Research into the smalltooth sawfish in Florida and The Bahamas is gradually revealing important information about this mysterious species. Perhaps the biggest question of all is whether marine national parks can provide sanctuaries in which its population can recover.
The sawfishes (family Pristidae) are a small but highly charismatic group of very large batoids, or rays. They are the only living batoids that possess a toothed rostrum, which they use for defence and to catch prey – imagine a shark-like ray up to five metres (16 feet) long that has a giant hedge trimmer at the end of its snout! Worldwide, there are five living species of sawfishes (1) and although they all reach a length of at least three metres (10 feet), these unusual creatures often go unnoticed, spending much of their lives lying on the bottom in the murky coastal and estuarine waters of the tropics. For decades, sawfishes didn’t receive the attention they deserved from scientists – or the conservation community either. That has changed over the past 15 years or so, as it became apparent that populations around the globe had collapsed.
In view of their imposing size, it is not surprising that, as adults, sawfishes are among the top predators in the ecosystems they inhabit. As we go about our research, it isn’t unusual to find the scales of large bony fishes impaled on the ends of their needle-sharp rostral teeth. And on multiple occasions we have caught an adult sawfish on our line that was not even hooked, but had swallowed a whole metre-long (three-foot) shark that had been caught on the line first! In attempting to remove the hook, I have been able to reach into the sawfish’s mouth and pull out the shark, hook and all, by the tail, removing it completely from the sawfish.
A study recently published by our research team has demonstrated that in Florida juvenile to adult smalltooth sawfishes occupy essentially the same position in the food web as bull sharks of the same life stage (2). Like many species of sharks, sawfishes have been the subject of myths, fear and misunderstanding. This is summed up in an article by Erwin Bauer titled ‘Mystery Monster’ that appeared in a 1959 issue of the US outdoor magazine Field & Stream. Bauer wrote, ‘Like an axe murderer, he’s been in the headlines… There have been assertions that sawfish stalk bathers… and cut them in two. According to old whaling journals, the saws hunted whales in packs and sliced them up like so much salami. There are even accounts of sawfish attacking men in dories after reducing the boats to driftwood.’
Of course, none of this is true. While I can attest that sawfishes are incredibly defensive when captured, and a large one can certainly inflict serious injury, there are no records of sawfishes attacking bathers, whales or men in boats!
The Field & Stream article went on to state that ‘Marine researchers are finding [sawfish] wholesale almost everywhere. It’s hard to predict where [they] will poke their lethal noses next.’ Oh, how we wish that were true today!
Sawfishes occur in tropical coastal waters where their large toothed rostrum renders them highly susceptible to entanglement in all types of fishing gear. Targeted commercial fishing in some regions, sport fishing, the collection of rostra for the curio trade and personal memorabilia, and by-catch in net and long-line fisheries have all led to declines in sawfish populations the world over. Like most large sharks, sawfishes have conservative life histories that limit how quickly they can recover if threats should abate. By 1996, four of the seven species of sawfish recognised at the time were designated as Critically Endangered or Endangered by the International Union for the Conservation of Nature (IUCN). All five of the currently recognised species (3) are now listed as Critically Endangered or Endangered, distinguishing the Pristidae as the most imperilled family of elasmobranch species, if not the most endangered of all marine fishes (4, 5).
The smalltooth sawfish Pristis pectinata is endemic to the Atlantic Ocean and is the only sawfish species native to the USA and the islands of the Greater Caribbean Basin. In the United States, its range contracted during the second half of the 20th century and its population was estimated to have been reduced by perhaps as much as 95%. The primary stronghold for the smalltooth sawfish in the western Atlantic is, and always has been, south-western Florida. Population declines in this region became apparent in the early 1990s, resulting primarily from the capture of the sawfishes in gill-net and trawl fisheries. This led to legislation listing the smalltooth sawfish as a protected species in Florida’s waters in 1992, and in 2003 it became the first native marine fish to be registered as Endangered under the US Endangered Species Act. In addition, all sawfish species are now listed on Appendix I of the Convention on International Trade of Endangered Species (CITES), which prohibits international commercial trade.
