(subdued music) - [Ross] 2023, that's a year we probably won't forget for a long time.

Seeing those temperature records being broken daily was pretty hard.

- [Matt] In late 2023 and then into the winter of 2024, fishermen started reporting that they were seeing spinning fish, abnormal behavior.

They were extremely disoriented.

- [Theresa] They're spiraling or they're swimming in circles.

Sometimes they'll splash or they'll do somersaults.

- [Mike] Some fishes will bob their head out of the water and kind of gasp.

- [Tom] It started off small, but then it increased to dozens of reports every day.

(phone chiming) - [Ross] But these reports just kept coming in.

- [Tom] By December, it increased.

By January, it doubled again.

By February, it doubled again, and it peaked in March.

- [Ross] In the peak of it, a lot of places became lifeless.

There's no fish.

As the waters warmed, the reports got less and less.

And then as we got into summer, it just vanished.

(gentle music) - [Narrator] While fish are known to spin when they are stressed, and there had been other reports of spinning fish in the past, the Florida Keys had never experienced a spinning event of this magnitude.

- [Ross] Ground zero was around Cudjoe Key, Big Pine Key, and Sugarloaf.

That's where it started.

And from there, it emanated east and west reaching as far north as lower Key Largo.

And as far west as about Key West.

It was on both sides of the Keys, ocean side and bay side.

- [Theresa] There were over 80 species of fish.

- [Tom] 70 of those were bony fish and 10 were the sharks and rays.

- [Narrator] The vast majority of fish species seemed to eventually recover, but it was devastating for one iconic South Florida animal: the endangered smalltooth sawfish.

- [Dean] These large sawfish basically swimming in circles with their rostrum up out of the water.

Some of them were beaching themselves and dying.

And it started as a trickle and then it quickly ramped up.

- [Narrator] By early summer of 2024, 56 smalltooth sawfish had been reported dead, with many more potentially unaccounted for.

- [Theresa] And that's alarming because they're critically endangered and their reproduction is slow as well.

- [Glenn] What's really causing it all?

That's the question.

I don't think anybody has the complete answer yet.

(subdued music) - [Narrator] Early on, a team of multidisciplinary scientists, together with the Lower Keys Guide Association, began collaborating to unravel this mystery.

- [Matt] None of us study the entire ecosystem.

We're all kind of focused on different aspects of it.

- [Theresa] And so we all came together and began this working group to determine the cause of the spinning fish.

- [Alison] The behavior was clearly showing neurological effects, so the major suspects there were going to be neurotoxins, or some anthropogenic pollutant that was producing neurotoxic effects.

- [Kate] We're trying to hone in on the specific organisms that we think are probably causing issues.

- [Mike] It's probably not a single toxin.

It's probably going to be a suite of toxins and maybe they have a synergistic effect where the sum is greater than each individual potency.

- [Kate] We just want to make sure we don't rule anything out.

- [Narrator] What might have led to the sawfish deaths and this abnormal spinning behavior?

And how are researchers trying to solve this complex puzzle?

(serious thoughtful music) - [Announcer] Major funding for this program was provided by the Batchelor Foundation encouraging people to preserve and protect America's underwater resources.

Additional funding was provided by the Parrot Family Endowment for Environmental Education.

(gentle music) - [Narrator] Florida Keys-based researchers with the Bonefish & Tarpon Trust were among the first to respond to this unusual spinning behavior.

- How we work is really built through collaborations with the fishing community.

So we work with them hand-in-hand to evaluate some fish impacts.

And that collaboration really enabled us to be the eyes and ears of this event.

We were able to detect it first, track it thoroughly throughout the Keys, and identify new hotspots.

We were going out three times a week, more or less, just going with fishing guides and asking them, just fish how you'd fish.

And then we put fish in an aquarium and then evaluate how long it took them to break out of the spinning fish behavior.

We found feeding rates were way down in sick fish versus healthy fish.

