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

The Rise and Fall of Crassostrea Virginica Oyster Reefs: The Role of Disease and Fishing in Their Demise and a Vignette on Their Management

We describe a model designed to simulate the shell carbonate budget of an oyster reef.We identify five parameters descriptive of basic characteristics of the shell carbonate budget of a reef that limit simulation accuracy. Two describe the TAZ (taphonomically-active zone) and the distribution of shell carbonate within it. One is the taphonomic rate in the TAZ. Two determine the volume contribution of shell carbonate and the taphonomic loss rate within the reef framework. For Mid-Atlantic estuaries, model simulations suggest that reef accretion only occurs if oyster abundance is near carrying capacity. Simulations further suggest that reef accretion is infeasible for any estuarine reach where dermo is a controlling influence on population dynamics.We forecast that the oyster disease dermo is a principal antagonist of reef persistence through its ability to limit shell addition. Model simulations suggest that reefs with inadequate shell addition ‘protect themselves’ by limiting the volumetric content of shell carbonate in the TAZ. Thus, a dominant process is the transient expansion and contraction of the shell resource, otherwise termed cultch, within the TAZ, rarely expanding enough to generate reef accretion, yet rarely contracting enough to foster erosion of the reef framework. The loss of framework carbonate thusly is curtailed during periods when the surficial shell layer deteriorates. Stasis, a reef neither accreting nor eroding, is a preferred state. Reef recession requires an inordinately unbalanced shell carbonate budget. Results strongly argue for expanded focus on the dynamics of the shell resource within the TAZ, as this likely fosters a feedback loop with abundance through recruitment, serves as the protective layer for the reef during periods of reef stasis, and establishes the threshold conditions for reef accretion and recession. Model simulations suggest that attaining maximum sustainable yield and maintaining a biomass capable of supporting sufficient shell production for reef accretion are irreconcilable goals over a large component of the oyster’s range. Reef stasis would appear to be the only achievable restoration goal in Mid-Atlantic estuarine reaches where dermo holds sway. Exploitation rates much above 5% of the fishable stock per year restrict availability of surficial shell and foster reef erosion. In contrast, in the Gulf of Mexico at the high-productivity end of the oyster’s range, an enhanced fishery and reef accretion may be compatible goals

Oyster food supply in Delaware Bay: Estimation from a hydrodynamic model and interaction with the oyster population

To evaluate oyster food supply, water samples were collected at fifteen sites in Delaware Bay nearmonthly in 2009 and 2010. Food was estimated as the sum of particulate protein, labile carbohydrate, and lipid. Delaware Bay shows a typical spring bloom, centered in March and April, with declining food supply thereafter into early fall, followed sporadically by a minor fall bloom. The geographic and temporal structure of food was more predictable in summer to early fall, and considerably less predictable in spring. Five variables each based on temperature and the spatial and temporal variability of temperature were significant contributors to a multiple regression (R2 = 0.28). Cluster analysis on residuals identified two large groups of sites, one comprising most sites on the eastern side of the bay including all of the sites on the New Jersey oyster beds downestuary of the uppermost beds and one including most of the sites along the central channel and waters west. Food values over the New Jersey oyster beds were often depressed by as much as 50% relative to the bay-wide mean. Food values did not follow an upestuary-downestuary trend anticipated from the salinity gradient. Rather, the differential was cross-bay and was distinctive throughout the estuarine salinity gradient, thus explaining the lack of significance of any salinity-related variable in the multiple regression. The consequence is that food supply cannot be sufficiently predicted or modeled based on observed environmental variables or those predicted from a hydrodynamic model. The cross-bay differential cannot be extracted from such datasets. The oyster reefs of Delaware Bay are dominantly sited on the New Jersey side, where food supply was most depressed and where passive particle residence times were longest. While not conclusive, this dataset suggests that oysters can influence food values on the New Jersey side of the bay at present biomass, and this would explain the cross-bay gradient in food values as an outcome of oyster feeding

Estimating the Sex Composition of the Summer Flounder Catch using Fishery-Independent Data

