Management strategy of the naked carp (Gymnocypris przewalskii) in the Qinghai lake using matrix population modeling.

Naked carp (Gymnocypris przewalskii) is the only fish species commercially harvested in Qinghai Lake, which is the largest inland saltwater lake in China. Multiple ecological stresses such as long-term overfishing, drying-up of riverine inflows, and decreases in spawning habitat caused the naked carp population to decrease from 320,000 tons before the 1950s to only 3000 tons by the early 2000s. We used matrix projection population modeling to quantitatively simulate the dynamics of the naked carp population from the 1950s to the 2020s. Five versions of the matrix model were developed from the field and laboratory information that represented different population states (high but declining, low abundance, very low abundance, initial recovery, pristine). Equilibrium analysis was applied to density-independent versions of the matrices and population growth rate, age composition, and elasticities were compared among versions. Stochastic, density-dependent version of the most recent decade (recovering) version was used to simulate the time-dependent responses to a range of levels of artificial reproduction (addition of age-1 from hatchery) and of the pristine version to simulate combinations of fishing rate and minimum age of harvest. Results showed the major role of overfishing in the population decline and that the population growth rate was most sensitive to the survival of juveniles and the spawning success of early-age adults. Dynamic simulations showed a rapid population response to artificial reproduction when population abundance was low and that if artificial reproduction continues at its current level, then population biomass would reach 75% of its pristine biomass after 50 years. Simulations with the pristine version identified sustainable fishing levels and the importance of protecting the first few ages of maturity. Overall, modeling results showed that artificial reproduction under conditions of no fishing is an effective approach to restoring the naked carp population. Further effectiveness should consider maximizing survival in the months just after release and maintaining genetic and phenotypic diversity. More information on density-dependent growth, survival, and reproduction, as well as on the genetic diversity and growth and migratory behavior (phenotypic variation) of released and native-spawned fish, would help inform management and conservation strategies and practices going forward.

Applying spatiotemporal models to monitoring data to quantify fish population responses to the Deepwater Horizon oil spill in the Gulf of Mexico.

Quantifying the impacts of disturbances such as oil spills on marine species can be challenging. Natural environmental variability, human responses to the disturbance (e.g., fisheries closures), the complex life histories of the species being monitored, and limited pre-spill data can make detection of effects of oil spills difficult. Using long-term monitoring data from the state of Louisiana (USA), we applied novel spatiotemporal approaches to identify anomalies in species occurrence and catch rates. We included covariates (salinity, temperature, turbidity) to help isolate unusual events. While some species showed evidence of unlikely temporal anomalies in occurrence or catch rates, we found that the majority of the observed anomalies were also before the Deepwater Horizon event. Several species-gear combinations suggested upticks in the spatial variability immediately following the spill, but most species indicated no trend. Across species-gear combinations, there was no clear evidence for synchronous or asynchronous responses in occurrence or catch rates across sites following the spill. Our results are in general agreement to other analyses of monitoring data that detected small impacts, but in contrast to recent results from ecological modeling that showed much larger effects of the oil spill on fish and shellfish.

Testing and applying a fish vitellogenesis model to evaluate laboratory and field biomarkers of endocrine disruption in Atlantic croaker (Micropogonias undulatus) exposed to hypoxia.

Recently, hypoxia has been shown to act as an endocrine disruptor. We used a model of vitellogenesis in a female sciaenid fish to simulate the effects of hypoxia and to determine if reproductive impairment observed in field-caught fish could be attributed to dissolved oxygen conditions at the sampling sites. The model is a set of coupled, ordinary differential equations that simulate major biochemical reactions from the secretion of gonadotropin to production of vitellogenin. Various intermediate variables in the model correspond to commonly measured biomarkers, and we assume a direct relationship between cumulative vitellogenin (VTG) and the gonadosomatic index (GSI). Model predictions were compared to results of laboratory studies that examined the effects of hypoxia on Atlantic croaker (Micropogonias undulatus) reproduction. When hypoxia was assumed to cause reduced gonadotropin and impaired aromatase activity, model predictions of VTG production were similar to laboratory-measured reductions in GSI. The model was then applied to reproductive biomarkers measured in fish from normoxic and hypoxic locations in Pensacola Bay (FL, U.S.A.). We simulated the relationship between reduced estradiol-17beta and VTG production under hypoxia, and we compared these results with field data. Good agreement between field and simulation results suggested that croaker collected from hypoxic sites in October were exposed to hypoxic conditions for an extended period during gonadal recrudescence and that hypoxia was a dominant cause for the reduced GSIs. Monte Carlo uncertainty analyses suggested that the maximum rate of free testosterone production is the most sensitive parameter. Our simulations demonstrated that the model can be used identifying the mechanism underlying endocrine disruption and for interpreting field-measured biomarkers in situations of multiple stressors.

