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Populations of the eastern oyster (Crassostrea virginica) have experienced declines from overfishing and disease throughout much of its US range, though development of maximum sustainable yield (MSY) management criteria has been elusive. This is due in part to the discordance between oyster spawning stock and recruits, as the classic stock–recruitment model does not account for the requirement of shell substrate on which recruits settle. This issue was recently addressed with the development of a surface area–recruitment model, which is herein incorporated into a simulation analysis to estimate MSY-based reference points for C. virginica in the Delaware Bay. Simulations demonstrate that at low natural mortality, fishing mortality (F) may be sustainable at values between 10% and 15%; however, if disease or other mortality-enhancing processes occur, the margin of error in fishing is small and may quickly lead to population and reef collapse, emphasizing a precautionary F < 10%. The MSY-based reference points generated here provide rebuilding goals for the oyster fishery and reef management on fished and unfished reefs and the framework from which shell-planting can be incorporated and optimized in the future.

Competing pressures imposed by climate-related warming and offshore development have created a need for quantitative approaches that anticipate fisheries responses to these challenges. This study used a spatially explicit, ecological-economic agent-based model integrating dynamics associated with Atlantic surfclam stock biology, decision-making behavior of fishing vessel captains, and fishing fleet behavior to simulate stock biomass, and fishing vessel catch, effort and landings. Simulations were implemented using contemporary Atlantic surfclam stock distributions and characteristics of the surfclam fishing fleet. Simulated distribution of fishable surfclam biomass was determined by a spatially varying mortality rate, fishing by the fleet was controlled by captain decisions based on previous knowledge, information sharing, and the ability to search and find fishing grounds. Quantitative and qualitative evaluation of simulation results showed that this modeling approach sufficiently represents Atlantic surfclam fishery dynamics. A fishing simulation showed that the captain's decision-making and stock knowledge, and the distribution of fishing grounds relative to home ports controlled the landed catch. The approach used herein serves as the basis for future studies examining response of the Atlantic surfclam fishery to a nexus of simultaneous, complex natural and anthropogenic pressures, and provides a framework for similar models for other resources facing similar pressures.

The Holocene, starting approximately 11.7 cal ka, is characterized by distinct periods of warming and cooling. Despite these known climate events, few temperature proxy data exist in the northwestern Atlantic Ocean. One potential record of past water temperatures is preserved in the marine fossil record. Shell growth of ocean quahogs ( Arctica islandica), a long-lived bivalve, can provide records of past environmental conditions. Arctica islandica habitat includes the Mid-Atlantic Bight (MAB), an area rapidly warming as a consequence of climate change. The Cold Pool, a bottom-trapped water mass on the outer continental shelf within the MAB, rarely rises above 15°C. Ocean quahogs inhabiting the MAB are confined to the Cold Pool as a consequence of an upper thermal limit for the species of ~15–16°C. Recently, dead A. islandica shells were discovered outside of the species’ present-day range, suggesting that the Cold Pool once extended further inshore than now observed. Shells collected off the Delmarva Peninsula were radiocarbon-dated to identify the timing of habitation and biogeographic range shifts. Dead shell ages range from 4400 to 60 cal BP, including ages representing four major Holocene cold events. Nearly absent from this record are shells from the intermittent warm periods. Radiocarbon ages indicate that ocean quahogs, contemporaneous with the present MAB populations, were living inshore of their present-day distribution during the past 200 years. This overlap suggests the initiation of a recent biogeographic range shift that occurred as a result of a regression of the Cold Pool following the Little Ice Age.

The ocean quahog, Arctica islandica, is a commercially important bivalve in the eastern USA but very little is known about the recruitment frequency and rebuilding capacity of this species. As the longest-living bivalve on Earth, A. islandica can achieve lifespans in excess of 200 y; however, age determinations are difficult to estimate and age variability at size is extreme. Objectives for this study included the creation of an extremely large age-composition dataset to constrain age at length variability, development of reliable age-length keys (ALK), and descriptions of sex-based population dynamics for the quasi-virgin A. islandica population at Georges Bank (GB) within the greater US Mid-Atlantic stock. Sexually dimorphic characteristics are clearly present, as females are larger than males within age classes and males tend to dominate the oldest age classes. A male represented the maximum age of 261 years and is older than the maximum age previously documented for this region. Sex-specific ALKs were robust and reliable but not interchangeable. This population had higher estimated natural mortality rates than presumed for other regions in the Mid-Atlantic, and females have the highest mortality rate. However, recruitment expansion was also occurring which would affect the age-frequency data used to derive mortality estimates and result in higher mortality. Age frequencies at GB suggest effective recruitment to the population each year since 1867 CE. Reduced recruitment periods are documented and likely attributed to fluctuating environmental conditions. Sex-based demographics are clearly divergent in regard to growth rate, maximum size, longevity and mortality rates.

