Fish carriers generally report on AIS because they are large enough to be required to carry an AIS transponder (i.e. above 300 tonnes on an international voyage or 500 tonnes regardless of their course).
When fish carriers meet a fishing vessel for transhipping or the exchange of goods and crew this is flagged as an encounter if the fishing vessel is also reporting on AIS.
In some cases, fishing vessels are not required to report on AIS because they do not meet the minimum size threshold. If a fish carrier encounters a fishing vessel that is not reporting on AIS this may appear as a loitering event.
Try it out: See a reefer vessel loitering near the Argentinian EEZ boundary where it is likely meeting another vessel for transhipment or exchange of goods and crew.