By Laura Louon,
Marine conservation student
Few would be surprised by the fact that fishing causes a reduction in the population of the targeted fish. That is a direct effect of fishing. But nothing in the ocean happens in a vacuum; if you decrease the number of individuals of one species, you are bound to see an effect on at least one other species, if not the entirety of the ecological community. When developing holistic management and conservation plans, it is therefore imperative that managers also consider the indirect effects of decreasing the population of a species in an ecosystem as to make the correct decisions. But how do you measure, and hence predict, these indirect effects?
Imagine you are a scientist studying the effects of predators on a coral reef ecosystem. One species of reef fish has four predators: two groupers (large-bodied and large-mouthed slow-swimming fish) and two jacks (fast-moving predatory fish that hunt above reefs). You devise a series of experiments where you alter the density of each predator and measure the effect on the population of the reef fish. For example, you may remove grouper A entirely from one test reef, double the population of grouper A on a second reef and maintain it at its natural density on a third. Then you could count the number of the coral reef fish in each scenario and determine the effect of that grouper as a predator. Simple, right? Sure… if the marine system was that simple. Expand this example to include the hundreds of species and interactions in a real ecosystem, and you can imagine how quickly this individual species approach becomes extremely complicated, time-consuming and not to mention expensive. So to simplify it, you group the groupers into one group of predators and the jacks into another. A lot of the research into the effects of similar predators both on land and in the ocean has taken this simplified approach. The assumption made is that the actual identities of the predators if they are morphologically similar with a common diet and habitat (i.e. in the same ecological guild) don’t matter: they have “substitutable” effects in their ecological community. But is this really true? Would fishing out one grouper of a community have the same effect as removing the other? A manager trying to control the effects of fishing has these species-specific concerns, but scientists often unfortunately do not provide this level of detail.
Christopher Stallings, a biological oceanographer and assistant professor at the University of South Florida, became concerned with these assumptions when considering the effects of the overexploitation of large predatory fishes. His research in the Caribbean had indicated that fishing caused a decline in large-bodied fish species, leaving behind coral reef communities that were dominated by small-bodied species (2009a). Therefore, Stallings wanted to determine if two predators that are commonly placed in the same guild, but have significantly different adult sizes did have substitutable effects on a coral reef ecosystem (2009b). He investigated how two groupers, Nassau grouper Epinephelus striatus and coney Cephalopholis fulva, affect the ability of larval coral reef fish to settle on the coral reef (known as recruitment). The Nassau grouper is a large-bodied fish, with an adult size of between 12 to 16 inches, whereas the smaller coney is typically 6 to 7 inches long. Both predatory fish are found throughout the greater Caribbean reef habitats and have similar diets of crabs and other invertebrates and fish. Due to these similarities, field studies testing the effect of groupers on the abundance of their prey have commonly grouped these two species together, making the above-discussed assumption that Nassau grouper and coney have substitutable effects on their communities