Global and local stressors kill corals, which can lead to the decline of reefs if algae replaces coral. Foraging by invertebrate and vertebrate herbivores removes macroalgae, preventing space preemption by algae, and promoting coral recruitment and recovery. Unfortunately, Hoey and Bellwood (2011) demonstrated that herbivorous fish forage less intensely on macroalgae where it is dense, potentially leading to a positive feedback loop toward algal dominance. The authors suggested that herbivores avoid dense algae stands because they fear that predators might be hiding in the structure of algal thalli. Since reef fish often prefer to associate with 3 dimensional structure whether they are predator or prey, we tested other hypotheses to explain the mechanisms underlying this foraging behavior. We tested three questions in Jamaica and San Salvador, Bahamas: 1) We used video recordings to determine whether herbivorous reef fish show different behavior near simulated dense algae stands compared to sparse stands. 2) We did algae transplant experiments to determine if herbivores remove less algae in patches where we simulated dense algae without adding 3 dimensional structure. 3) We used algae transplant experiments to determine if herbivores preferred algae that were grown in dense stands versus sparse stands. This would tell us whether there was something distaste full about the algae (e.g., secondary chemicals or distasteful epiphytes).
Results/Conclusions
Video observations of foraging during daylight hours showed no significant differences in behavior between dense and sparse patches. However, in consumption experiments, we have found that fish do not forage effectively against disruptive algae‐like backgrounds (with no 3 dimensional structure). When algae were presented against a simulated dense algae background, algae removal was not different from cage controls. Also, when algae from sparse locations are presented alongside algae from dense plots, herbivores selectively forage on sparse‐sourced algae. In summary, herbivorous fish remove less algal biomass when algae patches appear denser even when 3 dimensional structure is absent. These fish also show a preference for thalli that have grown in sparsely settled areas, suggesting that dense algae are distasteful. In future work, we will look for differences in the presence of unpalatable chemicals and epiphytes on algae growing in sparse and dense plots. This work may help explain why dense algal stands remain on Caribbean reefs (or even expand) when herbivorous fish are present.