AIS (Automatic Identification System) has been used since the 1990s to track all vessels over 300GRT - including fishing vessels. Initially the AIS coded VHF signal was only received by shore stations (with limited range for ocean-going vessels), but since ~2008 there has been satellite reception.
So theoretically, one should be able to track at least where these boats fish.
However, as with all things illegal - there are ways around these requirements - the simplest method is to turn off the AIS. Other more complex methods spoof the lat/longs.
So... w/o boots on the ground - tracking illegal fishing using AIS can have limited success - if not ineffective.
That's why these enforcement patrols are needed. BUT...
Any one country's jurisdiction only extends out to the EEZ - 200km, and there are many illegal fishing boats/activities well beyond that arbitrary line....