A grid-based, spatially-explicit Regional Organism Exchange (ROE) mode
l is presented as a framework for integrating aquatic ecosystems and f
ish population processes at the landscape level. Active fish movements
across a grid cell boundary were predicted, based on environmental to
lerance ranges. The model was designed to be easily modified for any a
quatic system, migratory life-stage, or trophic community. ROE was spe
cifically developed to understand how large-scale physical patterns (i
.e., tidal and freshwater intrusions) and landscape biological process
es (i.e., primary production and foraging behavior) control migration
of stenohaline fishes in the estuarine lagoon of Laguna de Terminos, M
exico. A migration response matrix for temperature, salinity, food ava
ilability, birth, and mortality was used to control cell-to-cell popul
ation movements. Internal cell processes included logistic population
growth, trophic interactions, and ecosystem feedback parameters. Outpu
t data maps from the ROE model showed how population spatial distribut
ions were linked to spatial and temporal patterns of water quality. Ho
wever, the most significant parameter affecting long-term population s
tability was birth rate; an internal cell variable. It was concluded t
hat the simulation of large, density-dependent, spatial processes such
as migration can be understood with a grid-based mechanistic ROE mode
l because its rule-based design for movement allowed organisms to resp
ond to ecological processes and adjust to changing environmental condi
tions.