Shrimp is the most consumed seafood item in Europe , but its production is still inadequate to meet consumer demand and high-quality domestic supplies are lacking. To facilitate intensive farming of brackish-water species, recirculating aquaculture systems (RAS) can be adopted . This allows farmers to control inputs and outputs, helps save water and salt costs, and prevents adverse environmental impact. This project focused on assessing the effects of two different types of RAS (clear-water and hybrid) , along with horizontal substrate on intensive Pacific white shrimp production and water quality. Clearwater (CW) RAS is an effective strategy for shrimp production but may have relatively high equipment and operational costs. Simple hybrid systems (HY) have some advantages as external biofilters maintain water quality, but solids filtration is relatively minimal. A rtificial horizontal substrate in RAS may help to enhance shrimp production by increasing the surface area for shrimp to graze and facilitating higher shrimp density, thereby justifying more intensive filtration.
In this study, two levels of each experimental factor were used : system type (CW vs HY) and presence of an artificial substrate (WS) versus absence of substrate (NS). Th is resulted in 4 treatments: HY-WS, HY-NS, CW-WS, and CW-NS each of them having four replicates randomly assigned to 1 m3 circular tanks. Ozone, a strong oxidizing agent, was introduced in CW tanks. CW tanks had a foam fractionator, a se ttling chamber, and a biofilter, while HY systems had only a settling chamber and a biofilter. Four layers of horizontal cloth net, each layer covering 0. 5 m2 area, 20 cm apart from each other, and square in shape were installed as substr ate in the eight circular tanks with substrate. Shrimp were stocked at 450 m -3 in the experimental tanks and reared for 53 days.
The HY-WS treatment had significantly higher individual shrimp weight at harvest, total biomass harvested m-3 , survival, and growth rate compared to the HY-NS and CW-NS treatments. The CW-WS treatment was not significantly different than HY-WS with regard to these parameters, but was also not significantly different than the HY-NS treatment. Solids and turbidity were significantly lower in the CW treatments, as intended. TAN and nitrite-N were significantly lower in HY systems and the presence of substrate also resulted in significantly lower TAN and nitrite-N. It is possible that the biofloc particles in the HY treatments and the surface area provided by the substrate both allowed for more nitrifying bacteria in the systems, thereby lowering TAN and nitrite-N concentrations. In further support of HY treatments having more nitrifying bacteria, these systems also had significantly greater average nitrate-N concentrations. Findings from this study suggest that inclusion of horizontal substrate in the intensive indoor shrimp culture system could enhance production and help in biofiltration.