most of the retention ponds in south florida are connected to canals through underground drains. its likely connected to a canal which eventually leads to saltier water.
Tarpon have a modified swim bladder, they can 'breathe' air. When they go into the mangrove swamps at low tide they clean house because all the baitfish are sluggish due to low oxygen levels. Cool fish, saw someone on here with them in a fish tank a couple weeks ago.