The bodies of six dead basking sharks, including five in one week, have been washed up on the coast of Cornwall.
Basking sharks are seen of the South West coasts every summer
The locations where they were found include Gerrans Bay, Coverack, Roseland Bay and in the Fal Estuary.
The Coverack shark was entangled in buoy ropes and drowned, but why the others died is a mystery and worrying, the Marine Conservation Society says.
Environmentalists fear they may have been fishing net victims. Fishermen deny being the cause of any deaths.
The latest body, found on Wednesday at Perranporth, was 6.1m long - the biggest of all the strandings.
Dr David Sims, a research fellow with the Marine Biological Association, said: "We've been studying shark movements for about 10 years.
"Anecdotally, there does seem to be a higher incidence of strandings this year compared with the previous 10."
Joana Doyle, of Marine Conservation Society, said: "Five strandings in the space of one week is highly unusual and very worrying.
"It highlights the importance of reporting strandings and entanglements as soon as possible so that we can determine what is going wrong."
Fishermen have already been blamed for dolphin deaths around the South West's coast because of pair trawling.
A volunteer from the Cornwall Wildlife Trust, Caroline Curtis, said she was sure fishing boats contributed to the shark deaths.
She said: "They didn't show any signs of illness or disease, and I believe they were all quite young animals. I can't see any other reason why they died."
However, pair trawler crews have denied they are responsible for such sea animal deaths.
Joana Doyle said: "Basking sharks are protected in the UK, but fishermen should understand that they have not committed an offence if they accidentally entangle a shark.
"We urge all sea users to report shark entanglements as soon as possible."