The Influence of Reef Depth on Parrotfish Sex Ratio, Abundance, and Diversity near Calabash Caye, Belize

We investigated the sex ratios of six species of parrotfish (Family Scaridae) at multiple reef sites off the coast of Calabash Caye, Belize, in the Caribbean Sea. Parrotfish are sequential hermaphrodites, which means they change their sex at some point in their life. All six species are protogynic hermaphrodites. They begin their lives as females and change to males. The female or juvenile phase is referred to as the “initial” phase, and males comprise the “terminal” phase. Parrotfish feed on large algae and sponges which grow on the reefs, and they graze directly on the coral itself, in order to consume the tiny symbiotic algae which live inside the coral. The large algae and tiny coralline algae compete for sunlight to grow. By feeding on large algae and sponges, parrotfish play a key ecological role in the health of coral reefs by reducing the growth of their competitors. Terminal phase males must consume more food than initial phase females in order to maintain their relatively large body size and meet the energy demands of male-male competitive behavior. Adult parrotfish have a harem social system, in which a single male actively guards and defends access to multiple females. We hypothesized that greater ratios of terminal males would occur at the deeper reefs. Deeper reefs have greater biomass, and deeper reefs are comprised of greater species diversity. Consequently, deeper reefs provide greater food supply. We selected four reef sites which varied in depth and species composition. The depths ranged from 5’ to 30’: one site was shallow; two had intermediate depths; and one was deep. We identified initial and terminal phase individuals on the basis of established coloration differences. At each site, we randomly selected 20m linear transects to survey. We spent the same total amount of time at each site. The greatest proportions (23-39%) of terminal males occurred at the intermediate and deep sites. Only 3% of the parrotfish at the shallow site were terminal males. Parrotfish were most abundant at the deep site and at one of the intermediate sites. The greatest parrotfish diversity, calculated by the Shannon Index, was observed at the two intermediate depth sites. The preponderance of terminal phase individuals at the relatively deeper sites supports our hypothesis that more males would be found on reefs which provide more food to meet their greater energetic demands. Five of the six species exhibited a predominance of initial phase individuals. This female predominance supports the observation that parrotfish have a harem social system. We found no initial phase Queen Parrotfish at two of the four sites, and only three and four terminal phase individuals at the two other two sites, respectively. This absence of females, who are the younger individuals, raises concern for the long-term survival of Queen Parrotfish and their critical role in coral reef ecosystems