In the early days, the state of Utah used to stock lakes with fish by hauling small fish in metal milk cans on horseback, then by off-road vehicles and trucks.
Image caption: To reach remote lakes, horses were loaded with metal milk cans filled with fish and water. Image credit: Mount Rainier National Park Archives
By the 1950s, small airplanes became the preferred method. In this Facebook post, Utah Division of Wildlife Resources posted a video clip of how 200 high-elevation lakes across the state were stocked with fish. They noted that these lakes weren't easily accessible by vehicle, so air drop of fish is the preferred method.
The fish dropped were between 1 to 3 inches long, so they "flutter down" slowly to the water.
An airplane can hold hundreds of pounds of water and 35,000 fish in a single flight without reloading.