Models that account for sex-specific behavior and population dynamics are becoming more common in the stock assessment of sexually dimorphic fishes. However, such models can be data intensive and require some knowledge or assumptions about the sex ratio of fishery landings. A recent stock assessment review of Summer Flounder Paralichthys dentatus identified the need to account for sex-specific fishing mortality in the assessment model; however, no data on the sex composition of the catch were available. Fishery-independent, sex-specific information for this species is collected annually by the National Marine Fisheries Service\u27s Northeast Fisheries Science Center during their bottom trawl survey. Sex at age from the survey could be applied to the fishery landings if the probability of landing a given sex at a given age is equivalent for fish collected by the survey and those in the landings. To generate the first regionally comprehensive database on the sex ratio of Summer Flounder landings and to determine the efficacy of using survey sex-at-age keys to estimate the sex of landed fish, we recorded the sex composition of the commercial and recreational catches of Summer Flounder (n = 31,912) in 2010 and 2011. When (1) trawl survey length data were left-truncated to simulate the minimum retention sizes in the fisheries and (2) age-length keys generated from fishery-dependent data were applied to length frequency distributions from the survey to simulate the growth rates of landed fish, the sex-at-age pattern in the survey-derived data closely resembled the patterns in the catch. However, statistically significant differences in sex at age remained between the catch and the survey-derived data. We hypothesize that these differences are attributable to differences in the spatiotemporal distributions of the sexes and of the survey and fishing effort

Change in larval fish assemblage in a USA east coast estuary estimated from twenty-six years of fixed weekly sampling.

Climate change is leading to significant alterations to ecosystems all over the world and some of the resulting impacts on fish and fisheries are now becoming apparent. Estuaries, which are highly susceptible to climate change because they are relatively shallow and in close proximity to anthropogenic stressors, provide habitat to many fish species at a critical time in the life history, after transport and just prior to settlement in nurseries. Despite this, the long-term impacts of climate change on larval fish at this critical location/stage in the life history are not well documented. The larval fish assemblage of a coastal estuary was sampled once per week for twenty-six years at a fixed location in southern New Jersey, USA. We used ordination and regression analysis to evaluate the whole assemblage, individual species/family occurrence, and trends in total density and diversity over that time. The larval fish assemblage changed significantly in response to warming water temperatures. In addition, approximately one quarter of the species/families in the assemblage exhibited a statistically significant trend in individual occurrence over time. Of these, all five of the five northern-affiliated species decreased in occurrence while 18 of 21 southern-affiliated species increased in occurrence. Finally, total fish density and species diversity increased over the course of the study. The non-uniform response of the species/families in this larval assemblage is similar to what has been documented in other studies that evaluated the temporal trend of open ocean juvenile and adult fish assemblages

Correction: Change in larval fish assemblage in a USA east coast estuary estimated from twenty-six years of fixed weekly sampling.

[This corrects the article DOI: 10.1371/journal.pone.0224157.]

Density-Dependent Capture Efficiency of a Survey Dredge and Its Influence On the Stock Assessment of Eastern Oysters (\u3ci\u3eCrassostrea virginica\u3c/i\u3e) in Delaware Bay

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

Oyster food supply in Delaware Bay: Estimation from a hydrodynamic model and interaction with the oyster population

To evaluate oyster food supply, water samples were collected at fifteen sites in Delaware Bay nearmonthly in 2009 and 2010. Food was estimated as the sum of particulate protein, labile carbohydrate, and lipid. Delaware Bay shows a typical spring bloom, centered in March and April, with declining food supply thereafter into early fall, followed sporadically by a minor fall bloom. The geographic and temporal structure of food was more predictable in summer to early fall, and considerably less predictable in spring. Five variables each based on temperature and the spatial and temporal variability of temperature were significant contributors to a multiple regression (R2 = 0.28). Cluster analysis on residuals identified two large groups of sites, one comprising most sites on the eastern side of the bay including all of the sites on the New Jersey oyster beds downestuary of the uppermost beds and one including most of the sites along the central channel and waters west. Food values over the New Jersey oyster beds were often depressed by as much as 50% relative to the bay-wide mean. Food values did not follow an upestuary-downestuary trend anticipated from the salinity gradient. Rather, the differential was cross-bay and was distinctive throughout the estuarine salinity gradient, thus explaining the lack of significance of any salinity-related variable in the multiple regression. The consequence is that food supply cannot be sufficiently predicted or modeled based on observed environmental variables or those predicted from a hydrodynamic model. The cross-bay differential cannot be extracted from such datasets. The oyster reefs of Delaware Bay are dominantly sited on the New Jersey side, where food supply was most depressed and where passive particle residence times were longest. While not conclusive, this dataset suggests that oysters can influence food values on the New Jersey side of the bay at present biomass, and this would explain the cross-bay gradient in food values as an outcome of oyster feeding