Simulating movement of highly mobile aquatic biota: foundation for population modeling in an ecosystem context

PURPOSE: This technical note describes an approach for incorporating population dynamics into ecosystem-level assessments. The tool described herein is termed the Coupled Eulerian- Lagrangian (CEL) Hybrid Ecological Model. The CEL Hybrid Ecological Model couples the kinds of models typically used by engineers (the Eulerian module) with the types of models typi- cally used by biologists (the Lagrangian module). The CEL Hybrid Ecological Model can be applied singly or in concert with other tools and methods that describe the physical environment and hydro-biogeochemical processes having ecosystem significance. INTRODUCTION: The Corps of Engineers is required to address ecosystem-level impacts and to develop plans that guarantee sustainable development. Many important aquatic species show behavioral responses to changes in hydraulic and water quality patterns in their environment, including changes resulting from the operation of dams and water intakes. These species may move to new habitats in the system, change their depth position, or redistribute in more complex ways. To predict the effects of operational changes, to design fish passage measures, or to assess aquatic resource management strategies at Corps projects, it is necessary to predict the relation- ship between project design or operation and the movement behavior of highly mobile aquatic biota, such as fish. Most critically, simulating the movement behavior of aquatic biota is neces- sary to link population biology to ecosystem processes. This linkage can be used to integrate population models and engineering models so that population level dynamics can be viewed in an ecosystem context. The linkage can be viewed as a two-step process. First, the effects of the proposed changes on physical factors directly influencing fish movement, such as hydraulic pat- terns and water quality, must be identified. Modeling tools for accomplishing this step are well-established. Second, however, is a more difficult step: accurately predicting fish movement behavior in response to predicted physico-chemical changes. In the absence of simulation tools for predicting fish response, designers and resource managers often have to rely on the ineffi- cient "build, operate, and test" paradigm for implementing changes at projects. PRINCIPLES OF THE MODEL: The CEL Hybrid Ecological Model was originally developed as a building block for predicting and assessing ecosystem level impacts of reservoirs on aquatic species, particularly large, highly mobile aquatic biota such as fish that are difficult to simulate in typical engineering models. The model provides a way of simulating the movement behavior of aquatic biota, but in an engineering framework. The model represents individual fish and groups of individuals as individual particles in space (Matuda, Liang, and Sannomiya 1993). The biological component of the model (the Numerical Fish Surrogate module) is based on a particle- tracking algorithm that moves passive, neutrally buoyant "particles" through the system created by a separate hydrodynamic and water quality simulation model. The particle-tracking algorithm