Beyond the Aquarium
Detecting Sawfish DNA
in Madagascar’s Mangroves
By Ruth H. Leeney
Protect Africa’s Sawfishes
Sawfish are amongst the most endangered of all sharks and rays worldwide.
A lack of knowledge about where sawfish populations still exist, outside of the remaining strongholds of Australia and Florida, greatly hinders conservation efforts.
My research in Madagascar began in 2015, when I spent three months traveling the north and west coasts of the country to interview fishers and fish traders. I wanted to know if they still encountered sawfish and, if so, where they were most often caught or seen in markets and fish landing sites.
Parts of Madagascar’s coastline cannot be reached by road, and this means that some of the country’s mangroves, which are important habitats for sawfish, are still in good condition. However, sawfish populations around Africa have declined dramatically in recent decades, and Madagascar has a long history of shark fisheries.
From my interviews with fishers and fish traders, it was clear that largetooth sawfish were still present in certain areas, but are much less common now than they were in the past. It certainly seemed a far more positive result than on the African mainland, and I suspected there might be an opportunity in Madagascar to protect this Critically Endangered species. But to really confirm the presence of sawfish in specific habitats, I knew I would need additional evidence.
In recent years, a new technique—called Environmental DNA, or eDNA—is vastly improving the ability of scientists to detect very rare or cryptic (difficult to find) species. In aquatic environments such as rivers, lakes, and even along coasts, the animals living there leave traces of their genetic material—their DNA—in the water. That DNA can be extracted from the water and used by researchers to identify which species live there.
Working with collaborators, including some local fishermen, I collected water samples from several sites in Mahajamba Bay and Bombetoka Bay in northwestern Madagascar. We filtered the water to extract all the DNA then added a fixative to the vials to preserve the DNA. The samples were then sent to the specialized eDNA laboratory at Stellenbosch University, where honors student Michelle Maberly analyzed them to see if any contained sawfish DNA. To check that the technique was working, I collected water samples from the tank containing a largetooth sawfish at Océanopolis Aquarium in Brest, France, and those samples were analyzed in the same way.
Through our collaboration with the Oceanographic Research Institute in Durban, South Africa, we also had samples of cartilage (the hard substance that sharks and rays have instead of bones) from several sawfish saws. By extracting DNA from the cartilage, Michelle was also able to test her analysis methods using DNA known to have come from largetooth sawfish.
A largetooth sawfish at Océanopolis Aquarium in Brest, France
In the laboratory and using the extracted DNA, Michelle targeted a small section of genetic code that is found only in sawfish. The positive samples from the cartilage, as well as the eDNA samples from the aquarium water, all worked well and showed that the eDNA method was able to detect sawfish from water samples.
For the samples collected in the rivers in Madagascar, the results were unfortunately inconclusive, with only a small number of samples indicating the presence of sawfish. The team repeated the process with samples from Madagascar many times, but unfortunately did not achieve consistent results, so at this stage it is not possible to confirm the presence of sawfish in the study area. Sawfish may well be present there, but at such low numbers that finding their DNA is more difficult than finding the proverbial needle in the haystack.
Sawfish may also only use these areas at certain times of the year, and, if this is the case, they are only detectable when they are in the area, as DNA breaks down after only a few hours or days. Samples from areas containing lots of sediment—like those collected in Bombetoka Bay, where the water looked like hot chocolate—also present additional technical challenges as sediment can prevent the procedures in the lab from working. Sampling will be repeated in these sites and will be continued in another large river in Madagascar later in 2020.
The reddish-brown water of the Betsiboka River
So, why is this work necessary? If we can identify areas where sawfish are still present, we can identify the specific threats faced by sawfish in those areas and work in collaboration with local communities to protect sawfish and encourage more sustainable fisheries.
However, eDNA analysis is no replacement for building strong relationships with communities and encouraging them to report sawfish catches, as this allows researchers to understand what drives people to catch sawfish, and thus provides essential information needed to design locally relevant conservation and management plans.
In early 2019, my colleague Rindra was contacted by a community leader, Mr. Sylvestre, who had accompanied us on my eDNA sampling trip in Bombetoka Bay. A fisher in his community had caught a sawfish, and because of Mr. Sylvestre’s rapid action, Rindra was able to visit the village and take photos of the catch. Sadly, the sawfish was killed and eaten, as is common in low-income countries where people have to put their own survival before conservation priorities.
Nonetheless, this is the first report of a sawfish catch, in near real time and accompanied by a photograph, to have been recorded since I began working in Madagascar, and shows the value of building relationships and sharing educational tools with communities and local NGOs.