The boreal bivalve Arctica islandica is an important fishery in the United States (US), yet very little is known about the resiliency of this species to fishing activity due to limited understanding of localized population demographics. Demographics including age frequency, recruitment patterns, mortality rates, and sexual dimorphism were evaluated for a population sampled off Long Island (LI, 40.09658°N 73.01057°W) and compared with samples from Georges Bank (GB, 40.72767°N, 67.79850°W) collected in 2015 and 2017, where GB was described in a previous study. This study supports evidence that this species is sexually dimorphic. Earlier assumptions of prolonged lapses in recruitment were not substantiated for either the GB or LI populations; yearly cohorts were observed for the past century, and both populations presented recruitment pulses in approximately 8-y periods. Estimated ages from this study are older than previously reported for the US Mid-Atlantic with the oldest animal represented by a 310-year-old male collected from LI. Simulated total mortality was higher at GB than LI, and higher for GB females than GB males, with simulated mean longevity estimates greater at LI than GB. The population sex ratio at GB was 1:1.1 (female:male), whereas the LI ratio was 1:1.4 and relatively deficient in large females. Recruitment into the populations occurs routinely with substantial hiatuses being rare and substantive year classes occurring at least decadally with lesser, but contributing, recruitment in most years in between. Routine recruitment may insulate this species from risks posed by overfishing to an extent not typical for other long-lived species.

The Atlantic surfclam (Spisula solidissima) fishery generates approximately USD 30 million in landings revenues annually, distributed across ports throughout the US Mid-Atlantic and Northeast. Overlap between areas of Atlantic surfclam harvests and offshore wind energy leasing make the fishery vulnerable to exclusion and effort displacement as development expands in the region. An existing integrated bioeconomic agent-based model, including spatial dynamics in Atlantic surfclam stock biology, heterogeneous captain behaviour, and federal management processes, was extended to incorporate costs and revenues for fishing vessels and processors and used to evaluate the potential economic effects of offshore wind development on the Atlantic surfclam fishery. Fishing activity and economic outcomes were simulated under different offshore wind energy development scenarios that impose spatial restrictions on Atlantic surfclam vessel fishing and transiting behaviour. Decreases in the number of trips and shifts in the spatial distribution of fishing effort reduced revenues for Atlantic surfclam fishing vessels and processors by ∼3–15% and increased average fishing costs by

Much of the contribution of bivalves to paleoecology is based on the fraction preserved whole. The use of fragments has been limited by the inability to reconstruct the whole shell. For this study, shells of selected species were broken, metrics obtained for shell reconstruction, and a stepwise model developed and tested to evaluate the potential of reconstructing whole shells from fragments. The model is based on the facts that shell thickness is well correlated with shell length, width, and weight, that fragments can be assigned to shell areas, and that each shell area can be matched to a potential donor shell by combinations of length, width, and weight. Simulations using the model show that shell reconstructions are feasible and that the size frequency can be reconstructed more accurately than the number of shells. If all fragments are preserved, shell number tends to be modestly overestimated. Assuming fragment loss, the shell number estimated will be a conservative estimate of original abundance. Reconstructed size frequencies did not differ significantly from the original size frequencies of the whole shells even if 40% of the fragments were not preserved. Biovolume, consequently, may also be estimable. Standardly, bivalve fragments are enumerated by counting intact umbos. If all fragments are preserved, this approach is preferable, as shell model reconstructions overestimate abundance under that circumstance. However, the expectation that fragments with intact umbos are uniquely well preserved is unrealistic and if fragment loss has occurred, relying on all fragments may provide an improved estimate of the original abundance and, in addition, an estimate of the size frequency and the latter proves to be robust even with 40% of the fragments lost.

We examined evidence for larval spillover (increased recruitment outside the closures) of Atlantic sea scallops (Placopecten magellanicus) due to rotational closures in the Mid-Atlantic Bight using a 40-year fisheries survey time series and a larval transport model. Since the first closure of the Hudson Canyon South (HCS) area in 1998, mean recruitment in the two areas directly down-current from this closure, Elephant Trunk (ET) and Delmarva (DMV), increased significantly by factors of about 7 and 2, respectively. Stock–recruit plots indicate that low biomasses in HCS were associated with reduced mean recruitment in ET and DMV. Simulations indicate that larvae spawned in HCS often settle in the two downstream areas and that model-estimated settlement (based on gonad biomass in HCS and year-specific larval transport between the areas) is correlated with observed recruitment. This study gives strong evidence that the rotational closure of HCS has induced increased recruitment in down-current areas.

