I work for a British agency that does this and I'm not entirely sure! (I do IT support).
I know that we develop tiny tags that can be attached to fish to study their (surprisingly unknown) movements, along with rewards for anglers that catch and return them.
I know that we have a lot of dedicated scientists genuinely passionate about protecting fish. We have records of fish studies going back to the 19th century from steamer research vessels that have resulted in enforcement of fishing law in Britain to protect fish - often unpopular in the UK press!.
Sorry, to go back to your original question as far as I know attaching micro tags to fish is the only way to track their movements along with long term historical records of observations, regular test trawling, sonar observation and behaviour observation in labs.