Immune system and production of pacific white leg shrimp on the different stocking density and sediment redox potential

Abstract

Pacific white leg shrimp is a desirable species for shrimp farmers, which has eral advantages such as the availability of specific pathogen free post larvae, i relatively fast growth, and high survival rate. Pacific white leg shrimp Li 'P,enaeus vannamei has become one of the most important species in aquaculture sectors, including in Indonesia. Indonesian shrimp production had en dramatically increased during the last decade, from 191 148 MT in 2003 to 593 280 MT in 2014 or about 213% increase. The increase of shrimp production is ostly due to new openings of farming area and by intensification by increasing the stocking density. However, despite the progressive increase in production, shrimp-farming industry still encounters some major challenges among which is disease outbreak. Disease outbreak and environmental quality in shrimp culture systems are closely related. It has been suggested that viral and bacterial diseases, together with poor soil and water quality, were the main causes of the reported shrimp mass mortality in several studies. The aim of this study is to evaluate the effects of stocking density and sediment redox potential on the shrimp production performance and non-specific immune response. This study consisted of three main research stages with the following objectives: first, to observe the behavioural activity of L. vannamei and its oxygen consumption, as well as sediment oxygen demand in relation to sediment redox condition; second, to evaluate the effect of shrimp density on the animals immune system and growth performance, as well as the water quality in the culture system; and third, to evaluate the effects of sediment redox potential and shrimp stocking density on the sediment and water quality parameters, shrimp production performance, and immune response and resistance upon white spot syndiome virus (WSSV) challenge The first experiment was conducted in semi outdoor laboratory using 18 units of aquarium with a dimension of 30 cm x 30 cm x 100 cm. Each aquarium was filled with filtered seawater and 5 cm deep soil substrate with different redox pote tial at the bottom of each tank. Shrimp were randomly distributed into the tank at a stocking density of 130 shrimps m·²• One hour after being transferred to the aguarium, the shrimp swimming behaviour was observed for 6 consecutive hours, during which no feed was given to the shrimp. At the initial and end of expe ·ment, dissolved oxygen was measured at different depth. The study showed that sediment redox potential did not influence L. vannamei behavioural profile. Shrimp stayed on the tank bottom for most of the time in all treatments and these demonstrate that these animals were unable to stay for a long time in the water co , . Dissolved oxygen decreased with the increase of the water depth incHcating that dissolved oxygen on the bottom was consumed by both shrimp and s i; ent with the more negative sediment redox potential the higher sediment en demand.

In the second experiment, a completely randomized experimental design was applied with four different stocking densities, i.e. 50, 150, 300 and 600 shr· s m·2 with four repetitions. Eighteen units of fibre tanks with a dimension of 48 cm x 48 cm x I 00 cm were used as the experimental containers. The tanks were fil ed with filtered seawater to a height of 90 cm. Healthy shrimp at intermolt stag ere selected and randomly distributed to each tank at the density according to ill treatments. The dissolved oxygen concentration, temperature, pH and sa· l}' of the culture water were measured daily. Water samples were collected wee: y to measure the concentrations of total ammoniacal nitrogen, nitrite, nitrate, and tal alkalinity. Shrimp growth performance comprising of average body wei t, specific growth rate, survival, as well as immune parameters including total ... aemocyte count, phenoloxydase, and respiratory burst were measured . The results of the experiment demonstrated that maintaining shrimp de sr_ at 300 shrimp m·2 and 600 shrimps m·2 led to significant reductions in

pro-d

ction performance, water quality and shrimp immunity. lie procedures for the third experiment were similar to those in the second

expe ent. In this experiment, however, sediment with different sediment redox potential (-65 mV, -108 mV and -206 mV) was added onto the bottom of each , stocking density (60 and 120 shrimps m"2), and a challenge test was performed at the end of the experiment. Sediment sample was collected from each experi ental tank at the initial and the final day of experimentation to measure the total or·anic C, total organic N, total P, S, Fe, and Mn concentrations in the sediment. The same manner as in the second experiment was carried out for the water samples, growth performance and shrimp immune parameter. The redox potentia of the sediment clearly affected the total concentration of some major metal elements, such as Mn, Fe, and S, which functioned as terminal electron accepto11s in subsequent anaerobic respiration in the sediment. The shrimp density strong! affected all water quality parameters measured in this study except H2S concentration. The redox potential of sediments also demonstrated significant effects on all water quality parameters except total amoniacal nitrogen concen ation. Furthermore, the results clearly showed that a redox potential of - 206 mVi significantly reduced the dissolved oxygen concentration in the sediment­ water interface and increased the generation of H2S in water column. Thereby, this red0x potential level is not advisable for shrimp culture system. Maintaining shrimp in -206 mV sediment redox potential led to significant reductions in production performance and resistance against WSSV. Stocking density did not affect e shrimp growth, but significantly reduced the production. When coupled with highly reduced sediment redox potential, increasing stocking density up to 12_0 shrunps m·2 significantly reduced the shrimp immunity and resistance against w'ssv. In conclusion, the present study showed that L. vannamei tend to stay at the pond bottom. The present study demonstrated that redox potential significantly affected the shrimp welfare including its susceptibility to disease and ultimately the , uction. In this regard, maintaining the pond sediment in an optimal