POPULATION-DYNAMICS OF THE FISH POECILIA-GILLII (POECILIIDAE) IN POOLS OF AN INTERMITTENT TROPICAL STREAM

(1) This study examines the population ecology of Poecilia gillii (Kner & Steindachner, 1863), a small algae- and detritus-feeding fish, inhabiting sixteen isolated, residual pools of a high-gradient, intermittent stream in Costa Rica following floods which severely reduced the number of fish in the stream. (2) Immediately after the floods, population sizes averaged 20% and pool surface areas 130% of their pre-flood values, periphyton production was relatively low and dissolved oxygen was relatively high. Among pools, population densities were not correlated with their pre-flood values, and the population growth rates showed a weak negative correlation with density, a marginal positive correlation with periphyton production and no correlation with oxygen. (3) Near the end of the 153-day post-flood study period, population sizes had risen to 50% of their pre-flood values, while pool areas had fallen to 73% of their pre-flood values. Periphyton production was relatively high and dissolved oxygen was very low. Among pools, population densities were correlated with their pre-flood values, and the population growth rate showed a strong negative correlation with population density and strong positive correlations with periphyton production and dissolved oxygen level. (4) These results suggest that in highly seasonal tropical environments, temporal patterns of population abundance in high-gradient streams will be very different from those in floodplains, where the rainy season is a time of increase and the dry season a time of decline. Spatial variation in environmental characteristics which control periphyton, detritus and oxygen availability may produce characteristic population densities in different pools. An increase in the proportion of males and a decrease in their size after the floods suggest social inhibition of sexual maturation during the late dry season.