Catch and release is the norm, during most of the year, but again, there are certain periods of time when a particular stretch of river may open for fishermen to keep what they catch. Due to low numbers, all cutthroat trout are catch and release only. Due to an overpopulation, a population that was illegally introduced, all lake trout are to be killed when caught. Your safety cannot be guaranteed. Bison are the most dangerous animal in Yellowstone and on average there are 3 deaths each year due to visitors approaching too closely.
Elk are also dangerous during the fall mating season and many visitors are injured each year.
High-powered spotting scopes and binoculars are a must when viewing wild animals. Keep a safe distance from all wildlife. When photographing wildlife, a lens in the 600 mm to 800 mm range is a must. Feeding any animal in Yellowstone is forbidden. With the focus on preservation through education.