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1/350 THE ADVOCATE OCTOBER 2001

NUTRITION

George Chamberlain, Ph.D.

Global Aquaculture AllianceSt. Louis, Missouri, USAgeorgec@integra.prserv.net

Yoram Avnimelech, Ph.D.Faculty of Agricultural Engineering TechnionIsrael Institute of TechnologyHaifa, Israel

Robins P. McIntoshBelize Aquaculture, Ltd.Belize City, Belize

Mario Velasco, Ph.D.

Bioexportables S.A.Guayaquil, Ecuador

Aerated microbial reuse (AMR) systemsuse intense aeration and mixing to produce yields of 10-30mt/ha/cycle with little or no water exchange. These systemsrecycle nutrients in the form of organic detritus, which con-sists of aggregates of organic particles coated with a filmof bacteria. The bacteria absorb nutrients from the particlesand surrounding water, which purifies the water. The en-riched particles serve as a nutritious source of food for de-trital feeders such as shrimp and tilapia. The compositionof the detrital material often called floc varies widely,but it is generally rich in protein and minerals. The pres-ence of a rich microbial floc can substantially reduce therequirement for formulated feed.

Reducing Feed ProteinTrials with both tilapia and shrimp have demonstrated

that feed protein levels can be reduced in these systems. In1992 trials in Israel, Avnimelech and coworkers comparedthe performance of 32 and 20% protein diets for tilapia inhighly intensive tanks with low water exchange. The low

protein diet resulted in higher yields, larger final weight,lower mortality, improved FCR, and greater profitabilitythan the higher protein diet.

In 1994, Hopkins and coworkers at the Waddell Mari-culture Center in South Carolina, USA, compared perform-

ance ofL. vannamei in aerated zero-water-exchange shrimpponds with 20% or 40% protein. They found no differencein production (all treatments ranged from 6 to 8 mt/ha).

In 2000, Velasco and coworkers at Texas A&M University conducted a series of trials with postlarvalL. vannameiusing protein levels ranging from 5 to 33% and lipid levelsranging from 3 to 11% in static aerated tanks as well as intanks receiving recirculated water from a common externafiltration system. Only those diets with

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Dilution of FeedsWith Carbohydrate

The first article in this series de-scribed the importance of maintainingthe proper balance of carbon to nitro-gen (C:N) for suitable bacterial uti-lization of waste. In order to achievethe appropriate C:N, conventionalfeeds can be diluted with carbohy-drate. In 1999, Avnimelech and cowork-ers calculated the percentage of carbo-hydrate to add directly to a 30% pro-tein tilapia feed to reduce C:N usingthe following assumptions:

Content of 30% protein diet =4.8% nitrogen.

Ammonia N available forrecycling after excretion = 50%.

C:N ratio of microbial tissue = 4. Carbon content of carbohydrate

= 50%. Efficiency of microbial protein

synthesis = 40%. Carbohydrate to add to diet =

4.8% * 50% * 4/(50% * 40%)

= 48%.

The same formula can be used toadjust other feeds for AMR systems.

Establishing Microbial FlocOne of the challenges of AMR sys-

tems is to design a diet that serves boththe target organism as well as the mi-crobial community. For example, atthe start of the production cycle, bio-mass and associated feed require-ments are quite small and protein re-

quirements are high. An appropriatefeeding rate for such a small biomasswould be inadequate to initiate a mi-crobial floc. On the other hand, it wouldbe prohibitively expensive to allocatea sufficient quantity of formulatedfeed to establish an early floc.

At the end of the cycle, the proteinrequirements are considerably less,

due to the need for a high C:N to pre-vent accumulation of inorganic nitro-gen. To avoid continually reformulat-ing diets, McIntosh (2000 b) utilizedinitial applications of an inexpensivegrain-based pellet with high C:N atthe start of the cycle to initiate thefloc. During the course of the cyclethis regime was adjusted transitionedto a lesser application of grain-basedfeed and a greater application of for-mulated shrimp feed (Figure 1). Thedisadvantage of this approach is that

juvenile shrimp grow faster on a highprotein feed.

