Turning red tides for Kemp’s ridley turtles
By Conservancy Research Manager Dr. Jeff Schmid
Kemp’s ridley sea turtles inhabiting the waters of Southwest Florida were hit especially hard in October 2019. According to Florida Fish and Wildlife Conservation Commission stranding data, a total of 56 marine turtles were recovered in Collier County during October through the beginning of November and, of this total, 49 (88%) were Kemp’s ridleys. There were no Kemp’s ridley strandings reported in the region during September or the latter weeks of November. The unusual increase in October strandings corresponded to a bloom of the harmful algae Karenia brevis, known as “red tide”, along the coast.
Blooms of red tide algae are a naturally occurring phenomena and take place annually, though the conditions and subsequent effects in Southwest Florida coastal waters vary from year to year. During late 2019, the harmful algae were first noted off Naples beaches in water samples collected in September. The bloom shifted southward during early to mid-October and there were medium (the concentration at which causes mortality in marine life) to high cell counts recorded from Marco Island to Cape Romano. The red tide algae were not present in water samples collected in eastern Gullivan Bay; however, sample collections are limited both spatially and temporally in this more remote region of the Ten Thousand Islands. The bloom shifted northward in late October and then persisted along the coast from Sarasota to Cape Romano.
Gullivan Bay, in the upper region of the Ten Thousand Islands archipelago, is an important feeding ground for critically endangered Kemp’s ridley turtles. Growing evidence suggests turtles become susceptible to the effects of red tide once the powerful neurotoxins (brevetoxins) become incorporated in the food web. Our ongoing diet studies in the Ten Thousand Islands indicated Kemp’s ridleys were consuming sandy-skinned tunicates (Molgula occidentalis, aka “sea squirts”) before and during the current red tide bloom. Tunicates are known to filter the harmful algae from the water and incorporate the toxins into their internal organs. The habit of opportunistically feeding on these sessile invertebrates may put turtles in harm’s way during a red tide event. Providing support for this hypothesis, a badly decomposed Kemp’s ridley recovered by staff at Rookery Bay National Estuarine Research Reserve appeared to have fed on sandy-skinned tunicates prior to its demise.
Our continuing satellite tracking studies of Kemp’s ridleys in the Ten Thousand Islands resumed in October 2019, at the time of the increase in turtle strandings and the onset of red tide conditions along Southwest Florida beaches. As indicated, turtles captured in the backwaters of eastern Gullivan Bay had been feeding on sandy-skinned tunicates and crabs but none of them showed any physical signs of red tide exposure (lethargy, lack of coordination, muscle spasms). That is not to say these turtles did not have the brevetoxins in their system. Studies of free-ranging turtles during red tide events indicated Kemp’s ridleys may have elevated levels of the toxins in their blood without showing obvious neurological symptoms. Nonetheless, the track of one of the Kemp’s ridleys named “Pax” suggested that turtles venturing to the western region of Gullivan Bay may have been exposed to harmful levels of the red tide toxins.
Pax was instrumented with a satellite transmitter and released in the Ten Thousand Islands on 12 October with two other Kemp’s ridleys, Queenie and Hudson, released the following day. The turtles remained in the northeastern region of Gullivan Bay for a week, inhabiting the island passes between Tripod and Turtle Keys. Pax briefly traveled westward on 17 October and then again on 19 October, intermediately after which began exhibiting some disconcerting behaviors. Subsequent locations for this turtle were clustered along the mangrove shoreline of a very shallow embayment. Tracked turtles in the Ten Thousand Islands typically occupy the deeper channels among the islands and bays.
Following Pax’s westward movement on 19 October, we began receiving location estimates with the highest level of accuracy. The transmitter sends radio signals (messages) to the satellites orbiting overhead whenever the turtle surfaces to breathe. The more messages received by the satellite, the more accurate the locational estimate. As such, the telemetry data indicated Pax began spending much more time at the surface and was possibly floating along the mangrove shoreline. We tried on a few occasions to locate the turtle among the tangle of roots but our efforts were unsuccessful. There was no further contact with the transmitter after 24 October and there are a number of possible reasons for loss of transmissions. Given our lack of direct observation and no reports of a stranding for this particular turtle, we can only infer the possible fate of Pax. Blood samples collected upon capture may offer insight to the exposure to red tide toxins prior to satellite tracking. These satellite data highlight the perils faced by Kemp’s ridleys inhabiting the waters of Southwest Florida and the potential for underestimating the number of turtle strandings along the mangrove coast of the Ten Thousand Islands.
Marine turtles are protected under federal and state laws and a permit is required to handle sick, injured, or dead animals. Our marine turtle research activities are conducted under NMFS permit #22123 and FFWCC permit #136. If encountering a stranded turtle, you are asked to contact the appropriate state hotline. In Florida call FWC’s 24-hour Wildlife Alert Number at 1–888–404-FWCC (1–888–404–3922) or dial *FWC or #FWC on a mobile phone. Be prepared to answer the following questions:
- What is the exact location of the animal?
- Is the turtle alive or dead?
- What is the approximate size of the turtle?
- Is the turtle marked with spray paint? (This may indicate that the turtle has been previously documented.)
- What is the location of the closest access point to the turtle?
If the stranded turtle is alive, be prepared to stay with it until help arrives.
Click here to read more about our Kemp’s ridley research.
