Accumulation and loss of critical elements in shrimp RAS
By Quentin Dodd
By Quentin Dodd
Indoor recirculating aquaculture systems (RAS) for the production of Pacific white shrimp (Litopenaeus vannamei) have attracted considerable research and economic interest because they are relatively environmentally friendly and bio-secure. They can also be located inland away from expensive coastal real estate, in temperate climates and in proximity to major markets. They can be grown in either clear-water systems or using biofloc technology.
Inland facilities that culture shrimp typically derive culture water by mixing synthetic sea salt with water from wells or municipal sources. Typical commercial production may replace between 0.5 and 10% of the makeup water on a daily basis.
The lower water replacement rates reduce the expense of deriving new synthetic saltwater. However, as water replacement rates approach 0%, the system can accumulate or lose critical elements (ions) that are required to maintain shrimp growth rate and survival levels for commercial production. These include (among others) Calcium, Chloride, Magnesium, Phosphorus, Potassium, Sodium and Sulphur, as well as other elements that are needed in smaller levels but are toxic when in excess. These include: Copper, Cadmium, Chromium, Lead, Mercury, Manganese, Selenium, and Zinc.
This issue was discussed in a paper presented at the International Conference on Recirculating Aquaculture held last summer at Roanoke, Virginia, by Dr. David Kuhn of Virginia Tech, Blacksburg, VA. He and his colleagues (Addison Lawrence, Jack Crocket, and Dan Taylor) have found that the concentrations of elements (and ions) in culture water are not the only critical factor since the ratios of these elements are also important. Fortunately these can be monitored during production and adjustments can be made by adding a particular salt either directly to the culture water or in the feed. The salts come in various affordable forms such as salts of the cations sulphate and chloride.
In order to describe element-management in shrimp RASs, Kuhn described two case studies. The first concerned potassium imbalances at an inland commercial clear-water facility growing Pacific white shrimp (Litopenaeus vannamei) in indoor shallow raceways using typical RAS filtration. A synthetic sea salt (Crystal Sea Marine Mix) was added to well water to give a nominal salinity of 10–12‰. Temperatures were maintained at approximately 30°C and dissolved oxygen levels at 5.0 mg/l or higher. Ammonia and nitrite were typically less than 0.75 mg/l, and nitrate-N accumulation less than 200 mg/l.
After a few months of the RAS being operated, a large number of the shrimp began exhibiting tail cramping after being handled, from which a large percentage of shrimp did not recover. The literature pointed to cramp tail syndrome (CTS) or cramp muscle syndrome (CMS) that could be caused by stressed due to high temperatures, Vibriosis, or toxins in the water. These potential causative agents were determined to not be the cause of CTS/CMS. However the potassium concentrations were determined to be about half of what would typically be available in freshly prepared culture seawater (50–75 vs 100-125mg/l). Calcium concentrations, by contrast, were typical of freshly prepared culture seawater.
The addition of potassium chloride to the water reduced, but did not eliminate the incidence of CTS/CMS, but increasing the KCl level in the feed from 0.7% to 1.2% KCl resulted in no further CTS/CMS incidence.
In a 14-day lab experiment in formulated saltwater containing concentrations of potassium ranging from 150% to zero, shrimp were fed a feed containing 1.25% KCl. Survival in the saltwater containing potassium levels from approximately 175 mg/L ppm to 55 ppm ranged between 100 and 87%, Meanwhile survival was barely 5% in the saltwater targeted to contain zero ppm K. Also, there was no incidence of CTS/CMS. In a second 60 day experiment in which the potassium levels ranged from 100% (approximately 115 ppm) to 10% (approximately 15 ppm) in the culture seawater with the shrimp fed a diet containing 1.25% K; the survival generally declined from 75 to 40% in proportion to the potassium content, and CMS appeared in the 10% K treatment after day 21.
The second case study, this time of a biofloc system, showed how manganese, a toxic element, accumulated in suspended-growth biological reactors (SBR) that generate ex-situ bioflocs used to supplement shrimp feed. Typically, SBRs are filled with wastewater from the shrimp RAS. Residual biofloc material from the previous batch seeds the incoming ‘dirty’ water. This seeded water is mixed and the biofloc allowed to grow and accumulate until treatment objectives, such as greater than 90% reduction of nutrients are met. Obviously, the longer the biofloc is allowed to grow before harvesting the more nutrients will be removed from the water. When the mixing system is switched off the bioflocs settle out. The treated, “clean” wastewater is decanted from the bioreactor and can be returned to the RAS, and the settled biofloc can be used to supplement the shrimps’ pelleted feed.
Typically SBRs are operated with a young (30-day) sludge-age before decanting, which yields good quality water for return to the RAS, and the biofloc used to supplement the pelleted feed usually gives a shrimp performance equal or better than that with pellets alone. Sludge age is the average time that a particle of biofloc remains in the SBR. However, an SBR operated with a 60 day sludge age yielded a much cleaner water for return to the RAS, but the resulting biofloc, when used as a feed supplement, suppressed shrimp growth by up to 30%!
Analysis of the older (60-day sludge-age) biofloc showed that manganese content was between 0.9 and 1.1%, which translated into approximately 0.1 to 0.3% Mn in the shrimp diet, depending on the biofloc inclusion level. While manganese is an essential element for shrimp culture, further experimentation showed that shrimp fed with feeds containing more that 0.02% Mn had significantly lower growth rates.
For more information contact Dr. David Kuhn at: firstname.lastname@example.org.