IONIC CHANGES DURING THE PRODUCTION OF Litopenaeus vannamei FED TWO FEEDS OF DIFFERING PROTEIN CONTENT IN A ZERO-EXCHANGE, BIOFLOC-DOMINATED SYSTEM
The ionic composition of culture water can have a significant impact on shrimp and bacterial performance. Some studies suggest that specific ions can be depleted or may accumulate over multiple production cycles in culture water, biofloc and in shrimp tissue in biofloc-dominated no water exchange systems. To better understand this process, changes in the ionic composition of culture water and shrimp tissue were measured during a study testing the performance of Litopenaeus vannamei fed two feeds with 35% and 40% crude protein in a biofloc-dominated, limited-exchange system.
A 48-day trial was conducted in four 40 m3 greenhouse-enclosed raceways (RWs), each equipped with a YSI 5500D inline dissolved oxygen monitoring system. L. vannamei juveniles (5.3 g) were stocked at 457/m3 and fed one of two commercial feeds: Zeigler Shrimp GR Hyper-Intensive 35 (35% protein) (HI-35) to shrimp in two RWs, and Zeigler Shrimp Exp-14 (40% protein) (EXP) to shrimp in another two RWs. Biofloc-rich culture water used for a prior 62-day nursery trial was reused for the grow-out study. No water exchange was conducted, except for freshwater addition to compensate for evaporation. Solids concentrations were controlled with a foam fractionator and settling tank. The RW water was aerated, circulated, and mixed with airlift pumps, air diffusers, a pump driven Venturi injector, and spray nozzles. Shrimp and culture water were sampled for ionic composition analyses at stocking, mid-trial, and harvest.
At the conclusion of the trial there was no significant difference (P<0.05) in shrimp performance between feed types. Survival was reduced and FCR increased by Vibriosis. Mean temperature, salinity, DO, and pH were 29.9oC, 30.3 ppt, 5.4 mg/L and 7.5, respectively. Mean alkalinity was significantly lower (P<0.05) in EXP than in HI-35. Strontium was depleted by 10.0-11.4%, and potassium, nitrate, and phosphate accumulated by 12.9-17.4%, 320.0-415.6%, and 113.6-244.9%, respectively in culture water over the trial, independent of salinity. Strontium and copper concentrations were higher in shrimp fed the higher protein diet. No significant accumulation of heavy metals was recorded in either culture water or shrimp tissue. Feed ionic composition and normal physiological changes appeared to be the main factors associated with changes in shrimp ionic composition. No detrimental changes in ionic concentrations were recorded over the 48-day culture period.