Extensive fish stocking was initiated by state and federal agencies starting in the late 1800s, and continues today in many of these lakes. Concern about the effects of these introduced fish, mostly trout, on native aquatic wildlife has resulted in two USGS studies at the North Cascades National Park Service complex and the Mount Rainier National Park in western Washington. Scientists have determined that high densities of introduced trout can change the food web of the lakes. Some types of large-bodied zooplankton become scarce or absent in the presence of trout, as is the case for some salamander larvae. Successful efforts to remove trout from a few lakes have resulted in an increased density of salamanders.
Research is ongoing to determine the full scope of the problem.