A model for death assemblage formation: Can sediment shelliness be explained?

A numerical model for carbonate (shell) accumulation in marine sediments is proposed. Sediment shelliness is controlled by carbonate addition, taphonomic loss, carbonate reorganizing processes, and sedimentation rate. Using representative rates of carbonate production, taphonomic loss, and sedimentary carbonate content, the model shows that insufficient carbonate is produced today in many environments to explain sedimentary carbonate content and that most produced carbonate must be preseIVed despite a generally high capacity for taphonomic loss. An anthropogenically-produced decrease in carbonate production over the last ∼100 yr may explain the former. Representative rates of burial and sedimentation, and a temporal and spatial offset between carbonate production and organic matter decomposition can permit most produced carbonate to be preserved in sediments where taphonomic capacity greatly exceeds the carbonate production rate. The requirement that most carbonate be preserved, despite the observation that most individuals are not, indicates that most adults are preserved and reinforces the finding that biomass is a valuable community attribute for paleoecologic analysis. The requirement that most carbonate be preserved indicates that taphonomic loss must be restricted to the nearsurface in most habitats rather than being distributed throughout the bioturbated zone. The distribution and concentration of carbonate in sediments are partially decoupled from preservational processes because many processes affecting carbonate distribution have little effect on preservation. The time scales of the two differ. Preservational processes usually occur on time scales too short to be recorded as variations in carbonate content with depth. Evidence of preservational processes probably resides solely in the taphonomic signature of shells, hence emphasizing the importance of taphofacies analysis

Utilización de buques comerciales para aumentar la información de campañas de evaluación: la distribucción de frecuencias de tallas