Skill Assessment for Coupled Biological/Physical Models of Marine Systems.

Coupled biological/physical models of marine systems serve many purposes including the synthesis of information, hypothesis generation, and as a tool for numerical experimentation. However, marine system models are increasingly used for prediction to support high-stakes decision-making. In such applications it is imperative that a rigorous model skill assessment is conducted so that the model's capabilities are tested and understood. Herein, we review several metrics and approaches useful to evaluate model skill. The definition of skill and the determination of the skill level necessary for a given application is context specific and no single metric is likely to reveal all aspects of model skill. Thus, we recommend the use of several metrics, in concert, to provide a more thorough appraisal. The routine application and presentation of rigorous skill assessment metrics will also serve the broader interests of the modeling community, ultimately resulting in improved forecasting abilities as well as helping us recognize our limitations.

Life-history constraints on maximum population growth for loggerhead turtles in the northwest Atlantic.

Conservation planning for protected species often relies on estimates of life-history parameters. A commonly used parameter is the instantaneous maximum population growth rate (r max) that can be used to limit removals and design recovery targets. Estimation of r max can be challenging because of limited availability of species- and population-specific data and life-history information. We applied a method proposed by Neil and Lebreton, originally developed for birds, to loggerhead turtles. The method uses age-at-first-reproduction and adult survival to estimate r max. We used a variety of datasets and matrix population models to confirm an allometric assumption required by the method, and to generate estimates of age-at-first-reproduction and adult survival. A meta-analysis was applied to parameters from reported growth curves, which were then combined with the size distribution of neophyte nesters to derive estimates of age-at-first-reproduction. Adult survival rates were obtained from an existing matrix population model. Monte Carlo simulation was then used to combine the estimates of the allometric coefficients, age-at-first-reproduction, and adult survival to obtain a probability distribution of approximate r max values. Estimated annual maximum population growth rates averaged 0.024, with a mode of 0.017 and a 95% highest density interval of 0.006-0.047. These estimates were similar to values reported by others using different methods and captured the variability in positive, annual change estimates across nesting beach sites for the northwest Atlantic loggerhead population. The use of life-history parameters has a long history in wildlife and fisheries management and conservation planning. Our estimates of r max, while having some biases and uncertainty, encompassed values presently used in recovery planning for loggerhead turtles and offer additional information for the management of endangered and threatened species.

Modeling larval fish behavior: scaling the sublethal effects of methylmercury to population-relevant endpoints.

Expressing the sublethal effects of contaminants measured on individual fish as cohort and population responses would greatly help in their interpretation. Our approach combines laboratory studies with coupled statistical and individual-based models to simulate the effects of methylmercury (MeHg) on Atlantic croaker larval survival and growth. We used results of video-taped laboratory experiments on the effects of MeHg on larval behavioral responses to artificial predatory stimuli. Laboratory results were analyzed with a regression tree to obtain the probability of control and MeHg-exposed larvae escaping a real predatory fish attack. Measured changes in swimming speeds and regression tree-predicted escape abilities induced by MeHg exposure were then inputted into an individual-based larval fish cohort model. The individual-based model predicted larval-stage growth and survival under baseline (control) conditions, and low- and high-dose MeHg exposure under two alternative predator composition scenarios (medusa-dominated and predatory fish-dominated). Under MeHg exposure, stage survival was 7-19% of baseline (control) survival, and the roughly 33-day stage duration was extended by about 1-4 days. MeHg effects on larval growth dominated the response under the medusa-dominated predator composition, while predation played a more important role under the fish-dominated predator composition. Simulation results suggest that MeHg exposures near extreme maximum values observed in field studies can have a significant impact on larval cohort dynamics, and that the characteristics of the predator-prey interactions can greatly influence the underlying causes of the predicted responses.

