Monkeys were left in an island after the completion of several Tarzan movies in the 1930’s. The Production company thought they would stay on the island and be contained. I guess no one told that rhesus monkeys can swim.
Thats one theory. Another is, an eccentric boat captain released them over 100 years ago.
They are rhesus macauqe monkeys from Southeast Asia.
There are days you can paddle all over the river and see none. Other days they can be spotted.