USE OF PROBIOTICS IN PENAEID SHRIMP GROWOUT
Probiotics can be defined as culture (single or mixed) of selected
strains of bacteria that are used in culture and production systems
(tanks, ponds and others) to modify or manipulate the microbial
communities in water and sediment, reduce or eliminate selected
pathogenic species of microorganisms, and generally improve growth
and survival of the targeted species. Microbes are very important and
have critical roles in aquaculture systems, including shrimp farming at
both the hatchery and the growout level, because water quality and
disease control are directly related and closely affected by microbial
activity. The use of probiotics in hatcheries has been intensively
researched and well documented, because of the importance to limit
the ocurrence of pathogenic bacteria in the very-susceptible-to-
disease-outbreaks, intensive systems used to produce shrimp
postlarvae in commercial hatcheries around the world. Garriques and
Arevalo (1995) recently reviewed the production and use of a live
bacterial isolate to manipulate the microbial flora in the commercial
production of Penaeus vannamei postlarvae in Ecuador, and suggested
that the use of Vibrio alginolyticus as a probiotic could significantly
increase growth and survival of P. vannamei postlarvae by the
competitive exclusion of potentially pathogenic bacteria, thus
reducing the need to use antibiotics and chemoterapeutants that can be
used indiscriminately to manage pathogenic bacteria and other
pathogens.
The use of probiotics is not limited to shrimp hatcheries, and is
implemented in the larval production of several commercially
important fishes and invertebrates such as Pacific oysters and turbot.
There are various ways through which probiotics may act in
aquaculture systems, including by competitive exclusion of pathogens,
by enhancing digestion through the supply of essential enzymes, by
moderating and promoting the direct uptake of dissolved organic
materials, by active production of pathogen inhibiting substances, and
other possible mechanisms. For some time now the use of bacterial
amendments has been recommended for use in aquaculture ponds to
obtain several benefits, including reducing blue green algae
populations and preventing off-flavor, reducing nitrate, nitrite,
ammonia and phosphate levels, increasing dissolved oxygen
concentrations and promoting decomposition of organic. The use of
probiotics in pond culture has much potential and can provide a
significant contribution to the development of successful and
sustainable, reduce or no water discharge, intensive systems, but the
use and management of pond probiotics is a young area that still
requires much research.
According to Bratwold et al. (1997) the specific ecological
applications of microbial ecology management in shrimp ponds
include the following: optimizing nitrification rates to keep low
ammonia concentrations; optimizing denitrification rates to eliminate
excess nitrogen from ponds as nitrogen gas; maximizing carbon
mineralization to carbon dioxide to minimize sludge accumulation:
maximizing primary productivity that stimulates shrimp production
and also secondary crops; and maintaining a diverse and stable pond
community where undesirable species do not become dominant.
In general, it is most accepted that probiotics in ponds probably act
through competitive exclusion. According to Moriarty (1996a) "the
species composition of a microbial community, such as that in a pond,
will be determined partly by chance, and partly by physiological
factors that allow a species to grow and divide more rapidly than
others, and thus dominate numerically. Chance favors those organisms
that happen to be in the right place at the right time to respond to a
sudden increase in nutrients, e.g. from the lysis of algal cells or the
decomposition of feed pellets that fall around them. So, the farmer can
manipulate the species composition by seeding large numbers of
desirable strains of bacteria or algae; in other words, by giving chance
a helping hand."
Boyd (1995) reviewed the chemistry and efficacy of several
compounds and amendments - including bacterial and enzymatic
preparations - frequently used or advocated for the treatment of
shrimp pond water and soil quality imbalances. He concluded that
applications of bacterial amendments or bacterial extracellular
enzymes which solubilize organic matter are unlikely to improve pond
soil and water quality conditions. He notes that "Bacteria are
ubiquitous; their spores and vegetative bodies occur in almost all
natural environments. The major factors affecting their abundance and
activity are temperature, pH, oxygen supply, moisture supply, and
amount and type of substrate. Bacterial activity is favored by warmth
(25-35 C), above neutral pH (7.5-8.5), plenty of oxygen, low C:N ratio
(<1:15), abundant substrate, and a substrate consisting of easily
decomposable organic matter. In shrimp ponds the major factor
affecting bacterial activity is usually the dissolved oxygen supply, but
in some ponds the pH may be too low. There is normally abundant,
readily-decomposable organic substrate consisting of remains of
aquatic biota, feces and feed. If bacterial activity is low, it is because
of poor environmental conditions for microbial activity. One would
not expect application of bacterial amendments or enzymes to enhance
bacterial activity and improve water quality."
On the other hand, Moriarty (1996a;b) strongly advocates the use of
microbial technology in pond aquaculture. An experiment conducted
by Dr. Moriarty at two farms in Indonesia (one used probiotics, the
other did not) to assess the value of adding selected Bacillus strains as
probiotics to control luminescent vibriosis in shrimp ponds produced
the following results (AquaFarm News, 1996): "The bacterial species
composition was different in the pond water on the two farms,
demonstrating that it is possible to change bacterial species
composition and improve shrimp production in large water bodies.
