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Marine taxa track local climate velocities. - Science (New York, N.Y.)
Organisms are expected to adapt or move in response to climate change, but observed distribution shifts span a wide range of directions and rates. Explanations often emphasize biological distinctions among species, but general mechanisms have been elusive. We tested an alternative hypothesis: that differences in climate velocity-the rate and direction that climate shifts across the landscape-can explain observed species shifts. We compiled a database of coastal surveys around North America from 1968 to 2011, sampling 128 million individuals across 360 marine taxa. Climate velocity explained the magnitude and direction of shifts in latitude and depth much more effectively than did species characteristics. Our results demonstrate that marine species shift at different rates and directions because they closely track the complex mosaic of local climate velocities.
Predicting climate effects on Pacific sardine. - Proceedings of the National Academy of Sciences of the United States of America
For many marine species and habitats, climate change and overfishing present a double threat. To manage marine resources effectively, it is necessary to adapt management to changes in the physical environment. Simple relationships between environmental conditions and fish abundance have long been used in both fisheries and fishery management. In many cases, however, physical, biological, and human variables feed back on each other. For these systems, associations between variables can change as the system evolves in time. This can obscure relationships between population dynamics and environmental variability, undermining our ability to forecast changes in populations tied to physical processes. Here we present a methodology for identifying physical forcing variables based on nonlinear forecasting and show how the method provides a predictive understanding of the influence of physical forcing on Pacific sardine.
Detecting causality in complex ecosystems. - Science (New York, N.Y.)
Identifying causal networks is important for effective policy and management recommendations on climate, epidemiology, financial regulation, and much else. We introduce a method, based on nonlinear state space reconstruction, that can distinguish causality from correlation. It extends to nonseparable weakly connected dynamic systems (cases not covered by the current Granger causality paradigm). The approach is illustrated both by simple models (where, in contrast to the real world, we know the underlying equations/relations and so can check the validity of our method) and by application to real ecological systems, including the controversial sardine-anchovy-temperature problem.
An index to assess the health and benefits of the global ocean. - Nature
The ocean plays a critical role in supporting human well-being, from providing food, livelihoods and recreational opportunities to regulating the global climate. Sustainable management aimed at maintaining the flow of a broad range of benefits from the ocean requires a comprehensive and quantitative method to measure and monitor the health of coupled humanâ€“ocean systems. We created an index comprising ten diverse public goals for a healthy coupled humanâ€“ocean system and calculated the index for every coastal country. Globally, the overall index score was 60 out of 100 (range 36â€“86), with developed countries generally performing better than developing countries, but with notable exceptions. Only 5% of countries scored higher than 70, whereas 32% scored lower than 50. The index provides a powerful tool to raise public awareness, direct resource management, improve policy and prioritize scientific research.
Pathways between primary production and fisheries yields of large marine ecosystems. - PloS one
The shift in marine resource management from a compartmentalized approach of dealing with resources on a species basis to an approach based on management of spatially defined ecosystems requires an accurate accounting of energy flow. The flow of energy from primary production through the food web will ultimately limit upper trophic-level fishery yields. In this work, we examine the relationship between yield and several metrics including net primary production, chlorophyll concentration, particle-export ratio, and the ratio of secondary to primary production. We also evaluate the relationship between yield and two additional rate measures that describe the export of energy from the pelagic food web, particle export flux and mesozooplankton productivity. We found primary production is a poor predictor of global fishery yields for a sample of 52 large marine ecosystems. However, chlorophyll concentration, particle-export ratio, and the ratio of secondary to primary production were positively associated with yields. The latter two measures provide greater mechanistic insight into factors controlling fishery production than chlorophyll concentration alone. Particle export flux and mesozooplankton productivity were also significantly related to yield on a global basis. Collectively, our analyses suggest that factors related to the export of energy from pelagic food webs are critical to defining patterns of fishery yields. Such trophic patterns are associated with temperature and latitude and hence greater yields are associated with colder, high latitude ecosystems.
Role of egg predation by haddock in the decline of an Atlantic herring population. - Proceedings of the National Academy of Sciences of the United States of America
Theoretical studies suggest that the abrupt and substantial changes in the productivity of some fisheries species may be explained by predation-driven alternate stable states in their population levels. With this hypothesis, an increase in fishing or a natural perturbation can drive a population from an upper to a lower stable-equilibrium population level. After fishing is reduced or the perturbation ended, this low population level can persist due to the regulatory effect of the predator. Although established in theoretical studies, there is limited empirical support for predation-driven alternate stable states in exploited marine fish populations. We present evidence that egg predation by haddock (Melanogrammus aeglefinus) can cause alternate stable population levels in Georges Bank Atlantic herring (Clupea harengus). Egg predation by haddock explains a substantial decoupling of herring spawning stock biomass (an index of egg production) from observed larval herring abundance (an index of egg hatching). Estimated egg survival rates ranged from <2-70% from 1971 to 2005. A population model incorporating egg predation and herring fishing explains the major population trends of Georges Bank herring over four decades and predicts that, when the haddock population is high, seemingly conservative levels of fishing can still precipitate a severe decline in the herring population. These findings illustrate how efforts to rebuild fisheries can be undermined by not incorporating ecological interactions into fisheries models and management plans.
