Fifteen years of searching for smalltooth sawfish
by NOAA Fisheries 17 Nov 13:15 UTC
Annsli Hilton performs surgery on a smalltooth sawfish to insert an acoustic tag to track the animal's movement © NOAA Fisheries / Reni Poston-Hymel
NOAA Fisheries scientists have traveled to South Florida multiple times a year for 15 years to study smalltooth sawfish. During these expeditions, the team spends several days capturing, tagging, and releasing sawfish to study their relative abundance, habitat use, and life history.
This endangered species, once found from Texas to North Carolina, is now almost solely found in Florida waters. These research efforts help scientists track whether their range is shifting due to climate change, species recovery, or habitat loss. This long-term monitoring can also help the team detect changes—either spikes or dips—in sawfish numbers.
The 15th Field Season
This year's field team, led by Dr. John Carlson and Annsli Hilton, included scientists from the Southeast Fisheries Science Center, Southeast Regional Office, and NOAA's Office of Protected Resources. During most trips, the scientists navigate a small boat through the shallow waters of Florida Bay and the Everglades. Fortunately, the weather was gorgeous on the latest trip—providing pristine conditions for successful sampling. The team captured and sampled 10 sawfish—including seven that were captured in one effort (they usually only capture one or two at a time)!
"It's not every day you get to work with an endangered species, and sawfish are one of my favorite animals—they are very majestic," said Annsli Hilton, co-investigator on the project. "I'm excited to co-lead this project to track their movement, monitor their recovery, and learn more about their biology—which we knew so little about only 10-15 years ago."
Data collected
Once a sawfish is captured, the team works quickly to get the data they need to ensure each animal is released in good condition. This information includes:
- Measurements of body size
- Sex and maturity
- Biological samples like tissue biopsies and vials of blood
- Notes on its health and condition
Before release, the team attaches an external tag and implants one or two internal tags into each animal. The type of internal tag varies depending on the animal's size and what kind of data the team wants to collect.
The scientists also take note of habitat characteristics where each animal is caught, including:
- Water temperature
- Salinity
- Dissolved oxygen
- Water depth
- Characteristics of the seafloor
- Nearby vegetation types
- Vegetation characteristics like mangrove tree prop roots in which small sawfish hide
Types of tags
The most common tag used for sawfish is a PIT (passive integrated transponder) tag, which is the same as a microchip for pets. These tags can be as small as the size of a grain of rice, up to about half an inch in length. They have no battery so, as long as they stay in the fish, they will be detectable when near a scanner. The fish are also marked with an external tag that lets anglers and other scientists know that the fish has been tagged before. It provides contact information to report it or find out more information.
Some of the larger sawfish are surgically implanted with 5- or 10-year acoustic tags. Acoustic tags are battery operated, so they are approximately 2.5 inches long. These tags send a unique acoustic signal every few seconds that can be detected by nearby acoustic receivers. Researchers throughout the U.S. Atlantic and Gulf of Mexico maintain arrays of these receivers and share detection data as part of the iTag network and Atlantic Cooperative Telemetry network.
An unforgettable experience
While 15 years of sampling is certainly worth celebrating, this trip achieved another important milestone. Annsli became one of the few people in the world to perform surgery on a sawfish to implant an acoustic tag. She started in this new role early this year and immediately began preparing for the sawfish trips. She practiced acoustic tag surgery on bananas and preserved shark specimens—learning where to make the incision, how large to make it, and how to suture.
Annsli described the experience stating, "While we were in the field, we caught a sawfish large enough for me to perform the surgery. I was very nervous but very excited. I wanted to make sure I did the procedure quickly and effectively. Thanks to all my practice and guidance from John, I was able to successfully release the sawfish in healthy condition."
Annsli expertly performed the procedure to insert a 10-year acoustic tag. This tag will allow researchers to track this young sawfish's movement around the southeast so the team can make inferences about habitat use, timing of movements out of the nursery, coastal migrations, and more.
Reflecting on 15 years of research
The data collected from this tagging effort over the last decade and a half has led to tremendous strides in our understanding of smalltooth sawfish. This information helps address NOAA Fisheries' smalltooth sawfish recovery plan objectives and inform management decisions. But we still have a lot to learn.
So far, we've learned more about where sawfish live, which habitats they use, and when they move between them. Fortunately, our team has seen a general increase in relative abundance of juvenile sawfish up to 2019—a good sign of recovery for this endangered species. However, restrictions on field sampling due to the Covid pandemic prevented sampling for 2 years—creating a gap in the abundance data time series. Results of newer genetic analyses have helped us identify individuals, and relationships among the animals, and estimate population size. As we look ahead, advances in technology are allowing us to learn more about how sawfish are moving around and revealing how big their home ranges actually are. For example, young sawfish less than about 5 feet in length appear to be moving around a lot more than previously thought.
"So far, we've been able to identify the 10,000 Islands and parts of Everglades National Park in Southwest Florida as important sawfish pupping areas, or where they give birth," said Carlson. "Where they're going after that, and what's driving these movements is still being researched."
Once born (at approximately 2.5 feet in length), the juveniles—or young sawfish—stay in the shallow, mangrove-lined coastal areas of Everglades National Park and the Ten Thousand Islands National Wildlife Refuge. There, they can hide from predators among the complex mangrove tree root systems and feed until they're about 3 to 4 feet long. Then they start to move and expand their home ranges.
Adult sawfish are most commonly observed in coastal waters such as the Florida Keys, sometimes out to depths of 230 feet. Sightings outside of this region of South Florida are uncommon. In recent years, they've been seen as far west as Mississippi in the Gulf of Mexico, and as far north as South Carolina in the Atlantic. This is a positive sign that the species is returning to its former geographic range.
While NOAA Fisheries led this year's field effort, this work wouldn't be possible without valuable partnerships, including with:
- University of Miami Cooperative Institute for Marine and Atmospheric Studies
- Florida State University
- Florida Fish and Wildlife Conservation Commission
- Havenworth Coastal Conservation
- Florida Museum of Natural History
Critical habitat gaps
In 2009, NOAA Fisheries designated two areas along the southwestern coast of Florida as critical habitat for the U.S. distinct population segment of smalltooth sawfish. These areas provide important juvenile nursery habitat, where young sawfish can find food easily and avoid predators. This designation was based on limited information particular to young sawfish. Knowing where the adults reside, their preferred habitat features in these sites, and movements among sites will help inform critical habitat designation for the larger life stages as well.
An unexpected year
Our partners at the Florida Fish and Wildlife Conservation Commission have been documenting reports of abnormal fish behavior, including spinning and whirling, in South Florida since December 2023. Smalltooth sawfish were hit particularly hard by this unusual event: 227 sawfish were observed with symptoms and 56 were found dead. Affected sawfish were all large juveniles or adults, raising concern about the reproductive stock of this species. The Commission continues to lead the effort to investigate the abnormal behavior and cause of death, which at this time is unknown.
"While our sampling trips were not specifically planned in response to the recent sawfish deaths, these data may help us detect if there are any changes to the recruitment of the juvenile population," added Carlson. "While the true number of sawfish mortalities is unknown, I'm hoping we don't see a significant decline in our relative abundance index over the next few years."
How you can help
Report sightings of healthy, sick, injured, or dead sawfish to the Florida Fish and Wildlife Conservation Commission's Sawfish Hotline. Include date, time, and location of the encounter, estimated length, water depth, and any other relevant details.
- (844) 4SAWFISH (844-472-9347)
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