Microbially Derived Co-Products from Zero-Discharge Aquaculture
Authors: Brune David E. 
Published: June 2021

Background:

Global seafood production has doubled over the last two decades with aquaculture
now contributing nearly 50% of supply. Pressure to reduce or eliminate water and waste discharge
from aquaculture increases with each passing year. In response to this pressure, producers have adopted
increasingly sophisticated technology, expanding fish and shellfish production from 2,000 kg/ha to over
40,000 kg/ha. While water discharge has been drastically reduced, waste solids production from intensive
aquaculture continues to pose a management challenge. One potential solution is to co-culture filterfeeding
aquatic organisms with higher-value aquaculture species as a technique to harvest and covert excess
bacterial and algal biomass into useful co-products and biofuels.

Methods:

Over a period of twenty years, the author and co-workers have designed and operated catfish
and marine shrimp production systems employing co-culture of tilapia (Oreochromis niloticus) and brine
shrimp (Artemia) to remove, concentrate, and convert microbial solids into animal biomass and biofertilizer.
Past system operations are reviewed, and additional methods and data are presented and discussed.
In the case of tilapia, a technique entitled “tilapia enhanced sedimentation” is evaluated for use in converting
algae into concentrated fertilizer and fish flesh. Alternatively, brine shrimp are used to harvest
and convert microbial solids into a potential fish-meal replacement.

Results:

Tilapia co-culture was shown to be cost-effective in controlling aquaculture system algal species
and density, selectively removing cyanobacteria from culture water promoting green algal dominance,
reducing off-flavor in cultured fish species. Tilapia co-culture at biomass levels from 20-25% of
targeted fish carrying capacity was required to reduce algal and bacterial levels, significantly reducing
oxygen demand and aeration requirements, Tilapia enhanced sedimentation was demonstrated to be effective
in removing suspended algal and bacterial solids, concentrating excreted biomass into rapidly
settling fecal pellets. Brine shrimp culture has been demonstrated at densities of 2,000-4,000 animals/
liter, corresponding to 4 gm/liter of dry weight animal biomass concentration. Brine shrimp are capable
of conversation efficiencies as high as 50% of microbial dry weigh to brine dry shrimp weight, as
opposed to <3% conversion with tilapia. However, successful Artemia culture necessitates unique
culture system design and management, requiring two-stage, multiple-batch cultures of uniformly
sized cohorts to yield maximum growth and conversion efficiency. Unique pH and ammonia toxicity response
of brine shrimp necessitates management protocols very different from typical aquatic animal
culture. Tilapia harvested algal sludge is limited to fertilizer application, yielding a value of $0.10/kg at
98% dry weight. On the other hand, brine shrimp biomass can be used as a potential fish-meal replacement
at a value of $ 1.50/kg dry weight.

Conclusion:

Utilization of co-culture of filter-feeder organisms such as tilapia and brine shrimp to harvest,
concentrate, and convert algal and bacterial solids into concentrated sludge or animal biomass offers
potential to provide value-added products from integrated aquaculture operations as a more environmentally
friendly practice.