I was just watching this episode and he keeps talking about medieval stories of children being taken by river monsters in Germany. He shows a couple late medieval illustrations.
He seems to find it plausible that these fish could at least drown a person, if not eat them. They grow far bigger than the one he caught. Two of the fish he caught in the episode did turn and try to bite him when they were released so they are aggressive toward people. You can find the series on the cable channel called Animal Planet.