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).

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.

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.

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 colleagues (5) 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.