Shell planting to enhance oyster recruitment and habitat in Delaware Bay, NJ was reinitiated in 2005 with 3 y of federal funding. Since then, annual plantings of locally available surf clam and ocean quahog shell have been funded by the oyster fishery. The cured shell is deployed directly onto sites already containing commercially viable numbers of market-sized oysters in areas that are routinely fished sustainably. The locations targeted to receive shell plants vary and are selected to provide the greatest benefit throughout the fished reef tract. Annual sampling that included shell plants from 2005 through 2017 permitted evaluation of their performance. New oysters set and survive preferentially on newly planted shell during the first season and later at the same level as native shell for at least 10 y so that the benefit of once-added shell is retained for over a decade. Recruitment to shell plants enhances “good” sets and makes a critical difference during years of “poor” set. Plant substrate, now with oysters of its own, significantly outperforms native substrate in recruitment and survival of oysters in the second and third years post-planting, with enhancement declining exponentially over the first 4 y. An estimated half-life for this decline is just under 1 y. Beyond 3 y, the plant-based substrate continues to perform at least as well as native substrate. Size–frequency analysis shows that first-year oysters on planted substrate are larger than those on native substrate and that this difference carries through the fourth year of a plant. The volume contributed by live and dead oysters on planted substrate increases over the first 5 y until the contribution to the reef from planted substrate volume matches that of native substrate, thus carrying the enhancement forward through time. Annual survey data from the first 4 y of shell plants show that on seven of eight beds, live plus dead oyster density (number m–2) was up to 2.8× greater on planted sites than on non-planted sites. Change in density averaged positive over time on both planted and non-planted sites but was greater on planted sites. Shell plants on beds in the Delaware Bay fishery provide 3–4 y of enhanced oyster productivity and continue to double the local native substrate productivity as they age, maintaining an active and productive fishery.

Ocean quahogs (Arctica islandica) are the longest lived bivalve on Earth. Individuals on the deep continental shelf off Georges Bank can survive for centuries, and in the colder, boreal waters of Iceland, ages over 500 y can be reached. Ocean quahog landings in the United States represent a $24 million industry, yet assessment models operate with no age data because of the substantial sample size required to develop adequate population age distributions for such a long-lived species, the unknown error associated with age estimates, and the extensive time and financial investment required to create production-scale age datasets. Inclusion of age data for this species requires precision metrics to evaluate aging uncertainty such as percent agreement, percent error, coefficient of variation, and tests of bias. To move forward using error-validated age-composition data, a 3-fold error protocol was developed using a large dual-reader dataset (n = 610) from Georges Bank. First, a proxy age-validation study was performed to corroborate an aging method, followed by error evaluation in the context of age-reader bias, precision, and error frequency. Error thresholds were established for each of the three error methods. Georges Bank samples ranged from 33 to 261 y of age and met the predetermined error thresholds for bias (conditionally because of significant and nonsignificant results), precision (average coefficient of variation less than 7%), and error frequency (less than 10%). Consequently, age estimates were deemed acceptable to support age frequency analyses. Precision and bias error were greatest for the youngest animals and, in the context of age-reader bias, error rates were higher for young male ocean quahogs than for young females. Improved age validation of young, sex-differentiated A. islandica will constrain aging error and guide refinement of both aging and age-error protocols.

Increased bottom water temperatures have caused a relocation and contraction of the range of the Atlantic surfclam, Spisula solidissima, in the Mid-Atlantic Bight (MAB). Consequences include declining stock abundance and landings per unit effort (LPUE) in southern portions of the range. A management strategy evaluation (MSE) assessed the potential of rotational closures to improve surfclam productivity and fishery viability under three levels of abundance. In simulations, fishing vessels harvest the stock under performance and quota constraints and captain behavioral proclivities. Management alternatives examined included the addition of area management to the current management plan using two closure location rules and three closure durations at two levels of incidental mortality. Simulations showed that area management increased stock abundance and fishery LPUE, particularly when surfclam abundance was low. Simulations suggest area management could help insulate the stock and commercial fishery from further shifts in range.

Two mass mortality events in 2016 and 2019 affecting the eastern oyster in the Mississippi Sound permitted an investigation of population dynamics during the collapse and recovery from systemic stressors capable of producing local extinction. In this study, metrics were reported as true population metrics with values included in proportion to the size class representation in the population. The approach was critical as trends in Dermo prevalence and weighted prevalence varied substantially between size classes, and this permitted elucidation of recovery trajectories not obvious from the population as a whole. The division of the population into larger and smaller size classes revealed a consistent differential in the time series for Dermo prevalence and weighted prevalence; these metrics did not return to levels observed before the 2016 mortality event in the 2-y interim between the 2016 and 2019 mortality events, and prevalence, in particular, remained low in the smaller size class. The time delay in the return of Dermo infection due to a substantial change in the size frequency of the population toward smaller initially uninfected animals is consistent with the differential in Dermo and young oyster cell doubling times and the required time delay, albeit relatively short, for transmission to occur. Gonadal thickness generally tracked with condition index and followed the expected seasonal cycle; any impact resulting from the 2016 mortality event on these metrics appeared to be inconsequential. The relationship between the adductor muscle diameter and shell height varied little over the time series and was much more stable than between the adductor muscle diameter and shell weight, suggesting a change in the oyster shell shape post-2016. By inference, shell weight is more variable among the three metrics in estimation of the condition index and standardization of other health indices. The ability to establish health metrics for both specific size classes and the population as a whole is essential to understanding the temporal impacts of a severe stressor or mortality event on oyster populations.

