Georges Bank Region Research Articles

Building confidence: Developing image-based methods to incorporate fishery-collected data in the American lobster stock assessment

The American lobster, Homarus americanus, supports the most valuable single-species fishery in North America; however to-date, a reliable and robust method to determine age does not exist, and thus some of the more traditional catch-at-age stock assessment methods cannot be used to determine status. In lieu of this, the Atlantic States Marine Fisheries Commission’s American Lobster Stock Assessment Model uses a probabilistic growth transition matrix to determine how lobsters of different sizes will grow and recruit into the fishery. Developing and updating the growth transition matrix to reflect current growth dynamics requires estimates of molt increment and interval. Tagging studies are an inexpensive method to collect this type of information, but relying on fishing industry reported recapture data and buy-in can be challenging, resulting in varied return rates and uncertainty in measurement accuracy. Here, we report on a subset of recaptured lobsters released in the Gulf of Maine and Georges Bank regions between 2015 and 2020, where in addition to recapture locations, harvesters were encouraged to submit images of recaptured lobsters alongside a standard lobster gauge as a scale to estimate carapace length. Some fishermen were able to provide both direct measurements and images. Images were analyzed in ImageJ to estimate individual lobster carapace length (CL); and each image was assigned a quality score. For all groups, image-derived lengths were correlated with measured observations, regardless of the overall image quality, with only a slight underestimation at the maximum end of CL. This image-based method for length estimation provides high-quality length predictions regardless of image quality and can significantly increase the likelihood of harvesters contributing data with broad potential to engage commercial fisherman in collaborative science. Using this method we were able to expand the dataset of length records from this project by 251 or 33 %, providing additional data at a rate similar to tagging an additional 6000 lobsters.

Spatio-temporal contrastive learning for acoustic data

Large volumes of water column sonar data have been generated from acoustic surveys of living marine resources. They provide valuable information about marine ecosystems. To leverage them for acoustic target identification, scarcity of annotations is usually an issue. While some annotations have been generated by scientists via manual scrutiny or limited automation, they are limited to certain species. To fill in this gap, we propose a spatio-temporal contrastive learning approach for acoustic data. This unsupervised deep learning technique leverages both acoustic and non-acoustic (spatial and temporal) information for acoustic target classification. We firstly employ the Simple Linear Iterative Clustering (SLIC) algorithm to extract superpixels from acoustic data. Then the spatial and temporal similarity between two superpixels is computed, and the proposed spatio-temporal contrastive learning approach is applied to learn a semantically meaningful representation for superpixels. Finally, the learned representations of superpixels are used in acoustic target classification. We demonstrate that this approach outperforms previous methods with acoustic data collected by the NOAA National Marine Fisheries Service in the Gulf of Maine and Georges Bank region over 10 years. This dataset is archived at NOAA National Centers for Environmental Information and accessed for free through AWS and NOAA Big Data Program.

A test of the provisioning hypothesis of recruitment control in Georges Bank haddock

The haddock (Melanogrammus aeglefinus) stock of the Georges Bank region of the US Northeast Continental Shelf displays a pattern of large, episodic recruitments. Among the hypothesized controlling mechanisms is the idea that recruitment events are related to provisioning of prespawning haddock by the fall bloom the year before. With the occurrence of a recent large recruitment event in 2013, it would be prudent to retest this hypothesis. Fall bloom magnitude was positively correlated (r = 0.645, p = 0.005) with haddock survivor ratio (recruits per spawning biomass), including data from the 2013 recruitment. This relationship identifies a pathway of bottom-up control of a resource species, thus focusing concern over recent changes in lower trophic-level productivity.

Selectivity of Two Commercial Dredges Fished in the Northwest Atlantic Sea Scallop Fishery

