Spatial ecology of herbivores

Exploring the spatial and behavioral patterns of herbivorous reef fishes in order to understand the extent and location of their impacts on reefs.

Herbivores play a critical role in maintaining reef health through grazing that keeps seaweed growth in check and potentially facilitates settlement of new corals by exposing bare surfaces. We are conducting an intensive study of the spatial and behavioral patterns of herbivory on Palmyra’s coral-dominated reefs, focusing on six species of relatively abundant herbivorous fish. Because different herbivore feeding strategies result in unique impacts on coral-algal interactions, our study species span three functional feeding groups: large excavators, scrapers, and grazers. Large excavators, like the steephead and bumphead parrotfishes, take big bites of coral that leave bare substrate, while scrapers, like the bullethead parrotfish and other smaller-bodied parrotfish species, take small scrapes of low-lying algae, and grazers like the surgeonfish and bristletooth pick and crop benthic algae. Through our work, we measure not only the rate at which these important species of herbivores feed but how their feeding is distributed across the reef.

Our research group uses snorkel, SCUBA and technological tools such as acoustic telemetry to conduct behavioral observations across multiple sites and habitat types on Palmyra’s reefs. By following fish wherever they go, we can measure how, exactly, these fishes use the reef, the rates at which they feed, and their abundances. In addition to simple visual behavioral observations of multiple species, we are conducting a long-term study of the steephead parrotfish, Chlorurus microrhinos, using acoustic telemetry techniques. We use two types of acoustic telemetry, manual or ‘active’ tracking and passive tracking. Active tracking involves tagging fish with acoustic ‘pingers’ and following them with a hydrophone (listening device) from a boat. This type of tracking provides very high spatial resolution data: we know where fish are to the meter! We have now tracked multiple individuals over the course of several months, measuring and mapping home range sizes in order to understand their impacts on the seafloor. The second technique is passive tracking, which provides less spatial resolution but a longer study time. Passive tracking involves tagging also involves tagging fish but instead of active following, fish are detected on a series of moored ‘listening stations’. By coupling these methods, we are learning about how home range patterns change (or don’t) over the course of years. The spatial and behavioral patterns that we are measuring provide insight into how herbivores affect interactions between corals and algae on reefs.

Early results have shown that while large-scale movement patterns are consistent among individuals, there is variability in small-scale foraging patterns that differ across habitats. Behavioral data suggest that competition may play a key role in determining parrotfish home range sizes and shapes. By combining our visual observations with acoustic telemetry, we are increasing our understanding of how herbivory operates on a pristine coral reef such as Palmyra. As part of the Reefs Tomorrow Initiative, our studies of herbivorous fishes provide new insight into MPA design, individual species protections, and the role herbivores plays in coral reef resilience.

This research is being led by Dr. Jennifer Caselle and Dr. Robert Warner at the University of California, Santa Barbara, whose team includes Peter Carlson, Katie Davis, and Laura Dee.