Hydrological and Hydrobiological study of sturgeon rearing ponds.

The hydrobiological conditions of Acipenser persicus rearing ponds in the Shahid Beheshti and Siyahkal Hatcheries were studied during two rearing seasons from 1996 through 1997. The rearing of A. persicus fingerlings in earthen ponds begins every year in March and continues for 30-52 days in different ponds. Six ponds were randomly selected in the two hatcheries (3 ponds from each hatchery) to study growth in sturgeon fingerlings. The physical and chemical parameters of the water column and bottom soil were studied. Abundance and biomass of phytoplankton, zooplankton and benthic species were studied in each pond throughout the rearing periods. Hepatosomatic index, condition factor and growth rate of sturgeon fingerlings were also determined in order to estimate production in each pond. Results obtained indicate that the soil bottom in all ponds was clayey. Total phosphate and nitrogen in bottom sediments in the Shahid Beheshti ponds were 12.8 and 0.069 ppm, respectively. Water transparency in ponds was mostly high and pH was alkaline. The biological production in water was lower than the required amount for good fish production. The phytoplankton species Melosira granulate showed the highest abundance followed by Scenedesmus sp., Schroederia sp. and Cryptomonas sp. during the rearing period in all the ponds studied. Highest production among zooplankton species belonged to nauplius of Cyclops and Cyclops. Biomass of zooplankton during both rearing periods varied from 2.49 to 23.6 g m-3 in the Siyahkal hatchery and from 6.21 to 55.42 g m-3 in the Shahid Beheshti Hatchery. Nauplis of Cyclops and Cyclops together comprised 67% and 82.5% of the mean total biomass of zooplanktons of the Shahid Beheshti and Siyahkal hatcheries, respectively. The number and biomass of chironomids in 1996 was greater than that of other benthic species. Total mean biomass of benthic organisms in the Shahid Beheshti and Siyahkal ponds were 13.1 and 1.01 g m-2, respectively in 1996 and 0.17 and 0.95 g m-2 in 1997. Examination of stomach contents of A. persicus fingerlings during the rearing period up to the time of their release indicates low natural production in these ponds as a result of poor physical and chemical conditions of water and pond bottom. Examination of feeding behavior in A. fingerlings showed that daphnia, which was usually introduced to the ponds, comprised 90% of the food consumed by these fingerlings during the rearing period. Thus growth rates in A. persicus fingerlings decreased by 40-50% during the rearing period due to the lack of natural food production in ponds and also because live food (daphnia) was not introduced into the ponds at the right time.