The terns currently around the Po Toi Rock are three species, Bridled, Black-naped and Roseate.
Most of the terns in your photo are Bridled (the dark ones), the white ones are probably mostly Black-naped but with some Roseate.
See my report on Po Toi this week here
http://www.hkbws.org.hk/BBS/view ... 1557&highlight=
I don't think the fishing boats off-shore disturb the terns - they are often in a wheeling mass at this time, nesting has mostly finished.