Sea Turtles and Red Tide

By Allen Foley

Red tide often kills fish, but when the concentration of Karenia brevis reaches around 100,000 cells per liter, it can also kill sea turtles, birds, dolphins and manatees. In Florida, we have been documenting stranded (i.e., dead, sick or injured) sea turtles since 1980. We have documented unusually large numbers of stranded sea turtles coincident with red tides primarily along the Gulf coast (especially in the southwest) but also along a portion of the Atlantic coast (Brevard County). These strandings are typically adult and large immature loggerheads and Kemp’s ridleys, and small immature green turtles and hawksbills. Stranding data modeling and sampling of strandings to determine brevetoxin concentrations all indicate that red tides mostly kill loggerheads and Kemp’s ridleys. There are almost no strandings attributed to red tide during some years but there are many hundreds attributed to red tide during other years.

The latest red tide event began in southwest Florida during November 2017. Since then, we have attributed 589 stranded sea turtles (252 loggerheads, 265 Kemp’s ridleys and 72 green turtles) to that red tide bloom — the largest number of stranded sea turtles we have ever attributed to a red tide. The next largest groups of these stranded sea turtles were documented during 2006 (N = 345), 2003 (N = 230), and 2005 (N = 223).