And we also found there's definitely more species that are sensitive to it than others.

For instance, the jack crevalles, about 75% of them, showed symptoms if captured.

- [Captain] They just swirl around on the surface.

- [Ross] For a mangrove snapper, it was about 10% or 20%.

The mangrove snapper were able to correct themselves and recover within about 10 minutes, where the jack crevalles just never were able to recover.

So that helps us brace for impact for these long-term consequences we may have for our fisheries down here.

(phone chiming) - Florida Fish and Wildlife Fish Kill Hotline.

This is Kyle speaking.

- [Narrator] Around the same time the Bonefish & Tarpon Trust became aware of this issue, concerned citizens were also reaching out to the Florida Fish and Wildlife Conservation Commission's Fish Kill Hotline.

- We on average probably get about 1,200 reports a year.

For this Keys event, we have over 700 reports.

So we started working with the staff in the FWC Keys Lab down in Marathon, Florida.

And they went out and collected fish for us and water samples, and we were able to conduct necropsies.

We'll weigh and measure all the organs to look and make sure that it's a healthy fish.

So we looked for communicable disease, which we did not find any.

The microbiome analysis did not find any viruses or unusual levels of any known pathogens.

- [Narrator] Cultures of the brains and kidneys were negative for bacterial infections, and various organs were also preserved for histopathology.

- So they're looking for cellular changes and any kind of abnormalities in the cells.

The histopathology did show some pathological changes in the tissues.

So we are continuing to investigate that.

Some of the histopathological changes indicate that there might be some changes due to a toxin.

- [Narrator] Meanwhile, the Florida Department of Environmental Protection tested water samples for contaminants.

- [Theresa] They didn't see anything that was abnormally high or above background levels.

- There were a lot of other pieces of the puzzle that we investigated.

Sewage treatment plants were overflowing down here.

Turbid water, cold water, all of these things can make fish lethargic or spinning.

So we ruled out an awful lot of stuff.

- [Narrator] Water samples were also analyzed by FWC's Harmful Algal Bloom Group.

- We have blooms of the red tide organism, Karenia brevis, that typically occur every year in the fall and winter.

And so when we started to get reports of abnormal fish behavior, our first guess was that this was probably the Karenia brevis bloom rearing its ugly head.

And while we did find trace levels of Karenia brevis present, the bloom never developed.

And so we continued on trying to identify whether or not there might have been some other harmful algae present that could have been causing the same sorts of behavior.

So we started to look at samples and we saw several different types of dinoflagellates.

One of the genera that we saw was gambierdiscus.

And that's really when the partnership with our colleagues took off.

- So gambierdiscus is a dinoflagellate, which is a single-cell microscopic algae.

It has flagella so it can swim around.

It has pigments, so it photosynthesizes like plants and other algae, but it likes to live on the bottom, and it usually likes to live on seagrasses or seaweeds.

- [Narrator] Dr. Mike Parsons is an expert on the taxonomy of gambierdiscus.

- They mentioned a thousand cells of gambierdiscus in the water column.

Now for red tide, that's nothing, that's background levels, but for gambierdiscus, which lives on the bottom, that's a lot.

The most we ever saw in 10 years was 200, and we average about 30.

So the fact that it was a thousand, I said, "Well, that's high.

There might be something going on there."

And so then that's when we decided to take some samples and look more closely at it.

(gentle music) - [Narrator] Mike and his team have been collecting samples and conducting research into gambierdiscus in the Florida Keys since 2010.

- [Mike] We know there's five or six different species of gambierdiscus that are present in the Keys.

We're still trying to get a handle on which ones were part of this event.

- And to do that, we went from the light microscope to using the scanning electron microscope.

And with that we were able to hone in on a lot of the features on the cell wall that allow us to then go a little bit further in terms of identification.

So we were able to rule out certain species of gambierdiscus based on the morphology that we saw.

- [Narrator] The researchers are also using molecular methods to identify species by their DNA.

