Evaluating various management strategies of biofloc systems and understanding the physiological basis behind the thermal tolerance of Pacific white shrimp, Litopenaeus vannamei in low salinity waters

Abstract

Biofloc technology (BFT), in its various types, has been known to be a realistic solution for efficiently managing water quality with low or no water exchange, enhancing shrimp growth performance and establishing an efficient and healthy shrimp culture with a better food conversion ratio in the shrimp aquaculture business. A series of laboratory-based trials were conducted at E.W. Shell Fisheries Center at Auburn University in Auburn, Alabama to evaluate the effects of applying different management strategies using prebiotics, probiotics, biofloc and synbiotic type systems in different production phases of Litopenaeus vannamei. Twenty-four 150 L polyethylene indoor tanks were used as static biofloc individual tanks for a nursery phase trial conducted for 28-days except for four tanks that were used as a clear water recirculating system (RAS; a reference). To evaluate the effects of using commercial probiotic products as a feed supplement and as water additive on the water quality, growth, and survival on nursery culture performance of Pacific white shrimp PLs. At the conclusion of the nursery trial, no significant differences existed in weight gain of shrimp post-larvae between treatments, however final biomass (g), and survival (%) of PLs were significantly higher between probiotic and clear water treatments. An 8-week indoor grow-out experiment was conducted to study the effect of culturing the Pacific white shrimp in “biofloc” and a “synbiotic” type system on the growth and immune responses of shrimp. The experimental system consisted of 24 static indoor circular polypropylene tanks (800-L water volume). At the conclusion of the grow-out study, it was detected that all treatments produced good survival, rapid growth, low FCR and physiological parameters indicating all are viable options. A higher level of total haemocyte count (THC) was noted in shrimp reared in the biofloc and synbiotic treatments as compared to the control, however, there was no significant differences between treatments. Additionally, a grow-out study was conducted to evaluate the performance of the Pacific white shrimp fed with four different protein-based extruded diets [plant-based (AP), 8% poultry by-product meal (PM8), 8% fishmeal (FM8) and 12% fishmeal (FM12)] while cultured in clear water and biofloc type systems. Results from the clear water experiment showed that shrimp fed with PM diet had the lowest final individual weight, biomass (g), and weight gain (g), and the highest feed conversion ratio (FCR). Results from the biofloc experiment showed that shrimp fed with AP diet had the lowest biomass (g), weight gain (g), and thermal growth coefficient and the highest FCR. No significant differences in survival rate were observed between the four diets in both experiments. The low inclusion of fishmeal, as well as the use of alternative protein sources in these diets, did not adversely affect final weight, weight gain, and percent weight gain of Pacific white shrimp. Consequently, the choice of how to manage the bacterial community should be based on available resources. Another major challenge facing the shrimp industry in inland, low-salinity ponds is a phenomenon called late-term mortality which is thought to be driven by thermal stress at the end of the growing season when water temperatures can reach or exceed 36 °C in shrimp production ponds. To investigate the physiological mechanisms behind upper lethal limits in shrimp, we evaluated linkages between empirically measured thermal limits and absolute aerobic scope (AAS), or ability to provide energy above that needed for basic maintenance. At each temperature, intermittent respirometry was used to estimate resting metabolic rate and lethal thermal tolerance by evaluating critical thermal maximum (CTmax) was directly measured. Additionally, the electron transport system assay was used to estimate maximum metabolic rate at temperatures from 9–45 °C. Small shrimp had a higher CTmax than large shrimp, with upper lethal limits of 40.6 and 39.0 °C, respectively. In this study, we tested whether thermal tolerance decreases with increasing shrimp age/size and whether AAS is a useful concept for understanding the physiological basis of thermal tolerance in shrimp. Two size classes of shrimp (small: 2.07 ± 0.86 and large: 24.64 ± 2.55 g) were exposed to increasing temperature at a rate of 1 °C/h from 28–42 °C. At the conclusion of the study, AAS reached its minimum (AASmin) at temperatures within 2 °C of CTmax for both size-classes. Reductions in AAS appear to be one of the underlying physiological drivers of thermal tolerance in L. vannamei and an indicator of increasing thermal stress. Changes in the temperature at which AAS reaches its minimum may be a useful predictor of shifts in thermal tolerance among shrimp size-classes.