An alligator gar floats near the surface in the outer moat of Nagoya Castle on April 28. (Provided by Shojiro Sakai)

NAGOYA--Constructed to ward off invaders more than 400 years ago, Nagoya Castle may have finally met its match in an elusive predator.

City officials have been unable to capture a massive alligator gar that was spotted on April 8 in the castle's outer moat for the first time since August.

The fish appeared to be about 1.5 meters long, about 20 centimeters longer than in August, according to Shojiro Sakai, 63, who snapped pictures and took video recordings.

In response to eyewitness reports, the city government installed a gill net twice in April in the moat but could not snare the gar.

Sakai, who searched for the fish from the ground through the winter months, didn’t spot it.

“An alligator gar might have remained still at the bottom of the moat due to the cold,” said Sakai, who also took photographs and footage last August.

An alligator gar was first spotted in the castle moat in 2009.

Speculation had spread that the fish may have been secretly caught by an angler at night because it had not been seen since last summer.

However, it is now believed that there may be two alligator gars lurking in the moat.

The Environment Ministry plans to designate the alligator gar, of North American origin, as an invasive alien species. The carnivorous fish, with its elongated mouth filled with sharp teeth, could prey on native species and pose a threat to the ecosystem.