There was the osprey, perched on a pole at Lobsterville, with its talons completely wrapped up in a snarl of monofilament fishing line. He should be catching fish and bringing them to his hungry chicks. But no, he was unable to get untangled, and this was the last time this adult osprey was observed.

Dick Jennings, who has been surveying ospreys on Martha’s Vineyard since 2005, reports that this is not an isolated event, that such needless mortality is becoming more common. In 2008 an unlucky juvenile at an Oyster Pond osprey pole was strangled after getting caught in tangled fishing line in its nest. More recently, an adult male at a Katama Bay pole perished with three chicks in the nest — its legs were so tangled that it could not even perch properly.

Such entanglements are not always fatal. Last year a male nesting on Cape Pogue was lucky; he managed to free himself from the tangled fishing line.

And in 2013 a Chilmark Pond female got entangled in a plastic grocery bag; she somehow managed to untangle herself.

So how does this happen? According to Mr. Jennings, osprey like to decorate their nests with debris, and sometimes that includes spent fishing line discarded by a careless fisherman. He believes that people fishing from shore generate most of this debris, since many boat fishermen use heavier line rather than the monofilament line.

An avid fisherman, David Nash, points to shore fishermen as the source of the discarded fishing line. “Every day I go fishing I find tangled and discarded line is more common as you get closer to popular fishing places,” he said. “That such human-generated trash causes this unnecessary mortality is just horrible.”

Both Mr. Jennings and Mr. Nash agree that most avid fishermen are responsible and safely dispose of their spent fishing line. The Martha’s Vineyard Surfcaster’s Association has a long history of trying to keep our beaches clear of spent fishing line. For a long time they have maintained boxes for spent fishing line at many popular fishing sites, which they empty regularly. And the surfcaster’s association distributed a poster in 2010 educating fishermen about the dangers of this spent fishing line.

Osprey populations have been increasing for the past 45 years. According to Gus Ben David, the noted naturalist and retired director of Felix Neck Wildlife Sanctuary, and Rob Bierregaard, a research associate of the Academy of Natural Sciences of Drexel Univeristy, who have studied Vineyard osprey since 1970, there were only two breeding pairs of osprey on the Island in 1971. Now there are at least 85 breeding pairs and the population continues to slowly increase.

Concluded Mr. Jennings: “There is no excuse for leaving spent fishing line on the beach, where it can needlessly catch and kill wildlife.”

Robert Culbert writes the Bird News column for the Gazette, leads Saturday morning tours and is an ecological consultant living in Vineyard Haven.