Toxic Algae in the Florida Keys | Environment

Since October 2023, Florida residents have witnessed 80 species of fish, including the endangered smalltooth sawfish, exhibiting erratic spinning behavior before succumbing to death. According to Jonathan Veach, communications specialist at the Fish and Wildlife Research Institute, the FWC has received and documented over 500 reports of symptomatic fish from partners and the public since November 2023. The majority of sawfish reports have been received from the Lower Keys; however, biologists have recorded cases up to the Florida panhandle. Over the past year, the bizarre and tragic phenomenon has claimed at least 54 sawfish, raising concern for the imperiled species. Experts suspect a toxic algal bloom as the primary cause, but much remains unknown. 

Dr. Michael Parsons, a biological oceanographer who specializes in harmful algal blooms and water quality, has been following the lead since March. Gambierdiscus- an HAB that produces a neurotoxin called ciguatoxin- is normally seen in the water column at around 30 cells per liter. When the FWC collected water samples of the bloom at an extremely elevated level (1,000 cells per liter) Parsons knew he’d never seen anything like it before. Shortly after, he focused his entire research efforts on uncovering whether gambierdiscus was the leading culprit behind the spinning fish mortalities. 

Marine biologists have long been aware that people who eat seafood infected with ciguatoxin can experience vomiting, nausea, and neurological symptoms. However, ciguatoxin’s role in the erratic behavior of fish is an unusual case that hasn’t been studied before. Parsons explains, “When we talk about ciguatera and gambierdiscus, it’s usually dietary. But in this case, the fish are being exposed to the toxin through their gills.” Dr. Dean Grubbs, an ecologist who has been studying both healthy and impacted sawfish in the Keys and other areas of Florida, builds on the theory. He suggests the symptoms of ciguatoxin in humans and fish may be similar, but the mechanism of transmission is different or limited. Grubbs adds, “Ciguatoxin bioaccumulates within humans when they eat an infected fish. But what is happening to the sawfish is not trophically linked- they’re being infected through the respiratory system.” 

Scientists are also questioning what made last winter different from the years before. Grubbs says, “Sawfish are bottom associated, so they probably come into contact with gambierdiscus frequently.” A working hypothesis Parsons is currently looking into is whether the abnormally hot summer of 2023 may have caused an impact that allowed gambierdiscus to increase in abundance when the winter months rolled around. “Some kind of control that was keeping gambierdiscus levels lower was removed, allowing them to grow more prolifically when the temperatures cooled,” Parsons adds. 

Although gambierdiscus is the strongest lead thus far, Parsons believes it may not be the sole factor, and a synergistic effect of multiple compounds may be at play. “We’ve ruled out many factors- known pathogens and pollutants, low oxygen, and red tide. We’ve also detected some bioactive compounds and toxins, but we aren’t seeing them at elevated levels like gambierdiscus,” Parsons explains. These compounds are not new or unusual, they’re almost always present in the water. However, the levels, ratios, and synergy of the compounds are what could be masking the unsolvable dilemma. “We don’t have the baseline data for the toxins,” Says Parsons, “We’ve ruled out a lot of the most common factors, but we’re dealing with something uncommon here.” 

The phenomenon is further complicated by another case of erratic behavior displayed by a sawfish in the Tampa Bay area. Grubbs was searching for the reported distressed sawfish back in June, but his team found it deceased only a few days later. This instance, along with a similar one reported in the Florida panhandle, had Grubbs focused on yet another theory. “It suggests that however this toxin is attacking the fish- the host probably goes through phases where they’re showing an erratic, distressed behavior- but then they somehow recover for a few days to a week,” Explains Grubbs, “Unfortunately, it still ends up killing them.” Grubbs proposes that the fish most likely acquire the toxin in the Lower Keys, and as they move through their migration routes, they progress through periods of increased health until the toxin eventually takes its toll and kills them. 

Reports from the public and known sawfish mortalities have flattened out in recent weeks, yet Parsons and Grubbs aren’t certain on whether the event has ended, nor how likely it will occur again. Grubbs explains, “We’re at the stage where a lot of the species that were affected, like sawfish, have migrated north for the summer.” Parsons adds, “There aren’t as many gambierdiscus cells in the summer, so we expected things to calm down because that’s what typically happens.” When fish return to the Keys and the temperatures decline, Parsons and Grubbs are prepared to continue their research. Veach also points out that although they expect the event is slowing down, it is difficult to predict whether it’s beginning to end. He suggests that the reports coming in maybe fewer because the public knows an investigation is ongoing and assumes there is no longer a need to submit a report. However, Veach conveys that it is critical for the public to report any sightings so that the FWC can continue to document the progression of the event. 

With 20 percent of tagged sawfish killed from the Florida Keys phenomenon, Grubbs is afraid an even larger portion of un-tagged sawfish had the same detrimental fate. “It suggests that a significant chunk of the population was affected,” Says Grubbs, “We won’t know the exact impacts for some time, but it’s concerning.”  Historically, there are records of sawfish as far north as the Carolinas and as far west as Texas, but nowadays, they mostly stay in Florida. Grubbs looks for criteria such as sawfish returning to areas they once occupied as a sign that the species is recovering, but climate change confounds that information. He suggests that sawfish will either shift their habitats north, move deeper, or die due to the increasing water temperatures. “My lab does the only survey on the abundance of adult sawfish,” said Grubbs. “As we continue the survey, we should be able to see in the data if the abnormal event had a significant effect on the population.” 

Grubbs believes it will take scientists a couple of years to fully understand how much of a setback this event has been for the endangered smalltooth sawfish. Despite the uncertainty, he and other experts are optimistic that their ongoing research will significantly advance the understanding of harmful algal blooms and their impacts. Marc Suddleson, an oceanographer and program manager in the competitive research program of the National Ocean Service, emphasized the progress made. “Not only are they ahead of where they were at the beginning of the issue, but they have some specific likely culprits and have formed a collaboration between institutions,” Suddleson says. “If there is another event, they won’t necessarily be able to stop it, but they are better prepared to respond to it.” 

“Gambierdiscus is the most promising lead so far, but this is such a new or rare phenomenon that there needs to be more research,” Suddleson continues. Alison Robertson, another key marine scientist on the case, is eager to conduct lab experiments using Gambierdiscus cultures to better understand the harmful algal bloom and its effects on sawfish. In addition to Robertson’s research, Grubbs’ team will be hiring a biologist to stay in the Keys throughout the winter to improve monitoring and response. This biologist will work closely with colleagues from the FWC, Bonefish Tarpon Trust, and other partners to detect any early signs of a reoccurrence before it escalates. Meanwhile, Parsons remains optimistic: “Hopefully, it doesn’t happen again, but if it does, we’ll at least have data to compare it to, and we’ll better understand what happened.” 

As water temperatures in the Keys prime for another algal bloom, all eyes are on the ocean, hoping to avoid a repeat of last year’s crisis. Although the cause of the abnormal behavior and mortality of fish remains uncertain, multiple agencies and experts have prioritized work to investigate the role of HABs for enhanced response to the event. Parsons adds, “This time, we’re more prepared to sample and respond. There’s still work to be done, but we’ll see what happens this year.” While the threat to the smalltooth sawfish is dire, the proactive steps taken by experts and partners offer a glimmer of hope for a species that teeters on the brink of survival.