The effect of heated effluent on egg production in the estuarine prawn Upogebia Africana (Ortmann)

Egg production of a population of Upogebia africana (Ortmann) in a pond containing water heated by a thermal power station at Knysna, South Africa was compared with that of the population in the adjacent estuary and in two other estuaries. The breeding cycle, proportions of ovigerous females, incubation time for eggs at different temperatures, number of eggs produced, sex ratios, and size-frequency of males and females were all investigated. The number of eggs produced/100 prawns was found to be nearly twice as high in the population exposed to heated effluent as compared with the population in the adjacent estuary. The increase was due to a combination of a change in sex ratio resulting in more females in the population and a greater proportion of larger females in the pond. Since the number of eggs produced/female is positively related to the size of the prawn, large females in warmer water produced more eggs than smaller females in the adjacent estuary. Exposure to heated effluent did not alter the initiation or duration of the breeding season, nor cause precocious maturity; nor did it alter the number of eggs carried per female at various sizes.