Research in Florida and The Bahamas
Since 2010, my colleagues, graduate students and I have been conducting research aimed at promoting the recovery of the smalltooth sawfish in the USA and assessing its status in The Bahamas. It’s a species that typifies the challenges of assessing population status, not to mention recovery, in widely distributed marine fishes. Its baseline population sizes are unknown and long-term relative abundance surveys do not exist, so we are unable to track declines. However, significant range contraction and regional extirpations suggest that drastic population declines occurred throughout this sawfish’s range during the second half of the 20th century. In the western North Atlantic, a large majority of the recent confirmed smalltooth sawfish records are from the USA, where the current range is restricted primarily to south-western Florida. At the time we began our work in 2010, research into juvenile sawfishes had been ongoing in Florida for a decade. It was hypothesised then that Florida could be the only region remaining with a reproducing population.
The Bahamas is the only other country with relatively frequent records of smalltooth sawfish and the research we have conducted since 2010 suggests that a viable population also exists here, mainly on the western side of the island of Andros (6, 7). Lying in one of the most remote and unspoiled parts of the Greater Caribbean Basin, Andros harbours as much mangrove habitat as Everglades National Park in Florida. Working here is logistically challenging, which is precisely why the island’s ecosystems remain intact. In 2015, we assembled data from a variety of sources, including biologists, fishing guides and divers, throughout The Bahamas in order to assess the probable distribution of the smalltooth sawfish among the more than 700 islands and cays that make up the archipelago. Not surprisingly, a large majority of the records were from the island of Andros. Bimini was the second most popular location, and together these islands on the Great Bahama Bank accounted for more than 80% of The Bahamas’ records of smalltooth sawfishes between 2002 and 2015 (7).
Juvenile sawfishes and their nursery habitat
Nearly all of the initial sawfish research in Florida has been on juveniles. Although a sawfish giving birth had never been observed, the areas of parturition in Florida are well documented by the occurrence of cohorts of newborn sawfishes from Florida Bay to Charlotte Harbor on the south-western coast. Our group and colleagues have conducted a lot of research on the patterns of habitat use by juvenile sawfishes in these nurseries and much of it suggests that they have a strong affinity for the shallow water along red mangrove shorelines and islands in bays, rivers and backcountry creeks for at least the first year of life (8, 9). Critical habitat for the species, designated in Florida in accordance with the US Endangered Species Act, is based on these patterns of juvenile habitat use (10).
In the Everglades, we often observe that small juvenile smalltooth sawfishes associate with very specific mangrove islands, ignoring others. They are generally found only among red mangroves, probably because the dense pneumatophores of black mangroves deter them. The density of red mangrove prop roots and the water depth appear to be critical factors influencing which red mangrove islands are inhabited. Small sawfishes are vulnerable to predation not only by bull and lemon sharks, but potentially also by American alligators and American crocodiles in this region. Prop roots need to be spaced so that the juvenile sawfishes can move between them for cover at high tide, but dense enough to exclude predators. When the tide falls and the sawfishes are forced from the mangroves, the adjacent mud flats or channels must be deep enough that they don’t dry out, but shallow enough to exclude larger sharks.
Active tracking in the remote Everglades backcountry by graduate student Lisa Hollensead showed that these young-of-the-year sawfishes have incredibly small daily activity spaces (0.07–0.17 square kilometres; 0.027–0.06 square miles) that aren’t static, but move slightly from day to day (8). Interestingly, we found that the space occupied by a juvenile sawfish is larger during the day than during the night, but the rate of movement is higher during the night than by day. We hypothesise that the higher rates of movement in a more confined area at night may be related to hunting and feeding. High prey densities in these nurseries enable smalltooth sawfishes to grow incredibly fast as juveniles, more than doubling in length from 0.7 metres (2 feet 3 inches) to 1.5 metres (4 feet 11 inches) in the first year and reaching more than two metres (6 feet 7 inches) by two years of age. Rapid growth leads to lower predation risk, thus allowing juvenile sawfishes to begin exploring other habitats such as deeper rivers, creeks and bays (9).