Economic ImplicationsHistorically, the process of intensi-

fication of aquaculture systems hasbeen based on increasing levels ofwater exchange. As intensity increased,the contribution of natural foods di-minished, due to increasing water ex-change and increasing predation asso-ciated with higher stocking densities.The diminishing contribution of natu-

ral foods required development of morenutritionally complete and more ex-pensive formulated diets for intensivesystems. Consequently, feed cost was

an economic barrier to intensificationThe development of AMR sys

tems, reverses this trend. In AMR systems, the contribution of natural foodsincreases with increasing density, whichspares the cost of formulated feeds. Ina 1994 study, Avnimelech and coworkers estimated that tilapia feed cost wasreduced by 30% in AMR systems. Giv

en the fact that feed is the largest operating cost in most animal productionefforts, AMR systems are expected toreduce the economic barriers to intensification.

Future ResearchAMR systems have demonstrated

remarkable potential, but many questions remain. One yardstick of successis nitrogen assimilation efficiencywhich is still only about 50% in commercial systems.

In-depth modeling is needed to improve researchers understanding othe contribution of various componentof the pond community to transfer ofenergy, nitrogen, and phosphorus with-in the system.

To optimize pond hydraulics andefficiency, the optimal levels of organ-ic loading and mixing must be defined. Inadequate loading limits formation of microbial flocs, but properloading with inadequate mixing leadsto sedimentation and anaerobiosis. Ex

cessive organic loading requires highlevels of turbulence, which are likelyto stress animals.

Small amounts of starter feed areinsufficient to establish microbial flocsat the start of a new production cycle.

Figure 1.A typical feeding program at Belize Aquaculture, Ltd. Vertical bars indicatedaily feed application rates during each week of the cycle.

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Community SuccessionTo reduce the difficulty of initiating a microbial floc at

the beginning of a cycle, it seems sensible to carry overflocs from one cycle to the next much like activatedsludge sewage treatment systems. This would facilitatehead starting new pond cycles with mature and efficientstages of community succession.

Management as Rumen

Several researchers have alluded to the possibility ofmanaging AMR systems like the rumen of a cow. The con-cept is to combine low value carbohydrates with inorganicnitrogen to encourage in situproduction of bacterial pro-tein. McIntosh tested this concept in a pond trial in Belizewhere only pelleted wheat grains, trace minerals, urea, anddiammonium phosphate were utilized. PostlarvalLP. van-namei stocked at 120/m2 grew to 7-8 g on only these inputsGrowth rates averaged about 0.5 g/week. Productionreached the equivalent of 8 mt/ha. This demonstrates thepresence of substantial nutritional value from bacterially-enriched detritus.

ProbioticsIt seems clear thatsome components of themicrobial communityare quite beneficial interms of growth-promot-ing qualities and nutri-tional value. An obviousopportunity would be tospecifically seed thesemicrobes to improvepond performance. Con-ceivably, bacteria could

be selected with charac-teristics such as bal-anced amino acid pro-file, proper agglutina-tion agglutinization, etc.An easy way to start thisprocess is simply to re-use water from highperforming ponds.McIntosh has started abiological library of various stocks that appear in ponds toallow future selection.

ConclusionAerated microbial reuse systems offer practical opportunities for improved efficiency and cost reduction throughreduction of feed protein levels and fishmeal inclusion. Formulas are provided for calculation of carbohydrate dilutionto achieve the proper balance of carbon to nitrogen for bac-terial utilization of waste. The recycling of nutrients in AMRsystems facilitates important savings in feed cost, which re-duce former barriers to intensification. Much opportunityexists for further refinements in AMR systems through im-proved hydraulics and mixing, probiotics inoculation, andmanipulation of the microbial community.

54 THE ADVOCATE OCTOBER 2001

AMR systems can be managed forin situproduction of bacterialprotein through the addition ofinorganic nitrogen and a source oforganic carbon such as molasses.