Biologists use two main factors to determine trout stocking dates: the mortality rates among wild and stocked fish and how well a stream can support the trout population. The stocking date also depends on water temperatures in rivers, lakes and streams; therefore, it can vary widely from state to state.
Biologists also choose which rivers to stock and when to stock them based on the demand among trout fishermen and fish catch rates. They gather this information through creel surveys, in which anglers share details about the trout they have harvested. Scientists collect this data and make their estimations twice a year: once during the early part of the season and once late in the season.
Trout survive best when their habitat is not too hot; therefore, most stocking dates occur during cooler weather in the spring and fall. Trout hatcheries offer different ages of fish at different times of year, with the youngest fish stocked in the fall and the older fish in the spring. This ensures the best survival chances for the trout population. Ice in the winter months prevents cold-weather stocking in some cases. There are fisheries that do not require stocking; these fisheries can support adequate wild populations of trout.