Evaluation of an alternative salt mixture and the levels of magnesium in low salinity water for Pacific white shrimp Litopenaeus vannamei

Abstract

One of the primary challenges faced by inland shrimp farmers today is the higher cost of reconstituted sea salts (RSS) which represents a considerable financial burden considering the volume of salt necessary for the acclimation process or nursery and growth phases in inland shrimp farming. As an alternative to replace expensive RSS, the current study was designed to test the efficacy of a low-cost salt mixture (LCSM) in different production phases of Pacific white shrimp at 3, 6 and 15 g/L salinities under laboratory and farm conditions. The low-cost salt mixture (LCSM) was formulated based on Parmenter et al. (2019) to yield Na+, K+, Ca2+ and Mg2+ concentrations closely comparable to those of diluted seawater using agriculture grade sodium chloride, magnesium chloride, magnesium sulfate, muriate of potash (potassium chloride), calcium chloride, and sodium bicarbonate. In addition to the validation of LCSM, the same mixture was modified by decreasing the Mg level to allow for different Mg levels in the culture medium (100, 78, 55, 30, 17, 13 and 12 mg/L) in order to determine the effect of different Mg levels in low salinity (3 g/L) water on growth, survival, hemolymph osmolality, cationic composition in hemolymph, carapace and whole-body of Pacific white shrimp. Numerous laboratory-based trials were conducted at the E.W. Shell Fisheries Center at Auburn University in Auburn, Alabama and at the Alabama Fish Farming Center (AFFC) in Greensboro, Alabama at 3, 6 and 15 g/L salinities to test the efficacy of LCSM to rear Pacific white shrimp. On-farm evaluation of LCSM along with salinity acclimation of PL was carried out in two tank-based systems installed on levees adjacent to shrimp production ponds at Greene Prairie Aquafarm (GPA), southeastern Greene County, Alabama. Salinity acclimation was done from 32 g/L to 1.5 g/L salinity (PL size = 0.009 ± 0.02g) within 3-days by pumping (flow rate~3.5 L/min) low salinity pond water (1.5 g/L) into each tank. At the conclusion of the on-farm nursery trial, no significant differences existed in either survival (89-94%) or growth of shrimp post-larvae between RSS and LCSM treatments, which was conducted for 21-days following the salinity acclimation. These data were confirmed by the laboratory-based 21-day nursery trials, which had no significant differences in either survival or growth of shrimp post-larvae between RSS and LCSM treatments at all salinities (2, 6 and 15 g/L). At the conclusion of the 42-day growth trials, no significant differences were observed in survival, growth, osmoregulation and levels of cations in shrimp hemolymph between RSS and LCSM treatments at all salinities (3, 6 and 15 g/L) examined. At the conclusion of Mg2+ trial, a subsequent reduction (P<0.05) was noted in final weight, weight gain, hemolymph osmolality, osmoregulatory capacity, Mg2+ concentration in hemolymph, carapace and whole body of shrimp in respond to the reducing levels of Mg2+ in low salinity (3 g/L) water. Reductions in hemolymph osmolality and Mg2+ concentration in hemolymph are likely indicative of stress, which is assumed to be due to the dysfunction of osmoregulation in shrimp caused by low levels of Mg2+ in culture water. Results reflect the potential use of LCSM to replace RSS which could be an excellent solution to reduce the cost of production for inland low salinity shrimp aquaculture, thereby helping to further stimulate industry growth.