Expect the radioman to hear targets within 40 km and less if you stop your engines and go a good amount below periscope depth, about 30 meters. It's important to change your sub aspect once in awhile so that all 'cones' have a chance to make a contact (your baffles +- 30° will not reveal contacts AFAIK).
A lot of fishing boats, yes but a lot of destroyers too, so stay alert because they can even hear you when you are surfaced...
If you want to get a target solution with hydrophones, there are 2 good tutorial videos I can refer you to:
4 bearing method for SH5 part 1-4.mkv by Stoianm
Hydro hunt tutorial SH5 part 1-2.mkv by Stoianm
Go to Youtube and search for them. It's really worth watching to learn functionality of crew commands.