Federal wildlife officials have opened an investigation into dolphin deaths off Southwest Florida, where a red tide is suspected of killing 41 dolphins in August alone amid widespread fish kills across five counties.
In a briefing Friday, the National Oceanic and Atmospheric Administration said it was declaring the deaths an "unusual mortality event," allowing the agency to corral resources to research the ongoing deaths linked to the algae bloom. Since the 1990s, the agency has declared four similar red tide dolphin die-offs in the Gulf. Three occurred in the Panhandle and one, during a 17-month long tide that ended in 2006, covered the entire west coast.
"As we go through this event, if it's truly red tide, you may see a shift in the type and what the strandings look like," said veterinarian Teri Rowles, coordinator for NOAA's Marine Mammal Health and Stranding Program.
Researchers at Mote Marine Lab in Sarasota said they confirmed evidence of a bloom nearly a year ago in October. Massive fish kills and sea turtle deaths followed, leaving beaches littered and canals clogged with carcasses by June. It's not unusual for dolphin deaths to lag behind turtles, manatees and other marine life because dolphins tend to be poisoned by the food they eat rather than the water, Rowles said.
"In red tide events, we know the animals often die acutely with high levels of brevetoxins in their bodies and in there stomachs," she said. "But....even when the bloom is gone we may see an increase in mortality."
Dolphin deaths stayed in the normal range through much of the bloom: one was reported in November and about two dozen more through July, said Erin Fougeres, NOAA's strandings coordinator for the southeast region. Then in July the number hit eight, double the historic average, followed by the 41 deaths in August.
Only ten carcasses have been examined so far, but all had high levels of red tide toxin, suggesting their deaths were linked to the algae bloom, Rowles said.
It's not clear what effect the tide will have over the long term on the Gulf's dolphin population, said Laura Engleby, the Southeast Region Marine Mammal Program Branch Chief. The Gulf has eight known sub populations, she said, but only one is coastal and well-documented. Dolphins live long, with some females as old as 60. But they are slow to reproduce: pregnancy lasts about 12 months and mothers nurse their babies for up to 20 months. They only give birth every three to six years.
"So it can have an impact," Engleby said.
In 2005 and 2006, the last time such an intense red tide blanketed the coast, 283 dolphins died in two red tides in the Panhandle and on the west coast. The Gulf also had a spike in the number of baby dolphin deaths after BP's Deepwater Horizon oil rig exploded in 2010. In 2016, Rowles and other researchers linked the deaths to the oil spill.
It may also take dolphins longer to recover if the fish they consume are wiped out by the tide.
"The food web needs to recover and then it takes the dolphins time to recover," Rowles said.
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