So this is something I work with directly, but for some other agency.
First off, a big reason as to why - is because the laws differ from country to country. Some countries, like Norway for example, require shipping vessels over a certain length to broadcast their positions through AIS, VMS.
This may not be the case for, say, UK.
There are arguments to be had from both systems as to WHY you wouldn't want to broadcast your positions at all times - a typical one is that other could easily infer your fishing fields (fishing patterns are trivial, if you've worked with this you can easily spot what tools they are fishing with, and likely what species they are going for - don't need ML-based systems for that. Any fisher or fisheries analyst can spot the patterns), and thus go after that.
The other is privacy.
A very typical thing is that ships turn off their AIS as soon as they enter international waters. There is no enforcement of that, and many developed countries have practically zero resources to fight illegal fishing, from a technological point of view. UNDP has a program which is aimed at helping developing countries with the tech and training to detect illegal fishing, but there's a long way to go. Developing countries desperately need the data, which is either owned by governments, or private actors. AIS data is either picked up by satellites, or base stations. VMS systems are expensive, but also allow for ships to transfer catch reports and similar - but is unfortunately not always enforced in a good way.
But tech is becoming better. Satellites with NAV/marine radar detectors are in orbit. Long-range drones with sensors are a thing. Countries are hammering through laws that force ships to have certain sensors on them. Lots of ML-assisted tools for automating detection and analysis is being introduced and used.
First off, a big reason as to why - is because the laws differ from country to country. Some countries, like Norway for example, require shipping vessels over a certain length to broadcast their positions through AIS, VMS.
This may not be the case for, say, UK.
There are arguments to be had from both systems as to WHY you wouldn't want to broadcast your positions at all times - a typical one is that other could easily infer your fishing fields (fishing patterns are trivial, if you've worked with this you can easily spot what tools they are fishing with, and likely what species they are going for - don't need ML-based systems for that. Any fisher or fisheries analyst can spot the patterns), and thus go after that.
The other is privacy.
A very typical thing is that ships turn off their AIS as soon as they enter international waters. There is no enforcement of that, and many developed countries have practically zero resources to fight illegal fishing, from a technological point of view. UNDP has a program which is aimed at helping developing countries with the tech and training to detect illegal fishing, but there's a long way to go. Developing countries desperately need the data, which is either owned by governments, or private actors. AIS data is either picked up by satellites, or base stations. VMS systems are expensive, but also allow for ships to transfer catch reports and similar - but is unfortunately not always enforced in a good way.
But tech is becoming better. Satellites with NAV/marine radar detectors are in orbit. Long-range drones with sensors are a thing. Countries are hammering through laws that force ships to have certain sensors on them. Lots of ML-assisted tools for automating detection and analysis is being introduced and used.