[mysterio]

Pleisaurs aren’t swimming around in lakes. For there to be one, there must be a population of them.

[JoeProBono]

"Above: Photos of Lake Champlain, at Burlington Vermont. We went on the boat tour from Burlington; on the tour they said this rock in Lake Champlain in the second photo was considered to be a deity by the indians. We did not see Champ, however, although on the boat they said that once several years ago Champ had been spotted during the boat tour. If you visit Burlington, be sure to go on this boat cruise, and also visit the lake aquarium in Burlington to learn about Champ. "

[Isabel C.]

Pleisaurs aren’t swimming around in lakes.
For there to be one, there must be a population of them.

Your two statements appear to be mutually exclusive. Perhaps there is a population of Pleisaurs swimming around in lakes.