Adjusting the Carbon/Nitrogen Ratio

Adjusting the carbon/nitrogen ratio can be tricky.  It’s done by feeding the bacteria exactly what they want: a diet with a carbon/nitrogen ratio of around 12:1, or higher, accomplished by adding molasses, wheat, tapioca or some other inexpensive carbon source to the water.  The shrimp supply the nitrogen.  The farmer supplies the oxygen.  The bacteria take the bad stuff out of the water and convert it to food for the shrimp.

But, you’ve got to get the carbon/nitrogen ratio of the pond water right to make it all work.

During the last quarter of 2004, the Shrimp List (below), a mailing list for shrimp farmers, carried a great discussion on the carbon/nitrogen ratio in shrimp ponds.  Here’s an excerpt from that discussion:

Sunil Kant Verma, a former employee of Hi-Line Aqua in India, asked: Could someone give me information on the carbon/nitrogen ratio in shrimp ponds?

Peter Van Wyk, then an aquaculture project planning specialist at Harbor Branch Oceanographic Institution in Florida, USA, and currently a research associate at the Southwest Virginia Aquaculture Research and Extension Center, in Saltville, Virginia, USA, responded [His comments were updated in July 2006.]:

The feeds used in intensive shrimp ponds typically have at least 35% protein and a carbon/nitrogen ratio of around 9:1.  With such a low C:N ratio, carbon becomes the limiting nutrient, and the bacterial populations don’t expand beyond a certain point; however, when the C:N ratio is increased, bacteria proliferate and shrimp growth takes off.

If the C:N ratio is increased, either by feeding lower protein feeds with a higher percentage of carbohydrate, or by adding a carbohydrate source such as molasses in addition to the regular feed, the increased availability of carbon allows the bacterial population to consume a higher percentage of the protein in the organic material.  This results in more complete digestion of the organic material in the pond by the bacteria.  As the C:N ratio increases, the bacteria resort increasingly to ammonia metabolism to meet their nitrogen requirements.  As C:N ratios are increased even further, a point is reached where nitrogen, rather than carbon, becomes the limiting nutrient.  This occurs when the C:N ratio reaches about 15:1.  At this point ammonia concentrations should be close to zero in the pond.  Manipulation of C:N ratios is an effective tool for managing ammonia levels in shrimp ponds.

Increasing the C:N ratio can be accomplished by either holding the feed protein level constant and supplementing the feed with carbohydrate, or by feeding a feed with a lower percentage of protein and a higher percentage of carbohydrate.  Both approaches will result in much higher bacterial counts in the pond.  The oxygen required to support additional bacterial biomass will increase proportionally with the increase in bacterial population.  Likewise, CO₂ production will increase, driving pH down.  If you are contemplating carbohydrate supplementation to increase C:N ratios, make sure that your pond is well aerated and circulated to keep the organic detritus suspended in the water column where there is sufficient oxygen for the bacteria.  Also, once you develop a dense population of bacteria, don’t discontinue the carbohydrate supplementation suddenly.  This will starve the bacteria of carbon, causing a die-off to occur and ammonia to spike.

Claudio Paredes, aquaculture business development manager for Agribrands Purina in Venezuela: Do you seed the pond with bacteria, or are they already there?

Peter Van Wyk: It is not necessary to inoculate a pond with commercial bacterial products to manage one of these systems.  This can be accomplished simply by maintaining a C:N ratio greater than 12:1, and supplying adequate aeration.

Claudio Paredes: What’s the best way to measure the C:N ratio in a pond?

Dallas Weaver, a water quality specialist and hatchery consultant in California, USA, responded: Measurement of C:N is only part of the story.  If you measure TOC (total organic carbon), some of that carbon can be refractory and does not help the bacteria soak up the ammonia.  Measuring TOC and BOD (biological oxygen demand) with and without ammonia oxidation inhibition, along with TKN (total Kjeldahl nitrogen) will provide some useful management information.

Kevin Healey, a supplier of bacterial remediation products from Australia, responded: This has been an interesting discussion on C:N ratios, and thanks to Peter for the time he’s taken to provide such clear explanations.  I’m in agreement with pretty much all he’s stated, in particular the usefulness of molasses in promoting a bacteria bloom in ponds and the value of using probiotics in hatcheries.

Peter Van Wyk: Measurement of C:N ratios in ponds is not a simple task because the carbon and nitrogen end up in a lot of different places: the feces, the organic floc, the bacteria, the water and the shrimp.  Researchers use labeled isotopes of carbon and nitrogen in the feed to study C:N budgets in ponds.  Of course this isn’t practical in a production pond.  Managing the C:N ratio in a pond is handled more easily by managing the C:N ratio of your feed.  I estimate the C:N composition of the feed rather than measure it.  I don’t have access to laboratory equipment to measure total organic carbon and total Kjeldahl nitrogen, nor do I have the budget to send out samples to a laboratory for analysis.

Carbon accounts for roughly 50% of the dry weight of most feeds.  This is a crude estimate, but carbon content is remarkably constant even for feeds with widely varying compositions.  The nitrogen content of the feed is calculated from the protein content.  Protein is approximately 16% nitrogen.  Although this method for calculating C:N ratios is admittedly crude, it provides a reasonably close estimate of actual C:N ratios.