This incredible video shows a frenzy of around 50 sharks swarming at RSPB Medmerry nature reserve near Selsey.
The unprecedented sight was captured on video on Wednesday morning by RSPB warden Peter Hughes.
He said: “It was astonishing! There were just these huge fish everywhere in knee-deep water.”
The sharks measured up to 5ft in length and were identified as smooth-hounds, a type of shark usually found offshore in coastal waters.
A school of sharks this size has never before been seen at Medmerry, though there were a couple of sightings in the area last month.
Up to two smooth-hounds were seen in Medmerry in mid-June, while six were seen in nearby RSPB Pagham Harbour around the same time.
And there is advice for anyone hoping to see the sharks for themselves.
“We do not know if the spectacle is going to repeat itself,” said Pete.
“Those wanting to look for the sharks can view from the new footpaths around the perimeter of Medmerry.
“Anyone new to the area should pick up a trail map and get advice from the RSPB Pagham Harbour visitor centre at Sidlesham.”
He advised visitors that there is no access from the paths to the water’s edge.
Smooth-hounds are a relatively common native shark species around British coastal waters, although they are rarely seen.
They are known to feed largely on crabs and other invertebrates, and often travel in groups, growing up to 5ft.
Medmerry is the largest open-coast managed realignment scheme ever in the UK, and since being created by the Environment Agency between 2011 and 2013 has been a huge success.