The USCG just busted a Chinese vessel about 400 mi W of Hawaii with illegal
drift nets. Items seized included "drift nets in excess of 10 miles each in
length and bouy transponders". Taiyo and others make them. They are pinged
by a coded HF signal and reply with a GPS position. If you are lucky maybe you
can buy the gear when it is auctioned by the govt. These transponders make the
old net buoy AM beacons obsolete. They used to operate just above the AM
broadcast band and generally below 2 MHz. ADFs were used to locate them. These
old beacons and the ADFs show up on eBay from time to time.