High biomass and production but low energy transfer effciency of Caribbean parrotfish: implications for trophic models of coral reefs
Quantitative data are presented to assess the trophic role of scarids on the fringing coral reef of Bonaire (Netherlands Antilles), with particular emphasis on the stoplight parrotfish Sparisoma viride. Average herbivore biomass on the reef was 690 kg ha−1, 22% of which was accounted for by S. viride. From data on relative gonad weights, daily spawning frequencies, and egg numbers obtained by stripping, with previous estimates of somatic growth and energy intake, a gross effciency (GE: somatic plus gamete production/ consumption) of 2.3% was obtained. This is a factor of five to seven lower than the GE suggested to be valid for most aquatic ecosystems, including coral reefs. To investigate one potential cause for our low estimate, overestimation of food intake, our intake estimates were compared with published values for other herbivorous coral reef fish. This yielded a relationship (daily C intake=0.0342#W 0.816; wet body mass W in g) with high correlation (r 2=94.6%, n=13), which shows that the intake estimates agree well with other published data. Averaged over the year, primary production at 0–3 m depth was 17.2 kg C ha−1 day−1 while herbivore consumption was estimated at 17.4 kg C ha−1 day−1, indicating an ecotrophic effciency (EE, the fraction of total production at one trophic level that is consumed by all predators) of 100%. This suggests strongly that the food intake estimates are realistic, since no changes in algal biomass were observed over the study period. The two scarids for which food intake was actually measured in our own study area, were estimated to consume 55% of the algal production in the shallow reef (S. viride, 20%; Scarus vetula, 35%). This is lower than expected if consumption were proportional to biomass (S. viride, 22%; S. vetula, 40% of herbivore biomass in the shallow reef). Consequently, a minimum estimate of 88–91% can be inferred for the EE of these two species. Multiplied by GE, this yields a transfer effciency (TE, the fraction of production passing from one trophic level to the next) of 2%. For coastal and coral systems the primary production required (PPR) to sustain fisheries was estimated to be 8.3%, which was based on a TE of 10%. The present estimates show that the TE of a major herbivore at our reef is at least a factor of five lower. Assuming that the estimate is representative for all scarids (comprising 70% of the herbivore standing stock), it can be concluded that the PPR to sustain coral reef fisheries may be as high as 40% of the total primary production. The low value reported before, might suggest that the effect of fishing mainly affects target populations but not the lowest trophic levels. It is argued that our estimate is more realistic for coral reefs supporting high scarid biomass and explains better the many reports of coral destruction due to algal overgrowth at exploited reefs.