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Red tide linked to royal tern deaths on causeway

By SCCF - | Feb 17, 2021

PHOTO PROVIDED A royal tern sits on the Sanibel Causeway bridge railing.

On Feb. 16, the Sanibel-Captiva Conservation Foundation reported that many residents and visitors have called SCCF, the city of Sanibel and Clinic for the Rehabilitation of Wildlife with concerns about royal terns over the past few weeks and the sight of their carcasses on the Sanibel Causeway bridges.

Although SCCF Marine Lab scientists have detected decreasing levels of Karenia brevis — the dinoflagellate that blooms into red tide — at island beaches over the last two weeks, the number of wildlife impacts has increased.

“There have been a large number of sick royal terns observed on our islands and the causeway bridges,” SCCF shorebird biologist Audrey Albrecht said. “Our staff covered all of Sanibel and Captiva on foot last week for the statewide winter shorebird survey and observed several dead royal terns on the beach. I would say across both islands there were several dozen, maybe close to 50 royal terns that appeared sick with possible brevetoxicosis.”

The SCCF reported that CROW has admitted 60 royal terns since Feb. 1, with 38 deaths, 20 still receiving care, and two birds that recovered and were released. In just a month and a half, CROW has admitted a total of 90 royal terns compared to a total of 49 in all of 2020.


PHOTO PROVIDED A royal tern in recovery after treatment at CROW.

Test results on deceased royal terns confirmed on Feb. 15 that they had high levels of brevetoxins, the neurotoxin associated with red tide. On Feb. 16, CROW had planned to send several birds to the Southeastern Cooperative Wildlife Disease Study for further analysis, but this was delayed due to bad weather conditions.

“We are planning to send out several birds to SCWDS for full necropsy and testing to see if it is only red tide or if there may be some other secondary disease process as well,” CROW Medical & Research Director Dr. Heather Barron said. “We are currently treating the terns for brevetoxins, and several are being treated for secondary infections since they’re already immunosuppressed.”

Preliminary results are expected in a couple of weeks and final results in about three to four weeks.

“They are very thorough,” she added. “The amount of testing that they do is incredible.”

The city is reminding callers that the Causeway Islands and roadway are outside of the city’s jurisdiction and are governed solely by Lee County personnel. Because the terns are being hit on the bridges, drivers are not encouraged to try and rescue injured birds due to traffic concerns.

PHOTO PROVIDED A royal tern splayed out on the beach on Captiva.


Though Karenia cell counts have been lower and absent in red tide samples collected from the beaches, SCCF Marine Lab scientists said that that does not mean there is not a patch out there somewhere.

“The royal terns are feeding on planktivorous fish that have been exposed to a patch of the red tide bloom,” lab Director Dr. Eric Milbrandt said. “The deaths on the causeway are because the birds are sick and resting on the rails or on the road. They can’t avoid the vehicles and are lethargic.”

The red tide that persisted for two months offshore Lee and Collier counties is having a lingering affect on Southwest Florida’s coastal ecosystems, according to the SCCF.

“Some planktivorous fish eat red tide. The terns pick them off when the fish start getting nervous system problems. And then the terns get sick,” research scientist Rick Bartelson said.

“We are working together with CROW and Florida Fish & Wildlife Conservation Commission to determine the cause of this sudden increase in sick and dying royal terns, and if possible sickness is leading to the sudden increase in royal terns being hit on the causeway bridges,” Albrecht added.


The SCCF sea turtle team has also documented five strandings of sea turtles over the past two weeks. All five washed up dead. Currently, there is no funding available to test the turtles for possible red tide poisoning. To learn more about donating, call Development Director Cheryl Giattini at 239-395-2768.