Well I've been thinking for a long time about how to track fish in canals here in England. Reason being they move around a tremendous amount and are generally very hard to find, although they are often present in large numbers when discovered. This makes the fishing very exciting for about 1% of the time. No way am I smart enough to invent or build anything which could do this, but I was wondering where I could obtain a suitable device from. It doesn't have to be mega hi-tech, nor does it have to transmit over big distances as I know the length of canal I'm looking at between locks and I'm prepared to walk the entire length of the stretch to find the fish. This means it only needs to transmit say 20 feet or so, maybe a bit more (say 50ft). It also needs to be very small so as not to hinder the fish in any way and ideally not require it's own power source. I really need one of those things which only transmits back when hit with a signal itself but if I had to put a small battery in so be it. I know these things are out there and are publically available so anyone know where I could start looking to get one? I know it's a weird question, but variety is of course the spice of life.