Adam Cook, who lives in Daventry, said the dramatic theme tune from Jaws was swirling in his head as he spotted massive fins emerging from the water late Wednesday afternoon.
He said: "It was my first time out on the reservoir after obtaining the correct permits off the council, so I was little cautious.
"I was sat out pretty much in the middle at the deepest part when all of a sudden I saw these large fins break the water and slowly move towards me, (theme tune of jaws was going off in my mind). But the creature slowly swam past at which point I began to follow, getting little snips of footage while I could.
"I've shown the footage to a lot of my angling pals who none of which, can 100% identify it. Possible suspects are trench, bream or a discarded large aquarium fish. I've been on the water every day since and am just getting ready to go up again this afternoon, but yet to spot the beast again."
Have you seen the 'beast' or can you identify it? Please email [email protected]