The likely source of the poisoning is believed to be live trout on which the bears were feeding.
(The likely source of the poisoning; is believed; ) (The likely source of the poisoning; to be; live trout on which the bears were feeding) (live trout; were feeding; the bears)
The likely source of the poisoning The likely source of the poisoning live trout
is believed to be were feeding
live trout on which the bears were feeding
the bears