Individual-Based Fish Population Modeling Using Reach- and Network-Scale Physical Information

Spatially explicit, individual-based population models can be useful for linking synoptic mapping and hydrodynamic modeling to population dynamics and management decisions. Incorporating realistic temporal and spatial variation in physical habitat is a key element of these models. Efforts to apply individual-based models to stream fish are benefitting from increasing capability and effort to capture temporal variation in key physical variables such as streamflow, temperature and turbidity. To capture spatial variation, individual-based models of stream fish commonly use a grain (habitat cell) size on the order of 1 to 10+ square meters and cover reach lengths from 100 - 1500 meters. Through their capability to accept habitat cells that can individually vary in shape and size, some models can use topographic and hydraulic information from hydrodynamic models or data interpreted via GIS. In addition, larger scale spatial variation can be incorporated through the ability to link model reaches into river networks. A recent exploration of the effects of habitat fragmentation on resident trout demographics utilized this network capability.

For stream fish, passage barriers are a particularly visible cause of fragmentation often targeted for remediation. We used an individual-based trout population model that represents barriers by assuming trout cannot move upstream past a barrier, and move downstream over a barrier only if habitat above it offers only low expected future survival of starvation and predation. We simulated the stream network and a trout population in a catchment of about 25 square kilometers over 78 years, varying barrier density and observing effects on population stability properties and demographics. Increasing barrier density decreased sub-population persistence in first-order tributaries but not larger streams. Barriers reduced overall abundance and biomass at intermediate and high densities, and caused a small but surprising increase in biomass at low density. Fish that passed over barriers contributed relatively little to downstream populations but, had they not moved, would have increased persistence and abundance of the tributaries they moved from. Finally, differences in life history characteristics emerged from habitat differences: compared to those in larger channels, fish in isolated tributaries had about 2.5X higher survival of eggs to age 1, smaller spawners and smaller size at a given age. The results were not highly sensitive to the tendency for fish to move downstream over barriers.