Bivalve molluscs represent a significant proportion of the diet of both Atlantic and Pacific walrus (Odobenus rosmarus rosmarus and Odobenus rosmarus divergens, respectively) and are pivotal to benthic–pelagic coupling and carbonate cycling in the Arctic oceans. The latter is of particular relevance in a period of seasonal ice retreat, freshwater release into associated surface waters, decreasing water pH, and possible undersaturation of Arctic waters with respect to aragonite. Using population estimates and predation rates for the walruses on bivalve molluscs, a conservative estimate of bivalve consumption in the regions of active walruses foraging is 2.0–3.0 × 106 tonnes y–1—a tonnage comparable to the landings for the largest U.S. commercial fishery, the walleye pollock fishery in the eastern Bering Sea. Predation loss to other apex predators such as bearded seals is discounted. Using production:biomass ratios comparable to other high-latitude bivalves, a conservative estimate of bivalve standing stock required to support walrus populations is 0.4–3.0 × 107 tonnes. Whereas predominant clam prey species exhibit longevity in the 30+ y range, sampled populations in the Bering and Chukchi seas are dominated by small, often less than 1.0 cm individuals. Large clams are rare to absent in samples, suggesting either rapid turnover of the population with high predation balanced by high recruitment and/or a bias in sampling that discounts larger, more sparse individuals. Walrus grazing contributes up to 4.0–6.0 × 106 tonnes y–1 of carbonate to buffering of near-surface sediments in Artic regions. Accurate estimates of bivalve biomass and, thereby, the carbonate budget of Arctic shelf clam species, are critical to understanding the stability of associated continental shelf communities with continued warming of these high-latitude systems and their associated tendency toward aragonite undersaturation.

Stock–recruitment relationships (SRR) are used by many resource managers as a metric to evaluate stock reproductive potential and rebuilding capacity when harvested. This relationship is more evident in traditional finfish stock assessment models where larvae are frequently limited by the abundance of spawning adults, whereas invertebrate resources are much more confounding and often do not express a clear spawning SRR. Oysters in particular appear to be limited more strongly by post-settlement mortality as larvae are regularly demonstrated to be present in high abundance in response to timely shell plants; yet unexpectedly, a brood SRR appears to be present. Because of the unique life history of oysters which provide their own habitat through reef accretion, a posit is that the quantity and quality of such habitat are crucial determinants of recruitment to the stock, and habitat quality acts as a surrogate for brood stock in the stock–recruit relationship. An extensive dataset from Delaware Bay with a 65-y time series was analyzed to assess the relationship between available Crassostrea virginica natural reef materials and subsequent spat (

The unique characteristics of marine ecosystems have pushed investigators to refine well-tested and widely applied epidemiological modeling methods to understand marine disease dynamics. This chapter begins by reviewing models used to quantify within-host parasite dynamics in open marine ecosystems where infection is near universal. These models are powerful tools for quantifying how diseases respond to changing environmental conditions and, when reliable environmental data are available, can forecast marine disease risks into the future. This chapter then describes epidemiological models that consider transmission processes and parasite life histories unique to marine systems, and then incorporates disease processes in fisheries assessment models. Finally, because disease dynamics vary across local host populations, this chapter concludes by overviewing ocean circulation models and their use in understanding parasite dispersal and spread in marine ecosystems.

Larval Atlantic sea scallops (Placopecten magellanicus) simulations in the Middle Atlantic Bight (MAB) from 2006 to 2012 were performed to investigate annual and inter-annual dispersal and connectivity patterns among stock regions. These simulations used a circulation model based on the Regional Ocean Modeling System (ROMS) and an individual-based larval model (IBM) that included larval behavior. The circulation model used realistic dynamical forcing (e.g., winds, tides, and open ocean boundary conditions), thermo-dynamical fluxes (e.g., solar radiations, sensible and latent heating), and hydrological forcing; the larval IBM included vertical swimming and sinking behaviour, temperature-dependent growth, and settlement. Simulated larvae that reach settlement size and suitable habitat in 45 days are considered ‘successful’, and two regions are considered ‘connected’ by larval dispersal when larvae successfully disperse from one region to the other. In general, simulated larval dispersal patterns varied seasonally (28% higher in September and October compared to May and June), among years (2007 through 2009 had 5% lower larval success during August and September compared to other years), and spatially, with larvae released from the northern regions like Long Island acting as a substantive larval source with 14% greater dispersal success and 15% greater connectivity with other regions than those released elsewhere. Over the seven years simulated, the MAB scallop stocks showed high rates of connectivity to regions to the south and more limited and variable connectivity to regions to the north. In species like sea scallops with limited adult mobility, larval dispersal supplies recruits, enables range expansion, and connects populations. Thus, appreciation of dispersal patterns are essential for fishery management of this economically valuable stock.

