Use an NFC tag for each fishing net, which would then be traceable (manufacture, sale, use, possible loss).
The complete lifecycle of the net could now be recorded as it is in a closed cycle.
If the net is lost, two possible effects are created. As the net’s last location of use was documented, its position can be tracked, facilitating targeted recovery. The deposit can also be motivation for other parties: this could encourage the removal of large numbers of nets from the water - with a profitable reward.