Declining oxygen in the global ocean and coastal waters.

Oxygen is fundamental to life. Not only is it essential for the survival of individual animals, but it regulates global cycles of major nutrients and carbon. The oxygen content of the open ocean and coastal waters has been declining for at least the past half-century, largely because of human activities that have increased global temperatures and nutrients discharged to coastal waters. These changes have accelerated consumption of oxygen by microbial respiration, reduced solubility of oxygen in water, and reduced the rate of oxygen resupply from the atmosphere to the ocean interior, with a wide range of biological and ecological consequences. Further research is needed to understand and predict long-term, global- and regional-scale oxygen changes and their effects on marine and estuarine fisheries and ecosystems.

Maternal body burdens of methylmercury impair survival skills of offspring in Atlantic croaker (Micropogonias undulatus).

Methylmercury (MeHg), the organic form of mercury, bioaccumulates easily through the food chain. Fish in high trophic levels can accumulate substantial levels of MeHg and transfer it to their developing eggs. Here, the effects of maternally derived MeHg on the planktonic larval stage of Atlantic croaker were investigated. Adult Atlantic croaker were fed MeHg-contaminated food at three levels: 0, 0.05, and 0.1 mg kg(-1) day(-1) for 1 month. Fish were then induced to spawn and MeHg levels in the eggs were measured (0.04-4.6 ng g(-1)). Behavioral performance of exposed and control larvae was measured at four developmental stages: end of yolk absorption (yolk), end of oil absorption (oil), and 4 and 11 days after oil absorption (oil+4 and oil+11). Behaviors analyzed included survival skills related to foraging and predator evasion: routine behavior (rate of travel, active swimming speed, net-to-gross displacement ratio, and activity) and startle response to a visual and a vibratory stimulus (responsiveness, reactive distance, response distance, response duration, average response speed, and maximum response speed). Maternally transferred MeHg induced concentration-dependent effects on survival skills. Statistical and simulation models applied to predict the ecological consequences of the behavioral effects suggested that maternal transfer of MeHg may substantially lower survival of planktonic stage larvae compared to unexposed larvae. These results also imply that larvae of top predatory fish species, such as blue marlin, may suffer mortality through maternal transfer of MeHg.

Modeling vitellogenesis in female fish exposed to environmental stressors: predicting the effects of endocrine disturbance due to exposure to a PCB mixture and cadmium.

A wide variety of chemical and physical environmental stressors have been shown to alter the reproductive processes in fish by interfering with endocrine function. Most endocrine indicators or biomarkers are static measures from dynamic hormonally-mediated processes, and often do not directly relate to reproductive endpoints of ecological significance. Adequate production of the yolk precursor protein, vitellogenin, is critical for the survival and normal development of the sensitive egg and yolk-sac larval fish life stages. We developed a model that simulates vitellogenesis in a mature female sciaenid fish. The model simulates the major biochemical reactions over a 6-month period from the secretion of gonadotropin (GtH) into the blood to the production of vitellogenin. We simulated the effects of two endocrine disrupting chemicals (EDCs) that have different actions on vitellogenin production: a PCB mixture and cadmium. Predicted changes in steroid concentrations and cumulative vitellogenin production compared favorably with changes reported in laboratory experiments. Simulations illustrate the potential utility of our model for interpreting reproductive endocrine biomarkers measured in fish collected from degraded environments.

Evaluation of alternative PCB clean-up strategies using an individual-based population model of mink.

Population models can be used to place observed toxic effects into an assessment of the impacts on population-level endpoints, which are generally considered to provide greater ecological insight and relevance. We used an individual-based model of mink to evaluate the population-level effects of exposure to polychlorinated biphenyls (PCBs) and the impact that different remediation strategies had on mink population endpoints (population size and extinction risk). Our simulations indicated that the initial population size had a strong impact on mink population dynamics. In addition, mink populations were extremely responsive to clean-up scenarios that were initiated soon after the contamination event. In fact, the rate of PCB clean-up did not have as strong a positive effect on mink as did the initiation of clean-up (start time). We show that population-level approaches can be used to understand adverse effects of contamination and to also explore the potential benefits of various remediation strategies.