Size selectivity curves were estimated for two commercial dredge configurations used in the United States Atlantic sea scallop (Placopecten magellanicus) fishery. Traditionally, the New Bedford Style dredge (New Bedford dredge) has been used by the commercial fleet; however, since 2013, the Coonamessett Farm Turtle Deflector Dredge (turtle dredge) has been required seasonally in the Mid-Atlantic region to minimize the capture of sea turtles. This analysis provides estimates of selectivity and relative efficiency for the turtle dredge and the New Bedford dredge. Selectivity information for the turtle dredge is currently unknown, and selectivity for the New Bedford dredge was originally assessed in 2008. The Share Each Lengths Catch Total method was used to model scallop catch-at-length data for each commercial dredge with data collected during the Virginia Institute of Marine Science fishery-independent surveys in 2015 to 2017, conducted onboard commercial fishing vessels. The surveys were conducted in the Mid-Atlantic and Georges Bank regions of the resource in three distinct areas. A paired study design was used, where a nonselective National Marine Fisheries Service sea scallop survey dredge and either the turtle dredge or New Bedford dredge were towed simultaneously at each survey station. To assess for time-varying changes in selectivity, results were compared with the 2008 New Bedford dredge analysis. Results indicated the 50% retention length was 98.2 mm, with a selection range of 28.2 mm and a relative efficiency of 0.83 for the turtle dredge. New Bedford dredge results estimated a 50% retention length of 107.4 mm, selection range of 50.5 mm and relative efficiency of 0.81. Comparing selectivity profiles for the turtle dredge and New Bedford dredge with the 2008 results indicated a shift toward increased retention probability for smaller size classes of scallops.

Comparing acoustic classification methods to estimate krill biomass in the Georges Bank region from 1999 to 2012

AbstractKrill are a critical component of northern‐latitude ecosystems, yet are poorly monitored in the Gulf of Maine and Georges Bank regions of the northeast United States continental shelf. We applied five acoustic classification methods (multifrequency single‐beam index [MFSBI], dB‐differencing, multifrequency index [MFI], and two variations of the Z‐score method) to acoustic data collected during surveys conducted annually in the Georges Bank region in order to map krill spatial distribution from 1999 to 2012. Material properties (sound speed and mass density) of krill and length data from net samples collected in the Gulf of Maine and Georges Bank regions were used in conjunction with distorted wave Born approximation (DWBA) predicted target strengths to scale acoustic data to estimates of biomass for the Georges Bank region. All methods require a priori information, ranging from training sets (i.e., empirically based classification) to validated acoustic models of target strength (i.e., theoretically based classification) for the species and acoustic frequencies of interest. Incorporating predicted backscatter from theoretical models can improve confidence in classification, especially when clean training sets are not available, but does come at the cost of a validated acoustic backscatter model. The biomass estimates from the four acoustic classification methods were comparable among each other, with the MFSBI method consistently estimating lowest biomass and the empirically based Z‐score consistently estimating highest biomass. Comparisons to net‐based estimates were inconsistent, most likely due to net inefficiency (low catchability), targeting of layers, and scaling point estimates to regions.

Two-hundred year record of increasing growth rates for ocean quahogs (Arctica islandica) from the northwestern Atlantic Ocean

Ocean quahogs [Arctica islandica (Linnaeus, 1769)] are the longest-lived, non-colonial animal known today, with a maximum life span exceeding 500 years. Ocean quahogs are a commercially important bivalve, inhabiting the continental shelf of the North Atlantic basin. We examined growth rates of ocean quahogs that were fully recruited to the commercial fishery (>80-mm shell length) from four sites covering the range of the stock along the east coast of the U.S. through analysis of annual growth lines in the hinge plate. Both geographic and temporal differences (on a scale of decadal or longer) in growth rates exist throughout the range of the stock. The age at which animals reached 60, 80, and 90 mm decreased significantly, and average growth rates to 60, 80, and 90 mm increased significantly with birth year at a New Jersey and a Long Island site, both located in the southwestern portion of the stock, since the late 1700s/early 1800s, likely in response to increasing bottom water temperatures. That is, growth rates vary temporally with birth date at the southwestern sites, with younger animals growing at a much faster rate in recent decades than those born many decades previously, whereas at the northern sites off southern New England and on Georges Bank, changes in growth rates through time are limited to older adult animals or absent altogether. Thus, at the southern portion of the range, variation in growth rate over time exists in all phases of ocean quahog life, whereas on Georges Bank, little evidence exists for any differential in growth rate over the last ~200 years. The fact that ocean quahogs record the rise in ocean temperatures after the Little Ice Age in the Mid-Atlantic Bight southeast of southern New England, yet demonstrate little evidence of such a rise in the southern New England and Georges Bank region, would suggest a differential response of ocean circulation and its control of bottom water temperature between the northern and southern portions of the Mid-Atlantic Bight over the last 200+ years.

