A 300m exclusion zone has been imposed around a whale which remains trapped in Larne Lough, County Antrim.
It will be an offence for any vessel to stray into the area where the 30ft mammal has been stranded for two days.
A rescue boat managed to get the whale out to open sea on Wednesday, but it returned confused and disorientated when two ferries left the harbour.
An Environment and Heritage Service spokesman said they would try to guide the whale out to sea on Thursday.
Police have asked owners of any type of vessel planning to launch at Ballylumford beach to stay on shore and not to sail.
Originally thought to be a minke whale, the mammal is now described as being from a species of large whales.
On Wednesday, hopes were raised that the animal had made its way back to sea after it had not been spotted for several hours.
However, it resurfaced at about midday further up the lough close to a cement works.
The animal is swimming in tight circles, which experts say is a sign of confusion.
Vets have been put on standby in case the whale beaches.
A team from the Irish Whale and Dolphin Group has been trying to guide the whale back out to sea.
Peter Steele of the group asked boats to stay away from the Larne Lough area as they fear noise will confuse the mammal further.
On Tuesday, the whale became trapped for a time behind some of the legs of Ballylumford power station, which is situated on the opposite side of the lough facing Larne harbour.
Scientists believe the current heatwave may be contributing to unusually large numbers of whales and dolphins off the UK coastline.
It was originally thought that the mammal was a minke whale, but the Irish Whale and Dolphin Group now believes it is more likely to be a fin or a sei whale.