In late July, a whale shark washed up dead at Sanibel Island in Florida. The young adult male shark was 26 feet long and floating in the surf zone as soft waves lapped around its body.
A biologist from the Florida Fish and Wildlife Conservation Commission sampled the shark's muscle, liver, intestines and stomach contents. The muscles and organs tested positive for brevetoxin, a neurotoxin created by a harmful algae called Karenia brevis. This is the first time the commission has had an opportunity to sample whale shark tissue for this toxin.
Scientists cannot be certain about the exact cause of the whale shark's death, but the timing and location implicate the harmful algal bloom, or “red tide,” as the most likely cause. “This whale shark was definitely exposed to the bloom, and we know brevetoxins” are deadly to fish, said Kelly Richmond, a spokeswoman for the commission.
Five shark biologists were interviewed for this report, and none had ever heard of a whale shark death related to algal blooms.
The bloom along the Florida coast stretches 100 miles from Sarasota County to Collier County. It has affected the coastline in different ways and areas for the past nine months. The commission has fielded hundreds of reports on its Fish Kill Hotline, and its biologists have collected thousands of water samples to test for the algae. On Friday, algal load in the water was still very concentrated, at millions of cells per liter of ocean water. A “low” concentration is about 1,000 cells per liter. Harmful algal blooms are often called "red tide," because some kinds of algae can make the water appear reddish.
Whale sharks are filter feeding, gentle giants with constellations of white polka dots on glimmering navy blue skin. They have small eyes set wide above a mouth that gapes to an enormous size to pull in the water from which they filter small items such as fish eggs, plankton and krill. They do not generally occupy coastlines, said Bob Hueter, a senior shark scientist from the Mote Marine Laboratory in Florida, and harmful algal blooms are commonly found close to the coast. This geographic separation may be part of why whale sharks have not been reported with red tides previously.
The second part of the mysterious shark death is how the shark became trapped in the bloom in the first place. Sharks have taste buds in their throats and mouths as well as a very good sense of smell, which should help them to avoid feeding in toxic areas, Hueter said.
Along with the Fish and Wildlife Conservation Commission, Gretchen Lovewell, manager of Mote's Stranding Investigations Program, has been responding to other animal strandings caused by the bloom. She said more than 100 turtle deaths have been recorded — 50 in the past month. Green sea turtles, loggerheads, and Kemp's ridley (a critically endangered species) are among the casualties.
After accidentally ingesting the toxic algae, turtles appear “intoxicated” and uncoordinated in the water and are unable to swim. Most turtles drown as a result. It is turtle nesting season in Florida, Lovewell said, so this may explain the large number of turtles affected. As baby turtles hatch, they will have to swim through the toxic bloom to reach open water. Because baby turtles do not feed right after hatching (they have a tiny belly-button-shaped yolk sac to nourish them in their infancy), scientists hope that they will make it through the algae bloom.
Manatees, dolphins and grouper have also been reported dead as a result of the bloom, Lovewell said. The animals are accidentally ingesting the algae along with their food, said Rachel Graham, a whale shark scientist and director of MarAlliance.
Toxic algal blooms can also harm humans. The toxin is accumulated in filter-feeding animals such as mussels and oysters and is toxic to humans if ingested. Authorities are monitoring beaches and seafood from the area. Swimmers are advised not to enter the water if dead fish are present. The algae “can be airborne in sea spray and cause people along beaches to have difficulty breathing. You start wheezing and coughing,” said Don Anderson, director of the U.S. National Office for Marine Biotoxins and Harmful Algal Blooms.
Algal blooms are a natural phenomenon that can be aggravated by human influences such as pollution or nutrient runoff from sewage or land development. They can occur in freshwater and saltwater. The algae are usually present in small amounts such as a few thousand cells per liter. Once the algae are triggered to “bloom,” or rapidly reproduce, they reach concentrations in the millions. The ocean has thousands of species of algae, and only about 100 that produce dangerous toxins, Anderson said. Algae in general are hugely important to marine life. Scientists do not know why this algal bloom is so concentrated and persistent.
The Florida Fish and Wildlife Conservation Commission is maintaining a red tide hotline and status report website.