- [Kate] And that allows us to get a more definitive identification of the species that are involved.

- We know that gambierdiscus has a huge capacity to produce natural toxins and a complex suite of those toxins.

And they can include the ciguatoxins that are associated with the global seafood illness called ciguatera poisoning.

That's a fish poisoning syndrome.

But they're also capable of producing other toxins that also affect the nervous systems.

(subdued music) - [Narrator] Dr. Alison Robertson is an ecotoxicologist who specializes in harmful algae.

- [Alison] It's only been recently that my group and our collaborators were able to work out the toxins that are being produced by gambierdiscus.

So thankfully we had that in our back pocket, and we were able to do targeted analysis to see if those toxins were in our environmental samples.

And that means in the water, in the sediment, in the fish, and in those algal samples.

And we found them everywhere.

- [Narrator] So what might have led to the elevated levels of gambierdiscus in late 2023, early 2024?

- We think that there must've been some kind of perturbation related to the really hot summer, that then released gambierdiscus from some kind of population control, if you will, that allowed it to bloom almost unchecked, as we moved into late fall and early winter.

Now, what that release was is something we're still trying to explore.

- [Alison] The factors that drive gambierdiscus in the environment are still a little bit elusive.

We have an enormous culture collection that we share between our collaborative partners, so that we can do a lot of experimental work on those isolates of microalgae.

And we will modify things like temperature and salinity and their nutrient regimes to see if that influences their toxin production.

- [Narrator] But it's not just gambierdiscus that may have been a culprit in this spinning fish phenomenon.

- [Kate] We saw some other types of dinoflagellates in the samples that can also be associated with harmful algal blooms or toxins.

- And so it's possible that combined together that matters, even though those other ones were not necessarily elevated, they're still at a detectable level and could still have an influence on fish behavior if they were exposed to them.

(gentle music) - [Narrator] To work out what toxins may be triggering the spinning, researchers collect a variety of samples in the field.

- The most important samples for us to collect were water samples, sediment samples, macroalgal samples, and fish samples.

So, we were interested in looking at the tissues of fish that were impacted to see if there was something that they had been exposed to that might explain this very strange behavior.

In doing so, we found very high levels of a complex suite of these neurotoxins in the muscle tissue, even higher in the liver tissue, and higher again in their gills.

So this leads us to believe that the most likely exposure route is in the water crossing their gills.

- [Narrator] While the toxin levels didn't put humans at risk of ciguatera poisoning, they certainly could have impacted the behavior of the fish.

- When we see toxins present in the gills, that's a fairly rapid absorption area.

Likewise, in the liver.

So the level of exposure, the timing of exposure and the location of exposure are all gonna matter as we try and understand the cause.

We use a lot of advanced analytical instrumentation and chemistry to help us in identifying these toxins.

And depending on what the sample is, we have a whole series of processes that we have to put them through.

So we will be preparing samples in 5 to 10 different ways to make sure that we capture everything.

And those 5 to 10 subsamples of one, will then be fractionated into up to a hundred different fractions that we would then be testing.

It's like looking for a needle in a haystack.

- [Narrator] To measure the bioactivity of potential new toxins that Alison has isolated from the various samples, her team exposes them to cultured neuronal cells.

- If a neurotoxin is present, we can detect them in those cells at very, very low levels, trace levels actually, and about 50 times more sensitive than a lot of the analytical instrumentation we have.

We have found a number of toxins, many of which we know gambierdiscus is capable of producing, but we still have to work out if it is in fact gambierdiscus producing them.

We've also identified quite a few novel toxins that have never been described before and we have identified toxins that we have not seen in the Florida Keys before, but they may have been identified elsewhere in the world being produced by other types of microscopic algae.

So we're trying to work out which species in the Florida Keys is producing those particular toxins.

We have at least 15 compounds that we are trying to elucidate at the moment that are all bioactive and seem to be involved and capable of causing the fish spinning.