When we began our work in The Bahamas, there were no known parturition sites or nurseries for smalltooth sawfishes outside Florida. We could find no records of small juveniles anywhere in the region, leading to speculation that perhaps sawfishes in The Bahamas were born in Florida. During our initial trips to Andros, we developed a strong relationship with the Flamingo Cay Lodge, the only establishment on west Andros, and its fishing guides. The guides reported seeing small juvenile sawfishes and with them we observed sawfishes that were probably young-of-the-year, assuming that Bahamian sawfishes grow at the same rates as their Florida counterparts. This assumption may not be valid, however, in view of research having shown that lemon sharks in The Bahamas grow much more slowly than those in Florida. Nevertheless, the occurrence of such small sawfishes suggested to us that Andros may be a pupping area.
We have made numerous expeditions to Andros since 2010, supported by multiple foundations and grants, with the Save Our Seas Foundation playing the dominant role. Although working in such a pristine place has been rewarding, the research was also frustrating. By the end of 2015 we had tagged nearly 50 adult sawfishes in Florida, yet over seven expeditions to Andros during the same period we had only managed to capture four sawfishes.
Finally in December 2016, during an SOSF-funded expedition based on the R/V Garvin operated by The Field School, we were rewarded with proof that sawfishes do indeed give birth in The Bahamas. We captured the largest sawfish we have caught to date in The Bahamas: a 4.29-metre (14-foot) female – and she was giving birth! This was the first time this has ever been observed in the wild and it provided the only evidence to date of pupping in the western Atlantic outside the USA.
The female delivered five pups, though more were still in the uteri when we released her. All the pups emerged rostrum first, with the rostral teeth covered in thick connective tissue.
At approximately 0.7 metres, these pups were the same length as the newborn sawfishes we frequently capture in Florida. All five of the newborns were tagged with microchips similar to those used on dogs and cats and we hope that future recaptures of them will provide data on juvenile growth that may be compared to data from Florida.
Are sawfishes in Florida and The Bahamas distinct populations?
A critical determinant of whether the smalltooth sawfish is likely to recover across its range is the amount of exchange (or degree of isolation) between population segments. The fact that the species has been extirpated from large parts of its range suggests that recovery throughout the range requires populations to be connected. If there is mixing of adult sawfishes from multiple population segments, then putting significant effort into promoting revival in the core population may lead to range-wide recovery. However, if populations are isolated, then serial depletion may put the species at greater risk of extinction unless recovery efforts are spread across all extant populations.
Pop-off archival satellite tagging is a valuable tool in determining whether adult and large juvenile sawfishes undertake seasonal migrations and if there is movement between the USA and The Bahamas. As part of a large research effort, we deployed more than 70 satellite tags on Florida sawfishes between 2002 and 2016, although data were collected from only about half of these. In addition, since 2010 we have successfully satellite tagged six large juvenile or adult sawfishes on Andros.
In Florida, our ongoing surveys and telemetry data show that adult male and female sawfishes use both the backcountry mangrove habitat and the deeper waters (40–70 metres; 130–230 feet) along the edge of the continental shelf. These deeper habitats are buffered from temperature regimes and appear to offer thermal refuge in winter and summer, as well as increased foraging opportunities for large sawfishes. Data from the only adult sawfish tagged in Andros to date suggest that Bahamian adult sawfishes may use similar habitats at the edge of the Great Bahama Bank. Importantly, these deeper shelf-edge habitats would be the start-off point for sawfishes that may cross between Florida and The Bahamas.
