There are a wide variety of methods for tracking and studying populations of animals, from satellite tagging to aerial surveys to tissue samples. However, a new effective means of studying rare marine animals has arisen, and was recently used to study whale shark populations. Seawater environmental DNA (eDNA), or DNA collected in thesurrounding seawater, was used to predict population sizes and relations amongwhale sharks. Scientists from Denmark and Qatar collected water samples and then used DNA sequencing machines to decode the mitochondrial DNA found in the samples. The amount of DNA and the differences between them can be used to determine population sizes and to distinguish between related and unrelated whale sharks. The scientists then compared the DNA from the water samples to DNA previously taken from whale shark tissue samples, and used them to match haplotypes together.
This study took 20 seawater samples off of the coast of Qatar to identify whale shark populations. It discovered that whale sharks will group together between the Arabian Gulf and Indo-Pacific region, but will not group with Atlantic whale sharks. Sharks in the Arabian Gulf and Indo-Pacific region were more closely related due to similar genes, while Atlantic whale sharks had vastly different genes from the ones found around Qatar. The scientists also collected eDNA from mackeral tuna, and discovered that the amount of mackeral tuna eDNA in seawater was proportionate to the amount of whale shark eDNA. The more tuna there are, the more whale sharks that will be present, while fewer tuna results in fewer whale sharks. This indicates that the whale sharks likely feed on the spawn of mackeral tuna.
Though the methods of estimating population size are far from perfect, it is a good start. The predicted range of population size of whale sharks is very large, and ocean currents can move the eDNA to different locations. However, this method of study is beneficial to both humans and whale sharks, as it avoids the stress and potential damage towards whale sharks from tagging and tissue samples all while still allowing scientists to study population sizes. With the continuing advancement of technology and study techniques, the population size range can be narrowed down further and further until it reaches greater accuracy with little variation.