I've fished in Brazil, in China, and now in the United Stated. I've seen big fisheries suffer from over-harvesting (not even by poachers) and eventually dying over the years. My father always tells me on the phone: "some of our wonderful spots in Brazil are now empty. There's no fish left." I've seen non-point source pollution happen on a daily basis, polluting streams and rivers until no life was present anymore. Even more tragic -- I've even seen "dams of trash" blocking the water of streams, back in the days.
Note that you can also just type in "Riacho Grande" on your Google Earth, and locate the portion of the map below, so you can have a magnified view of it. If you do so, you will notice that there are 4 pumps just below the number "2" in the map, which pump water from Billings to Guarapiranga and vice-versa, so the levels of water between both reservoirs are closely the same (notice that there's land below that bridge; therefore, the water between both reservoirs are divided). The neat fact about this little fishing place is that the water would always overflow after heavy periods of rain, meaning that the pumps would get entirely covered, and fish from Guarapiranga and Billings were able to pass from one side to the other! I guess my father was an interesting person even when it came to choosing his fishing spots.