State Environmental Police and federal fisheries regulators are trying to figure out who dumped the headless fish, which had to be hauled out of the trees by a tow truck.
Authorities won't say exactly when the tuna was found or who tipped them off.
But Ally Rogers, a spokeswoman for the National Oceanic and Atmospheric Administration Fisheries Office for Law Enforcement, tells the Gloucester Daily Times that the fish was illegally harvested.
The tuna season runs from early June to November.
Maj. Patrick Moran of the Environmental Police says he's never before had to investigate a tuna in the woods.
Information from: Gloucester (Mass.) Daily Times, http://www.gloucestertimes.com
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