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Grand Cayman: Feeding of stingrays alters behaviour

Southern stingrays, Dasyatis americana, have been provided supplemental food in ecotourism operations at Stingray stingray City Sandbar (SCS), Grand Cayman since 1986, with this site becoming one of the world’s most famous and heavily visited marine wildlife interaction venues. But what are the effects of such activities on the focal species and their ecosystems?

A new study, for which acoustic tags were used, showed that least 164 stingrays, over 80% female, utilized the small area at SCS for prolonged periods of time.

“We saw some very clear and very prominent behavioral changes, and were surprised by how these large animals had essentially become homebodies in a tiny area,” Nova Southeastern researcher Mahmood Shivji said.

The tourist-fed stingrays swapped their normal nighttime foraging for daytime feeding, abandoned the species’ normal solitary behavior to crowd together, and showed signs of aggression, biting each other more than their wild counterparts, he said. Normally, wild stingrays are active during the night and move over a larger area; the fed stingrays stayed close to the feeding site.

That suggests that human-provided food can dramatically change how even large, highly mobile ocean animals behave, with potentially serious consequences, the researchers said.

“There are likely to be some health costs that come with these behavior changes, and they could be detrimental to the animals’ well-being in the long term,” Shivji stated.

Is ‘Eco’ always ‘eco’?

The increased density of stingrays at SCS appears to have led to a much higher frequency of interactions and physical contact, and has been shown to result in increased disease transmission, parasite loading, altered blood chemistry, injuries and overall poorer body condition. Furthermore, the authors speculate that under such long-term, highly crowded conditions there is the potential for alteration of the stingray mating system and reproductive patterns, as well as increased inbreeding.

The supplemental feeding has strikingly altered movement behavior of the stingrays and their distribution over the area. The fact that female stingrays are larger than the males, could explain the fact that 80% of the caught rays was female: the smaller males are chased away and have to forage elsewhere. Competition between genders for ecotourism provided supplemental food was also observed at a stingray feeding site in Australia, where the larger Dasyatis female stingrays behaved aggressively towards smaller conspecific males.

Shark diving is mostly done with ‘bait’: a relatively small quantity of food, not enough to fully feed an animal.

Stingray City generates a lot of tourist income for the Cayman Islands, researchers said, making understanding the affect on wildlife important, especially with the continuing expansion of ‘interactive ecotourism’.

“Measuring that impact is important because there’s a lot of interest in creating more of these interactive ecotourism operations, but we know little about the life histories of the animals involved or how they might change,” study co-author Guy Harvey of Nova Southeastern said.

Supplemental Feeding for Ecotourism Reverses Diel Activity and Alters Movement Patterns and Spatial Distribution of the Southern Stingray, Dasyatis americana.
Authors: Mark J. Corcoran, Bradley M. Wetherbee, Mahmood S. Shivji, Matthew D. Potenski, Demian D. Chapman, Guy M. Harvey

Read the full study at:
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0059235

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