The farmers from the Midwest in the United States are located in the drainage area of the Mississippi River system. The Mississippi River gets all the water from the region into the Gulf of Mexico where it ends its flow. Because for the farming the farmers are using lot of synthetic fertilizers, pesticides, herbicides, and fungicides, they are managing to pollute the water, be it the surface one, or the ground one. All of that polluted water ends up in the Gulf of Mexico.
The pollution affects the marine life very badly, including the fishes, so their numbers drop significantly because they are dying out from the pollution. That will lead to a situation where the fishermen from the Gulf of Mexico will be left without fish for fishing, thus without profit and without an important food source.