Several of our San Diego-based colleagues have been making the argument that fishing is the ultimate cause of coral bleaching, disease and loss in a series of stimulating papers. One purported underlying mechanism is that by reducing the density of herbivores, fishing is causing increases in macroalgae and concentrations of Dissolved Organic Carbon (DOC), thereby fueling microbes that lead to coral diseases or make corals more susceptible to warming.
Evidence supporting this idea stems from somewhat artificial laboratory studies and a field survey of four remote reefs in the central Pacific (see my post on this work here). After reading these papers, three UNC undergrads working in my lab asked “why doesn’t someone do a field experiment to test this idea?” So they did. Working with Ernesto Weil and I in Puerto Rico they performed three field manipulations to test the general hypothesis that overfishing and the subsequent alteration of coral reef trophic dynamics are a cause of coral epizootics. Specifically, they asked whether the presence of macroalgae can influence within- and among-colony spread rates of Caribbean Yellow Band Disease in Montastraea faveolata.
They placed macroalgae in small pouches next to infected and healthy, adult and small coral colonies to measure effects on disease spread rate, coral growth and coral survival. Surprisingly, the addition of macroalgae did not affect disease severity or coral fitness. Their results (published yesterday in PLoS One; Vu et al. 2009) suggested that macroalgae have no effect on the severity and dynamics of Caribbean Yellow Band Disease, a critical coral epizootic.
There are several unresolved issues though. First, could other algal species or other combinations of algae cause or exacerbate yellow band disease? What about other host species and other coral disease syndromes? Could there be other unexplored factors that are also necessary for infection to occur (e.g., high temperature)? And how is the concentration of relevant forms of DOC related to fish, fishing and algae? All of these questions remain unanswered. Leaving lots of room for future field experiments by ambitious students.