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Ecology

Manta ray movement and motivations

A picture is worth a thousand words

Why we care

“Charismatic megafauna” are the larger, cute or cool animals that attract the public’s attention. It’s hard not to be jealous of scientists who work with them sometimes- if you’re on a plane next to a tourist, do you think they’d be more excited to talk to you about sharks or algae?

A good scientist can make any organism interesting, but there aresome stigmas attached to charismatic megafauna that can’t be denied. For example, I’ve met a number of people that assume that since bigger animals are so large, they’re easier to keep track of. Or that we know all that we want to about animals that catch the public’s eye.

Manta alfredi, a fine example of charismatic megafauna. Is it just swimming or is it preparing to give you a hug? Photo in Jaine et al., 2012 (doi:10.1371/journal.pone.0046170.g002).

In fact, information about larger, popular animals can be elusive. Large marine organisms have the potential to move far, fast, and deep. Equipment that transmits accurate location information and stays attached to a fast-moving organism without hurting the animal is expensive. Without knowing where an animal goes, however, we can’t know much about the basic elements of its life history: how it interacts with others of its species or where it spends its time for particular activities.

Manta rays are key components of tourism industries and are classified as vulnerable to extinction, but they are relatively rarely observed in the water. To protect them, we must start with good information about where they are.

Methods

The authors tagged 10 Manta alfredi rays with satellite tags designed to automatically detach from the rays after 90 to 120 days. Basic data was transmitted automatically several times a day on all 8 working tags, while detailed data was obtained from 6 tags recovered after they washed ashore. Tags recorded location, light levels, and sea temperature and pressure so that accurate depth information could be included in movement models.

Random movement simulations were compared with real movement data from the rays to determine if they moved deliberately toward a feature of interest (the Capricorn Eddy, a hypothesized rich source of food for the rays). The study used a behavior-switching model, which combines the different types of data gathered to classify movement types, to differentiate transitory movement from movement associated with foraging. This particular model has been used and refined a number of times in other studies and used correlation between measured parameters (depth and speed, specifically) to classify ray movements as belonging to either of the two categories named above (note that any behavior not determined to be movement was therefore classified as foraging based on the constraints given to the model).

Results

Data was gathered on ray movements for an average of 92 days (range 37 to 120 days). The average ray traveled 1169 km during this time (the distance between Charlotte, NC and Miami, FL).

Movement tracks for the 8 manta rays successfully tagged during the course of this study. The Capricorn Eddy, a hypothesized feeding site, is located just offshore of Lady Elliot Island.

Movements indicated 2 foraging hotspots: one off the continental shelf near where the mantas were tagged (Lady Elliot Island) and another off the continental shelf considerably further south (near Moreton Island) and based off of data from just 1 ray. Random movement simulations indicated that the rays spent a significant amount of time in the Capricorn Eddy, presumably foraging on the dense plankton associated with this oceanic feature.

Behavior models categorized 53.5% of ray movements as belonging to a “foraging” state, while “transiting” comprised the remaining 46.5% of the rays’ time (all data were put into one of these 2 categories). Rays spent almost all of their time foraging while in the Capricorn Eddy.

The Lady Elliot Island foraging area, where all manta rays studied in this project were initially tagged.

The data also show that Manta alfredi travel long distances to frequent productive offshore environments specifically to forage. However, rays have a high degree of site fidelity, returning to particular coastal areas over time.

High coastal site fidelity has driven previous discussions of manta ray behavior. This study is one of the first to provide evidence that offshore, highly productive areas significantly influence ray movement and behavior. And while coastal ray aggregation sites are well known in eastern Australi, this study shows for the first time the path of (and therefore the currents used by) a ray moving between aggregation sites. This movement identifies what might be a previously unknown foraging hotspot and suggests that ray migrations may follow high-productivity areas where food is abundant.

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