Forecasting the dynamics of a coastal fishery species using a coupled climate--population model. - Ecological applications : a publication of the Ecological Society of America
Marine fisheries management strives to maintain sustainable populations while allowing exploitation. However, well-intentioned management plans may not meet this balance as most do not include the effect of climate change. Ocean temperatures are expected to increase through the 21st century, which will have far-reaching and complex impacts on marine fisheries. To begin to quantify these impacts for one coastal fishery along the east coast of the United States, we develop a coupled climate-population model for Atlantic croaker (Micropogonias undulatus). The model is based on a mechanistic hypothesis: recruitment is determined by temperature-driven, overwinter mortality of juveniles in their estuarine habitats. Temperature forecasts were obtained from 14 general circulation models simulating three CO2 emission scenarios. An ensemble-based approach was used in which a multimodel average was calculated for a given CO2 emission scenario to forecast the response of the population. The coupled model indicates that both exploitation and climate change significantly affect abundance and distribution of Atlantic croaker. At current levels of fishing, the average (2010-2100) spawning biomass of the population is forecast to increase by 60-100%. Similarly, the center of the population is forecast to shift 50 100 km northward. A yield analysis, which is used to calculate benchmarks for fishery management, indicates that the maximum sustainable yield will increase by 30 100%. Our results demonstrate that climate effects on fisheries must be identified, understood, and incorporated into the scientific advice provided to managers if sustainable exploitation is to be achieved in a changing climate.
Rebuilding global fisheries. - Science (New York, N.Y.)
After a long history of overexploitation, increasing efforts to restore marine ecosystems and rebuild fisheries are under way. Here, we analyze current trends from a fisheries and conservation perspective. In 5 of 10 well-studied ecosystems, the average exploitation rate has recently declined and is now at or below the rate predicted to achieve maximum sustainable yield for seven systems. Yet 63% of assessed fish stocks worldwide still require rebuilding, and even lower exploitation rates are needed to reverse the collapse of vulnerable species. Combined fisheries and conservation objectives can be achieved by merging diverse management actions, including catch restrictions, gear modification, and closed areas, depending on local context. Impacts of international fleets and the lack of alternatives to fishing complicate prospects for rebuilding fisheries in many poorer regions, highlighting the need for a global perspective on rebuilding marine resources.
Periodic versus constant harvesting of discretely reproducing fish populations. - Journal of biological dynamics
We use a single-species discrete-time model to demonstrate changes that introduction of the strong Allee mechanism and periodic exploitations have on compensatory and overcompensatory stock dynamics through comparison with corresponding models that lack such constraints. Periodic and constant exploitations simplify complex overcompensatory stock dynamics with or without the Allee effect. Both constant and periodic exploitations force a sudden collapse to extinction of fisheries systems that exhibit the Allee mechanism. However, in the absence of the Allee effect, fisheries systems decline to zero smoothly under high exploitation.
Spatially discrete metapopulation models with directional dispersal. - Mathematical biosciences
We use an age-structured discrete-time metapopulation model linking two sub-populations through larval transport and directed movements of adults to study the implications of linkages among subpopulations for the stability and resilience of exploited species. Our two-habitat model, a generalization of Fogarty's inshore-offshore lobster population model, includes isolated habitats under compensatory (monotone) or overcompensatory (oscillatory) dynamics [M.J. Fogarty, Implications of migration and larval interchange in American lobster (Homarus americanus) stocks: spatial structure and resilience, in: G.S. Jamieson, A. Campbell (Eds.), Proc. of North Pacific Symposium on Invertebrate Stock Assessment and Management, Can. Spec. Publ. Fish. Aquat. Sci. 125 (1998) 273]. Pre-migration local dynamics are selected from general classes of functions that capture the effects of competition for resources via contest (compensatory) and scramble (overcompensatory) intraspecific competitions. We explore the implications of these mechanisms on the long-term survival of exploited species. In particular, we use threshold parameters R(d)1 for Habitat 1 and R(d)2 for Habitat 2 together with precise mathematical definitions to prove that species persistence is possible at high levels of fishing in one habitat and low to moderate levels of fishing in the other. Our results support Fogarty's conclusion that conservative management of larval source populations could contribute to the resilience of exploited species.
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