Malaysia detain 6 Chinese fishing vessels

[ombugge]

6 Chinese fishing vessels has been detained off Johore for being in Malaysian waters:

Malaysia detains 60 Chinese nationals, 6 vessels for trespassing

It appears that they were not fishing but on lawful passage through Singapore Strait:

The MMEA said the vessels, which had no cargo when detained, were believed to have been en route to Mauritania but had to stop due to some malfunction.

When are fishing vessels just ordinary vessels on passage?