In search of sharks and mermaids

That was Abel Valdivia’s talk title at #BEM2012 (not really). He just posted a nice piece on The Abaco Scientist (AKA Dr Craig Layman) explaining a poster he gave recently on his work that Iv’e reposted without permission below.  Thanks Craig and Abel!

I am currently a PhD graduate student in the John Bruno Lab at the University of North Carolina, Chapel Hill studying the role of top predators in the structure and functioning of coral reefs. Originally from Havana Cuba, I moved to Florida in 2004 after the culmination of my master of science in Marine Ecology from the University of Havana. While in Cuba I studied the impact of pollution on the dynamic and structure of urban reef systems. In the US I worked for several seasons at the Bermuda Institute of Oceans Science teaching Marine Biology. Before my enrollment in UNC-Chapel Hill I was a contractor for NOAA Southeast Fisheries Science Center studying the population dynamic of the endangered elkhorn coral species Acropora palmata in the Florida Keys and other regions of the Caribbean. At NOAA I was fortunate to participate as aquanaut in a ten days mission inside the Aquarius Underwater Research Laboratory in the Florida Keys.

My current research interest have taken me to places like Belize, Cuba and Bahamas looking for those rare reef sites with relative high fish biomass to understand the ecological importance of top predators. In 2010 Craig Layman and I started to collaborate in this project in Abaco with the logistic support of Friends of the Environment and the Bahamas National Trust. Abaco is an ideal place because the total reef fish biomass is relatively higher than most of the areas of the Caribbean thanks to the relatively lower fishing pressure around the island.

The first section of our project have focused on answering how do the reef fish trophic structure and ecological processes such as predation vary along a regional gradient of fishing pressure. The trophic structure (relative biomass of predators, carnivores, herbivores, etc) of many aquatic ecosystems has been altered by decades of fishing. Today, most large predators are functionally extinct in coral reefs. The direct effects and degree of this over-exploitation have been widely debated, yet the consequences of predators loss in coral reefs remains unclear. Our results are revealing. Among 26 reef sites (8-15m deep) that include marine reserves and unprotected areas across Cuba, Belize and Abaco we found a 15 fold difference (500gm-2) in total fish biomass. Abaco falls above the average. We used this gradient as a proxy for fishing pressure. In general, highly vulnerable fish groups (e.g. sharks and large groupers) showed a positive or threshold positive trend with increasing total fish biomass. However, most secondarily targeted species (e.g. parrotfish, triggerfish and hogfish) exhibited a humped-shaped trend along the fishing pressure gradient. This indicates susceptibility to fishing at the lower end of the gradient and probably susceptibility to predation in the higher end of the fish gradient. Over 80% of the variation of the biomass in the fish community structure was associated with the gradient of total fish biomass. Thus, we could safely say that while fishing have altered fish assemblages at most sites, predation may control lower trophic levels when the biomass of top predators is relatively high. An inverted biomass pyramid, where the biomass of higher trophic levels dominates the ecosystem, seems to be a natural feature of unperturbed coral reefs which has also be found in isolated and “pristine” reefs of the central Pacific.





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