Why would a farmer fertilizing his field on a hillside concern a fisherman that works on a lake kilometres away?

This is due to the farm runoff, or fertilizer run off

Explanation:

Various fertilizers like nitrogen and phosphorus are very essential for plant growth but it also increases the chances of the algal growth in the water bodies when washed away. This happens when intense rainfall occurs and washes away the nutrients and chemicals to the lakes, and many water bodies that has not been taken by the plants. these chemicals and nutrients affects the aquatic ecosystem. Due to toxicity various fishes die, hence a threat to the fisherman.