(water bubbles) (serious music) - [Narrator] Once toxins have been isolated and identified, Alison's lab conducts multi-stressor experiments on fish to see if they can induce spinning behavior.

- So we can do combined exposures of different toxins at different levels, but we can also then factor in changing temperature regimes.

We can change the salinity, we can change the nutrients, we can change the light levels and other factors, and we can combine all of those factors to see which is contributing to the effects that we see.

We know that ciguatoxins alone don't cause fish spinning.

And the other toxins that we have found, when exposed to fish, also does not cause spinning.

But in our early bioassay work, we found that the combined exposure of multiple toxins does cause that spinning.

And if we remove one of the factors, they stop.

The behavioral effects we see in our exposure studies are transient though, so that tells us that the waterborne exposure, again, seems to be the likely route there.

So, it means a lot to us to get to the answer as quickly as possible.

However, the complexity of all of this is quite significant.

So we have to do a lot, and we have to be sure.

(bait splashes) (gentle music) - [Matt] All right, there's a blacktip.

Just go neutral, Alina.

So we'll bring this one onto the back.

- [Narrator] It's not just bony fishes that were impacted by this event.

Researchers with the Bimini Shark Lab have been studying the abundance of different shark species in the Lower Keys since 2022.

- Probably a pregnant one.

Do you wanna get the ultrasound?

And what that's actually allowed us to do is have a long-term data set, actually even prior to some of the spinning fish and sawfish mortalities that were going on.

Most of the species that that we come across are things like great hammerheads, bull sharks, blacktip sharks, sometimes tiger sharks, lemon sharks.

It was coming up to the surface right here.

So there's a whole big diversity of sharks that are right in these coastal waters.

In 2024 during the peak of the spinning fish condition, we were actually seeing much fewer sharks in that hotspot area.

So that was especially pronounced for some of the like more resident coastal sharks, like blacktips or juvenile lemon sharks that kind of reside in those shallow water areas.

Everybody ready?

Fortunately in 2025, it's actually started to rebound a little bit.

So hopefully that's good news.

The few juvenile lemon sharks and blacktips that we did catch in the winter of 2024 were actually showing some physiological impairment.

So it's kind of akin to what would look essentially like a lock jaw or really, really cramped up muscle.

They couldn't relax it once they were actually on the line and on the boat.

We think that it might be because these species tend to spend a lot of time in that really, really, shallow, near-shore seagrass habitat, very similar to where a lot of the conditions for spinning fish or sawfish mortalities were also taking place.

(soft music) - [Narrator] As part of their ongoing research, the scientists collect muscle and blood samples from the sharks.

This allows them to look for the presence of toxins, and they will be able to compare samples to those from previous years to see if there are any physiological changes taking place.

- [Matt] We all good on data?

- [Narrator] Some of the sharks captured are also outfitted with an acoustic tag that lets researchers track their movements.

- All right.

Tag's in.

There's a whole array of what's called acoustic receivers spread throughout Florida and then up and down the east coast and the Gulf of Mexico.

And so anytime an animal that has that acoustic tracking tag passes by one of these receiver sites, it'll actually then pick up that individual tag.

And then every tag has a unique serial number and ID associated with it.

And then we get that data back about once a year.

So one thing we're gonna look at is, you know, how have movements in these habitats changed over time.

- You got the coordinates on that receiver?

- [Narrator] Researchers studying all kinds of species own and maintain receivers as part of this network and share the data with each other.

Data from one receiver located at the epicenter of the spinning event showed some interesting findings.

- When this event really intensified, we went from detecting four or five different types of animals over a couple-day period, on average, to zero for about a whole month.

No living animal with a tag swam by that device, which was completely uncharacteristic for that area, especially that time of year.

So there definitely seemed to be some avoidance to that, which is good.

It's great to know that these fish have some capacity to detect it and stay away from it.

- [Narrator] Another species that researchers have been tracking with acoustic tags is the smalltooth sawfish.

