An important difference between most pools/tanks that are used to raise the shrimp is that the research reports are for tanks that are indoors in a controlled environment. We are trying to raise the shrimp outdoors where there are many more factors that can effect the grow-out environment.
Algae are normally considered to the a benefit in controlled environments. They “fix” the nitrogen-containing waste that the shrimp release (ammonia predominantly) to keep the levels at the suitable range for optimal shrimp growth. They release oxygen during photosynthesis and contribute to the dissolved oxygen in the water. All good, right?
This works in a controlled temperature environment. But when the water temperature in the pool is the same as the outside temperature, algae begin to have a potentially fatal effect at night. At high temperatures (>85o F), the maximum dissolved oxygen can lower to the minimum level for the shrimp to survive. Any additional oxygen demand can lower levels to those fatal to the shrimp. In temperatures in this range, algae will “bloom” at night. During this growth phase, the algae consume oxygen. When this happens, the dissolved oxygen level can plummet to levels lower than are sustainable for healthy shrimp.
During the first week of JUN18, we stocked our juvenile shrimp in a 2000 gal pool that had some lower level of algae. Ambient temperatures were higher than normal (highs in the upper 80’s to lower 90’s) and the maximum dissolved oxygen level was about 5-6 ppm. This is at the lower end of the suggested range for shrimp growth. I noticed that on the third day, the water had turned a cloudy green. On the fifth day, the water was a darker green and I was unable to scoop up any shrimp. I did notice a pink mass near the water pump intake. I asked James (the guy that I bought the stocking shrimp from) about this and he indicated that when shrimp die, they form a ball that will develop a pink color nail they completely decompose. Clearly, the algae had bloomed and dropped the oxygen level to kill the shrimp.
How do we find this? The pools came with filter water pumps that are normally used to keep the water clear for swimming. Installing the filter pump removed the algae suspended in the water within 3 days (with a filter cleaning after the first day). In normal use, it is suggested that the pump only needs to run for 3-4 hours to clean the water. I have the pump on a timer and am running the pump at night when the maximum oxygen demand is greater.