Water Quality in Inland Saline Aquaculture Ponds and its Relationships to Shrimp Survival and Production

Abstract

Survival and production of shrimp (Litopenaeus vannamei) in low-salinity pond waters at Greene Prairie Aquafarm (GAF) near Forkland in Green County Alabama varied greatly among ponds, but averages were higher in 2013 than in 2014. Examination of historical data (2001-2014) for this farm revealed that survival and production were extremely low in 2001, but following the adoption of potassium (K) augmentation in 2002, survival and production improved. Magnesium (Mg) augmentation also has been used since 2003, but the main benefit to survival and production is accrued from K augmentation. Nevertheless, much unexplained variation in survival and production occurred among ponds during a given year, in the same pond across years, and for the entire farm across years. By making weekly analyses in 20 ponds at GAF in 2013 and 2014, a large amount of data for concentrations of K, Mg, sodium (Na), calcium (Ca) and total alkalinity (TA) was acquired. The concentrations of these variables differed considerably among ponds on individual sampling dates, and for a given pond, across sampling dates. Simple linear regression revealed significant relationship (P<0.05) between K concentration and survival in 2013 and between Ca concentration and survival in 2014. Production was positively correlated with increasing K and TA concentration in 2013 and with salinity, sodium, and TA concentration in 2014. Variation in salinity, cations, and TA concentrations was not clearly related to rainfall patterns during the two years. Ordinary least squares analysis provided equations that used concentrations of the four major cations and TA as explanatory variables to account for about 49% and 55% of the variation in survival and production, respectively. However, Ca and TA concentrations had the greatest influence on the predictability of both survival and production by the OLS equations. The seawater equivalent concentrations of the cations, and the Na/K, Ca/K, Ca/Mg, and Alk/K ratios were not found to be reliable indicators of shrimp survival and production. Two other low-salinity shrimp farms located near GAF were included in the study in 2014. Fifteen ponds of these farms had relatively similar average concentrations of cations and TA, but there was considerable variation in concentrations of these variables among ponds on each sampling date as also occurred at GAF. Salinity and concentrations of cations were higher than those found in ponds at GAF, but TA concentration was lower. The lower TA concentration may have resulted from greater calcium carbonate (CaCO3) precipitation in waters with greater Ca concentration than found at GAF. Unfortunately, the pond owners did not provide records on stocking rates, survival, or production necessary for comparison with shrimp performance at GAF. Potassium supplementation of pond water at GAF is extremely critical, because of the low concentration of this cation in the water supply. Magnesium concentration also is low in the water supply, but the importance of Mg augmentation is unclear. In ponds of especially low salinity, Na augmentation would possibly be beneficial. The possibility for increasing Ca and TA concentrations in ponds at GAF is questionable, because water often is at saturation with CaCO3. Compounds like calcium sulfate (CaSO42H2O) and sodium bicarbonate (NaHCO3) being highly soluble may be used to increase Ca and TA, however there are chances that Ca will precipitate in either case.