The most direct way to solve this would be to put a small GPS receiver, along with a microprocessor and a medium-range wireless transmitter of some sort, on each buoy — or on enough of them to get a sense of the shape of the net.
Each unit would have to be battery-powered, and power management could be "interesting". It would partly depend on how much latency you can tolerate. Can you stand to wait, say, 10 minutes before detecting the fish? If so, put the entire system in a low-power state for as long as possible.
At your camp, you would have a receiver, and you'd plot the coordinates that the buoys are transmitting on your laptop (or whatever). The differential errors among the receivers should be small enough to let you distinguish the shape of the net. Common-mode errors don't really matter in this application.