03 Jul 2019
The Danger that Lurks in the Night: Low Dissolved Oxygen in Pond Aquaculture
Written by Zach Stein

We are building an entirely new kind of water quality monitoring system for aquaculture and the biggest fear we hear from farmers, especially those without aeration, are drops in dissolved oxygen at night. Not only can this leave the animals in stressful conditions where they will grow more slowly and be more susceptible to disease, but farmers often show up to a pond in the morning to find the animals dead.

In this post we’ll explore the dangers of dissolved oxygen falling at night and what farmers today are doing about it.

What Causes Dissolved Oxygen to Fall at Night

Dissolved oxygen drops when oxygen is not being produced in a pond. In algael, extensive systems this occurs at night as they are often not aerated. The algae, who have been busy photosynthesizing and producing oxygen all day switch modes and behig respirating.

Not only does this produce carbon dioxide, which can reduce pH in a poorly buffered pond, but it means that any remaining oxygen dissolved in the pond water will begin to off-gas and be consumed by the other animals, plants, and microorganisms in the pond until the sun rises again.

We’ve heard about farmers who are willing to roll the dice and live with dissolved oxygen as low as 1.5 mg/L for extended periods of time. This is only in highly extensive ponds, of course. We’ve also heard of farmers checking their dissolved oxygen three times per night to catch and low DO scenarios so emergency hydrogen peroxide can be rushed to the pond and provide a boost of 1.0 mg/L until the next morning.  

Monitoring Options to Save Your Ponds

In extensive systems, the vast majority of farmers monitor their ponds with handheld sensors at 1-3 times per night. They walk from pond to pond take a reading, write it down on a clipboard, and go to the next. If something is very off, they walkie-talkie back to the office to report the number.

The issue with such a system as the motivation to make up the next number after such a rotation of ponds is high. Or just say it didn’t change much from reading to reading.

To combat this farmers are looking to continuous sensors that can report immediately if a pond’s dissolved oxygen level falls below desired parameters. The problem with such systems is that they are very expensive and difficult to keep clean and calibrated, so farmers don’t yet trust them (we’re changing that).

How to Avoid Low Dissolved Oxygen at Night in Your Pond

Monitoring is a start, but how to avoid low dissolved oxygen levels? One choice is to stock less densely. This puts less pressure on the pond as a whole to produce and will mean that less feed has been introduced so overall the pond is less biologically active than in a more intensive system.

Aeration is another option. The bonus with such an investment is that it opens up the possibilities for significantly higher stocking densities as increased aeration is needed to allow the animals to consume the levels of feed required.

Finally, higher water exchange and updated pond canal designs can help bring in a slow, but steady trickle of higher oxygenated water where the shrimp can gravitate towards at night.

Conclusion

Low dissolved oxygen is a threat every night but with careful investment design and forthcoming technology advances, waking up to find an entire pond lost will be a thing of the past.