How does it reproduce? It's been around for an awfully long time.
Alten, on that program, said that eels live 80-100 years. He said that mariners from Columbus to others came across what they would sometimes call "sea monsters" in the Sargassos Sea (which isn't too far from Scotland) which Alten surmised were these giant aggressive eels.
He believes one may have somehow become locked within the loch. One of the theories as to why it was an eel, he said, is a reputed tooth found in a half-eaten deer on a loch beach. A picture of the tooth found, when compared to an eel tooth, apparently was similar. Such eels, said Alten, apparently "lock on" to an object when bitten to. [There, I've managed to use the work "lock" twice and "loch" twice!]