Shark watchers are hoping a tag like this will be found
Beachcombers could earn themselves £200 if they find a satellite tag which has fallen off a 7m-long (23ft) basking shark called Colin.
A shark watch organisation is offering the reward after its tag came loose from Colin about 20 miles from Skomer island, off the Pembrokeshire coast.
They say the tag, which costs £2,000 and had been attached to Colin for 77 days, can be put on another shark.
The tags help scientists learn about basking shark migration patterns.
The Manx Basking Shark Watch, which is based on the Isle of Man, issued a Google map of the last known position of the basking shark's tag.
Jackie Hall, who runs the shark watch, said they believe it is very close to the mouth of Milford Haven estuary.
"The battery is fading and the position it is giving us is no longer accurate," she said.
"It may be lying on its side on a beach."
The shark watchers' website said the tag was still transmitting its data to a satellite and would tell them where Colin had been over the last three months.
"It will also tell us how deep he has been and what the water temperature was," said Ms Hall.
"This information is of great interest to us as we can find out more about basking shark migration patterns.
"This will help us to develop more effective protection for these endangered animals.
"It will until its battery runs out in about 7-10 days. We will then download the data and find out where Colin has been for the last 77 days."