Swathes of dead fish, mostly bait fish called menhaden, began washing ashore last week at Sebastian Inlet State Park, TCPalm.com reported.
Carli Segelson, a spokeswoman for the state's Fish and Wildlife Research Institute in St. Petersburg, said researchers were trying to determine if the low oxygen levels were caused by a proliferation of marine algae.
The institute dispatched an inspection team from its research field station near Melbourne to collect water samples at the state park for analysis.
Vero Beach resident Mary-Frances Womack, who went to the park with her brother Tuesday, said the smell of decaying fish was so strong "it irritated my throat."
Womack and her brother estimated the number of fish as being in the millions.
Since the first reports of dead fish last week, the die-off has not appeared to have increased, Park Manager Terry Coulliette said.
There are no plans to remove the fish that have washed up on shore or are floating in the waters, Coulliette said.
"Nature is taking its course," she said.