Experts say the algae bloom known at red tide has showed up in high concentrations on the beach at South Padre Island.
The Texas Parks & Wildlife Department said water samples show lesser concentrations near the South Padre Island convention center.
Fish kills were confirmed Monday in the area, plus at Padre Island National Seashore, Boca Chica beach and Port Mansfield.
South Padre Island, a year-round beach spot and spring break destination, was excluded from Saturday's coastal beach cleanup for safety reasons due to red tide.
The reddish bloom can kill fish and lead to shellfish poisoning in humans who consume it. Red tide aerosols, kicked up through wave action, can cause breathing troubles in humans.
Signs of red tide were also confirmed near Pelican Island and at Port Aransas.
