AUTOTROPHIC AND HETEROTROPIC TREATMENT IN SEMI-INTENSIVE, INTENSIVE AND SUPER-INTENSIVE FISH AND SHRIMP CULTURE

Aquaculture productivity has progressively increased over the last 50 years. In the 1960's un-aerated aquaculture ponds, referred to as extensive systems, yielded 1,000-2,000 lbs/ac of fish or shrimp in freshwater or marine systems. Feed rates averaged 10-20 lbs/ac-d, the limiting factor being wind-driven re-aeration rates. Algal photosynthesis suppresses ammonia levels at algal productivities of 0.5-1.0 g-C/m²-d at water column volatile suspended solids (VSS) levels of 10-20 mg/l. Beginning in the 1980's farmers added 1-2 hp/ac supplemental aeration to marine shrimp and freshwater fish-ponds, pushing yields to 4,000-7,000 lbs/ac, at average feed rates of 50 lbs/ac-d and maximum of 100 lbs./ac-d. At this loading, algal photosynthesis of 2-3 g-C/m²-d matches ammonia generation rate. Exceeding this level of production requires enhanced algal productivity, or pond flushing with discharge of ammonia and VSS to the environment.

In Asian marine systems, flushing of ammonia-laden water and increased pond aeration to 10-20 hp/ac allowed shrimp production in excess of 10,000 lb/ac-cycle. In the U.S. discharge of water was increasingly prohibited; beginning in the 1990's U.S. aquaculturists turned to enhanced photosynthetic systems such as partitioned aquaculture systems, split-ponds, in-pond raceways, or intensively aerated ponds to increase carrying capacity. Fish yields of 10,000-19,000 lbs/ac were achievable with feed loadings of 100-250 lbs/ac-d. Under these conditions, algal photosynthesis peaks at levels of 6-12 g-C/m²-d.

To further increase production to "super-intensive" levels, farmers turned to alternative biological treatments techniques, fixed-film nitrifying reactor (trickling filters), or suspended-culture microbial reactors (biofloc). Trickling filters are economically favored in most finfish culture because of potential for higher volumetric density in fish as opposed to shrimp culture. In general, biofloc treatment is economically favored in shrimp culture, using nitrifying systems (NS) or heterotrophic systems (HS).  HS require external organic supplementation in addition to feed application. Optimal microbial levels in the range of 300-400 mg/l keep pace with needed ammonia removal while not imposing excessive oxygen demand.  Shrimp production in excess of 40,000 lbs/ac-cycle is achievable. NS require aeration power of 50-60 hp/ac, whereas HS aeration can exceed 60-80 hp/ac. The major advantage of HS culture is rapid growth rate of heterotrophic microbes providing nearly instantaneous control of ammonia levels. HS culture requires organic supplementation of 40-60% of feed and removal of as much as 10% of microbial mass per day. Microbial solids production from NS is typically 10% that of HS, and no additional organic supplementation is required. Enterprise budgets suggest shrimp production costs in NS and HS at 40,000 lbs/ac-120d are similar at $8.00/kg-shrimp. An added advantage of super-intensive aquaculture is the potential to provide "zero-discharge" systems, eliminating water and sludge discharge by converting microbial biomass production to useful byproducts.