., 1987. The significance of photoperiodicity, water temperature and an inherent endogenous rhythm for the production of viable eggs by the African catfish, Clark gariepinus, kept in subtropical ponds in Israel and under Israeli and Dutch hatchery conditions. Aquaculture, 63: 169-185.A comparison was made between the fecundity of female African catfish, Clarias gariepinus, transferred from their natural habitat in Northern Israel to nearby fish ponds and an indoor hatchery respectively, and conspecifics reared and kept in an indoor hatchery in The Netherlands.The results indicate that an inherent endogenous rhythm rather than the natural light periodicity determines the cyclical changes in ovarian activity, and that this internal rhythm is determined by environmental factors at an early stage of development. High water temperatures and the presence of males seem to enhance ovarisn activity and to shorten its resting period. The significance of gonadotropin secretion by the pituitary for the ovarian cycle is discussed.It is concluded that for propagation of C. gariepinus throughout the year, independent of the seasons, broodfish should be reared and kept in hatchery tanks at a water temperature of 25"C, with proper feeding, in the presence of males, and at any local light periodicity.

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Rice production, ricefield environment and the feeding ecology offish were studied in an experiment conducted at a rice‐fish station in the Mekong Delta, Vietnam. In total, six treatments (three replicates) were investigated: four different polyculture combinations of small sized silver barb, Puntius gonionotus (Bleeker), Nile tilapia, Oreochromis niloticus (L.), and common carp, Cyprinuso carpio L.; one treatment with pre‐grown fingerlings; and a control treatment in which no fish were stocked. No insecticides or fungicides were utilized before or during the experiment. Frequent fertilization of the water and a low rice plant biomass during the early vegetative growth phase stimulated the development of phytoplankton and zooplankton. The total weed biomass was low (maximum = 5.3 g dry weight m−2) and not significantly (P < 0.05) different between the treatments. A major component of the silver barb diet consisted of rice plants and accessible grains. However, the introduction of silver barb only had a significant effect on the number of rice tillers in the ratoon crop and not on the paddy yield. The quantitative differences in the diets of tilapia and common carp were minimal: both species fed mostly on detritus. Ricefields without silver barb produced the highest paddy yield (3120 kg ha−1). The total yield of introduced fish increased after increasing the stocking density of silver barb from 319.9 to 494.1 kg ha‐1. The highest fish yield (541.8 kg ha‐1) was obtained by stocking pre‐grown fingerlings in the ricefields.