Other Organisms: Relative to other organisms, a variety of piscivorous fish eat trout. We would further research the number of animals that eat trout, and how they would be affecting the population of trout. In the year 1940 piscivorous fish eating trout was a problem.
Food: Trouts eat a host of aquatic insects, terrestrial insects, other fish, crustaceans, leeches, worms, and other foods. This could have happened in the 1950s, researching the population of their food supply will definitely help in the case of population growth and decrease.
Environment: Trouts need cold water, clean water, food to eat, places to hide from predators, and clean gravel to lay their eggs to survive. In 1960s, people might have destroyed their environment, so the population decreased. We could find how and why their natural environment was destroyed.
Reproduction: Female usually give birth to their young between November and January when the water is cold and carrying lots of oxygen, because that is when eggs need to hatch. The population could have increased, in 1970, or decreased due to climate change. Studying a massive change in climate will further benefit our research.