INDIVIDUAL-BASED MODEL OF SYMPATRIC POPULATIONS OF BROWN AND RAINBOW-TROUT FOR INSTREAM FLOW ASSESSMENT - MODEL DESCRIPTION AND CALIBRATION

This paper describes an individual-based model of sympatric population s of brown and rainbow trout in a stream habitat. The model provides a tool for projecting flow and temperature effects on trout populations by linking the hydraulic component of the instream how incremental me thodology/physical habitat simulation system (IFIM/PHABSIM) to an indi vidual-based population model. PHABSIM simulates the spatial distribut ion of depth and velocity at different flows, and indirectly, the avai lability of spawning habitat, cover and feeding station. The individua l-based model simulates reproduction, growth and mortality of individu al trout as a function of flow and temperature. Population dynamics ar ise from the survival and reproduction of individual trout. The spatia lly explicit nature of the model permits evaluation of behavioral resp onses used by fish to changes in physical habitat. The model has been calibrated to a stream segment in the North Fork Middle Fork Tule Rive r, California. Selected parameters were adjusted to calibrate the mode l for length and abundance (including production of a new year class) at the end of 1-year simulations for each of 9 years. Predicted and ob served lengths were in good agreement, although neither varied appreci ably among years. Predicted and observed abundances were not in as goo d agreement, and differed considerably for some years. These differenc es reflect a combination of uncertainties in the held data and uncerta inties in the model structure and parameter values. Fifty-year simulat ions indicated that model projections of length and abundance were sta tionary, although abundance values fluctuated considerably. Seven adva ntages for using simulation models of this type are emphasized. How to most effectively interpret results from such simulation models as par t of instream flow environmental assessments remains a challenge. Vari ability and uncertainty in both field data and replicate model simulat ions are realities that have implications for scientists, resource man agers, and regulators in projecting growth and abundance responses of fish populations to alternative flow or temperature regimes. (C) 1998 Elsevier Science B.V. All rights reserved.