The RSPCA has been left heartbroken after a rescue team was called to help a swan caught in illegal nets in a lake near Thetford, but couldn't save its mate.
Both birds were caught in netting at Lynford Lake, Mundford, which is on Forestry England land in March 3.
They were spotted in the middle of the lake by members of the public who reported one swan appeared entangled and distressed.
Firefighters from Norfolk Fire and Rescue used their water rescue boat to reach the stricken swan which was uninjured.
But the second swan which was also caught up in the netting had drowned.
"It is really upsetting that another swan has likely died as a result of this netting in the water - especially given that it should never have been there," RSPCA Inspector Jon Knight said.
"The netting was of some considerable size and was extremely heavy - which is why the poor swans could not escape," he added.
The police are investigating and urging members of the public to be vigilant in the area.
"The area of Lynford Lakes is protected under the SSSI scheme (Site of Special Scientific Interest) and as such, no genuine organisations would use nets like these for any purpose," PC Les Maguire said.
"We suspect the nets have been placed there in efforts to illegally poach fish. We will be increasing our patrols in the area and would appeal to anyone with information to come forward to help identify and trace those responsible," he said.