Scientists often spend time studying the various biotic and abiotic components that make up the ecosystems. One important element of an ecosystem that scientists may be interested in is animal populations. They may want to know how many individuals of a particular species reside in a specific habitat.
Use this PowerPoint with "An Ecosystem Study: Determining Trout Populations," which may be found in my store. It provides visuals that can be used with the activity "Determining Trout Populations: Capture, Count, Release."