Mortality estimates from repeated visual censuses of a parrotfish (Sparisoma viride) population: demographic implications |
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Authors: | J M van Rooij J J Videler |
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Institution: | (1) Department of Marine Biology, University of Groningen, PO Box 14, 9750 AA Haren, The Netherlands, NL |
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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.
Received: 2 January 1997 / Accepted: 27 January 1997 |
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