Where are the whale sharks going? Guest Blogger Ana Sequeira

This week is Seaweek and guest blogger Ana Sequeira describes how whale shark distribution might be shifting according to seasonal environmental predictors.

Ana Sequeira is a PhD student at the University of Adelaide. Her main research interests are to develop models applied to the marine environment to describe key environmental processes, species distribution patterns and ecological interactions.

The main objective of her PhD thesis is to investigate behavioural ecology of whale sharks. She is now trying to understand which environmental variables may affect whale sharks distribution.

The whale shark (Rhincodon typus, Smith 1828) is the largest fish in the ocean and can reach more than 12m in total length. Although little is known about their habitat selection or migration patterns, the whale shark appears to be a highly mobile species. They predictably form near shore aggregations in some coastal locations (e.g. off Ningaloo reef in Western Australia) what makes them the subject of highly lucrative marine ecotourism industries. Also, artisanal and small-scale fisheries for the species still exist in many parts of the tropics.

Since whale shark is classified a Vulnerable species (IUCN, 2010), understanding their migratory behaviour became of chief importance as they can be travelling from regions where they are protected to regions where they are still harvested.

To identify Whale shark’s possible distribution patterns in the Indian Ocean, we developed multivariate distribution models of seasonal whale shark sightings – opportunistically collected by the tuna purse-seine fishery.

17-year time series of whale shark observations, bathymetric information, chlorophyll-a concentration and sea surface temperature data extracted from satellite images, where used as inputs for generalized linear and mixed effects models.

Sea Surface Temperature was identified as a key component for whale shark distribution, being a forewarning that large shifts in the current aggregation locations are to be expected under the upcoming climate change scenarios.

Our modelling approach can be used to predict the Whale shark appearance timings at specific sites (e.g. at sites where they are still currently fished) and to identify the underlying hypotheses for the current population decline.

Guest Post by Ana Sequeira. Contact the Environment Institute if you would like to contribute as a guest blogger.

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4 Responses

  1. I find this study is so helpful for the country in concern ..Egypt for example… Few months ago,we faced a very unusual behavior of the SHARKS in the Red Sea (at South of Sinai ) SHARM ELSHEIKH..some sharks attacked the tourists and killed them..and this is the first time,we see Sharks attack the swimmers ..of course I am not qualified to give the reason of this attack,but I think it is something to deal with the environmental condition in this area,or maybe something else irritated the the Sharks

    Ashraf Soliman
    Egyptian Mechanical Air-conditioning Engineer

  2. Ana says:

    Hi Ashraf,

    Thanks for your comment.

    It is indeed very important to understand the movement patterns of sharks, and it is very likely that changes to those movements are mainly related to environmental changes as you suggest.

    Although these sharks I am studying (whale sharks) are filter feeders – therefore representing no harm to humans, and their behaviour can be very different from other sharks.

    Best regards,

  3. Lyndall Bensley says:

    Hi Ana

    can you tell me why whale sharks dont generally migrate on the east coast of Australia? I know of a couple of sightings around 2006 I think, so why have they been past S.E Qld but not as a general migatory path?

    Lyndall Bensley

    • The Environment Institute says:

      Hi Lyndall,

      Yes, I also heard about some sightings off the east coast, though your question does not have an easy answer 
      Unfortunately, the migratory patterns of whale sharks are not yet known… actually, we are still trying to understand if they have one!

      For instance, in Ningaloo (WA) they are seen between March and June every year, although we don’t really know where they come from or where they are heading to after.
      Sea surface temperature seems to be one of the main predictors for their occurrence, although there are other variables that might play an important role when it comes down to “path choice”.