The trend towards use of commercial vessels to enhance survey data requires assessment of the advantages and limitations of various options for their use. One application is to augment information on size-frequency distributions obtained in multispecies trawl surveys where stratum boundaries and sampling density are not optimal for all species. Analysis focused on ten recreationally and commercially important species: bluefish, butterfish, Loligo squid, weakfish, summer flounder, winter flounder, silver hake (whiting), black sea bass, striped bass, and scup (porgy). The commercial vessel took 59 tows in the sampled domain south of Long Island, New York and the survey vessel 18. Black sea bass, Loligo squid, and summer flounder demonstrated an onshore-offshore gradient such that smaller fish were caught disproportionately inshore and larger fish offshore. Butterfish, silver hake, and weakfish were characterized by a southwest-northeast gradient such that larger fish were caught disproportionately northeast of the southwestern-most sector. All sizes of scup, striped bass, and bluefish were caught predominately inshore. Winter flounder were caught predominately offshore. The commercial vessel was characterized by an increased frequency of large catches for most species. Consequently, patchiness was assayed to be higher by the commercial vessel in nearly all cases. The size-frequency distribution obtained by the survey vessel for six of the ten species, bluefish, butterfish, Loligo squid, summer flounder, weakfish, and silver hake, could not be obtained by chance from the size-frequency distribution obtained by the commercial vessel. The difference in sample density did not significantly influence the size-frequency distribution. Of the six species characterized by significant differences in size-frequency distribution between boats, all but one was patchy at the population level and all had one or more size classes so characterized. Although the variance-to-mean ratio was typically higher for the commercial vessel, five of the six cases that were otherwise were among the species for which the size-frequency distribution differed between the two vessels. Thus, the origin of the significant differences observed between vessels would appear to lie in the spatial pattern of the species as it interacts with the tendency for one vessel to obtain large catches more frequently for some size classes. One consequence of differential distribution and catchability is that more large fish were present in the commercial vessel catches than in the survey vessel catches in cases where the two vessels obtained different size-frequency distributions. Application of commercial vessels to the evaluation of size frequency hinges on understanding how to interpret differences among boats, gear, and sampling design. Here we show that key ingredients to this understanding are the degree of nonlinearity in catchability across a range of size classes, the interaction of varying spatial arrangements among size classes and the sampling design, and the interaction of varying spatial arrangements with differential catchability.La tendencia hacia la utilización de buques comerciales para incrementar y optimizar los datos de campañas de evaluación requiere la valoración de las ventajas y limitaciones de las distintas opciones para su uso. Una aplicación consiste en aumentar la información referente a distribuciones de frecuencias de tallas obtenidas en campañas de evaluación de pesquerías de arrastre multiespecíficas, en las que los límites de los estratos y la densidad del muestreo no son óptimas para todas las especies. El presente análisis se centró en diez especies importantes, tanto desde el punto de vista recreacional como comercial: Pomatomus saltatrix, Peprilus triacanthus, Loligo pealei, Cynoscion regalis, Paralichthys dentatus, Pleuronectes americanus, Merluccius bilinearis, Centropristis striata, Morone saxatilis y Stenotomus chrysops. El buque comercial realizó 59 lances en la zona muestreada al sur de Long Island, Nueva York, y el buque de investigación, 18. C. striata, L. pealei y P. dentatus presentaron un gradiente desde la costa hacia mar abierto tal que los individuos de menor talla fueron capturados desproporcionadamente en aguas costeras y los ejemplares de mayor talla a mayor distancia de la costa. P. triacanthus, M. bilinearis y C. regalis se caracterizaron por un gradiente sudoeste-nordeste tal que los ejemplares de mayor talla se capturaron desproporcionadamente al nordeste del sector más sudoccidental. Todas las tallas de S. chrysops, M. saxatilis y P. saltatrix fueron capturadas predominantemente en aguas costeras. P. americanus se capturó predominantemente en aguas alejadas de la costa. El buque comercial se caracterizó por una mayor frecuencia de grandes capturas para la mayoría de especies. En consecuencia, la agregación en áreas de alta densidad se mostró superior en el buque comercial en casi todos los casos. La distribución de frecuencias de tallas obtenida por el buque de investigación para seis de las diez especies (P. saltatrix, P. triacanthus, L. pealei, P. dentatus, C. regalis y M. bilinearis no pudo ser obtenida por azar a partir de la distribución de frecuencias de tallas obtenida por el buque comercial. La diferencia en densidad del muestreo no influenció significativamente la distribución de frecuencias de tallas. De las seis especies caracterizadas por diferencias significativas en la distribución de frecuencias de tallas entre buques, todas menos una mostraron agregaciones en áreas de alta densidad a nivel poblacional y todas presentaron una o más clases de talla caracterizadas de esta manera. Aunque la relación varianza-media fue típicamente superior para el buque comercial, cinco de los seis casos en que no fue así se dieron entre las especies en las que la distribución de frecuencias de tallas fue distinta entre los dos buques. Así, el origen de las diferencias significativas observadas entre buques radicaría en la pauta espacial de las especies al interaccionar con la tendencia de un buque a obtener grandes capturas con más frecuencia para algunas tallas. Una consecuencia de la distribución diferencial y capturabilidad es que más individuos de gran tamaño estuvieron presentes en las capturas del buque comercial que en las capturas del buque de investigación en casos en los que los dos buques obtuvieron distintas distribuciones de frecuencias de tallas. La utilización de buques comerciales para la evaluación de frecuencias de tallas depende de la comprensión sobre cómo interpretar las diferencias entre buques, artes de muestreo y diseño de muestreo. Mostramos aquí que ingredientes clave para esta comprensión son el grado de no-linearidad en la capturabilidad a lo largo de un rango de clases de talla, la interacción de distribuciones espaciales distintas entre clases de talla y el diseño de muestreo, así como la interacción de distintas distribuciones espaciales con la capturabilidad diferencial.  

An assessment of scup (Stenotomus chrysops) and black sea bass (Centropristas striata) discards in the directed otter trawl fisheries in the Mid-Atlantic Bight

