PhD ceremony Mr. Y. Afeworki: Population ecology of the rusty parrotfish Scarus ferrugineus , a dominant grazer on a seasonal coral reef

PhD ceremony: Mr. Y. Afeworki

Dissertation: Population ecology of the rusty parrotfish Scarus ferrugineus, a dominant grazer on a seasonal coral reef

Promotor(s): prof. J.J. Videler, prof. J.H. Bruggemann

Faculty: Mathematics and Natural Sciences

The thesis of Yohannes Afeworki investigates the effects of seasonality on the population ecology of a common grazer, the rusty parrotfish (Scarus ferrugineus), on a southern Red Sea reef.

Tropical seas under the influence of the Asian monsoon show significant seasonality in rainfall, temperature and irradiance. The influence of these changes on coral reef fish communities is generally considered small but has rarely been studied. The extreme summer temperatures in this region are the main cause of seasonal changes in the composition and biomass of benthic algae, inducing changes in the vertical distribution of grazing fishes. Seasonal changes in the availability of turf algae, its preferred food type, lead to significant changes in intake rates. Higher food intake in spring results in peak body condition, liver mass and growth rate in early summer. Unexpectedly, reproduction is more intense during the cool winter season. In this regard S. ferrugineus behaves like a capital breeder: it utilizes energy stored during the warmer parts of the year to fuel reproduction in the cool season. Feeding rate in large males slows down in late summer during the hottest time of year when they experience higher natural mortalities. This observation suggests that the high summer temperatures (34°C) are close to the critical temperatures of the largest individuals of S. ferrugineus. High male mortality sets an upper age limit for sex change and as a result, females reaching 4 years and beyond do not gain in fitness by changing to male.

Large individuals are closer to their upper critical temperatures in tropical areas such as the southern Red Sea. Climate change and increases in temperature may thus influence size structure and age structure of reef fish communities. That the decision to change sex in sequential hermaphrodites is flexible and is dependent on demographic parameters among other factors. Higher male mortality can set an upper age limit for sex change. In populations where male mortality is higher than that of females, older females will not change sex.