Ruzicka James J.

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James J.

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  • Preprint
    Constructing end-to-end models using ECOPATH data
    ( 2011-03) Steele, John H. ; Ruzicka, James J.
    The wide availability of ECOPATH data sets provides a valuable resource for the comparative analysis of marine ecosystems. We show how to derive a bottom-up transform from the top-down ECOPATH; couple this to a simple NPZD web with physical forcing; and use the end-to-end model (E2E) for scenario construction. This steady state format also provides a framework and initial conditions for different dynamic simulations. This model can be applied to shelf ecosystems with a wide range of physical forcing, coupled benthic/pelagic food webs, and nutrient recycling. We illustrate the general application and the specific problems by transforming an ECOPATH model for the Northern Californian Current (NCC). We adapt results on the upwelling regime to provide estimates of physical fluxes and use these to show the consequences of different upwelling rates combined with variable retention mechanism for plankton, for the productivity of fish and other top predators; and for the resilience of the ecosystem. Finally we show how the effects of inter-annual to decadal variations in upwelling on fishery yields can be studied using dynamic simulations with different prey-predator relations. The general conclusion is that the nature of the physical regimes for shelf ecosystems cannot be ignored in comparing end-to-end representations of these food webs.
  • Article
    Analysis of energy flow in US GLOBEC ecosystems using end-to-end models
    (The Oceanography Society, 2013-12) Ruzicka, James J. ; Steele, John H. ; Gaichas, Sarah K. ; Ballerini, Tosca ; Gifford, Dian J. ; Brodeur, Richard D. ; Hofmann, Eileen E.
    End-to-end models were constructed to examine and compare the trophic structure and energy flow in coastal shelf ecosystems of four US Global Ocean Ecosystem Dynamics (GLOBEC) study regions: the Northern California Current, the Central Gulf of Alaska, Georges Bank, and the Southwestern Antarctic Peninsula. High-quality data collected on system components and processes over the life of the program were used as input to the models. Although the US GLOBEC program was species-centric, focused on the study of a selected set of target species of ecological or economic importance, we took a broader community-level approach to describe end-to-end energy flow, from nutrient input to fishery production. We built four end-to-end models that were structured similarly in terms of functional group composition and time scale. The models were used to identify the mid-trophic level groups that place the greatest demand on lower trophic level production while providing the greatest support to higher trophic level production. In general, euphausiids and planktivorous forage fishes were the critical energy-transfer nodes; however, some differences between ecosystems are apparent. For example, squid provide an important alternative energy pathway to forage fish, moderating the effects of changes to forage fish abundance in scenario analyses in the Central Gulf of Alaska. In the Northern California Current, large scyphozoan jellyfish are important consumers of plankton production, but can divert energy from the rest of the food web when abundant.
  • Preprint
    Dividing up the pie : whales, fish, and humans as competitors
    ( 2013-04-29) Ruzicka, James J. ; Steele, John H. ; Ballerini, Tosca ; Gaichas, Sarah K. ; Ainley, David G.
    Similarly structured food web models of four coastal ecosystems (Northern California Current, Central Gulf of Alaska, Georges Bank, southwestern Antarctic Peninsula) were used to investigate competition among whales, fishes, pinnipeds, and humans. Two analysis strategies simulated the effects of historic baleen and odontocete whale abundances across all trophic levels: food web structure scenarios and time-dynamic scenarios. Direct competition between whales and commercial fisheries is small at current whale abundances; whales and fisheries each take similar proportions of annual pelagic fish production (4 - 7%). Scenarios show that as whale populations grow, indirect competition between whales and fish for zooplankton would more likely impact fishery production than would direct competition for fish between whales and commercial fisheries. Increased baleen whale abundance would have greater and broader indirect effects on upper trophic levels and fisheries than a similar increase in odontocete abundance. Time-dynamic scenarios, which allow for the evolution of compensatory mechanisms, showed more modest impacts than structural scenarios, which show the immediate impacts of altered energy pathways. Structural scenarios show that in terms of energy availability, there is potential for large increases in whale abundance without major changes to existing food web structures and without substantial reduction of fishery production. For each ecosystem, a five-fold increase in baleen whale abundance could be supported with minor disruptions to existing energy flow pathways. However, such an increase would remain below historical population levels for many cetaceans. A larger expansion (20X) could be accommodated only with large reductions in energy flow to competitor groups. The scope for odontocete expansion varies between ecosystems but can be restricted because they feed at higher, less productive trophic levels.
  • Thesis
    Integrating bioenergetics and foraging behavior : the physiological ecology of larval cod (Gadus morhua)
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2004-06) Ruzicka, James J.
    How do larval cod, Gadus morhua, balance foraging effort against the high cost of swimming in a viscous hydrodynamic regime? A respirometry system was developed to measure the activity metabolism of individual larvae. The cost of swimming was modeled as a power-performance relationship (energy expenditure as a function of swimming speed) and as the cost of transport (the cost to travel a given distance). The cost of transport was high relative to juvenile and adult fish, but larvae swam more efficiently as they grew and became better able to overcome viscous drag. A large-volume observation system was developed to record foraging behavior in three dimensions. There are two phases of the saltatory search cycle used by larval cod: the burst which serves to position larvae within a new search volume and the pause when larvae search for prey Burst characteristics did not change under different prey treatments, but pause duration increased while foraging capacity and swimming activity decreased when prey were absent. Longer pause durations could reflect greater effort to visually process each search volume when prey were difficult to find. Reduced swimming activity could also be an energy conservation strategy under unfavorable foraging conditions. By applying the cost of swimming model to the observed swimming intensity of freely foraging larvae, foraging activity was estimated to account for up to 80% of routine metabolism. A trophodynamic model was developed incorporating observed foraging behavior and swimming costs to estimate the prey density required to cover all metabolic demands. Small larvae (5mm) can survive on typical mean Georges Bank prey densities in mildly turbulent conditions. Larger larvae (>6mm) can survive even at high turbulence levels. Simulated alternative foraging strategies predict that when predator-prey contact rates are high, the greatest net energy gain is realized with short pause durations. When predator-prey contact rates are low, larvae should achieve greater net energy gains by remaining at rest for extended periods. Larvae observed foraging in the absence of prey do not change behavior as much as the simulation model predicts, suggesting that they use a prey encounter maximization strategy rather than an energy conservation strategy.