This study was undertaken to re-assess the level of scup (Stenotomus chrysops) discards by weight and to evaluate the effect of various codend mesh sizes on the level of scup discards in the winter-trawl scup fishery. Scup discards were high in directed scup tows regardless of codend mesh — typically one to five times the weight of landings. The weight of scup discards in the present study did not differ significantly from that recorded in scup-targeted tows in the NMFS observer database. Most discards were required as such by the 22.86 cm TL (total length) fish-size limit for catches. Mesh sizes ≤12.7 cm, including the current legal mesh size (11.43 cm) did not adequately filter out scup smaller than 22.86 cm. The median length of scup discards was about 19.83 cm TL. Lowering the legal size for scup from 22.86 to 19.83 cm TL would greatly reduce discard mortality. Scup discards were a small fraction (0.4%) of black sea bass (Centropristis striata) landings in blacksea-bass−targeted tows. The black sea bass fishery is currently regulated under the small-mesh fishery gearrestricted area plan in which fishing is prohibited in some areas to reduce scup mortality. Our study found no evidence to support the efficacy of this management approach. The expectations that discarding would increase disproportionately as the trip limit (limit [in kilograms] on catch for a species) was reached towards the end of the trip and that discards would increase when the trip limit was reduced from 4536 kg to 454 kg at the end of the directed fishing season were not supported. Trip limits did not significantly affect discard mortality

Onshore–Offshore Trends in the Size-Frequency Distribution of Death Assemblages: Northwestern Gulf of Mexico

The size-frequency distributions of death assemblages were compared at three sites on the inner continental shelf of Texas by means of three descriptor variables, numerical abundance, paleoproduction (biomass at death), and paleoingestion (lifetime ingestion, a measure of energy flow). These death assemblages were then compared with six other death assemblages covering a transect from the estuary (Copano Bay, TX) to the continental slope. Typically, size-frequency distributions are based on abundance and size classes are set proportional to the largest individual in the collection. Restriction to this one analysis would have identified few of the important trends observed in this study. The evaluation of size frequency on the basis of species\u27 maximum size as well as assemblage maximum size and the comparison of a suite of assemblages on the basis of the largest maximum size provide important new inferences into community dynamics. The distribution of measures of energy flow across the size-frequency spectrum provided an additional, valuable source of information on community structure and habitat optimality. Within-habitat variability was consistently less than between-habitat variability. The autochthonous continental slope assemblages were the most diverse in their size-frequency spectra. Comparison between habitats showed that the continental slope assemblages had the largest proportion of adult individuals. The continental shelf assemblages were dominated by juveniles. The chemoautotrophic and heterotrophic assemblages in Copano Bay and on the continental slope were similar in most respects despite substantial differences in their trophic structure. Similarity existed in the proportion of adults, in the tendency toward bimodality, and in the degree to which species reached maximum size. The shapes of the size-frequency spectra were controlled in large measure by (a) the relative loss of juveniles through taphonomy, (b) the degree of survivorship to adulthood, probably predominately determined by predation, (c) the food and space resources present that control species size, and (d) the optimality of the habitat that allowed animals to approach maximum size. The habitats on the continental slope had the highest proportion of individuals near maximum size. The Copano Bay assemblages were also characterized by a large proportion of adult individuals; however, these normally did not reach sizes above 70% of species\u27 maximum size. The largest individuals were found at the petroleum seeps and in the heterotrophic assemblages from Copano Bay. Continental slope habitats should be temporally most stable, and our information supports that expectation. Food supply should be greatest in estuaries and in cold seeps where chemosynthetic processes dominate. Our data support this expectation

Why Oyster Restoration Goals In The Chesapeake Bay Are Not And Probably Cannot Be Achieved

Efforts to restore the native oyster in the Chesapeake Bay enjoy enormous public support and have consumed and continue to consume vast, some would argue unreasonable and unjustifiable, amounts of funding. Despite this support the stated goals of restoration efforts are poorly defined and consequently provide no realistic measures of success in terms of time, space, or biomass. Quantitative approaches used successfully in management of and rebuilding plans for other marine and estuarine species have not been appropriately applied. Basic information in oyster population dynamics and ecology has been inadequately appreciated in defining the quantitative problem. Given these limitations it is not surprising that little success has been achieved despite the massive investment. We note a lack of ability to predict recruitment, and limit the ingress and impact of disease. Without control of both of these functions, populations cannot be managed in a self-sustaining rebuilding mode within the footprint that they either currently occupy or formerly occupied. Sustained expansion of that footprint through substrate provision is prohibitively expensive, beyond the limits set by availability of substrate material, and futile in the presence of disease and susceptible oysters. Without attaining a substantially increased and rebuilding population, ecological services will be limited. Water quality impacts will, in reality, be modest, local and seasonal, and still subject to being overwhelmed by periodic storm events. Coherent and rational evaluation of biological limitations will lead to more realistic, and indeed very modest goals for ecological restoration. We must accept the fact that efforts to date to restore native oyster populations have failed and the prognosis for improvement of this situation is continued failure. The argument is proffered that stabilizing the present bed footprint with a realistic and sustainable population and the promotion of aquaculture to increase commercial yield is a more predictable and stable economic investment. Each of these options is consistent with the most realistic ecological outcome and should take priority in future efforts