This review focuses on the eastern oyster Crassostrea virginica and its pathogen Perkinsus marinus, but the reference points considered are applicable to a wide range of sessile hosts suffering significant mortality from disease. The biological characteristics and imperatives that constrain the deduction of reference points are first reviewed. This is followed by a review of the various models and their reference points that have been implemented or considered in an effort to sustainably manage C. virginica stocks. The last section evaluates the application of reference points addressing maximum sustainable yield (msy) as management tools. Characteristics and imperatives include the preference of abundance over biomass or biovolume as the primary metric for the stock and the bipartite partition of the mortality rate into natural mortality and the additive portion contributed by disease that depends upon the distribution of the stock over the salinity gradient. Other contributing factors include female biases in natural mortality and fishery landings, application of a Ricker model for the broodstock-recruitment relationship, the influence of disease on the surplus production trajectory of the stock, and the importance of managing shell as well as live oysters. Reference points may be developed to manage the disease or the diseased stock. All reference points used in management at this writing are of the latter type. One is volume based; the remainder are abundance based. Constant-abundance reference points have received most attention. Reference points focused on msy have received little use. Constant-abundance reference points stabilize the stock at a known (usually the present) state. Constancy reference points can be applied to the management of the stock and also to the management of the shell resource critical to maintenance of reef habitat. The alternative msy approach is used to stabilize the stock at a state maximizing sustainable production. Though little used, consideration of the behavior of surplus production formulations suggests that msy-based reference points may be feasible and that this approach could then be used to identify management goals other than the present state.

The Atlantic surfclam (Spisula solidissima) is a dominant member of the biological community of the Middle Atlantic Bight continental shelf and a commercially harvested species. Climate warming is affecting the biology and distribution of this species, which provides an opportunity to investigate the processes and conditions that are restructuring this fishery and the implications for ecological and socioeconomic systems. A Management Strategy Evaluation (MSE) developed for the surfclam fishery provides a mechanistic description of the surfclam's response to climate change and understanding of the cascade of effects initiated by changes in oceanographic conditions that ultimately appear as social and economic effects. This understanding in turn informs development of management policies for the resource. This overview considers the components of the surfclam MSE, relevant results, and implications for management and policy. The lessons learned from the surfclam MSE provide a basis for applying si...

A reliable measure of gear capture efficiency is required to calculate unbiased estimates of population size and fishing mortality from survey data in a stock assessment. However, capture efficiency can vary spatially and temporally due to changes in abundance, stock area, the environment, and the sampling gear itself. Therefore, periodic reassessment of this parameter is necessary to ensure that the catchability coefficients being applied accurately reflect the capture efficiency of the survey sampling gear, especially when catchability is being estimated outside of the stock assessment model. Using data from field experiments conducted in 1999, 2000, 2003, and 2013, we evaluated spatial and temporal variability in capture efficiency for a commercial dredge used to conduct a fishery-independent survey of the eastern oyster (Crassostrea virginica) population in Delaware Bay, USA. A spatial gradient in capture efficiency was detected, but no temporal trend. Capture efficiency was a function of the density of oysters in the sampled area. To our knowledge this is the first time density-dependent capture efficiency has been identified for a sessile invertebrate stock survey. Since density dependence in capture efficiency leads to hyperstable catch-per-unit-effort, caution is advised when deriving oyster abundance from dredge survey catch-per-effort data, especially at low oyster density and when high spatial resolution estimates of survey dredge capture efficiency are not available.

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.

Few empirical studies have focused on the effect of sample density on the estimate of abundance of the dominant carbonate-producing fauna of the continental shelf. Here, we present such a study and consider the implications of suboptimal sampling design on estimates of abundance and size-frequency distribution. We focus on a principal carbonate producer of the U.S. Atlantic continental shelf, the Atlantic surfclam, Spisula solidissima. To evaluate the degree to which the results are typical, we analyze a dataset for the principal carbonate producer of Mid-Atlantic estuaries, the Eastern oyster Crassostrea virginica, obtained from Delaware Bay. These two species occupy different habitats and display different lifestyles, yet demonstrate similar challenges to survey design and similar trends with sampling density. The median of a series of simulated survey mean abundances, the central tendency obtained over a large number of surveys of the same area, always underestimated true abundance at low sample densities. More dramatic were the trends in the probability of a biased outcome. As sample density declined, the probability of a survey availability event, defined as a survey yielding indices >125% or

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.

