Aquaculture of finfish, crustaceans

Aquaculture of finfish, crustaceans, mollusks, and algal
plants is one of the fastest-growing food-producing sectors,
having grown at an annual rate of almost 10% from 1984 to
1995 compared with 3% for livestock meat and 1.6% for capture fisheries production (97).
Disease outbreaks are being increasingly recognized as a
significant constraint on aquaculture production and trade,
affecting the economic development of the sector in many
countries. For instance, disease is now considered to be the
limiting factor in the shrimp culture subsector (65, 124). So far,
conventional approaches, such as the use of disinfectants and
antimicrobial drugs, have had limited success in the prevention
or cure of aquatic disease (124). Furthermore, there is a growing concern about the use and, particularly, the abuse of antimicrobial drugs not only in human medicine and agriculture
but also in aquaculture. The massive use of antimicrobials for
disease control and growth promotion in animals increases the
selective pressure exerted on the microbial world and encourages the natural emergence of bacterial resistance (World
Health Organization antimicrobial resistance fact sheet 194,
http://www.who.int/inf-fs/en/fact194.html). Not only can resistant bacteria proliferate after an antibiotic has killed off the
other bacteria, but also they can transfer their resistance genes
to other bacteria that have never been exposed to the antibiotic. The subtherapeutic (prophylactic) use of antibiotics related to those used in human medicine or the use of any
antimicrobial agent known to select for cross-resistance to antimicrobials used in human medicine could pose a particularly
significant hazard to human health (146).
According to the World Health Organization (fact sheet 194
web site), much needs to be done to reduce the overuse and
inappropriate use of antimicrobials. The emphasis in disease
management should be on prevention, which is likely to be
more cost-effective than cure. This may lead to less reliance on
the use of chemicals (antimicrobials, disinfectants, and pesticides), which largely treat the symptoms of the problem and
not the cause (92).
Several alternative strategies to the use of antimicrobials in
disease control have been proposed and have already been
applied very successfully in aquaculture. The use of antimicrobial drugs in a major producing country such as Norway has
dropped from approximately 50 metric tons per year in 1987 to
746.5 kg in 1997, measured as active components. During the
same time, the production of farmed fish in Norway increased
approximately from 5 3 104 to 3.5 3 105 metric tons. The
dramatic decrease observed in the consumption of antimicrobial agents is mainly due to the development of effective vaccines (66, 124), which illustrates very well the potential effectiveness of the procedure. Enhancing the nonspecific defense
mechanisms of the host by immunostimulants, alone or in
combination with vaccines, is another very promising approach
(96, 111). Third, Yasuda and Taga (148) already anticipated in
1980 that bacteria would be found to be useful both as food
and as biological control agents of fish disease and activators of
the rate of nutrient regeneration in aquaculture. Vibrio alginolyticus has been employed as a probiotic in many Ecuadoran
shrimp hatcheries since late 1992 (49). As a result, hatchery
down time was reduced from approximately 7 days per month
to less than 21 days annually, while production volumes increased by 35%. The overall antibiotic use was decreased by
94% between 1991 and 1994. The addition of probiotics is now
also common practice in commercial shrimp hatcheries in
Mexico (101). According to Browdy (14), one of the most
significant technologies that has evolved in response to disease control problems is the use of probiotics. Considering the recent successes of these alternative approaches, the Food and
Agriculture Organization of the United Nations (124) defined
the development of affordable yet efficient vaccines, the use of
immunostimulants and nonspecific immune enhancers, and the
use of probiotics and bioaugmentation for the improvement of
aquatic environmental quality as major areas for further research in disease control in aquaculture. The results of this
research will undoubtedly help to reduce chemical and drug
use in aquaculture and will make aquaculture products more
acceptable to consumers.
This review aims to provide an overview of the work done on
bacteria as biological control agents for aquaculture environments, with a critical evaluation of the results obtained so far
and a detailed description of the possible modes of action
involved. Furthermore, a rationale for the search for probiotics
is presented and directions for further research are proposed.