Artificial sea salt mixtures are required for shrimp inland production, which can be a substantial portion of the production costs. This article presents results that indicate that using the low-cost artificial sea salt (LCS) formulation reduces artificial sea salt cost significantly, up to 15%, while having no significant impacts on shrimp production and water quality compared to a commercial marine salt mixture.
Due to high demand and exhausted natural fisheries stock, the aquaculture sector has grown rapidly, with aquaculture products currently representing more than 50% percent of seafood production for human consumption (FAO, 2020).
Although most aquaculture is pond based, indoor aquaculture is growing in popularity, especially in inland areas where fresh seafood products are often difficult to acquire. Such indoor operations primarily utilize recirculating aquaculture systems (RAS).
“RAS are contained production systems that provide substantial environmental control, including solids filtration, biofiltration, temperature control, and other control mechanisms based largely on animal needs and climate considerations.”
RAS also require much less space than pond-based aquaculture through higher production densities, allowing them to be situated in a variety of building types. Due to the costs of RAS, high-value species are being investigated for their suitability in RAS production as the Pacific white shrimp (Litopenaeus vannamei).
One limiting factor in RAS marine shrimp production is the need for salt to create marine or brackish water. Most inland areas do not have direct access to saltwater; therefore, shrimp production operations must use an artificial marine salt mix.
Most commercial salt mixes attempt to replicate the mix of elements found in natural seawater, including trace minerals such as Fe, I, Zn, and Mn.
Some of these trace elements have been shown to play a role in physiological functions when included in the shrimp diet. These commercial marine salt mixes can represent a significant portion of production costs for inland shrimp producers.
“Simplifying these salt mixes down to only essential elements may reduce costs for producers by reducing the number of ingredients and facilitating inhouse made salt mixes.”
A previous experiment found that there were no significant differences in shrimp production when using five salt mixtures: containing 100% commercial sea salt (CSS), 75% CSS and 25% of a least cost salt mix (LCS), 50/50% CSS/LCS, 25/75% CSS/LCS, and 100% LCS (Tierney et al., 2021).
Although there were no significant differences in that study, shrimp survival in the 100% LCS treatment was low at 57%, whereas the other treatments averaged 70% survival.
The purpose of the current experiment was to further examine salt mixtures in the range of 75% LCS to 100% LCS due to the low survival found in Tierney et al. (2021) and definitively determine if a LCS can result in adequate shrimp production, maintain acceptable water quality levels, and reduce salt cost in intensive, indoor shrimp aquaculture systems.
Materials and Methods
Experimental design and operation
This experiment took place in the Kentucky State University (UK), Sustainable Aquaculture Development Lab (SADL). The SADL is a 174 m2 insulated and climate-controlled building used for indoor aquaculture research.
Five treatments were developed for this experiment, each with 4 replicated tanks for a total of 20 tanks. Each treatment used different combinations of two salt mixes to reach the target salinity.
The two salts were a commercial sea salt mixture, Crystal Sea Marine Mix (CSS) (Marine Enterprises International, Baltimore, MD, USA); and a least-cost salt mixture (LCS) made from sodium chloride (NaCl), magnesium sulfate (MgSO4 ), magnesium chloride (MgCl2 ), calcium chloride (CaCl2 ), potassium chloride (KCl), and sodium bicarbonate (NaHCO3 ) (Table 1).
The individual treatments in the experiment were 75/25% LCS/CSS, 80/20% LCS/CSS, 90/10% LCS/ CSS, 95/5% LCS/CSS, 97.5/2.5% LCS/CSS, and 100% LCS. The specific brands and purity of each ingredient of the LCS are listed in Table 2.
Salinity in all systems was kept at 15 g L-1 throughout the experiment. Any water loss due to evaporation was replaced with dechlorinated municipal water.
When pH fell below 7.8, this was adjusted with additions of 35 g of sodium bicarbonate. Temperature, dissolved oxygen (DO), pH, and salinity were all measured twice daily at approximately 08:00 and 16:00 h. Total ammonia nitrogen (TAN), nitrite, and turbidity were measured once weekly during this study.
The shrimp used were purchased from American Mariculture, Inc. (St. James City, FL, USA). Upon arrival, the shrimp were raised in two 3.4 m3 nursery tanks for 37 days before being stocked into the experimental tanks.
The salinity in the nursery tanks was started at 30 g L-1 and was lowered to 15 g L-1 over the course of the nursery period. The shrimp were fed 6 different rations throughout the nursery.
“The shrimp were fed on 24-hour automatic belt feeders for the majority of the nursery stage to ensure continuous feed availability.”
The post-nursery shrimp were stocked into the experimental tanks at 262 shrimp m-3
The cost of salt in USD for all treatments was generated by calculating the total cost of each salt mix to reach 15 g L-1 salinity and the percent of each salt used in each treatment. In addition, the cost of salt m-3 and the shrimp production m-3 in each treatment were combined to calculate the cost of salt kg-1 of shrimp.
Results There were no significant differences in temperature, TAN, nitrite, TSS, or VSS (p > 0.05, Table 3).
