footprint along the offshore edge of the shelf-break which has greatly increased the abundance of warm salty water within the Slope Sea. The increased ring occupancy footprint along the shelf follows from the near doubling in annual Warm Core Ring formation rate from the Gulf Stream. The increased occurrence of intrusions is likely driven by a combination of a larger number of rings in the slope sea and the northward shift in the GS position which may lead to more interactions between rings and the shelf topography. These results have significant implications for interpreting temporal changes in the shelf ecosystem from the standpoint of both larval recruitment as well as habitability for various important commercial species.

Asymmetry in the rate of warming and the phenology of seasonal blooms in the Northeast US Shelf Ecosystem

AbstractPredicting the impact of marine ecosystem warming on the timing and magnitude of phytoplankton production is challenging. For example, warming can advance the progression of stratification thereby changing the availability of nutrients to surface phytoplankton, or influence the surface mixed layer depth, thus affecting light availability. Here, we use a time series of sea surface temperature (SST) and chlorophyll remote sensing products to characterize the response of the phytoplankton community to increased temperature in the Northeast US Shelf Ecosystem. The rate of change in SST was higher in the summer than in winter in all ecoregions resulting in little change in the timing and magnitude of the spring thermal transition compared to a significant change in the autumn transition. Along with little phenological shift in spring thermal conditions, there was also no evidence of a change in spring bloom timing and duration. However, we observed a change in autumn bloom timing in the Georges Bank ecoregion, where bloom initiation has shifted from late September to late October between 1998 and 2020—on average 33 d later. Bloom duration in this ecoregion also shortened from ∼7.5 to 5 weeks. The shortened autumn bloom may be caused by later overturn in stratification known to initiate autumn blooms in the region, whereas the timing of light limitation at the end of the bloom remains unchanged. These changes in bloom timing and duration appear to be related to the change in autumn thermal conditions and the significant shift in autumn thermal transition. These results suggest that the spring bloom phenology in this temperate continental shelf ecosystem may be more resilient to thermal climate change effects than blooms occurring in other times of the year.

Larval transport pathways from three prominent sand lance habitats in the Gulf of Maine

AbstractNorthern sand lance (Ammodytes dubius) are among the most critically important forage fish throughout the Northeast US shelf. Despite their ecological importance, little is known about the larval transport of this species. Here, we use otolith microstructure analysis to estimate hatch and settlement dates of sand lance and then use these measurements to parametrize particle tracking experiments to assess the source–sink dynamics of three prominent sand lance habitats in the Gulf of Maine: Stellwagen Bank, the Great South Channel, and Georges Bank. Our results indicate the pelagic larval duration of northern sand lance lasts about 2 months (range: 50–84 days) and exhibit a broad range of hatch and settlement dates. Forward and backward particle tracking experiments show substantial interannual variability, yet suggest transport generally follows the north to south circulation in the Gulf of Maine region. We find that Stellwagen Bank is a major source of larvae for the Great South Channel, while the Great South Channel primarily serves as a sink for larvae from Stellwagen Bank and Georges Bank. Retention is likely the primary source of larvae on Georges Bank. Retention within both Georges Bank and Stellwagen Bank varies interannually in response to changes in local wind events, while the Great South Channel only exhibited notable retention in a single year. Collectively, these results provide a framework to assess population connectivity among these sand lance habitats, which informs the species' recruitment dynamics and impacts its vulnerability to exploitation.

Environmental drivers and trends in forage fish occupancy of the Northeast US shelf

Abstract The Northeast US shelf ecosystem is undergoing unprecedented changes due to long-term warming trends and shifts in regional hydrography leading to changes in community composition. However, it remains uncertain how shelf occupancy by the region's dominant, offshore small pelagic fishes, also known as forage fishes, has changed throughout the late 20th and early 21st centuries. Here, we use species distribution models to estimate the change in shelf occupancy, mean weighted latitude, and mean weighted depth of six forage fishes on the Northeast US shelf, and whether those trends were linked to coincident hydrographic conditions. Our results suggest that observed shelf occupancy is increasing or unchanging for most species in both spring and fall, linked both to gear shifts and increasing bottom temperature and salinity. Exceptions include decreases to observed shelf occupancy by sand lance and decreases to Atlantic herring's inferred habitat suitability in the fall. Our work shows that changes in shelf occupancy and inferred habitat suitability have varying coherence, indicating complex mechanisms behind observed shelf occupancy for many species. Future work and management can use these results to better isolate the aspects of forage fish life histories that are important for determining their occupancy of the Northeast US shelf.

