John Swenson
Who I am
I don’t know why or where it came from, but I have felt drawn to the ocean and captivated by elasmobranchs (sharks and rays) for as long as I can remember. Over the years, as I studied elasmobranch biology and sought opportunities to interact with the animals, my casual interest transformed into a passion; the more I’ve learnt, the more I’ve come to realise how exceptional sharks and rays are. Their uniqueness arises from their evolutionary history. Sharks and rays (and chimaeras) took their own evolutionary path relative to other vertebrates about 450 million years ago. This does not mean that they are ‘ancient’ or that they are any less evolved than land animals; rather, such a divergent evolutionary history implies that sharks and rays are just very different. The accumulation of millions of years of small evolutionary differences manifests in the present day as the behaviour and biological attributes that make sharks perpetually fascinating to people. Unfortunately, the same distinctions make them likely to be misunderstood. Having had many personal encounters with sharks and rays, I’ve consistently been struck by their gentle curiosity, while only once have I felt threatened. Common perception would have expected the opposite. The broad goal for my PhD research (and beyond) is to use science to decode these enigmatic animals in ways that can inform how they are managed and ensure their persistence for the indefinite future.
Where I work
This project focuses on a population of whitespotted eagle rays off the coast of Sarasota, Florida. Eagle rays in the eastern Gulf of Mexico are largely protected from fishing pressure – one of the primary drivers of population declines worldwide – but remain susceptible to habitat loss, prey loss and toxic algal blooms. Fortunately, researchers have been collecting non-lethal tissue samples from the population for more than a decade and during this time have gathered ample data to inform the construction of a close-kin mark-recapture model that we can use to estimate the abundance of the population and better understand the magnitude of the threats it faces.
What I do
Close-kin mark-recapture combines recent advances in molecular technology, statistical methods and computational power to effectively ‘count’ the number of individuals in a population based on a representative sample of that population. A close-kin mark-recapture project can be broadly split into three phases: field work, lab work and data analysis. Field work involves collecting tissue samples from individual animals, which can mean taking small biopsies from animals in the water or taking fin clips from animals that have been caught by fishermen. Once tissue samples have been collected and transferred to the lab, we expose the tissue samples to enzymes at a high temperature, which breaks the tissue down and exposes the DNA. We isolate the DNA with microscopic magnetic beads and wash away all the non-DNA material from the solution. After confirming that the DNA is high quality (DNA can break down over time), we shear the DNA with sound waves and give it a unique synthetic ‘barcode’, which enables us to trace each DNA sequence back to the individual animal it came from. Following various preparatory steps, we send the DNA from all the sampled individuals to a sequencing centre, which returns to us a series of very large text files with the DNA sequences of every individual. At this point we transition into the data analysis phase. Data analysis entails using supercomputers to analyse the very large text files with the DNA sequence data and working out who’s related to whom. These relationships are used in a statistical algorithm that reports the most likely population size that would have given rise to the number of relatives we observed among our samples. So, my work involves a little bit of field work, a moderate amount of lab work and a whole lot of computer work!