Sawfish CSI: using DNA to unravel the past and protect the future

Dr Nicole Phillips and Kelcee Smith

The most distinct part of a sawfish is undeniably its saw. But this saw is also the Achilles heel of the sawfish, because it makes this iconic creature so susceptible to capture in fishing lines or nets and valued as trophies by recreational fishers. It is largely agreed on by experts that sawfishes have undergone substantial declines in range and abundance due to human activities, but the evidence for these declines is largely anecdotal. There is little information regarding past abundances of most sawfish populations, and it would seem impossible to go back in time to determine how large sawfish populations may have once been. It also seems impossible to know how ‘healthy’ these past populations were compared to the sawfish populations of today. But is it impossible? And why is it important to compare past and present populations of sawfishes?

 

A historic photo of a large number of sawfish captured by fishers.

Declines in abundance are typically also accompanied by declines in genetic diversity. Genetic diversity, in essence, is variations in DNA sequences, and when talking about genetic diversity in populations, this refers to animals in a population with different DNA sequences. Maintaining moderate to high levels of genetic diversity in a population is important because it allows a population to change and adapt to changes in the environment. Populations that have undergone drastic and sudden declines in abundance tend to have lower levels of genetic diversity, which may be less ‘fit’, more susceptible to disease, and less able to adapt in a changing environment.  If a population is very small, there may also be a risk of inbreeding, which is when close relatives mate and produce offspring that could have reduced fitness and possible birth defects, such as deformities.  

These peppered moths are the same species but differences in their DNA sequence has resulted in different physical appearances, making each more 'fit' or better suited for different environments. Genetic differences within a population may be relatively small and are not always observable.

It is important to have an understanding of the levels of genetic diversity in present day sawfish populations so we can identify and protect populations that are genetically less diverse. It is also important to understand how the levels of genetic diversity in present sawfish populations compare to those in the past to determine if these levels have declined over time. We are fortunate to have old sawfish saws available in museums, and sometimes in private collections, as these saws can be used as a resource for studying past sawfish populations. Some of these saws date back to the 1800’s and are the only remaining evidence that sawfish once existed in certain locations around the world. These old saws not only provide insights into the levels of genetic diversity of past populations, but the DNA can even be used to estimate past population sizes, which can be compared to current day sawfish population sizes to look for evidence of declines in abundance.

A sawfish saw that was kept as a fishing trophy.

Such research is not without its own set of challenges. Finding adequate numbers of samples is the first major hurdle; working closely with many researchers and museums is key. Once the samples are collected, the next set of challenges researchers are faced with is generating usable data from old saws that were not stored for DNA research. While at Murdoch University in Western Australia, and with some help from an honors research student, Dr. Nicole Phillips spent a number of years working out the best type of tissue to sample on a saw. This work included testing various ways to extract the DNA and testing different genes as well as different sizes of genes, since the DNA is often broken down into many small fragments. A lot of time was spent during this developmental phase so other researchers could take samples from rare saws efficiently and effectively. Nicole’s research into levels of genetic diversity of past sawfish populations has focused on the largetooth sawfish (Pristis pristis) and the green sawfish (Pristis zijsron) and she will be continuing this research at the University of Southern Mississippi later this year with a number of other researchers. This research will compare levels of diversity in past populations of the largetooth and green sawfishes to those of current populations to determine if there has been substantial declines in diversity (and abundance) and what may have been lost in terms of genetic diversity from regionally extinct populations around the world. 

Dr. Phillips taking tissue samples from an old sawfish saw in northern Australia.

Dr. Sabrina Taylor and Kelcee Smith at Louisiana State University are currently tackling the question of whether there has been a loss of genetic diversity, and by extension, effective population size in the smalltooth sawfish (Pristis pectinata) in Florida. They are collaborating with scientists from the National Marine Fisheries Service (NMFS) and the Field Museum of Natural History. The smalltooth sawfish is currently the only species of sawfish found in U.S. waters and is listed as Endangered (since 2003) under the Endangered Species Act. If this research finds that there has been a substantial loss in genetic diversity over time, then management efforts should focus on maintaining and increasing abundances of these sawfish, particularly in a variety of locations and habitats across their distribution in Florida.

These research projects may demonstrate that genetic diversity in sawfish populations have declined, indicating that there have been substantial decreases in abundance of sawfishes. Alternatively, we may find that the current levels of diversity have been maintained throughout the 1900’s despite the anecdotal evidence of large declines during this time period. Regardless, the opportunity to use sawfish saws in genetic analyses provides us with a glimpse into the past and provides information vital to the conservation and recovery of these sawfish species.