Estimates of Sea Scallop (Placopecten magellanicus) Incidental Mortality from Photographic Multiple Before—After-Control—Impact Surveys

After several decades of stock decline, the Atlantic sea scallop (Placopecten magellanicus) resource has rebounded to become one of the most valuable fisheries in the United States. The continued sustainability of this fishery is supported by catch limits determined by annual stock projection models. Incidental mortality is an important term in these projection models, but is historically difficult to measure. Current estimates are derived from field experiments that relied heavily on qualitative observations and as a result are based on limited data with low precision. To better quantify incidental mortality, a multiple before—after-control—impact experimental design was used to measure the effect of scallop dredging on the disposition of sea scallops that remain uncaptured on the seafloor following dredging. An autonomous underwater vehicle was used to collect color photographs and side-scan sonar images of the seafloor before and after controlled dredge treatments in the mid-Atlantic and Georges Bank regions. Approximately 170,000 photographs were annotated for instances of mortality. Dredge-induced incidental mortality of 2.5% and 8% was estimated for the mid-Atlantic Bight and Georges Bank sites, respectively, a difference that is likely attributable to the relatively harder substrate of the scallop habitat on Georges Bank that results in greater physical trauma to the uncaptured scallops. This study provides a quantitative estimate of incidental mortality using a noninvasive platform that offers precise mission navigation for repeated surveys. The spatial scale and distribution of the study sites are broad relative to past incidental mortality studies, and the substrate types at each are common in the sea scallop fishery. Estimated mortality rates are lower than the values currently used in fishery stock models and suggest the existing values are conservative, but likely appropriate estimates for management purposes.

The death assemblage as a marker for habitat and an indicator of climate change: Georges Bank, surfclams and ocean quahogs

A comprehensive dataset for the Georges Bank region is used to directly compare the distribution of the death assemblage and the living community at large spatial scales and to assess the application of the death assemblage in tracking changes in species’ distributional pattern as a consequence of climate change. Focus is placed on the biomass-dominant clam species of the northwest Atlantic continental shelf: the surfclam Spisula solidissima and the ocean quahog Arctica islandica, for which extensive datasets exist on the distributions of the living population and the death assemblage. For both surfclams and ocean quahogs, the distribution of dead shells, in the main, tracked the distribution of live animals relatively closely. Thus, for both species, the presence of dead shells was a positive indicator of present, recent, or past occupation by live animals. Shell dispersion within habitat was greater for surfclams than for ocean quahogs either due to spatial time averaging, animals not living in all habitable areas all of the time, or parautochthonous redistribution of shell. The regional distribution of dead shell differed from the distribution of live animals, for both species, in a systematic way indicative of range shifts due to climate change. In each case the differential distribution was consistent with warming of the northwest Atlantic. Present-day overlap of live surfclams with live ocean quahogs was consistent with the expectation that the surfclam's range is shifting into deeper water in response to the recent warming trend. The presence of locations devoid of dead shells where live surfclams nevertheless were collected measures the recentness of this event. The presence of dead ocean quahog shells at shallower depths than live ocean quahogs offers good evidence that a range shift has occurred in the past, but prior to the initiation of routine surveys in 1980. Possibly, this range shift tracks initial colonization at the end of the Little Ice Age.

Use of acoustic estimates of euphausiid biomass in trophic dynamics and ecosystem models of the Georges Bank/Gulf of Maine Region from 1999 to 2012

Euphausiids are a key link between primary production and higher-level predators in the Gulf of Maine, but are not well sampled during standard fisheries surveys. Multifrequency acoustic data may provide useful estimates of euphausiid distribution and biomass, as long as automated classification of acoustic backscatter is reliable and robust. Estimates of euphausiid biomass in the Georges Bank region of the Gulf of Maine were derived from annual acoustic/midwater trawl surveys from 1999 through 2012. Acoustic data were collected continuously with Simrad EK500 and EK60 echo sounders operating at 18, 38, and 120 kHz. Four different methods were used to classify euphausiids from the acoustic data: multifrequency single beam imaging, “dB-differencing” of 120- and 38-kHz volume backscatter, multifrequency z-score, and a multifrequency index. Scattering model predictions of euphausiid target strength and biological metrics were used to scale acoustic data to biomass for each classification method. Biomass estim...