Ocean quahogs [Arctica islandica (Linnaeus, 1769)] are the longest-lived, noncolonial animal known today, with a maximum life span exceeding 500 y. Ocean quahogs are a commercially important bivalve, inhabiting the continental shelf of the North Atlantic Basin. Although considerable information exists on the growth and physiology of A. islandica, limited information is available regarding recruitment; accordingly, sustainably managing the fishery is a challenge. To investigate long-term recruitment trends, the age of ocean quahogs from Georges Bank which were fully recruited to the commercial fishery (>80 mm shell length) was determined by analysis of annual growth lines in the hinge plate. Ages of animals representing the fully recruited size range were used to develop an age—length key, enabling reconstruction of the population age frequency. The population age frequency showed that the Georges Bank population experienced an increase in recruitment beginning in the late 1890s. Initial settlement, documented by a few ocean quahogs that were much older, occurred much earlier, in the early 1800s. Following the late 1890s increase in recruitment, the population expanded rapidly reaching carrying capacity in 20–30 y. Recruitment was more or less continuous after this expansion, consistent with maintenance of a population at carrying capacity. Unusually large year classes were not observed, nor were significant periods of high recruitment interspersed with periods of low recruitment. The relationship of growth rate with age for the oldest clams was assessed using the time series of yearly growth increments and the resulting relationship fitted to three models (von Bertalanffy, Gompertz, and Tanaka's ALOG curve). The ALOG model was clearly superior because it allows for persistent indeterminate growth at old age, rather than the asymptotic behavior of the other two and because it allows for a rapid change in growth rate at what is presumed to be maturity.

Dermo disease caused by the protistan Perkinsus marinus in Eastern oysters Crassotrea virginica is an important source of mortality impacting oyster population dynamics resulting in substantial losses in fisheries and aquaculture. The rapid transmission and spread of the disease minimized the importance of transmission models and past models (proliferation-based models) assumed simple density-dependent transmission or rapid infection post-settlement. This approach is a good approximation only for low population densities. A transmission model was developed for P. marinus in Eastern oysters that accounts for the seasonal change in disease dynamics and density-dependent foraging (of suspended particles) interference among hosts. The model, verified and evaluated against field observations, incorporates parasite release to the water column from live and dead individuals, parasite consumption by living oysters, the diffusion of parasites in the water, body burden-based dose-dependent transmission, recruitment, and disease-caused mortality. The model returns a basic reproduction number R 0 for Dermo much greater than unity ( R 0 = 90) in accordance with the current persistence and pandemic nature of this disease in oysters. No population density is obtained that is low enough to suppress R 0 below 1 (i.e. disease extinction). R 0 is also estimated for high oyster densities (>300 individuals m −2 ) and particularly for relatively large oysters (∼90 mm), today rare but once common before generalized overfishing occurred on healthy oyster reefs. In this scenario, R 0 drops below 1, indicating that high oyster density can limit disease invasion through foraging interference and depletion of parasites in the water column. High intensity recruitment events allow the oyster population to attain such densities and limit the development of epizootics. These results provide insight into the transition from past populations, where Dermo is inferred to have been limited in its impact, to the current persistent and pandemic nature of this disease. Further coupling of this model into metapopulation and hydrodynamic models could be a promising tool to support management decision-making for bivalve populations impacted by Dermo disease.

WOS:000447083800041; International audience

A mid-water otter trawl fishery targeting Illex illecebrosus operates in the northwestern Atlantic Ocean. The majority of I. illecebrosus are captured during mid-June to early September. Diel migratory behavior limits the fishery to daylight hours, but does the time-of-day affect catch? Illex illecebrosus were collected from each tow from a subset of the trawler fleet fishing on the outer continental shelf of the Mid-Atlantic Bight over the fishing season to determine the influence of time-of-day on catch. The male-to-female ratio was not influenced by time-of-day of capture. The size–frequency​ distribution of I. illecebrosus varied between tows within the same day of capture. Time-of-day of capture influenced average weight and, to a lesser degree, mantle length. Shorter and lighter squid were caught in the middle of the day. Larger and heavier squid were caught during the morning and afternoon. Data collection on commercial fishing vessels required that squid be frozen onboard, thawed onshore, and then weighed and measured. Differences between fresh and frozen squid were significant, but small, which permitted combining fresh and frozen measurements from different boats to track temporal and spatial trends in squid size. As time-of-day differences in catch were significant, obtaining unbiased size data from fishing vessel-based reports requires that samples be collected from the morning, afternoon, and evening tows during the trip.

