Pond Bottom Management — A Black Bottom Means You Are Late

Some ponds read normal on all six water quality parameters and still lose shrimp steadily. That happens when you are only watching the water. Shrimp live on the bottom, and the accident starts there. ■ The bottom keeps accumulating Uneaten feed, waste and dead plankton settle every day. The rate rises as the cycle goes on, because the feed rate rises. Breaking that down takes oxygen. Once the layer thickens, oxygen no longer reaches inside it. Bacteria that work without oxygen take over, and from that point the products of decomposition change. ■ Hydrogen sulfide — a different order of magnitude What forms on an oxygen-free bottom is hydrogen sulfide, the gas that smells of rotten eggs. Keep it under 0.01 mg/L. For white shrimp, harm is reported at concentrations as low as 0.002 mg/L. Put that number beside the others. Ammonia is discussed in units of 1 mg/L, and so is nitrite. Hydrogen sulfide causes accidents more than a hundred times below that. So "there is a little" is not a meaningful phrase here. ■ Lower pH is more toxic — the opposite of ammonia Ammonia gets more toxic as pH rises. Sulfide is the reverse: the toxic fraction rises as pH falls. Near pH 8 the toxic form is under a tenth of total sulfide, and that share climbs steeply as pH drops. This is why bottom trouble compounds after heavy rain, or on ponds running low alkalinity. A falling pH and a bottom problem should never be looked at separately. ■ Prevention — get oxygen to the bottom Hydrogen sulfide only forms where there is no oxygen. Put the other way: it does not form as long as oxygen reaches the bottom. The working target is 3 mg/L or more near the bottom. That is what aerator placement is for. Aerators must lift bottom water and mix it, not just churn the surface. In summer, when stratification sets in at midday, this matters most. And the cheapest method is to let less accumulate in the first place. Check the feed trays and adjust the ration by what is left. Uneaten feed is both a cost and a load on the bottom. ■ Checking — colour and smell Few farms measure hydrogen sulfide daily. Check the bottom directly instead. Scoop bottom mud with a pole or net and look at the colour and smell it. Blackened and smelling of rotten eggs is conclusive — sulfide is being produced right there. It varies by spot within one pond. It appears first in corners the current does not reach and far from the aerators. Do not judge from one place; sample several. ■ If the dead shrimp are on the bottom If mortalities are found mostly on the bottom, if shrimp try to move to the edges, and if all six water parameters read normal, suspect the bottom. Keep measuring only the water and you will spend days without finding the cause. ■ Records matter more than thresholds Noting the bottom's colour and smell every few days is enough. Put feed rate, aerator hours and bottom checks side by side on the same date and you will see the point where the bottom started to go. Shrimp365 logs dissolved oxygen, pH and feed rate per tank so you can see the bottom deteriorating before it costs you. Free to use.