Objectives and Scope


This report presents the results of a modeling study of the transport of larval suckers from spawning grounds in the Williamson River, through the Williamson River Delta, and into Upper Klamath and Agency Lakes. Water currents simulated with a calibrated hydrodynamic model for the Upper Klamath and Agency Lake system (Wood and others, 2008) were used in this study to transport the larvae through the combined Upper Klamath Lake/Agency Lake/Williamson River Delta system. This modeling approach was used to address three objectives. First, the effect of opening up both sides of the Delta on larval transport and on larval catches at the mouth of the Williamson River and along the shorelines in Upper Klamath and Agency Lakes was described. Second, the implications of nighttime-only drift behavior for sampling strategies at larval catch sites within the Delta were examined. Third, the accuracy of the model assumptions was assessed by comparing model simulations to measurements of larval density. Qualitative and quantitative (using rank order correlation analysis) comparisons were made between the simulated results and larval catches. Several examples of qualitative comparisons between simulated fish densities and model simulations were found in the literature (Ellien and others, 2004; Arnold and others, 2005; Petrone and others, 2005, Mariani and others, 2010); however, attempts to quantitatively assess the performance of a model in simulating fish densities by comparing to measurements from net catches as done in this study appear to be rare. 


In pursuing these objectives, we used a density (advection-diffusion equation) approach. The density approach is limited to describing the advective transport of larvae by currents, in combination with passive diffusion, and therefore cannot be used to simulate the active dispersal resulting from horizontal swimming. Nighttime-only drift was incorporated, even though it is non-passive behavior, by “freezing” the density in the river channel during the day. The advantage of this approach is that it yields concentration information at all locations within the numerical grid at fine temporal resolution, and therefore allows the comparison of simulated results directly to larval catch densities. 


First posted April 2, 2012