Hi Enox! Hope you and your family are well during this time!
That´s a question wich very few people in the world knows, mostly for low salinity environments and freshwater vannamei culture. I´m sure you are one of them, since I had learn with you the first steps for water correction in freshwater shrimp farming, so thank you for the question!
Differently from literature and several presentations regarding water quality in shrimp farming, the alkalinity have it´s value underestimated. In my opinion, alkalinity is the main parameter which will drive all other water parameters in your farm, to allow you grow shrimp or not (with salinity range from 0ppt to 100ppt).
The hardness it is a bonus to enhance your growth and survival dependind the density you are working with, and can allow you culture shrimp in places with lower alkalinity. Salinity is part of hardness, so naturally "the experts" always will recomend an "ideal salinity" to work with.
"You can grow shrimp with a good hardness, but you can´t with no alkalinity "
You have ask about 4-5ppt metrics, but you can use this principle for all salinities. Please do not consider the genetic agressive strains stockings, wich will eat each other in freshwater culture - must be regular poslarvae. These are one average of some freshwater farms in americas, europe and asia, which I have manage the water correction, in the last 20 years:
|Growth capacity and survival rate for 50-750 pcs/m3 - Alkalinity variation POV|
|Growth capacity and survival rate for 50-750 pcs/m3 - Hardness variation POV|
As much higher the alkalinity you get on you water, better will be the results in low salinity or freshwater culture. As higher hardness you get and increase using chemical fertilizers, better will be the survival rate and growth. In RAS systems and indoor facilities, if you don´t know how to correct alkalinity and hardness, the only number you will see will be the bankrupcy bill in your office after fail several times trying crack the code.
That´s because in the rainny season, several saltwater farms have bad results or disease outbreaks. The rain fall lower the alkalinity. That´s because saltwater farms have more probability to grow shrimp than freshwater - alkalinity is quite low in the river and lakes. That´s because in high salinities the shrimp stops to growth, not due salt concentration, but by the lower alkalinity in the dry season.
I´m sure that we can make a correlation between these two factors to predict survival and weekly growth, but so far, I couldn´t go throught it.
Best regards Enox,
Ps: i forget to mention about alkalinity! That's why several hatcheries loose their batch trying to acclimate larvae to freshwater, or the larvae dies after one week of stock at pre growout in the farm.