Analyses of the satellite telemetry data (11) suggest, however, that sawfishes tagged in Florida stayed in Florida and those tagged in The Bahamas stayed in The Bahamas. The sawfishes in both regions spent most of their time in waters shallower than 10 metres (33 feet). The deepest depth recorded by any tagged sawfish was 68 metres (223 feet) and they rarely ventured into waters below 20 °C (68 °F; the coldest experienced was 18 °C, or 64.4 °F). Crossing from Florida to The Bahamas requires crossing depths of more than 800 metres (2,625 feet) in the Florida Straits, where the bottom temperatures are less than 6 °C (42.8 °F). Therefore, movements between Florida and The Bahamas would require about 80 kilometres (50 miles) of pelagic swimming by a batoid that spends its life on the bottom.
Over the past several years, large arrays of acoustic receivers have been deployed by collaborative groups of researchers all over the world. Hundreds of receivers are currently deployed along the US Atlantic and Gulf coasts and throughout The Bahamas. We have added receivers to arrays in the Florida Everglades and on the west side of Andros. In 2016, we began implanting long-term acoustic transmitters in sawfishes in both regions. Since these tags will remain active for 10 years, this technology promises to provide a wealth of information about sawfish movements and habitat use.
In a little over one year, we have received thousands of detections of our tagged sawfishes on more than 80 receivers in the USA. These preliminary data indicate that adult sawfishes in Florida move frequently between Florida Bay in Everglades National Park and the Florida Keys reef tract. Many tagged sawfishes appear to use the deeper shelf edge habitats in this region throughout the year, although in summer some adults migrated 500–900 kilometres (310–560 miles) north from the Florida Keys. Importantly, no sawfishes tagged in Florida have been detected in The Bahamas.
Similarly, data from our first year in The Bahamas suggest that sawfishes are probably resident year round in the mangrove backcountry and creeks of north-western Andros until they approach maturity. The only adult sawfish tagged to date at Andros was the pregnant female from December 2016. Interestingly, when our colleague Kevin Feldheim analysed her genetics, he discovered that he already had a sample from her in the database. She had been caught and sampled in Bimini in 2002, nearly 15 years before we captured her at Andros, and had grown from 2.6 metres (8 feet 6 inches) to 4.29 metres (14 feet), providing an important indication of growth rates.
Taken together, our data suggest that The Bahamas and the USA contain isolated populations of the Critically Endangered smalltooth sawfish. The large collaborative receiver arrays and long-term transmitters will enable us to determine this with more certainty in the next few years.
Adult sawfishes: movements, habitat use, fisheries interactions
One of the major goals of our work to investigate the habitat use and migration of adult sawfishes was to delineate the areas with highest potential interactions with different fisheries, particularly commercial fisheries where most mortality occurs. If these habitats are discrete and sawfishes aggregate to them predictably, time-area closures for specific fisheries could be effective measures to decrease mortality.
Our findings suggest that adult sawfishes do aggregate. We have captured nearly 60 adults in Florida on scientific long-line sets. Of the one-hour sets, 30% caught more than one sawfish, often on successive hooks. On two occasions, six sawfishes were caught together on the same set. One aggregation area is the northern part of Florida Bay in Everglades National Park. Adults occur in this region year round, but large numbers of adult males aggregate here from March until August (12). Large sawfishes are frequently caught by recreational and charter fishers in this region, but mortality is believed to be low.
We also discovered that adult sawfishes aggregate in shelf-edge habitats 40–70 metres (130–230 feet) deep along the entire Florida Keys. Divers have reported similar aggregations along Florida’s east coast at shallower depths (15–25 metres; 50–82 feet). In the USA, these shelf-edge habitats are where most sawfish by-catch mortality occurs in shrimp trawl and demersal long-line fisheries.
The sawfish aggregations comprise both males and females and, based on our surveys and telemetry data, they do not appear to be seasonal. In fact, we captured seven adult sawfishes (five females and two males) together in this region in January and six adults (four females and two males) together in July. Our satellite telemetry data suggest that use of these deeper habitats is ephemeral, as sawfishes move back and forth from shallower creeks and backcountry regions to the deep shelf edge. The function of these movements is unknown, but may be related to foraging and thermoregulation.
