The stoplight parrotfish (SPARISOMA VIRIDE) is a large and common herbivore on many Caribbean coral reefs. Until recently, hardly any quantitative data were available on the uptake and transfer of nutrients and energy by scarids, despite their important role in these "oases of diversity." This project was set up to construct a detailed field budget for a population of S.VIRIDE at Bonaire, the Netherlands Antilles. The main question addressed in this trophodynamic context is how efficiently it converts its food into growth and gamete production.

Over 1600 hours of underwater observation were spent studying the behaviour and energetics of individually recognized fish and the structure and density of the population. Some new methods and equipment were developed for direct estimation of the major energy expenditures, viz. stereo photography to measure growth, a hoop net for field collection of eggs, an underwater event recorder for simultaneous records of behaviour, bite rates, fin beats, and ventilation frequency, and establishment of the relationship between ventilation frequency and oxygen uptake (allowing field estimates of metabolic rates) and between fin beat frequency and swimming speed (as a measure of swimming activity). This detailed approach resulted not only in an accurate estimate of the total amount of energy, carbon, and nitrogen processed by the population, but also revealed a high degree of intrapopulation variability that could be related to behavioural ecology and life history theory. In this context, the adaptive significance of territorial behaviour and of early sex change have been investigated.

Comparison of total energy expenditure with independent estimates of energy intake and absorption obtained in a parallel study, shows that S.VIRIDE can live on a strictly algal diet. In fact, total production as a fraction of intake yielded quite a low energy conversion efficiency (1.8%), whereas nitrogen was processed more efficiently (18%). However, expressed as fraction of the assimilated energy (9%) or nitrogen (40%), food conversion compares well with values reported for carnivorous fish. It is concluded that an important fraction of the excavated algal material is not used by S.VIRIDE itself, but transferred to micrograzers and/or the microbial web.

A major distinction could be made between territorial males and males sharing common home ranges. The latter showed much higher growth rates, despite a relatively poor food supply. This could be explained by the high activity of territorial males related to territory defence, resulting in elevated metabolic rates. In return, they had guaranteed access to a number of harem females and attained much higher spawning frequencies. Calculations of the expected lifetime reproductive output (based on size-dependent mortality and spawning rates) indicated that individual differences in the timing of sex change and in the lifetime pattern of growth and reproduction are adaptive. The capacity to flexibly adapt its behaviour and physiology in response to an unpredictable environment, may well explain the success of S. VIRIDE.