Seasonal Prediction of Bottom Temperature on the Northeast U.S. Continental Shelf

AbstractThe Northeast U.S. shelf (NES) is an oceanographically dynamic marine ecosystem and supports some of the most valuable demersal fisheries in the world. A reliable prediction of NES environmental variables, particularly ocean bottom temperature, could lead to a significant improvement in demersal fisheries management. However, the current generation of climate model‐based seasonal‐to‐interannual predictions exhibits limited prediction skill in this continental shelf environment. Here, we have developed a hierarchy of statistical seasonal predictions for NES bottom temperatures using an eddy‐resolving ocean reanalysis data set. A simple, damped local persistence prediction model produces significant skill for lead times up to ∼5 months in the Mid‐Atlantic Bight and up to ∼10 months in the Gulf of Maine, although the prediction skill varies notably by season. Considering temperature from a nearby or upstream (i.e., more poleward) region as an additional predictor generally improves prediction skill, presumably as a result of advective processes. Large‐scale atmospheric and oceanic indices, such as Gulf Stream path indices (GSIs) and the North Atlantic Oscillation Index, are also tested as predictors for NES bottom temperatures. Only the GSI constructed from temperature observed at 200 m depth significantly improves the prediction skill relative to local persistence. However, the prediction skill from this GSI is not larger than that gained using models incorporating nearby or upstream shelf/slope temperatures. Based on these results, a simplified statistical model has been developed, which can be tailored to fisheries management for the NES.

Resource Occurrence and Productivity in Existing and Proposed Wind Energy Lease Areas on the Northeast US Shelf

States in the Northeast United States have the ambitious goal of producing more than 22 GW of offshore wind energy in the coming decades. The infrastructure associated with offshore wind energy development is expected to modify marine habitats and potentially alter the ecosystem services. Species distribution models were constructed for a group of fish and macroinvertebrate taxa resident in the Northeast US Continental Shelf marine ecosystem. These models were analyzed to provide baseline context for impact assessment of lease areas in the Middle Atlantic Bight designated for renewable wind energy installations. Using random forest machine learning, models based on occurrence and biomass were constructed for 93 species providing seasonal depictions of their habitat distributions. We developed a scoring index to characterize lease area habitat use for each species. Subsequently, groups of species were identified that reflect varying levels of lease area habitat use ranging across high, moderate, low, and no reliance on the lease area habitats. Among the species with high to moderate reliance were black sea bass (Centropristis striata), summer flounder (Paralichthys dentatus), and Atlantic menhaden (Brevoortia tyrannus), which are important fisheries species in the region. Potential for impact was characterized by the number of species with habitat dependencies associated with lease areas and these varied with a number of continuous gradients. Habitats that support high biomass were distributed more to the northeast, while high occupancy habitats appeared to be further from the coast. There was no obvious effect of the size of the lease area on the importance of associated habitats. Model results indicated that physical drivers and lower trophic level indicators might strongly control the habitat distribution of ecologically and commercially important species in the wind lease areas. Therefore, physical and biological oceanography on the continental shelf proximate to wind energy infrastructure development should be monitored for changes in water column structure and the productivity of phytoplankton and zooplankton and the effects of these changes on the trophic system.