Between 1997 and 2011, The National Marine Fisheries Service conducted 50 depletion experiments to estimate survey gear efficiency and stock density for Atlantic surfclam (Spisula solidissima) and ocean quahog (Arctica islandica) populations using commercial hydraulic dredges. A model formulated for this purpose, the Patch Model, was used to estimate gear efficiency and organism density. The range of efficiencies estimated is substantial, leading to uncertainty in the application of these estimates in stock assessment. A simulation protocol was developed to examine sources of uncertainty in Patch Model estimates. Analysis of simulations showed that uncertainty in the estimates of gear efficiency from depletion experiments was reduced by higher numbers of dredge tows per experiment, more tow overlap in the experimental area, a homogeneous as opposed to patchy distribution of clams in the experimental area, and the use of gear of inherently high efficiency. Stock density was of lesser importance, though still contributing to estimated uncertainty. Simulations suggest that adapting the experimental protocol during the depletion experiment by adjusting tow number and degree and dispersion of tow overlap may substantively reduce uncertainty in the final efficiency estimates. Simulations also suggest that the pattern of population dispersion in the experimental area is, and will likely remain, an important source of uncertainty, which may, however, be mitigated by updating experimental design during the course of the experiment.

A survey of the adjacent waters east of Nantucket, Massachusetts provided an opportunity to evaluate the epibiota attached to cobbles, rocks, boulders, and Atlantic surfclam shell in a region of high tidal current velocity and sand scour where burial, exhumation, and scour may limit epibiont coverage on exposed, and thus otherwise highly preferred for attachment, substrates. Such conditions may confute the expectation that substrate complexity always adds significantly to ecosystem value by expanding the range of habitat options and consequently increasing species richness and trophic linkages. Sedimentary particles potentially providing good attachment substrate for erect sessile epibiota included surfclam shells, abundant at many locations, cobbles, nearly ubiquitous, rocks, routinely encountered, and occasional boulders. The attached epibiota fell into three categories based on their biases for particle types. Some preferred the largest particles or evidence of their occupation was best preserved on these particles: these included sponges, mussels, and barnacles and their scars. Some preferred intermediate and smaller terrigenous particles; these included tunicates and encrusting bryozoans. Some preferred surfclam shells, namely the slipper shells and erect hydroids. Slow-growing attached epibionts were exceedingly rare and all soft-bodied attached epibionts were rare. Only barnacles and their taphonomic scars and hydroids were common. The frequency of barnacle scars relative to intact barnacles suggests sediment scour under a high-flow regime. Mussels were rarely attached to larger sedimentary particles such as cobbles and rocks, though commonly occurring locally as mussel beds on sand and pebble, further supporting the ephemerality of exposed unscoured attachment sites. The absence of attached epibionts demonstrates that edaphic processes minimize the importance of cobbles, rocks, boulders, and shells in community structure in some subtidal high-energy regimes, defying expectations from their contribution to substrate complexity. Their apparent contribution to habitat complexity belies their resultant much more minor role in determining community composition, thereby limiting their ecosystem value.

Mid-Atlantic Bight (MAB) warming accompanied by a decline in recruitment has slowly reduced surfclam abundance. Simulations examined fishery dynamics during an extended period of low recruitment followed by stock recovery after a high-recruitment event. The model assigned performance characteristics to each vessel and gave captains defined behavioral proclivities including a tendency to search, to communicate with other captains, to use survey data, and to integrate variable lengths of past-history performance in targeting fishing trips. During the simulated excursion in abundance, LPUE (landings per unit effort) declined as lower abundance required an extended time at sea to catch a full load. Captains expanded their geographic range of interest steaming farther from port in an effort to maintain their performance. Net revenue declined. Use of survey data significantly improved performance. About equal in positive effect was moderate searching. Other behaviors incurred penalties. Communication failed to improve performance because both poor and good information was transferred. Reliance on a long period of catch history failed to improve performance because information was out of date during a time of rapidly changing conditions. In these simulations, no captains' behaviors prevented a collapse in vessel economics at low abundance, but certain behaviors limited the degree and duration of economic dislocation.

