MANATEE — A large patch of red tide off Charlotte and Sarasota counties is moving north, causing fish kills over the last couple of days, a scientist said Wednesday.

A couple of samples taken in Tampa Bay showed low concentrations of the red tide organism, said Alina Corcoran, research scientist for the Florida Fish and Wildlife Conservation Commission in St. Petersburg.

However, the biggest patch is off Sarasota County, and as visible in satellite images,it is moving north, said Corcoran.

“In Sarasota County, we are seeing medium concentrations there,” she said, adding that there are reports of “a rather massive redfish kill.”

Red tide blooms result when the water temperature and salinity reach a certain point, she said. In enough concentration, it can cause respiratory distress, which people have reported in both Sarasota County and around Anna Maria Island, she said.

Asked whether she expected the large patch of red tide to migrate to Manatee County’s shores, Corcoran said, “It has been moving north pretty quickly.”

“We’ll have more information tomorrow, once samples are processed.” Sara Kennedy, Herald reporter, can be reached at 941-745-7031. Follow her on Twitter @sarawrites.com.