Multiple Linear Regression Models for Reconstructing and Exploring Processes Controlling the Carbonate System of the Northeast US From Basic Hydrographic Data

AbstractIn the coastal ocean, local carbonate system variability is determined by the interaction between ocean acidification and local processes. Sporadic observations indicate that biological metabolism, river input, and water mass mixing are dominant local processes driving carbonate system variability in northeast US shelf waters. These processes are also reflected in the variability of observed temperature (T), salinity (S), oxygen concentration (O2), and nitrate concentration (NO3−). Therefore, regionally specific empirical models can be developed, which relate carbonate system parameters to a combination of basic hydrographic parameters. Here, we develop multiple linear regression models that represent the processes that drive carbonate system variability in the Mid‐Atlantic Bight and Gulf of Maine using observations obtained on three hydrographic surveys in summers between 2007 and 2015. The empirical model equations reveal the observation‐based relationships between carbonate parameters and basic hydrographic variables. Unlike other regions where empirical models have been developed, salinity appears in all models. T is the most important parameter for predicting aragonite saturation state (ΩAR), while S and O2 are most important for predicting pH on total scale (pHT). The basic hydrographic variables explain over 98% of the variability in total alkalinity (TA), dissolved inorganic carbon (DIC), and ΩAR and 89% of the variability in pHT in the calibration data. We recommend applying models that depend on T, S, O2, and NO3− as predictors, which reproduce TA and DIC with R2 &gt; 0.97, ΩAR with R2 &gt; 0.93, and pHT with R2 &gt; 0.77, to reconstruct carbonate system parameters in the region.

Sensitivity of sand lance to shifting prey and hydrography indicates forthcoming change to the northeast US shelf forage fish complex

AbstractNorthern sand lance (Ammodytes dubius) and Atlantic herring (Clupea harengus) represent the dominant lipid-rich forage fish species throughout the Northeast US shelf and are critical prey for numerous top predators. However, unlike Atlantic herring, there is little research on sand lance or information about drivers of their abundance. We use intra-annual measurements of sand lance diet, growth, and condition to explain annual variability in sand lance abundance on the Northeast US Shelf. Our observations indicate that northern sand lance feed, grow, and accumulate lipids in the late winter through summer, predominantly consuming the copepod Calanus finmarchicus. Sand lance then cease feeding, utilize lipids, and begin gonad development in the fall. We show that the abundance of C. finmarchicus influences sand lance parental condition and recruitment. Atlantic herring can mute this effect through intra-guild predation. Hydrography further impacts sand lance abundance as increases in warm slope water decrease overwinter survival of reproductive adults. The predicted changes to these drivers indicate that sand lance will no longer be able to fill the role of lipid-rich forage during times of low Atlantic herring abundance—changing the Northeast US shelf forage fish complex by the end of the century.

Dynamics and functional diversity of the smallest phytoplankton on the Northeast US Shelf

Picophytoplankton are the most abundant primary producers in the ocean. Knowledge of their community dynamics is key to understanding their role in marine food webs and global biogeochemical cycles. To this end, we analyzed a 16-y time series of observations of a phytoplankton community at a nearshore site on the Northeast US Shelf. We used a size-structured population model to estimate in situ division rates for the picoeukaryote assemblage and compared the dynamics with those of the picocyanobacteria Synechococcus at the same location. We found that the picoeukaryotes divide at roughly twice the rate of the more abundant Synechococcus and are subject to greater loss rates (likely from viral lysis and zooplankton grazing). We describe the dynamics of these groups across short and long timescales and conclude that, despite their taxonomic differences, their populations respond similarly to changes in the biotic and abiotic environment. Both groups appear to be temperature limited in the spring and light limited in the fall and to experience greater mortality during the day than at night. Compared with Synechococcus, the picoeukaryotes are subject to greater top-down control and contribute more to the region's primary productivity than their standing stocks suggest.

Cold range edges of marine fishes track climate change better than warm edges.

Species around the world are shifting their ranges in response to climate change. To make robust predictions about climate-related colonizations and extinctions, it is vital to understand the dynamics of range edges. This study is among the first to examine annual dynamics of cold and warm range edges, as most global change studies average observational data over space or over time. We analyzed annual range edge dynamics of marine fishes-both at the individual species level and pooled into cold- and warm-edge assemblages-in a multi-decade time-series of trawl surveys conducted on the Northeast US Shelf during a period of rapid warming. We tested whether cold edges show stronger evidence of climate tracking than warm edges (due to non-climate processes or time lags at the warm edge; the biogeography hypothesis or extinction debt hypothesis), or whether they tracked temperature change equally (due to the influence of habitat suitability; the ecophysiology hypothesis). In addition to exploring correlations with regional temperature change, we calculated species- and assemblage-specific sea bottom and sea surface temperature isotherms and used them to predict range edge position. Cold edges shifted further and tracked sea surface and bottom temperature isotherms to a greater degree than warm edges. Mixed-effects models revealed that for a one-degree latitude shift in isotherm position, cold edges shifted 0.47 degrees of latitude, and warm edges shifted only 0.28 degrees. Our results suggest that cold range edges are tracking climate change better than warm range edges, invalidating the ecophysiology hypothesis. We also found that even among highly mobile marine ectotherms in a global warming hotspot, few species are fully keeping pace with climate.