Atlantic herring (Clupea harengus) demographics in the Gulf of Maine from 1998 to 2012

Atlantic herring (Clupea harengus) in the Georges Bank, Gulf of Maine, and southern New England regions were sampled from early September into mid-November by the Northeast Fisheries Science Center during their annual randomly-stratified bottom trawl and systematic acoustic/midwater trawl surveys. Atlantic herring were randomly selected for length distributions and systematically subsampled for biological metrics (e.g., weight, age, maturity, and sex). Broad similarities in Atlantic herring biological metrics between midwater and bottom trawl catches suggest both gear types provide comparable sampling of herring demographics in the Gulf of Maine and Georges Bank regions during autumn. Annual mean lengths and weights and temporal patterns of herring size were consistently similar between gear types. Similarity in age structure between Georges Bank and the Gulf of Maine suggest a similar response to intrinsic and extrinsic factors between herring stocks in these regions, but apparent asynchrony in spawning timing suggests some level of independence between these stocks. Overall similarities can mask interesting distinctions such as midwater trawls seem to sample younger, smaller, but heavier fish than do bottom trawls. Sampling of historical spawning sites appears to characterize the biological state of Atlantic herring in the Georges Bank region and could be utilized to design an efficient sampling scheme for Atlantic herring in the Gulf of Maine.

Distribution of Atlantic herring (Clupea harengus) in the Gulf of Maine from 1998 to 2012

Atlantic herring (Clupea harengus) in the Gulf of Maine were surveyed using fisheries-independent systematic and stratified-random surveys conducted annually by the Northeast Fisheries Science Center from 1998 to 2012 during autumn. The systematic surveys were dedicated to surveying Atlantic herring and were designed to be synchronous with pre-spawning, whereas the stratified-random surveys were broad-scale, multi-species surveys. Both types of surveys collected multi-frequency acoustic and trawl data. These surveys were analyzed to evaluate their individual effectiveness as surveys, whether results could be combined, or whether either survey could be used alone to estimate Atlantic herring abundance. With the exception of two years, more Atlantic herring in the water column were observed in the Georges Bank region during the dedicated systematic survey than later in the season by the stratified-random survey in the Georges Bank and northern Gulf of Maine regions. These observations suggest that the systematic survey was not out of synchrony with Atlantic herring spawning in the Georges Bank area, and the majority of spawning in the off-shore Gulf of Maine region occurred in the Georges Bank area. Additionally, the area backscattering time series showed no dramatic changes among surveys conducted with different vessels, suggesting that the Atlantic herring did not react substantially to changes in vessels and/or reacted similarly to the presence of any of the four vessels used.

BAYESIAN SPATIAL-TEMPORAL MODELING OF ECOLOGICAL ZERO-INFLATED COUNT DATA.

A Bayesian hierarchical model is developed for count data with spatial and temporal correlations as well as excessive zeros, uneven sampling intensities, and inference on missing spots. Our contribution is to develop a model on zero-inflated count data that provides flexibility in modeling spatial patterns in a dynamic manner and also improves the computational efficiency via dimension reduction. The proposed methodology is of particular importance for studying species presence and abundance in the field of ecological sciences. The proposed model is employed in the analysis of the survey data by the Northeast Fisheries Sciences Center (NEFSC) for estimation and prediction of the Atlantic cod in the Gulf of Maine - Georges Bank region. Model comparisons based on the deviance information criterion and the log predictive score show the improvement by the proposed spatial-temporal model.

Nutrients and water masses in the Gulf of Maine–Georges Bank region: Variability and importance to blooms of the toxic dinoflagellate Alexandrium fundyense