Is Oyster Shell a Sustainable Estuarine Resource?

The decline of the eastern oyster (Crassostrea virginica) as an estuarine resource is well documented for many estuaries on the United States east coast. This decline is often associated with a decline in the shell resource and ultimately the disappearance of the shell bed. We develop a model that expressly and conjointly evaluates oyster abundance and surficial shell quantity and examine whether stability in the stock and the habitat can be simultaneously achieved. Simulations suggest that a steady-state shell content exists for any set of recruitment and natural mortality rates and that the amount of shell present at steady state varies over a wide range as recruitment and natural mortality vary. Shell mass is maximized at a natural mortality rate near the rate observed in unfished populations unimpacted by disease. A species dependent on the maintenance of hard substrate for survival, as is the oyster, might have a life span adapted to maximize the accretion of carbonate; thereby sustaining the substrate on which it depends. Relatively small changes in the recruitment rate produce large changes in abundance and consequently shell mass and the scale of variation dwarfs that of natural mortality or fishing. Only variations in the rate of shell loss or the average size of animals at death produce equivalent excursions in shell mass. In comparison, the ambit of natural mortality imposed by the disease process fortuitously occurs in a range that restrains the change in carbonate mass, probably because increased mortality reduces abundance but also increases the death rate, thus adding more shell. Simulations covering a range of fishing rates indicate that no fishing rate exists that is likely to be sustainable of the shell resource over the long term. Fishing will always abet the taphonomic and depositional processes conspiring to debilitate the oyster bed. Successful management of the oyster shell resource is obstructed by the simple fact that no additional mortality, whether imposed by disease or through fishing, can occur that will not result in habitat loss at some rate. The shell resource is maximized when the population is at predisease natural mortality rates and unfished. Thus, if fishing is to be permitted or if disease has increased persistently the natural mortality rate, the only recourse of the manager is the perpetual addition of shell in compensation to the loss or the acceptance of the degradation of the shell bed

Application of a Gene-Based Population Dynamics Model to the Optimal Egg Size Problem: Why Do Bivalve Planktotrophic Eggs Vary in Size?

The presumption is that egg quality influences larval survival and that egg size influences egg quality. Thus, larger eggs should be favored by selection. Counterweighing the tendency for egg size to increase is the number of eggs that can be produced if egg size remains small. We examine how egg size and egg number counterbalance in Crassostrea oysters, resulting in an average egg size near 50 mu m. Simulations imposing a diversity of ranges in larval survivorship-from little advantage for large eggs relative to small eggs to a great advantage-yield some anticipated outcomes in which genotypes generating larger eggs are favored. In other simulations, however, genotypes generating smaller eggs became increasingly common. In these cases, egg size declines, as does the likelihood of survival of individual larvae: the antithesis of expectation. Few simulations identify preferred egg sizes near the size typically observed, suggesting that, under most field conditions, a selective advantage exists for smaller or larger eggs than those typically spawned. However, the extremes in egg size are rarely advantageous. Most simulations resolve an optimal intermediate egg size. Thus, observed egg size is a balance between the chanciness of larval survival enhanced by the production or a larger number of eggs and the genetically predisposed, but environmentally modulated, individual probability of larval survival that is a function of egg size, with environment determining the optimal size. The 50-mu m size observed likely represents the median outcome of a range of larval survivorship probabilities, each selecting for relatively larger or smaller eggs, imposed stochastically over multiple generations. In this scenario, each year the population is pulled toward smaller or larger egg sizes, but in the next year the impetus is independent of the previous year. Reduced generation time, by disease or fishing, modifies the extent, but not the direction of trend. Thus, environmental stochasticity retains preeminence in stabilizing a balance between the probabilities of survival modulated by egg number and by egg size. The influence of shortened generation time-by disease, for example-is unlikely to be manifest in a modification in egg size and hence egg number

