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 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.