We report here the results of ten oceanographic survey cruises carried out in the Gulf of Maine–Georges Bank region of the Northwest Atlantic during the late spring to summer period in 2007, 2008 and 2010, for which we examine and characterize relationships among dissolved inorganic nutrient fields, water mass dynamics and cell densities of the toxic dinoflagellate Alexandrium fundyense. Nutrients are supplied to continental shelf waters of the Gulf of Maine–Georges Bank region by inflows of deep offshore water masses; once in the Gulf they are transported with the residual circulation and mix with surface waters, both in the Gulf and on the Bank. Those fluxes of offshore water masses and their nutrient loads are the major source of nutrients for phytoplankton production in the region, including annual blooms of A. fundyense in the Gulf and on Georges Bank. This much is already known. We suggest here that the locations and magnitude of A. fundyense blooms are controlled in part by variable nutrient fluxes to the interior Gulf of Maine from offshore, and, those interior Gulf of Maine waters are, in turn, the main nutrient source to Georges Bank, which are brought onto the Bank by tidal pumping on the Northern Flank. We present evidence that nitrate is the initial form of nitrogenous nutrient for A. fundyense blooms, but it is quickly depleted to limiting concentrations of less than 0.5µM, at which time continued growth and maintenance of the population is likely fueled by recycled ammonium. We also show that phosphate may be the limiting nutrient over much of Georges Bank in summer, allowing recycled ammonium concentrations to increase. Our temperature–salinity analyses reveal spatial and temporal (seasonal and interannual) variability in the relative proportions of two deep source waters that enter the Gulf of Maine at depth through the Northeast Channel: Warm Slope Water (WSW) and Labrador Slope Water (LSW). Those two source waters are known to vary in their nutrient loads, with nitrate concentrations about 50% higher in WSW than LSW, for example, and as such the proportions of these two water masses to one another are important determinants of the overall nutrient loads in the interior Gulf. In addition to these deep slope water fluxes, we show evidence here of episodic fluxes of relatively fresh and low-nutrient shelf waters from the Nova Scotian Shelf, which enter the Gulf in pulses at depths between the surface and approximately 150m, displacing deep slope waters, and consequently they significantly dilute the Gulf's interior waters, reducing nutrient concentrations and, in turn, affect the magnitude of A. fundyense blooms.

Geospatial analysis of habitat use in yellowtail flounder Limanda ferruginea on Georges Bank

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 468:279-290 (2012) - DOI: https://doi.org/10.3354/meps10035 Geospatial analysis of habitat use in yellowtail flounder Limanda ferruginea on Georges Bank Jose J. Pereira1,*, Eric T. Schultz2, Peter J. Auster3 1NOAA Fisheries, Milford Laboratory, 212 Rogers Avenue, Milford, Connecticut 06460, USA 2Dept. of Ecology and Evolutionary Biology, University of Connecticut, Storrs, Connecticut 06269-3043, USA 3Department of Marine Sciences at the University of Connecticut, and Sea Research Foundation—Mystic Aquarium, Groton, Connecticut 06340, USA *Email: jose.pereira@noaa.gov ABSTRACT: Three theories of habitat use proposed for marine fishes—the constant density model, the proportional density model, and the basin model—make contrasting predictions of how the geographical range, local density, and fitness change as population size changes. We tested model predictions with survey data on yellowtail flounder Limanda ferruginea from the Georges Bank region, where abundance changed by a factor of 4 over a decade. Surveys took place in spring and fall, and data on individual length, mass, sex, and reproductive status were available. Analysis of spatial pattern revealed that the overall area occupied by flounder increased by a factor of 2 when abundance was high, and local density increased predominantly in high quality habitat that had been closed to commercial fishing. Condition, which served as a proxy for fitness, was lower in females when abundance was high. Geospatial analysis revealed mesoscale variability in condition, over 10s to >100 km, except in the spring season during low abundance periods. Spatial autocorrelation explained as much as 25% of the variability in condition, indicating that site dependence was a factor in explaining the spatial distribution that we observed. These results are most supportive of both the constant density model and the basin model. This approach detected an important population center for yellowtail flounder and determined its extent using only measures of abundance, location, and condition of individual fish, data commonly collected during routine fishery assessment surveys. Here we demonstrate that analyses linking population responses to variation in such measures at local spatial scales can have significant implications for identifying areas of important fish habitat and suggest greater use of geospatial approaches in conservation and management of exploited species. KEY WORDS: Spatial autocorrelation · Kriging · Yellowtail flounder · Georges Bank · Habitat Full text in pdf format PreviousNextCite this article as: Pereira JJ, Schultz ET, Auster PJ (2012) Geospatial analysis of habitat use in yellowtail flounder Limanda ferruginea on Georges Bank. Mar Ecol Prog Ser 468:279-290. https://doi.org/10.3354/meps10035 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 468. Online publication date: November 14, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Exploring the population dynamics of winter skate (Leucoraja ocellata) in the Georges Bank region using a statistical catch-at-age model incorporating length, migration, and recruitment process errors