Environmental Effects on the Growth and Development of Eastern Oyster, Crassostrea virginica (Gmelin, 1791), Larvae: A Modeling Study

The effects of temperature, food concentration, salinity and turbidity on the growth and development of Crassostrea virginica larvae were investigated with a time-dependent mathematical model. Formulations used in the model for larval growth are based upon laboratory data. Simulations were done using temperature conditions characteristic of Laguna Madre, Galveston Bay, Apalachicola Bay, North Inlet and Chesapeake Bay. These simulations show that the duration of the planktonic larval phase, which is determined by larval growth rate, decreases at lower latitudes in response to warmer water temperatures. Also, oysters in the more southern locations have a longer spawning season during which the oyster population can produce more larvae. Simulations were done for Galveston Bay and Chesapeake Bay using idealized time series of food supply that included higher concentrations in the spring, summer or fall. Additional simulations considered the effects of increased food supply in both spring and fall seasons. The results show that shifting the period of enhanced food supply from March-April to April-May, when temperatures are warmer, reduces the minimum larval planktonic period from 44 to 34 days. Shifting the fall bloom from August-September to September-October, however, does not appreciably change the minimum larval planktonic period. The final set of simulations considered the effect of low salinity events and turbidity on the planktonic period of the larvae of Crassostrea virginica. By imposing a simulated low salinity (5 ppt) event of one month duration in August, the larval planktonic time is increased by about 39% over normal August salinities. Turbidity concentrations less than 0.1 g l-1 result in slightly decreased planktonic times. These model results show clearly the importance of ambient environmental conditions in determining the planktonic time of larvae of Crassostrea virginica, and hence their ultimate recruitment to the adult oyster population

The “Challenge of Depletion: Why the Oyster Fishery is not Self-Regulating

The possibility that the economics of the oyster fishery impose a self-limitation on overharvesting has been proffered on occasion. The inefficiency of harvesting by the fishery has been evaluated and estimates of the exploitation rate permissible under conditions of maximum sustainable yield have been obtained in previous studies. The question becomes to what extent does the inefficiency of harvest interact with the economics of the fishery to compromise ready detection of overfishing? This study explores the possibility that the constraint of economics on the fishery occurs at oyster exploitation rates that are higher than maximum sustainable yield, leading ineluctably to overfishing if unconstrained and to the appearance of unduly limited fishing if properly constrained. A model is developed that simulates oyster harvesting by dredging. This model tracks vessel behavior and fishery performance in economic terms (CPUE) under varying stock densities and dredge efficiencies. Simulation results show that stock density and on-deck culling speed have the strongest effect on time required, profitability, and effectiveness of harvest, whereas dredge efficiency has a lesser influence. Evaluation of simulations shows that overfishing occurs at a stock density that provides near-optimal economic returns. The oyster fishery does not perceive a decline in the stock under sustainable conditions, as the on-deck processing capacity enables the catch rate to remain relatively stable until the stock declines well below sustainable levels. The consequence of setting fishing regulations such that a decline in catch is perceived is to assure routine and substantive overfishing, thereby creating a potential conflict between apparent and real sustainability. This conflict may explain the inability of state regulatory authorities to impose limitations consistent with long-term resource stability. The perception that a decline in the rate of catch should be observed under standard effort-based regulatory controls is a principal challenge that must be overcome if sustainability is to become normative in the U.S. oyster fishery

The Case of the ‘Missing’ Arctic Bivalves and The Walrus: The Biggest [Overlooked] Clam Fishery on the Planet

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 3 106 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 3 107 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 4.0–6.03106 tonnes y–1 of carbonate to buffering of near-surface sediments in Arctic 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