Water Quality in Inland Ponds for Low-Salinity Culture of Pacific White Shrimp Litopenaeus vannamei

Abstract

Four investigations were performed in inland, low-salinity shrimp farms in Blackland Prairie Region in western Alabama. Three of these investigations were related to performance of shrimp responses to variation of water quality variables in ponds. The forth investigation was performed to address the concern of environmental impacts as a result of utilization of saline water for aquaculture purpose in inland region. Common water quality variables in nine ponds exhibited wide variation in concentration among ponds and overtime. Shrimp performance varied significantly among ponds as follows: survival, 16 to 128%; production, 928 to 5, 950 kg/ha; feed conversion ratio, 1.18 to 2.89. None of the water quality variables were at the lethal concentrations but few variables were occasionally outside optimum ranges for shrimp production and may have stressed shrimp. Survival and production were positively correlated (P < 0.05) with increasing concentration of methyl orange alkalinity, total alkalinity, and calcium hardness. Production was negatively correlated with higher pH and temperature but these correlations may have resulted from lower temperature and pH during final days of the crop in ponds harvested in October rather than from actual effect of temperature and pH on growth. Variables that were correlated with shrimp survival and growth or outside optimal ranges deserved to be further investigated to ascertain whether or not they are causal and harmful. Concentrations of 18 trace elements tended to be greater than concentration in normal seawater except for molybdenum, boron, and silicon. Concentrations of most trace elements varied greatly among ponds and sampling dates. The analytical method used in this investigation measured total concentrations of trace elements, includes free ionic forms, hydrolysis products, ion pairs, and coordination compounds (chelated forms), but only the free ionic forms that are toxic to aquatic organisms. Although concentrations of iron, aluminum, zinc, and selenium were occasionally higher than typical, safe concentration limits for continuous exposure to aquatic life, it is doubtful that free ionic concentrations in these waters were great enough to harm shrimp. Positive correlations (P < 0.05) between shrimp survival and production, and increasing concentrations of zinc, cobalt, and iron should be further investigated to ascertain if additions of these elements to ponds might improve shrimp performance. Monthly, mean water temperature among eight ponds differed by 3.40oC in May and 2.83 oC in September, but there was less than 1 oC difference among pond in June, July, and August. Differences in water temperature among ponds were not related to pond water surface:volume ratio. The differences in water temperature among ponds probably were related mainly to variation in plankton density. The negative correlations (P < 0.05) between water temperature and shrimp production and survival are thought to have resulted from variation in crop duration and therefore are not causal. However, differences in water temperature among ponds in May and September were great enough to have possibly influenced shrimp survival and production. Investigations of effluent volume and pollutant loads of an inland, low-salinity shrimp farm were conducted in 2008 and 2010 and the draining effluent at harvest of both years were 50.4% and 57.9 % of impounded water, respectively. Farm effluent caused a notable increase in concentrations of all variables other than pH at sampling station downsteam of the farm outfall. But only chloride concentration exceeded 230 mg/L─ upper standard for freshwater streams set by the Alabama Department of Environmental Management. Average pollutant discharge per tonne of shrimp was 444.9 kg total suspended solids, 51.3 kg 5-d biochemical oxygen demand, 15.2 kg total nitrogen, 1.33 kg total phosphorus, and 4,402 kg chloride. Concentrations of turbidity, settleable solids, total suspended solids, total phosphorus and possibly other variables in effluent could be lessened by installation of a settling basin to remove medium and coarse solids before final discharge. Release of less water from the farm during harvest, and improvement in feed conversion ratio also would lessen pollutant loads per tonne of shrimp.