Winter skate ( Leucoraja ocellata ) of all length classes increased dramatically in abundance on Georges Bank in the 1980s following the decline of many groundfish species. We present a full population model of winter skate to better understand the population dynamics of the species and elucidate the mechanisms underlying their increase in abundance in the 1980s. Specifically, we developed four statistical catch-at-age models incorporating length-frequency data with the following model structures: (i) observation error only (base model R1); (ii) observation and recruitment process errors (model R2); (iii) adult migration modeled as a random walk in adult mortality (model R3); and (iv) observation and recruitment process errors and adult migration (model R4). Akaike’s information criterion values indicated that models R3 and R4, which both included adult migration, were the most parsimonious models. This finding strongly suggests that the winter skate population increase on Georges Bank in the 1980s was not solely a result of increases in recruitment but likely involved adult migration (i.e., it is an open population). Finally, recent predicted fishing mortalities exceeded FMSY for all models.

Dispersal and settlement of sea scallop larvae spawned in the fishery closed areas on Georges Bank

Abstract Tian, R. C., Chen, C., Stokesbury, K. D. E., Rothschild, B. J., Cowles, G. W., Xu, Q., Hu, S., Harris, B. P., and Marino II, M. C. 2009. Dispersal and settlement of sea scallop larvae spawned in the fishery closed areas on Georges Bank. – ICES Journal of Marine Science, 66: 2155–2164. Three fishery closed areas in the Georges Bank (GB) region were implemented in 1994 to protect depleted groundfish stocks for population replenishment. However, the drift and ultimate destination of larvae spawned in the closed areas have not been analysed specifically within the framework of ocean currents. To assess the efficiency of the closed areas as population replenishment sources, we conducted a simulation-based analysis on the dispersal and settlement of sea scallop larvae spawned in the closed areas from 1995 to 2005 using circulation fields computed by the Finite-Volume Coastal Ocean Model, scallop survey data, and a population dynamics model. Closed area I located in the Great Southern Channel (GSC) had a persistently high rate of larval retention (86% on average). For closed area II located on eastern GB, a considerable quantity of larvae was dispersed out of the domain. For the Nantucket Lightship Closed Area located on Nantucket Shoals, larvae consistently drifted away from the region during the 11 years simulated. Our simulation revealed three high-retention regions that are the most suitable for closed-area selection and rotational fishery management in terms of larval supply to the GB–GSC region.

Sensitivity analysis of sea scallop (Placopecten magellanicus) larvae trajectories to hydrodynamic model configuration on Georges Bank and adjacent coastal regions

AbstractThe previous larval‐trajectory modeling studies on Georges Bank were assessed through process‐oriented Lagrangian‐tracking comparison experiments using the high‐resolution Gulf of Maine/Georges Bank Finite‐Volume Coastal Ocean Model (GOM‐FVCOM). The results indicate that in a strong nonlinear system such as Georges Bank, the passive tracer movement is driven by a fully three‐dimensional Lagrangian flow field that varies in space and time due to large tidal excursion and steep bottom topography. The particle‐tracking methods developed based on the assumption of weak nonlinearity of the flow field are not applicable to Georges Bank. The results of previous larval transport studies driven by circulation fields constructed under the weak‐nonlinearity assumption need to be interpreted with caution. In the present work, the influence of model physical setups on sea scallop larval dispersal and settlement on Georges Bank and adjacent shelf regions is examined. Distinct differences in the spatial distribution of the passive larvae predicted by the model under various physical conditions suggest that a fully nonlinear model driven by realistic spatially and temporally varying forcing should be employed for Lagrangian‐based studies of fishery population dynamics on Georges Bank.