Much of the basic biology and ecology of sawfishes was a mystery when we began our research, and unfortunately much of it remains a mystery. There are still so many questions: at what age and size do sawfishes reach sexual maturity? Where does mating take place? Does mating occur in aggregations or in solitary pairs? How often do female sawfishes give birth? How many sawfish pups are born in each litter?
Over the past few years we have begun to answer some of these questions, though often our findings only spawn new questions. My colleague Jim Gelsleichter has been examining the cycling of sex steroid concentrations in the blood plasma of the large sawfishes we capture to determine at what size they reach maturity as well as when, where and how often mating and parturition take place. We couple this with ultrasonography of females to assess pregnancy. Males appear to reach sexual maturity at about 3.6 metres (11 feet 9 inches) total length and females at about 3.8 metres (12 feet 6 inches).
No one has observed sawfishes mating, but given the toothed weapon on the rostrum, we predict it is a raucous affair resulting in scars in the form of parallel scrapes and punctures. Such scars are not seen on the aggregated sawfishes we encounter in the deeper habitats and we confirmed that some of the females were already pregnant. However, we hit a string of sampling luck in April 2017 when, in two different regions, we captured adult males and females together with fresh mating scars on both sexes! These were in the far backcountry of Everglades National Park and in a creek in the lower Florida Keys National Marine Sanctuary, and hormone analyses confirmed that mating had probably been taking place.
Interestingly, the potential mating aggregation in the backcountry occurred in the same habitat where parturition is known to occur; in fact, newborns were caught there the day the adults were seen. This would be expected in a species where the females undergo vitellogenesis during pregnancy and are ready to mate following parturition. Many small coastal sharks with annual reproductive cycles as well as many deep-sea sharks do this. In most large elasmobranchs with multi-year reproductive cycles, vitellogenesis takes place during the resting year, so mating and parturition don’t occur in the same habitat. Multiple lines of evidence indicate that sawfish gestation is one year and mating takes place every two years. The observation of apparent mating in the primary nursery habitat is therefore surprising and begs additional work.
National parks as ‘lifeboats’?
Habitat loss from urban development, agriculture, dredging and the diversion of freshwater has exacerbated the declines in sawfish populations worldwide and in some cases may be the greatest hindrance to recovery. For smalltooth sawfish in The Bahamas and the USA, there are large tracts of primary habitat that are protected from development. Unfortunately, there are also large areas of sawfish habitat that are heavily altered by development in both countries.
In The Bahamas, the west side of Andros Island is one of the most remote and unaltered tropical systems in the Atlantic Ocean. Recently proclaimed the Andros West Side National Park, it has the potential to protect these pristine habitats in a country where a tourism-based economy has traditionally led to gross overdevelopment and habitat degradation. Indeed, large sections of mangrove habitat on the island of Bimini, which once supported at least transient sawfishes, have been destroyed to build a large marina, casino and resort, and further development is planned.
In Florida, large tracts of sawfish habitat are protected in the remote and relatively pristine Everglades National Park and the Ten Thousand Islands National Wildlife Refuge, but Charlotte Harbor and the Caloosahatchee River, the other major sawfish nursery region, is heavily urbanised. Graduate student Bianca Prohaska has shown that juvenile sawfishes living in the heavily degraded nursery region in Florida may suffer from chronic physiological stress compared to those residing in the pristine habitat (13).
Nick Dulvy and his colleagues5 have coined the term ‘lifeboats’ for regions such as Florida and northern Australia, where there is hope that sawfish populations may recover and their extinction be prevented. In the western Atlantic, the Everglades National Park in the USA and the Andros West Side National Park in The Bahamas potentially represent 6,070-square-kilometre (1.5 million-acre) and 5,260-square-kilometre (1.3 million-acre) ‘lifeboats’ respectively for smalltooth sawfish recovery. In Florida, due to the protection of sawfishes instituted in 1992 and a 1994 ban on entanglement (gill) nets that decreased by-catch mortality, several lines of evidence, including our survey data, suggest that the US population of sawfishes is slowly increasing. The status of the sawfish population in The Bahamas is unknown and currently sawfishes are not protected in that country. Fortunately, no targeted fishery or large-scale commercial fishery that would take sawfish as by-catch exists there.