Significant differences were detected in DO, pH, salinity, and turbidity between treatments (p < 0.05). The DO concentration tended to increase as CSS concentration decreased, with 100% LCS and 97.5% LCS treatments having significantly higher dissolved oxygen levels than the other four treatments.
The 100% LCS treatment had significantly lower overall pH than the 90% and 80% LCS treatments. Turbidity was significantly higher in the 100% LCS treatment compared to all other treatments except the 90% LCS treatment.
There were no significant differences detected between treatments in all tested shrimp production metrics, including average weight shrimp-1, growth rate week-1, FCR, kg of shrimp m-3, and survival (p > 0.05, Table 4).
All average shrimp weights were between 20.7 g, and 22.2 g and average growth rates were 1.4–1.6 g week-1, FCRs ranged from 1.4 to 1.6:1, shrimp production ranged from 4.3 to 4.7 kg m-3, and survival averaged 81% across all treatments with a range of 76.7–84.3%.
The cost of salt m-3 at 15 g L-1 salinity was different between all treatments, and cost decreased as LCS percentage increased (Table 5).
The cost of salt kg-1 of shrimp produced was found to be lowest on average in the 97.5% LCS tanks, and significantly lower than the 80% and 75% LCS treatments (Table 6).
Total ammonia nitrogen and nitrite levels were both maintained within acceptable ranges for shrimp production throughout this experiment.
Although there were significant differences between treatments in DO levels, pH, salinity, and turbidity, these minute differences likely had little effect on the overall performance of the shrimp and were all within acceptable ranges.
There were no significant differences found between treatments in temperature, TSS, or VSS. Importantly, pH levels were maintained when using the LCS, even though a single source of alkalinity is used in the mixture (sodium bicarbonate).
“A complete sea salt mixture would likely include multiple buffers, such as calcium, potassium, and magnesium carbonate compounds.”
The lack of significant differences in shrimp production between treatments has important implications for shrimp producers. The increased concentration of LCS used in production appears to have no detrimental impact on shrimp performance.
Overall average survival was just above 80%, average FCR was 1.5 across all treatments, and the average growth rate was 1.5 g week-1, all comparable to or exceeding recent shrimp studies using reduced cost salt mixtures and commercial mixtures at similar salinities (Tierney et al., 2021; Galkanda-Arachchige et al., 2020; Pinto et al., 2020).
Shrimp in this study reached an average of 21.6 g individually at 86 days, a size that is within the range preferred by consumers in North America, Europe, and other regions.
This production time scale falls within a competitive harvest schedule and the shrimp size is at the highest of the range recommended by Zhou and Hanson (2017) in their economic model, suggesting that the results of this study are commercially relevant.
“These results further demonstrate the utility of this reduced-cost salt mix across several stages of shrimp growth, as the study by Galkanda-Arachchige et al. (2020) used an identical low-cost mixture and found equal performance of post-larval and juvenile shrimp between both low-cost and commercial salt mixes. “
The results of this study found use of the LCS resulted in high survival, exceeding the results in Tierney et al. (2021) who noted shrimp jumping out of the tanks which may have resulted in the discrepancy in shrimp survival.
The similar shrimp performance between treatments, regardless of LCS concentration, influences the economics of shrimp production operations.
The cost difference in salt between the 75% LCS and 100% LCS was just over $4 USD m-3, which could lead to significant cost savings for shrimp producers, especially those operating at large scale. The lower salt cost also reduced the cost of production kg-3 of shrimp by $0.75 USD, which represents a 15% decrease in production cost over the CSS formulation.
“The economics of pond-based shrimp farming are well studied; however, the feasibility of high intensity, indoor shrimp production is still unclear due to substantial upfront costs and variable system designs and production strategies.”
Any reduction in production cost may have significant impacts on this relatively new shrimp production style.
The salts that were used to make the LCS formulation in this study were each purchased in 23 kg bags that were shipped several hundred kilometers. However, in a commercial setting it is more likely that farmers would purchase these in bulk quantities and from local vendors if possible.
This commercial-scale strategy would likely reduce the cost of the mixture even further. In fact, Maier (2020) points out that scale is one of the biggest factors influencing the profitability of indoor shrimp farming. He goes on to note that using the same LCS formulation tested in this study can significantly improve profit potential for farmers.
The study shows the feasibility and cost savings of a low cost, easily made salt mixture in high-intensity indoor shrimp production. The use of the LCS mixture should be considered by shrimp producers due to the significant decrease in production costs, similar shrimp performance, and water quality compared to a commercial marine salt mixture.
This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “USING ALTERNATIVE LOW-COST ARTIFICIAL SEA SALT MIXTURES FOR INTENSIVE, INDOOR SHRIMP (LITOPENAEUS VANNAMEI) PRODUCTION” developed by LEO J. FLECKENSTEIN; THOMAS W. TIERNEY; JILL C. FISK; ANDREW J. RAY- Kentucky State University.
The original article was published in AQUACULTURE REPORTS, in May
The full version, including tables and figures, can be accessed online through this link: https://doi.org/10.1016/j.