Always there, its just now your standing up you can see them. My dad told me stories about surfing Byron on his mal in the 60's and seeing schools of small sharks and a few larger ones when catching those really long rides. I think there was a meat factory further north in the bay that keep them in the area though.
I often see schools of fish and the thought always goes through my head that something bigger is going be lurking about trying to eat those fish. Keep your eyes about you but don't let it become a paranoia.
Historically, sunrise, sunset and murky water are the danger times. Avoid those and you should be fine.....but no guarantee

Got to feel safer standing up then all those guys on shortboards or lids though