Whale shark exposed to red tide, but cause of death still unknown
Tissue samples from the body of a whale shark that recently washed ashore on Sanibel indicate the animal had been exposed to a red tide bloom, but it was not necessarily the cause of its death.
On July 22, the nearly 26-foot young male shark was discovered near the Island Inn. Staff from the Florida Fish and Wildlife Conservation Commission responded to the scene, collecting samples off the body before the city of Sanibel helped FWC dispose of the carcass by digging a hole and burying it.
“The muscle, liver, intestine – tissue and contents – and stomach contents of the whale shark all tested positive for the red tide brevetoxin – Karenia brevis,” Melody Kilborn, spokeswoman for FWC’s Southwest Region Office, said. “The whale shark was exposed to the red tide bloom, but this does not necessarily indicate this is why it died.”
FWC scientists have measured similar and higher levels in live fish caught from bloom areas.
“Given the high Karenia brevis concentrations in the area and the large number of recent fish kill reports, it’s fair to say that it may have been red tide-related, but we cannot know for sure,” she said.
Staff with the city’s Natural Resources Department have been communicating with FWC about the matter. Dr. Leanne Flewelling, with FWC’s Fish and Wildlife Research Institute, explained to them that the samples were analyzed using an antibody-based screening assay for the presence of brevetoxins.
Brevetoxins are the neurotoxins produced by Karenia brevis, also known as red tide.
“This fish was definitely exposed to the bloom, and we know brevetoxins are very ichthyotoxic,” Flewelling said. “We’re in the process of collating all the information obtained from this stranding.”
She also pointed out the brevetoxin levels in the tissue samples.
According to the analysis results, the liver contained 696 parts per billion, the stomach contents had 148 parts per billion, the intestine contained 75 parts per billion and the muscle had 44 parts per billion.
“The levels in the whale shark tissues are actually lower than many we’ve measured in live fish – including other species of sharks – during blooms,” Flewelling said. “However, this is the first time we’ve had the chance to analyze tissues from a whale shark for brevetoxin, and all species are different.”
Dr. Gregg Poulakis noted that FWC staff plan to run a toxicity test on the liver.
“To see if that helps inform us on the cause of death, but a definitive cause may not be established,” he said. “In general, there was nothing obvious that we noticed during the necropsy.”
Holly Milbrandt, environmental biologist with the Sanibel Natural Resources Department, reiterated that the initial results are only an analysis of the four samples and further research will be required.
“This isn’t really meant to be a final report,” she said.
Milbrandt echoed that the whale’s exact cause of death may never be revealed.
“But I can say this is a contributing factor,” she said, referring to the existence of brevetoxin in the tissue samples. “Red tide definitely played a part.”
Milbrandt noted the recent sea turtle strandings on the islands and other local sea life being impacted, pointing out that the area is in the midst of a red tide bloom that has been lingering since October.
“This is a really challenging time for our beaches and the Caloosahatchee,” she said. “I think it’s evident that things are not right in our shore waters.”
Mote Marine Laboratory Dr. Bob Hueter explained that the organization was alerted when the body was discovered and he monitored the situation via phone through the Bailey-Matthews National Shell Museum. Museum staffers were conducting a public beach walk the morning the animal was found.
“In mid-June, we tagged two whale sharks about 25 or so miles off,” Hueter said.
With one being a young male, Mote staff were concerned upon learning about the carcass.
“We were concerned that it might be the one we tagged,” he said.
“It was not,” Hueter added. “We confirmed that.”
FWC publishes a red tide status report on Wednesdays and Fridays that can be found online at www.myfwc.com/redtidestatus.
For sick, injured or dead sea turtles contact the Sanibel-Captiva Conservation Foundation Sea Turtle Hotline at 978-SAVE-ONE (728-3663) or FWC’s Wildlife Alert hotline at 888-404-FWCC (3922). The FWC Wildlife Alert hotline can also be used to report on any sick, injured or dead marine mammals.
To report a fish kill, contact FWC’s Fish Kill Hotline at 800-636-0511 or submit a report online. The public can also download the free FWC Reporter app to their mobile device.
To report a bird mortality, visit legacy.myfwc.com/bird.
For more information on how red tide effects fish and other marine animals, visit online at myfwc.com/research/redtide/general/marine-animals.