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New Paper - Satellite-tracking east Australian reef manta rays

September 5, 2014

My latest scientific paper is out:

 

 

This paper is one of the highlights of the research I conducted as part of my PhD research with The University of Queensland’s Project Manta. It took a lot of funding and sweat in the making but I am thrilled that it is finally out!

 

This paper represents one of the first detailed movement studies on manta rays. Using satellite-tracking technology (and quite a few statistical models and analyses), my co-authors and I explored movements of reef manta rays visiting Lady Elliot Island, a key aggregation site for the species on the east coast of Australia. Over the years – and during our previous studies – we had examined sighting trends of reef manta rays at the site, and our previous paper published in PlosONE had identified several environmental parameters as key influences for their occurrence at the site. More importantly, that study had highlighted the importance of food – zooplankton – for the species at the site, and the potential role of the nearby-forming cyclonic (clockwise) Capricorn Eddy in supplying the Lady Elliot reef with plankton-rich waters. By documenting the movements and distributions of reef manta rays in that same region, this paper provides new insight into the importance of Lady Elliot Island as well as the role of the Capricorn Eddy for the species.

 

Eight satellite-tagged reef manta rays moved quite extensively along the coastline, with the longest track recording 2441 km travelled in 118 days and 294.5 m the deepest depth recorded. Some great data that would already help support future conservation efforts for the species. But there was much more to get out of the data…

 

LEFT PANEL: Typical oceanographic setting for the region, pictured with NASA’s MODIS satellite imagery. RIGHT PANEL: Movement tracks of the eight satellite-tagged reef manta rays – each colour is a different individual/track, dots along the tracks represent estimated daily positions for the tagged individual.

 

 

The bigger part of the study – and the more interesting bit for me – was to understand the drivers and reasons behind the observed movement patterns. We ran random walk simulations (i.e. we simulated randomised movement tracks based on the properties of the real recorded tracks) to examine whether reef manta rays spent significantly more time in the offshore region characterised by the  Capricorn Eddy than would be expected by chance. And the answer was “yes, they do”. In other words, there was something interesting in this eddy region that made the mantas spend time there… Yet it is another state-of-the-art analysis, a behaviour-switching state-space model, that that completed the story – the result of that analysis suggested that, based on the movement patterns of the tracked mantas, this area may be an important feeding ground for the species. And having been in the field in that region for over 5 years now, it would make sense! There is definitely something special about the food – zooplankton – at Lady Elliot Island and in the surrounding area… Hopefully all of the plankton samples and oceanographic data that I collected over the years will come handy at some stage in the near future to make the link between fine-scale dynamics around the reef and regional circulation patterns.

 

LEFT PANEL: Daily maximum diving depths for tagged reef manta rays, showing deeper dives off the shelf in Capricorn Eddy region. RIGHT PANEL: Output from the behaviour-switching state-space analysis suggesting a major offshore foraging area in the vicinity of Lady Elliot Island.

 

 

For me the big highlight of this paper is the fact that all animals ventured off the continental shelf, in offshore waters, some venturing as far as 155 km off the shelf into the deep ocean. This is very important as it was previously thought that this species Manta alfredi was mainly an inshore species that would not spend much time offshore. But here you go! Manta rays never cease to amaze us! They do come and go from such aggregation sites, can spend extended periods of time out at sea (i.e. off the shelf) and can use offshore waters to move between aggregation regions (like we observed in this study). And if that’s the case then this aspect of their ecology needs to be considered when implementing management strategies for the species. I hope that this study will be useful to getting reef manta rays more protection in the future!

 

I hope you enjoy the reading – the full article can be found here.

 

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