Decadal changes in the productivity of New England fish populations

The Northwest Atlantic continental shelf is a large ecosystem undergoing rapid environmental changes, which are expected to modify the productivity of natural marine resources. Current management of most fished species assumes stationary production relationships or time-invariant recruitment rates. With linear state-space models, we examined the evidence of dynamic productivity for 25 stocks of the Northeast US shelf. We expanded the suite of options available within the state-space approach to produce robust estimates. Fifteen of the stocks exhibited time-varying productivity or changes in their maximum reproductive rate. Few productivity time series are related across the whole region, though adjacent stocks of the same species exhibited similar trends. Some links to region-wide environmental variables were observed. We demonstrate that fish recruitment can often be better predicted over a short-term horizon by accounting for dynamic productivity, which could be valuable for fisheries management. Improving predictions by incorporating environmental covariates or covariance among the stocks must be considered case by case and with caution, as their relationships may change over time.

Economic and Ecosystem Effects of Fishing on the Northeast US Shelf

Modeling tools that can demonstrate possible consequences of strategies designed to operationalize ecosystem-based fisheries management (EBFM) should be able to address tradeoffs over a wide suite of considerations representing the scope of marine management objectives. Coupled ecological-economic modeling, where models for ecological and economic subsystems are linked through their inputs and outputs, allows for quantification of such tradeoffs. Here, we link the harvest output from fishery management scenarios implemented in an end-to-end ecosystem model (Atlantis) to an input-output regional economic model for the Northeast US to calculate changes in socio-economic indicators, including the consequences of management action for regional sales, wages, and employment. We implement three simple scenarios (maintain, decrease, or increase current fishing effort), and compare model-projected values for systematic and sector-specific indicators. Systematic indicators revealed different ecological and economic outcomes, with large ecological responses and clear tradeoffs among the catch and biomass of species groups. Economic indicators for the region responded similarly to fishery yield, however changes in total sales did not match those in landed catch. Under increased fishing effort, a lower proportional increase in sales relative to total landed catch arose due to increased yield from lower value species groups. Average fisheries income changed little among scenarios, but was highest when effort was maintained at current levels, likely a reflection of fleet and catch stability. Our results serve to demonstrate that consequences of management may be felt disproportionately among species through the region and across different fisheries sectors. With our coupled modeling approach of passing Atlantis ecosystem model outputs to an input-output economic model, we were able to assess effects of fisheries management across a broader suite of indicators that have relevance for policymakers across multiple objectives.

Spatio-temporal dynamics of summer flounder (Paralichthys dentatus) on the Northeast US shelf

Summer flounder (Paralichthys dentatus) are an economically and ecologically important fish on the Northeast U.S. shelf. There is evidence that their spatial distribution has shifted over time. However, there are conflicting reports on the importance of various potential drivers of the shift. Here, we investigate whether the stock has shifted and the extent to which this can be attributed to changes in abundance, size-structure, environmental variables, and fishing. We do so using a vector-autoregressive spatio-temporal model that incorporates data from two seasonal bottom trawl surveys that together span the nearshore and offshore Northeast US shelf over the past 41 years. We find that the summer flounder distribution has shifted north and east in both the spring and fall. The shift is observed in both recruits and spawners, with recruits shifting northward faster than spawners, suggesting that increased spawner abundance may not be driving the shift in recruits. We find that only a small portion of the variability in distribution can be attributed to changes in abundance, fishing, or environmental covariates. Instead, the shift is most strongly attributed to unidentified factors.