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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002

http://dx.doi.org/10.4067/S0717-71782002030100086 

Bacterial Community Composition in a
Semi-Intensive Shrimp Culture of
L. vannamei
during
El Niño 1997-98 and La Niña 1999

Roberto Retamales

Maria Laura's Shrimp Farm, Bahía de Caráquez,
Avda. S.D.Ballen 603, Manabi-Ecuador,
E-mail: rretamales@easynet.net.ec

The microbial community plays a major role in pond culture, particularly with respect to productivity and nutrient cycling. Gut flora may enhance shrimp growth and survival by contributing exogenous enzymes, out-competing pathogenic bacteria and supplying essential compounds lacking in the shrimps' diet.

The aim of the present study was to assess the bacterial composition (e.g. total heterotrophic aerobic bacteria (THAB) and presumptive vibrios (PV)) from the shrimp intestines and its relation to shrimp performance indices in a semi-intensive shrimp culture during the El Niño and La Niña events of 1996,1997,1998 and 1999.

The study site was a commercial L.vannamei (Boone) farm, located at the Chone river estuary, Ecuador. Diversity and equitability indices were applied to microbial data in order to assess the variation in composition during the different periods of culture.

Twelve genera of heterotrophic aerobic bacteria were identified from the intestines of L. vannamei of culture ponds: Xanthomonas, Vibrio, Pseudomonas, Providencia, Moraxella, Klebsiela, Flavobacterium, Cytophaga, Citrobacter, Alcaligenes, Aeromonas, Acinetobacter.








During the periods of culture in the ENSO transition times: La Niña to El Niño Nov 96-Mar 97, El Niño to La Niña Sep98-Feb99 and during the La Niña period March 99-Jul 99, a high ratio between the number of PV and THAB was observed, particularly in the final phases of cultures, as well as low diversity (2-2.5) associated with a low bacteria genus number and low equitability (1.1-1.5). The Vibrio species isolated were mainly V. harveyi and V.parahaemolyticus, and disease outbreaks were observed during these culture periods.

Table 1 Shrimp performance indices in pools stocked with L. vannamei from different culture periods.
( Different superscripts in common columns denote statistical significance (P<0.05) ). *Data log transformed prior to statistical analysis. **Data arcsine transformed prior to statistical analysis

In contrast, during the periods of Feb 97- Jun 97 and Jul 97- Jan 98, (El Niño), high diversity indices (2.6 - 3.0) and equitability (2.2 - 2.5), associated with a low percentage of Vibrio genus bacteria and low ratio between the number PV and THAB were observed. No disease outbreaks occurred in this period and more stable temperatures were observed.

No disease outbreak in shrimp cultures during transition and La Niña periods suggest that vibriosis was the cause of mortalities. Environmental factors such as low salinity increase the virulence of V. harveyi in P. monodon, resulting in significant mortalities (Prayitno et al., 1995). V. parahaemolitycus is frequently found in surface sediment when the surrounding water column conditions are unfavourable, i.e low temperature (Joseph et al., 1983; Kancho & Colwell, 1973; Kancho & Colwell, 1978).

According to shrimp performance indices survival, yield, final weight and feed conversion ratio, El Niño 1997-1998 represented better conditions than transition or La Niña periods.

In Ecuador, shrimp farmers encounter two distinct growing seasons, characterized as a dry season and a rainy season. Large differences in shrimp production between these seasons are typical. Despite this there is no information available from literature in Ecuador. Teichert-Coddington et al. (1994) showed that temperature is the major environmental factor determining the seasonal production in Honduran shrimp farms.

In this study, temperature, salinity and interaction were important factors determining the decrease or increase in bacteria species diversity and in practice, a farmer thus alerted could then take appropriate and timely steps to alleviate the effects of environmental change.

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