- Between my research group and the State of Florida, FWC, we have upwards of around 200 large, tagged sawfish out there carrying long-term acoustic transmitters.

And so we've been tracking these animals now since 2017 basically.

- [Narrator] Dr. Dean Grubbs and his team have been studying adult sawfish in the Florida Keys and Everglades National Park since 2011.

- And we're the main group doing work on the adults.

There aren't too many people that work on adult sawfish.

They're not easy to work on.

(chuckles) A lot of what we're doing now is to, not only to collect a lot of the blood samples and other samples that might be used for looking at the toxicological and physiological effects, but also to tag more animals and collect tagging data.

We have an idea of what proportion of the tagged ones we hear from every year.

And so what we're hoping to do is use the rates of detection and that may give us a more robust estimate of what proportion of the population was hit by this.

It could be substantial, but we don't have an answer for that yet.

- [Narrator] Smalltooth sawfish, which in the U.S. once ranged from North Carolina to Texas, are now primarily limited to South Florida due to historic overfishing, habitat destruction, and accidental bycatch.

- Smalltooth sawfish was listed as endangered under the Endangered Species Act in 2003.

And so in 2023, in April, which was its 20-year anniversary, we as the Sawfish Recovery Team, we were celebrating the fact that we were starting to see recovery of the species.

We were all optimistic that we may be able to down-list sawfish at some point in our careers.

(gentle music) - [Narrator] Scientists estimate the number of adult sawfish to have been in the hundreds to a few thousand prior to the spinning event.

- So 56 animals dying like this is obviously a huge concern.

So those are the ones that FWC retrieved and that we know died.

They responded to, I think, more than 250 calls of individual sawfish exhibiting the behaviors so we're certain that the mortality was higher than that.

So it's pretty, pretty sad.

We're trying to come up with theories, why are sawfish hit so hard by it?

Other species seem to recover, whereas the sawfish don't seem to really recover.

- Sawfish have the potential to have a higher exposure because they live on the bottom.

And where we find these dinoflagellates producing neurotoxins, they're also on the bottom.

So they may have dietary exposure as well as exposure across their gills, just through the nature of their lifestyle.

And through the work of FWC on the histopathology, it looks like there's a chronic exposure happening in sawfish.

- This episode I think could have set us back decades in terms of our conservation and recovery of the species.

And so if it happens again, you know, it's just gonna be compounded.

I think we're all very nervous about the future of sawfish right now.

- [Narrator] Fortunately, in the winter and spring of 2024/2025, reports of dead sawfish and spinning fish in the Florida Keys were much lower than in the previous season.

- It's substantially, considerably less than it was the year prior.

- [Narrator] As scientists keep working to figure out what caused this event, their eyes are also to the future.

- There's still a lot of work to be done.

So you answer one question and then another question comes up, then another one and another one.

So while you learn more, you also still have more questions.

- I think we want be really careful to continue to follow along to see if it was the combination of algal toxins that may have been involved or if maybe there was something else going on.

I feel like it's important to just continue to ask that question.

If it's not algae, what could it be?

- This has been a great example of how science should be which is a huge collaborative group of people that are open and willing to work together.

We're much closer to finding the answers together than we ever would be working independently.

- The collaboration is why we'll be able to do this.

Some of the fishermen I knew had reported this to me 10 and 20 years ago, but we weren't able to identify what it was.

We didn't have the expertise.

- I think everybody is concerned that it can happen again.

- Climate change is gonna make these extreme events more intense, and this is going to be a consequence that we're going to have to live with.

In addition to this, we gotta clean up our waters and fix our habitats.

Improving our water will reduce the impact of these events when they inevitably occur again.

- And that's not being an alarmist, that's just being a realist.

(subdued music) (subdued music continues) - [Announcer] Major funding for this program was provided by the Batchelor Foundation, encouraging people to preserve and protect America's underwater resources.

Additional funding was provided by the Parrot Family Endowment for Environmental Education.

(stirring music)