Answer:
Rise in temperature.
Step-by-step explanation:
Increase in temperature is responsible for the decline in the population of trout because trout needs low temperature for better growth and survival. Due to more consumption of fossil fuels in vehicles and industries, more carbondioxide gas is released which causes air pollution. This air pollution increases the temperature of the earth surface known as global warming due to which the cold areas become warm and as a result the trout population decreases.