Fishermen made the horrific find of hundreds of dead fish and sharks tangled in an abandoned net drifting off Grand Cayman on Monday.

The “ghost net” may have been floating for several months, trapping and killing everything in its path, according to marine researchers.

The Cayman Islands Department of Environment scrambled the mosquito plane for a reconnaissance mission and issued an alert to all boaters Tuesday in an effort to relocate and potentially recover the net.

Dominick Martin-Mayes, one of the fishermen who made the initial discovery, said there were potentially 30 or more sharks trapped in the tangle of weighted netting that spanned 40 feet across and an estimated 40 feet deep. He said some of the animals caught in the net were so badly decomposed, it was impossible to tell what species they were.

Tim Austin, deputy director of the Department of Environment, said the marine growth on the net and the amount of creatures caught up in it suggested it had been floating for some time.

“If we can locate it, we can assess whether we can recover it or tow it somewhere to secure it till it can be dealt with safely,” he said.

“We don’t want it to end up in the reef and we need to stop its deadly ghost fishing.”

An image, taken by Mr. Martin-Mayes from beneath the surface, clearly shows an Oceanic whitetip shark trapped in the outer web of the net. He said there were also jacks, triple tails, big ocean turbots and a variety of pelagic fish caught inside.

The dead and trapped animals create a bloody chum slick which attracts sharks and other predators, and unless the net is recovered or makes landfall, it could drift indefinitely trapping yet more marine creatures.

“It is heartbreaking,” said Mr. Martin-Mayes, who is also a dive instructor on Grand Cayman.

“The fish come and eat what is there and get caught up in it themselves, so it just snowballs and becomes this gigantic floating net of death.”

The fishermen made the find around four miles off Grand Cayman, just before 5 p.m. Monday.

Mr. Martin-Mayes and his friends free-dove on the net and freed some of the fish that had been recently trapped. But they were unable to continue with the operation because of the risk of getting caught in the net themselves or being attacked by an injured shark or another predator drawn to the chaos.

The fishermen attempted to drag the net back toward land, but it was too heavy for them to move. Mr. Martin-Mayes said he had informed the Department of Environment of the coordinates in the hope that they could recover it.

Researcher and conservationist Guy Harvey was also out on the ocean, Monday, tagging Oceanic whitetip sharks as part of a conservation project.

He said he had not been informed of the net until much later and missed the opportunity to go to the scene.

From his examination of the photographs, he said the barnacle-covered net could have traveled for hundreds of miles.

“They are so destructive because when the equipment is lost, it continues to kill. From my assessment of the conditions of the net, it has been out there a long time.”

He said the strength and speed of the currents would make the net difficult to locate again and it could continue to drift, causing serious damage to marine life.

Mr. Austin said it was not clear what type of net it was. He said it had floats on and appeared to be fairly coarse, suggesting it may be a kind of seine net, used to encircle large schools of fish which are then pulled on board a boat.

He said, “Nets are regularly dumped at sea when they wear out so it’s difficult to determine if this is lost gear or dumped.”

The Mosquito Research and Control Unit, which has a light aircraft used for spraying for mosquitoes, loaned its pilots and its plane to the Department of Environment Tuesday for a reconnaissance flight, which was still ongoing at press time.

Mr. Austin said the department had also alerted marine police, the Port Authority and the boating community in an effort to locate the net.

Mr. Martin-Mayes said the find was horrifying but he hopes it will draw attention to the dangers of both unscrupulous fishing practices and the dumping of discarded nets and other dangerous trash in the ocean.

“When you see this sort of thing on television, on documentaries,” he said, “you think that’s terrible, but when you see it firsthand, it is genuinely heartbreaking. The sad thing is it is not likely to be the only one out there.”