Suspended Solids Effects In Shrimp Biofloc Systems

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When a production cycle begins with a well-stocked biofloc inoculum, the concentrations of ammonia and nitrite are relatively low, but concentrations of suspended solids tend to be higher in this phase. Generally, nitrite concentrations rise as suspended solids levels increase. At dissolved-oxygen concentrations above 5 mg/L, excess suspended solids were not a problem for shrimp respiration.

Marine shrimp aquaculture systems that incorporate biofloc technology often experience a high density of suspended solids within the systems. The injection of air at the bottoms of culture tanks promotes both the diffusion of oxygen in the water column and mixing of suspended material. The use of lined tanks increases the confinement of biofloc in the culture environment and restricts the organic particles in the water column from being recycled completely.

Suspended Solids

Suspended solids consist mainly of organic matter, which is comprised of microbial forms that when decomposing exert a high demand for oxygen. This demand can decrease the dissolved-oxygen concentrations in the culture system to reach levels below the recommended concentration for the cultivated species. An increase in suspended solids can also reduce the water quality within the system. Overall, these less-than-optimal conditions reduce system performance.

These conditions can occur throughout the different stages of the marine shrimp production cycle, depending on whether the system starts a cycle with mature biofloc inoculum. However, the use of biofloc inoculum relates to the suspended solids concentration in the water column.

At the beginning of a cycle, interactions among the nitrogenous compounds, ammonia and nitrite are most notable when biofloc is forming in the culture. The reduction of ammonia occurs with the establishment of ammonium-oxidizing bacteria, which does not require the use of organic carbon, and with the absorption of heterotrophic bacteria. Therefore, the nitrite accumulation occurs due to the slow growth of nitrite-oxidizing bacteria.

Total suspended solids levels constantly increase, and consequently nitrite concentrations increase. In contrast, when a cycle begins with a well-stocked biofloc inoculum, the concentrations of ammonia and nitrite are relatively low. However, the concentrations of suspended solids are higher in this phase. Both situations require management of suspended particulate matter within the culture system.

Experimental Work

In experiments conducted with Litopenaeus vannamei shrimp in biofloc systems at the Marine Aquaculture Station of the Federal University of Rio Grande in southern Brazil, the best growth performance occurred in a study where suspended solids were removed for the maintenance and control of the total suspended solids concentrations in culture inoculated with biofloc.

In another experiment, concentrations of nitrogen compounds differed when compared to different total suspended solids (TSS) concentrations during the formation of biofloc. Higher TSS concentrations resulted in higher concentrations of nitrite.

In another experiment that used the biofloc inoculum, it was observed that when the concentration of dissolved oxygen was maintained above 5 mg/L, the suspended solids excess was not a problem for the respiration of the reared shrimp. Considering the interactions of water quality parameters listed above, maintenance of the suspended solids levels is required for better water quality.

Monitoring, Intervention

Measurements of suspended solids were made using the gravimetric method, which measures total suspended solids and settleable solids within an Imhoff cone. Various techniques can be applied to reduce and maintain suspended solids concentrations, such as the use of a settling chamber or clarifier to remove solids. Set up in straightforward settling chambers, clarifiers rely on gravity to move particles to the bottom.

The radial water flow in the settling chamber can be adjusted based on prior analysis of biofloc sedimentation in an Imhoff cone, increasing the efficiency of the method. This method allows the control of TSS and keeps the concentrations near the recommended values. Another advantage of this method is maintaining a constant flow during the application of settling, so a small amount of water is sufficient for removing suspended solids.


Mouth Vision: Blind Fish Suctions Water to Navigate

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By Laura Poppick

The Mexican blind cavefish does not have eyes, but it can “see” obstacles in dark caves by puckering its mouth and producing bursts of suction, according to a new study. The research describes this unique form of navigation for the first time.

Scientists previously thought the eye-less Mexican cavefish navigated by sensing changes in water pressure produced by waves sent off from the fish’s own body. But when the researchers examined the fish, they found some problems with this explanation. For example, larger fish, which would presumably produce larger waves, should be able to identify objects from farther away than smaller fish. In fact, larger fish detected objects at about the same distance as smaller fish did.

Researchers at Tel Aviv University in Israel decided to investigate the sightless navigation further, conducting an experiment in which they counted the number of times the fish opened and closed their mouths when near objects the fish were familiar with. The researchers then moved the objects and observed changes in the fishes’ mouth movement in the unfamiliar environment.

The fish opened and closed their mouths more than twice as frequently in unfamiliar surroundings, and more frequently when approaching an object than in the open, with no objects nearby, suggesting this behavior plays a role in detecting the fish’s environment.

Through further analyses, the team determined the suction sent off by this mouth motion produces a signal similar to echolocation — a system in which animals, like bats and dolphins, emit sound waves and detect the distance of an object based on how long the sound takes to bounce back. Instead of measuring time, however, the cavefish appear to measure the magnitude of the pressure change produced by their mouth suction, study co-author Roi Holzman told Live Science.

“In this sense, it is different from echolocation, but it is similar because you have an animal that is purposefully emitting pressure waves to locate obstacles,” Holzman said.

The team does not know if other fish use this form of navigation. But some likely do, since all fish have the ability to produce suction waves with their mouths, and all have receptive organs along the sides of their bodies that can detect changes in water pressure. Both adaptations can be traced far back in the evolutionary history of fish, said Holzman.

“It’s a [newly discovered] mechanism made out of ancient material, and it just makes sense that other fish would have it,” Holzman said. “We haven’t tested it yet, but I’d really like to.”

The fish may also passively gather information produced by body waves when they swim through water, as previous studies have suggested, the team speculates. But the cavefish likely use both navigation methods in tandem, similar to how submarines rely on both active and passive sonar, Holzman said.

The researchers are now using a hydrophone to study how the fish modulate the suction signal depending on their distance from an obstacle, Holzman said.