Whale Sharks: Where Do They Come From, Where Will They Go?

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Research by Claire Prebble in the faculty of Ocean and Earth Sciences, Southampton, has revealed insights into the lives of whale sharks, elucidating our knowledge of their geographic range and where they choose to feed.

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Whale sharks, despite their threatening name, are filter feeders and can vacuum up as much as 2.8kg of plankton: marine worms, algae, crustaceans and krill per hour. Clare Prebble, a PhD student and project manager at Marine Megafauna Foundation, studied whale sharks in Mozambique, Tanzania, and Qatar, and found that their movement rarely surpasses 100km north or south of their range from these areas.

This research was done by means of creating a ‘biological passport’, a log of the internal levels of marine isotopes of nitrogen and carbon, which vary throughout the habitat naturally, giving us an idea as to where they have been, to better predict where they might go. We know that heavier isotopes, for example, are found closer to the shore than in deep-water, and as they are passed up the food chain from primary producers such as algae to apex predators, the ratios of nitrogen and carbon stay the same. By turning these into some pretty cool graphs (if you like graphs as much as me), we can see where the isotopic ratios differ in each of the three locations.

Previously, the standard way to tag large sharks involved attaching an electronic tag to them, which would only last weeks or months, but with Prebble’s method, we are able to get the information from skin tissue over a longer period of time (years), and they can provide information over a short time period before sampling. The results from the study tell us that we have to deal with sharks at each site separately, to ensure proper long-term management.

This data can also help us with providing new approaches to conservation in these areas of the Western Indian Ocean and the Arabian Gulf, where we can provide greater protection from threats such as becoming by-catch in fishing boats, and for increasing their stability, which in turn brings eco-tourism to the area.

The study ‘Limited latitudinal ranging of juvenile whale sharks in the Western Indian Ocean suggests the existence of regional management units’ (DOI: 10.3354/meps12667) is published in Marine Ecology Progress Series.

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