Mortality estimates from repeated visual censuses of a parrotfish (Sparisoma viride) population: demographic implications

Abstract

Repeated visual censuses of different categories (juveniles, females, territorial and group males) of the stoplight parrotfish (Sparisoma viride, a protogynous hermaphrodite) over a 3-year period indicated a relatively stable size and structure of the adult population. This allowed estimates of size-specific mortality, sex change, and territory acquisition probabilities from previously reported growth rates. Comparison of the predicted number of survivors, sex changers, and territory take overs with field observations indicates that our estimates are quite reliable. However, rather large differences in mortality are obtained for the largest three size classes (>25 cm), which may be due to reduced accuracy of length estimates of large fish. A pooled mortality for these classes is therefore suggested as a more realistic estimate. The life-history implications of our findings are investigated by comparing the predicted survival and future reproductive success of fish that change sex at different sizes. Ten percent of the adults reach an age of ca. 17 years, once a length of 20 cm is attained, as predicted from the pooled estimates. Calculations based on these estimates indicate that the predicted reproductive output of a 15-year-old fish (2500 matings) is the same for early (i.e. at 20 cm) and late (>30 cm) sex changers and for an average fish subject to the estimated daily sex change and territory acquisition probabilities. These findings suggest that S. viride individuals are able to flexibly adjust the timing of sex change in an adaptive way to unpredictable local conditions. However, independent mortality estimates are needed to corroborate our present findings.