An Ecosystem Approach for Assessment Advice and Biological Reference Points for the Gulf of Maine–Georges Bank Atlantic Herring Complex

AbstractThe biomass of Atlantic herring Clupea harengus in the Gulf of Maine–Georges Bank (GOM–GB) region declined sharply in the mid‐ to late 1970s, when fishery harvests ranged from 300 to 400 kilo–metric tons (kt) per year. During this same period, the consumption of Atlantic herring by fish and marine mammal predators was very large. Consumption has greatly exceeded landings from 1985 to the present. Previous research showed that four groups of predators (demersal fishes, marine mammals, large pelagic fishes, and seabirds) together annually consumed over 200 kt of Atlantic herring during the 1970s and over 300 kt during the late 1990s and early 2000s. Our analyses indicate that Atlantic herring predation mortality rates are related to the abundance of both Atlantic herring and predators. Predation mortality rates were relatively low during the 1960s, when Atlantic herring were abundant, but increased in the late 1970s and early 1980s, when Atlantic herring declined. Predation mortality rates declined in the 1990s as Atlantic herring abundance increased. Biological reference points for the Atlantic herring complex (composed of two stocks) are significantly different when predation mortality is included. Several methods were used to assess the available fishery yield given that predator requirements have first been satisfied. In general, all of the approaches indicated that the maximum sustainable yield harvest level for the GOM–GB Atlantic herring complex is lower than that estimated from the single‐species assessment in which predation effects are not explicitly accounted for.

NEW HYPOTHESIS HELPS EXPLAIN ELASMOBRANCH “OUTBURST” ON GEORGES BANK IN THE 1980s

Regime shifts are a feature of many ecosystems. During the last 40 years, intensive commercial exploitation and environmental changes have driven substantial shifts in ecosystem structure and function in the northwest Atlantic. In the Georges Bank-southern New England region, commercially important species have declined, and the ecosystem shifted to one dominated by economically undesirable species such as skates and dogfish. Aggregated abundance indices indicate a large increase of small and medium-sized elasmobranchs in the early 1980s following the decline of many commercial species. It has been hypothesized that ecological interactions such as competition and predation within the Georges Bank region were responsible for and are maintaining the "elasmobranch outburst" at the heart of the observed ecosystem shift. We offer an alternative hypothesis invoking population connectivity among winter skate populations such that the observed abundance increase is a result of migratory dynamics, perhaps with the Scotian Shelf (i.e., it is an open population). Here we critically evaluate the survey data for winter skate, the species principally responsible for the increase in total skate abundance during the 1980s on Georges Bank, to assess support for both hypotheses. We show that time series from different surveys within the Georges Bank region exhibit low coherence, indicating that a widespread population increase was not consistently shown by all surveys. Further, we argue that observed length-frequency data for Georges Bank indicate biologically unrealistic population fluctuations if the population is closed. Neither finding supports the elasmobranch outburst hypothesis. In contrast, survey time series for Georges Bank and the Scotian Shelf are negatively correlated, in support of the population connectivity hypothesis. Further, we argue that understanding the mechanisms of ecosystem state changes and population connectivity are needed to make inferences about both the causes and appropriate management responses to large-scale system change.

Geolocation of Atlantic cod (Gadus morhua) movements in the Gulf of Maine using tidal information

AbstractInformation derived from archival tags (digital storage tags, DSTs) were used to backtrack the migration of 11 tagged Atlantic cod (Gadus morhua) during 2001 in Massachusetts Bay, the Gulf of Maine, and Georges Bank. The DST tags continuously recorded time, temperature and depth. To geolocate fish positions during its time at large, we first extracted the tidal signal from the pressure recordings on the DST tags, and then compared the resulting data to data predicted with a Massachusetts Bay tidal model that provided us with geographical coordinates at a given date and time. Using least‐squares criteria within an estimated geographical region of confidence that was constrained by biological and statistical information (e.g. swimming speed, known release and recapture location, and bottom depth) we were able to geolocate the fish. The resultant geolocated migration tracks indicate a large degree of movement of Atlantic cod in the region and an elevated importance of the Great South Channel (GSC) migration corridor between Massachusetts Bay and the western Georges Bank and Nantucket Shoals region. This observation contrasts strongly with inferences of limited movements by Atlantic cod based on conventional tag recapture methods (mean of 1200 km traveled versus 44 km traveled as measured by conventional tagging and geolocation, respectively). This study demonstrates that geolocation methodologies applied to archival tag studies hold great promise of becoming an important new tool for fisheries managers to quantify the movements of fishes. It also points out the need for greater collaboration between fisheries scientists and oceanographers, and particularly for the development of improved tidal models to cover stock regions more accurately and with higher precision.