ScubaBoard - Yellow Stingrays

a DIVE TIME column
by Tim Grollimund

While I was doing some background research for a previous column, I went off on a tangent and began collecting information on ecological issues facing the Keys. One topic that caught my eye was a study about the changes in the population of yellow stingrays in various parts of the Caribbean. So why should we care about yellow stingrays?

In this study, published in the scientific journal Environmental Biology of Fishes, Reef Environmental Education Foundation data on the yellow stingray population were used to study the abundance of the species, as observed by REEF divers, from 1999 to 2007. Several noteworthy items emerged from the work, which you can see on the REEF web site (www.reef.org/db/publications).

First, as we discussed before, volunteer work to the organization has great value. Over 83,000 observational surveys were used in the study. Imagine that! Eighty-three thousand times volunteers gave their time and effort to advance science (and that was only in the Caribbean; it’s over 150,000 in total). While the accuracy and consistency of the observations may be subject to some scrutiny, the study authors stated the training requirements of REEF did support sufficient consistency for validating the study results. And citizen science is becoming a widely accepted practice. Again, my hat goes off to all who give of themselves for the benefit of others. And to REEF for emphasizing proper training so their results may be used for important studies such as this.

The main thrust of the work details the decline in abundance in yellow stingrays in many, but not all areas. The Florida Keys had the largest decline, while Jamaica had an increase. Why is that significant?

Several possible reasons were cited for the population changes. First, maybe they just moved out. But that reason is discounted because yellow stingrays are benthic (they live on the bottom) and stay in a rather small area, which has been independently verified in other studies. Habitat degradation from loss of seagrass cover, declines in fish densities, coral diseases, coral bleaching and water quality degradation seem at first blush to be reasonable causes - but if that were true, why the increase in Jamaica? Direct exploitation from the aquarium trade and the research industry are also noted. But again, why the increase in Jamaica, where habitat protection is minimal?

The last reason, and the one I feel is the most compelling, is due to changes in trophic interactions - eating or being eaten. As the yellow stingray population declined, the large grouper population increased - or is it the other way around - the proverbial chicken or egg question. The goliath grouper is protected and has made a significant comeback in our waters. Contrast this with Jamaica, where there is a stark absence of larger predators, most notably, groupers. When groupers were protected here, yellow stingrays declined... but is that a return to normalcy, and did the stingrays just “fill the gap” when the grouper population suffered? Now I’ll grant you, from the standpoint of the local economy, that groupers will always get the nod over yellow stingrays, but in a perfect world who is to say what the value of one species is relative to another?

What are the implications of imbalance? I found an example of this with sharks along the east coast. In a report from Oceana (www.oceana.org), as 11 species of large sharks declined over a 15 year period, the population of skates, rays and smaller sharks increased dramatically, up to ten times their number in some cases. The result? The collapse of the bay scallop industry as the dramatically increased numbers of cownose rays feasted on scallops, and had no predators to curb their growth rate. Now, without the bivalves to act as water filters, more algae grows and the water quality declines. And on and on it goes.

Local case in point: Lionfish. Can we ultimately overcome the introduction of such an invasive species? What will nature do to adjust? Even if we could kill all the lionfish, how has the balance of species been affected in the mean time, and what are the long-term consequences on our fisheries?

As divers, it is vitally important for us to appreciate how difficult it is to maintain the balance of an ecological system from a resource management point of view. And who truly knows the answer to the question of the optimal balance of species? Do resource managers have all the answers? And if they don’t, how close are they? All the more reason for the diving community to support our researchers, scientists and policy makers. We may not always agree with their decisions, or understand the reasons, but there is one thing I do know. The situation will change and adjustments will be made based on newly acquired knowledge.

During my conversation with Dr. Christy Pattengill-Semmens, one of the co-authors of the study, and the Director of Science for REEF, she taught me that adaptive management was the key to ongoing success in conservation. Scientists and resource managers have a very difficult job. The ecosystem is, quite literally, a moving target. The impact of different stressors, such as changes in water temperature - remember our fish kill last winter - yield a different set of circumstances than conservation managers could have foreseen when policies were put in place. Christy says that there are no easy answers, and as the coral ecosystem becomes less resilient due to increased and varied stressors it makes it harder for nature to adjust. In the words of “Kay” from Men in Black, “Imagine what you'll know tomorrow.”

Tim Grollimund is a freelance photographer and PADI divemaster based in Key Largo. He can be reached at tim@timgimages.com or through his web site at www.timgimages.com. Keep tabs on his activity for the Coral Restoration Foundation at www.timgimages.com/crlogbook.

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