The Texas Parks and Wildlife Department offered the estimate after a flyover Thursday in the Galveston area.
Tens of thousands of dead fish began washing up on beaches last weekend. Water samples collected Monday confirmed the red tide at various sites in Galveston Bay.
Wildlife experts Thursday assessed about 20 miles of beach shoreline, including Bolivar (BAW'-luh-vuhr) Peninsula, Galveston and Surfside. Monitoring continues in areas adjacent to the confirmed red tide outbreak.
Parts of Galveston Bay have been closed to shellfish harvesting because of the algae bloom, which can cause respiratory problems.