The National Oceanic and Atmospheric Administration (NOAA) Status and Trends Mussel Watch Program sampled the largest oysters in nearly every major US coastal lagoon and estuary in the Gulf of Mexico once during the winter from 1986 to 2010. This contribution examines trends in the principal oyster disease in the Gulf of Mexico, Dermo, caused by Perkinsus marinus, and some related population dynamic characteristics for its host, Crassostrea virginica. During the 1986–2000 period, P. marinus prevalence and infection intensity and oyster population dynamics followed the El Nino-Southern Oscillation (ENSO) cycle, responding to variations in salinity caused by variations in rainfall and freshwater inflow. The ENSO signal in the oyster population effectively ceased circa 2002. Beginning around this time, wintertime P. marinus prevalence and weighted prevalence began a decadal decline, as did the length of the largest oysters and the fraction of these largest animals that were female. The trends in Dermo disease, oyster length, and oyster sex ratio are all consistent with the following hypothesis: increasing temperature during the 2000s resulted in an increase in P. marinus infection intensity sufficient to increase the mortality rate in late summer and fall in the larger animals. This simultaneously reduced Dermo prevalence and infection intensity in the winter at the time of sampling and also resulted in the decline in the length of the largest animals targeted by Mussel Watch. Coincident with the decline in length is the expected decline in the fraction female, such that the percent female in the largest animals dropped to ≤50 % throughout much of the Gulf of Mexico. The decline in length leading to fewer large animals reproducing and the loss of females are predicted to have reduced oyster population reproductive capacity substantially during the 2000s. The early 60 % of the Mussel Watch time series took place during a period of negative Atlantic Multidecadal Oscillation (AMO) indices. The AMO moved into positive territory circa 2000. A positive AMO index is consistent with observed warmer water temperatures, and increased water temperature is consistent with an increase in Dermo-induced mortality.

Larval transport and settlement have been shown in various studies to be essential in determining population abundance and connectivity for benthic invertebrates. This transport is influenced by both the physical environment and biological behavior. The Atlantic surfclam, Spisula solidissima , is a commercially important benthic invertebrate fishery species along the U.S northeastern coast. In this study, a physical circulation model is coupled to a surfclam larval model to investigate the dynamics of larval transport and settlement within the Middle Atlantic Bight (MAB) shelf in 2006. The main physical mechanisms causing variability in larval transport and settlement are also examined. Model results show that surfclam larvae released from July to early October experience relatively larger settlement rates, due to higher average temperatures experienced by larvae. Larval along-shore transport exhibits a mean down-coast pattern following the coastal current from the northeast to the southwest, with most high-frequency (period of 2–10 days) variations caused by fluctuations in the along-shore surface wind stress, and with seasonal variations speculated to be driven mainly by changes in the across-shelf density gradient. Larval across-shelf movement is highly correlated with the along-shore surface wind stress mediated by coastal upwelling and downwelling episodes, but the correlation is further dependent on the vertical distribution of the larvae, particularly their position relative to the thermocline. Most surfclam larvae released from the Middle Atlantic shelf stay below the thermocline and experience a net onshore transport during the summer-stratified season when upwelling-favorable wind forcing dominates. A proposed critical value of water temperature at the thermocline successfully regulates the observed patterns of vertical distribution of surfclam larvae and their across-shelf movement off the New Jersey and South Virginia shelves; that is, when the water temperature at the thermocline is above the critical value (19.0 °C), surfclam larvae tend to escape the warm surface layer to concentrate below the thermocline and follow the across-shelf movement of bottom water, and vice versa . These results provide an important insight into the general mechanism of how physical environmental factors interact with biological behavior of the larvae to influence larval transport, connectivity and population dynamics, and also indicate the potential impact of large-scale climate change on benthic species and coastal ecosystems.

Benthic suspension-feeders can accumulate substantial numbers of microparasitic pathogens by contacting or filtering particles while feeding, thus making them highly vulnerable to infectious diseases. The study of disease dynamics in these marine organisms requires an innovative approach to modeling. To do so, we developed a single-population deterministic compartmental model adapted from the mathematical theory of epidemics. The model is a continuous-time model, unstructured in spatial or age terms, and configured to simulate the dynamics of diverse dose (body burden)-dependent infectious disease transmission processes in suspension feeders caused by susceptible individuals contacting or absorbing (filtering) infectious waterborne pathogens. Different scenarios were simulated to explore the effect of recruitment, filtration rate, particle loss, diffusion-like processes in the water column and non-focal hosts (i.e. non-susceptible in terms of disease) on disease incidence. An increase in recruitment (i.e. new disease free susceptibles) can reduce the prevalence of infection due to the dilution effect of adding more susceptibles, but the disease can spread faster for the same reason. Lower infective particle accumulation rates or increasing particle loss rates in the environment reduce the prevalence of infection. This effect is trivial when the water is saturated with infective particles released by infected and/or dead animals. Diffusion of particles from the local pool available to suspension feeders to the adjacent remote pool, prompted by a large remote volume and high particle exchange, limits epizootic development. Similarly, the likelihood of an epizootic can be constrained in a large susceptible population when competition for pathogens, more ‘active’ in active filter feeders than in passive suspension feeders, reduces the per capita infective particle accumulation rate. In passive suspension feeders, decreasing the area of the feeding surface has the same effect in constraining disease development. The effect of competition for infective particles in essence diluting the infective particle concentration in the water column is magnified when the susceptible population is part of a community with non-focal filter feeders, and is particularly effective in limiting disease development in high infective dose systems. At the same time, this active foraging strategy makes filter feeders more vulnerable to epizootics. The model is a suitable framework for studying the disease dynamics and determinants of disease outbreaks in benthic suspension feeders.