The number of vibrios, especially Vibrio harveyi, was low in ponds
were a large number of specially selected Bacillus species was
maintained in the water column. Vibrio counts were also low in
sediments and no luminous vibriosis outbreak occurred in sediments
where the probiotic Bacillus was used."
Further support for the use of pond probiotics is found in recent
publications describing the use of probiotics in shrimp growout ponds
and resulting benefits. One such article is by Suhendra et al. (1997),
and describes a management program which incorporates the routine
use of commercial probiotics (selected strains of naturally occurring
Bacillus spp.) in a shrimp farm in West Java (Indonesia). This
program reportedly reduces the incidence of Vibrio and viral
outbreaks that have hit shrimp farms in Indonesia hard, and results in
enhanced environmental conditions, reduced organic matter
accumulation, improved water quality and increased shrimp size and
total production. The program emphasizes strict monitoring of water
quality and promotion and maintenance of a healthy phytoplankton
population, active removal of organic sludge, and careful control of
feeding to prevent overfeeding and avoid stressing animals. According
to the authors, the production costs in ponds treated with probiotics
were higher than control ponds, but higher yields (total production and
larger shrimp size) in treated ponds resulted in increased profits,
making probiotic management more cost effective than traditional
production procedures. Suhendra et al. (nd) further discussed the use
of probiotics in intensive shrimp pond culture to address various
problems, including organic matter accumulation in pond bottoms
which leads to environmental degradation and unfavorable conditions
for shrimp growth. Intensive shrimp ponds are often subject to strong
fertilization and feeding management programs that accelerate pond
bottom deterioration and overall water quality conditions that hamper
shrimp production.
There are approximately 15 species of Bacillus, which are probably
the main component of commercial probiotic products for pond
aquaculture, and are generally the recommended microorganisms to
use as pond probiotics.
According to Moriarty (1996b), the successful use of probiotics to
promote sustainable aquaculture will be dependent on various factors
such as defining ecological process to be affected and naturally
dominant species vs. desirable alternative species of microorganisms
to be added or promoted. Further, "Whether we can change a bacterial
community depends on knowing enough about the ecological factors
that govern species composition, including specific growth rates,
nutrient composition and concentration, inhibitory interaction and the
types of bacteria already present. It is necessary to both manipulate
physico-chemical factors to alter microbial species composition and to
alter rates of metabolic activity by adding selected species to carry out
particular functions at faster rates than those present in a given system.
"Management of pond microbial ecology is an area where applied
research can lead to important breakthroughs to improve the
productivity and environmental "friendliness" of the shrimp farming
industry worldwide, particulary in view of recent negative publicity
regarding environmental impact of shrimp farms. Pond microbial
communities are a critical and often overlooked component of
aquaculture ecosystems in general, and are particularly important in
penaeid shrimp ponds. Microbial populations have critical roles in
ponds, including the recycling of nutrients and primary production,
and in the regulation, maintenance and remediation of water and
bottom quality.
Probiotic bacteria can offer possibilities to control virus diseases,
according to Moriarty et al. (in press). One possibility is a preventive
treatment where water quality is maintained by using probiotics to
stimulate degradation of waste organic matter under aerobic
conditions, thus preventing stress to shrimp. Another possibility is the
addition of bacteria that will specifically attack the virus or prevent its
infection of shrimp by producing antiviral compounds.
According to Browdy and Bratwold (1997), there are several
approaches to management of pond microbial communities that are
currently being studied in the laboratory and used in farms, including
selective addition of nutrients, expansion of habitats and culture
additions. Supplementation of limiting nutrients can prove natural
productivity, and enrichment of detrital food chains and stimulation of
selected primary producers could result in better yields and reduced
feed protein needs and improved feed conversion. Selective habitat
expansion can stimulate beneficial microbial processes such as
nitrification, while reducing processes - such as sulfide production -
that have negative effects on shrimp growth. These authors also state
that, in theory, culture additions may cause shifts in the microbial
community that can potentially result in faster carbon and nitrogen
cycling and removal, eventually reducing ammonia and sludge
accumulation. Further, in relation to pond trials, they report that
alternative microbial community management strategies typically lack
adequate controls, and that practical application of smaller scale
studies may be limited. They conclude that "successful application of
new management approaches will depend upon contined basic
research into shrimp pond microbial ecology and a continued
commitment of the industry to improving sustainable farming
practices."
There is ongoing research at shrimp farms in several countries on the
use of probiotics; bioremediation or bioaugmentation technology
using probiotics is a promising area to improve the efficiency of
aquaculture systems in general, and the technology has the potential to
be a significant factor for sustainable shrimp aquaculture. Pond
probiotics will be particularly important in improving shrimp yield in
high intensive systems, in the implementation and optimization of
low- and zero-water exchange production systems, and to manage and
improve the quality of pond effluents. As with several other areas,
additional research is still needed to realize the substantial potential
that pond probiotics appear to have to improve shrimp farming
efficiency and its viability as a sustainable, environmentally friendly
industry.
(excerpts from article by D.E. Jory in Aquaculture Magazine,
January/February 1998)
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