The East River’s “Swan Lake” story has ended in tragedy.

A rock-wielding, real-life “Rothbart” is believed to have ruthlessly murdered one half of the beloved swan couple that calls the river home — raising concerns about his bride as the birds mate for life, The Post has learned.

The majestic waterfowl, named Swanny, was euthanized late last month after veterinarians determined he had suffered extreme and irreversible head trauma, animal experts said.

“That is very easy prey for sick individuals or irresponsible — all the adjectives you could put — because I don’t forgive ignorance,” said Rossana Ceruzzi, founder of the Wildlife Freedom Foundation on Roosevelt Island.

Ceruzzi said she was worried about how Swanny’s mate, Odette, will fare with her other half now gone.

“It’s painful,

See Full Page