One of the challenges to assessing recovery in an endangered species is developing recovery criteria that can be assessed and are biologically meaningful. Often, baseline densities and environmental carrying capacities are unknown. Such is the case with the smalltooth sawfish in Florida and The Bahamas. We don’t know how many sawfishes were in the population when it was at carrying capacity before the decline began. Due to deterioration in available habitat, prey resources and the populations of predators and competitors, the current carrying capacity for sawfishes in these regions is probably very different from what it was a century ago, so recovery to pre-decline numbers or densities is unrealistic.
Evidence of a population increase like the one we are seeing in Florida is very encouraging. Should that increase halt, indicating that the population had reached a plateau, the challenge would be to distinguish a stop in recovery from a population reaching its new carrying capacity. The former would call for stronger conservation actions, whereas the latter would indicate the species could be downlisted to a less threatened category. Assessments of range expansion and spill-over, or increased abundance in areas outside the core, will help distinguish between these possibilities.
Everglades National Park and Andros West Side National Park have similar areas of available mangrove habitat for sawfishes, but the characteristics of the habitat – abiotic fluctuations, relative primary productivity and the available prey resources – are very different. These two important areas are likely to have different sawfish carrying capacities, which must be considered when assessing conservation success. There are probably other areas that may be important to the recovery and maintenance of genetic diversity in the western Atlantic smalltooth sawfish. We have received photographs of small sawfishes on the Little Bahama Bank, which suggests there may be a second Bahamas ‘population’ there. We also know there are contemporary records of sawfishes in Cuba. As movement between any of these four regions (Florida, Great Bahama Bank, Little Bahama Bank and Cuba) requires crossing water deeper than 600 metres (1,700 feet), exchange between them may be limited. However, whereas the distance between Florida and the Great Bahama Bank is 85 kilometres (53 miles), only 25 kilometres (15 miles) separate the Great Bahama Bank from Cuba. Perhaps in the future we will find these are additional ‘lifeboats’ to sawfish recovery.
This article summarises some of the research conducted by a large team of scientists and volunteers. I thank all my collaborators, including John Carlson, Andrea Kroetz and Dana Bates, Shelley Norton, and Adam Brame of NOAA Fisheries; Jim Gelsleichter of the University of North Florida; Mark Bond, Yannis Papastamatiou and Demian Chapman of Florida International University; George Burgess and Lindsay French of the Florida Museum of Natural History; Gregg Poulakis of the Florida Fish and Wildlife Conservation Commission; Kevin Feldheim of the Field Museum; Grant Johnson and Tristan Guttridge of the Bimini Biological Field Station; Tonya Wiley of Haven Worth Consulting; Sonja Fordham of Shark Advocates International; University of North Florida graduate students Brenda Anderson, Arianne Leary, Amanda Brown, Sam Ehnert and Chelsea Shields; Florida State University graduate students Lisa Hollensead, Bianca Prohaska, Johanna Imhoff, Cheston Peterson, Matt Kolmann, Bryan Keller and Brian Moe; and the many volunteers who assist our research. I also thank Charles and Cindy Bethell and the guides from the Flamingo Cay Rod and Gun Club, The Field School and the associated crew of the R/V Garvin, and Michael Scholl and the Save Our Seas Foundation for facilitating the Andros work. Research in Florida has been supported by funding from the NOAA Office of Protected Resources, NOAA Section 6 Program, Department of the Navy and the Northern Gulf Institute. Research in Andros has been supported by the National Geographic Society, the Moore Charitable Foundation, Ann Luskey, the Rackley Family Foundation and the Save Our Seas Foundation.
Sawfishes are rapidly disappearing from our seas, so when a healthy population was discovered off Andros Island in The Bahamas, the area became a very important place. Dean aims to understand this rare community of sawfishes in order to protect them.