Low-input Modified Extensive Shrimp Culture Systems for Penaeus monodon Restrain Vibriosis  

Sheryl Oliveira Fernandes1 , R.A. Sreepada2 , Shantanu S. Kulkarni2 , Sheetal V. Karekar2 , Resha R. Shirodkar2 , Christian Vogelsang3 , P.A. Lokabharathi1
1. Marine microbiology laboratory, CSIR-National Institute of Oceanography, Dona Paula, Goa - 403 004, India
2. Aquaculture laboratory, CSIR-National Institute of Oceanography, Dona Paula, Goa - 403004, India
3. Norwegian Institute for Water Research (NIVA), Brekkeveien 19, Kjelsaas-N-0411 Oslo, Norway
Author    Correspondence author
International Journal of Marine Science, 2013, Vol. 3, No. 40   doi: 10.5376/ijms.2013.03.0040
Received: 13 Jun., 2013    Accepted: 16 Jul., 2013    Published: 17 Aug., 2013
© 2013 BioPublisher Publishing Platform
This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Preferred citation for this article:

Fernandes et al., 2013, Low-input Modified Extensive Shrimp Culture Systems for Penaeus monodon Restrain Vibriosis, International Journal of Marine Science, Vol.3, No.40 319-332 (doi: 10.5376/ijms.2013.03.0040)

Abstract

It was hypothesized that an outbreak of vibriosis can be restrained in ponds with a low stocking density (~5 post-larvae m-2) till harvest. Therefore, during a 135 day production cycle of Penaeus monodon at an aquaculture facility, we monitored (i) physico-chemical parameters (ii) abundance of total heterotrophic bacteria (THB), total Vibrio like organisms (TVLO) and luminescent bacteria (LB) (iii) alterations in environmental factors controlling Vibrio abundance and (iv) changes in composition of the Vibrio community. Physico-chemical parameters were within the optimum range recommended for culture of P. monodon. In source and pond water, THB abundance remained steady at 103 CFU mL-1 throughout the culture period whereas in sediments, they varied between 102-4 CFU g-1. The abundance of TVLO and LB in pond components were below the threshold levels of 104 CFU mL-1 and 102 CFU mL-1 respectively. THB, TVLO and LB were more abundant in the shrimp hepatopancreas (105 CFU g-1) yet apparently below the threshold for the outbreak of vibriosis in the current set up. Changes in bacterial abundance were seemingly independent of changes in environmental parameters suggesting that the systems maintained TVLO below disease-causing threshold. The Vibrio community was represented mainly by V. metschnikovii, V. fluvialis, V. mimicus and the closely related Aeromonas spp.. The vibrios in shrimp hepatopancreas were similar to other pond components with no dominance of particular species. A disease-free environment prevailed and shrimp yield was sustainable within the set framework perhaps due to the low abundance of vibrios represented by innocuous strains. 

Keywords
Abundance; Bacteria; Environmental conditions; Species diversity; Shrimp culture; Vibriosis

 
In the recent years aquaculture has emerged as the world’s fastest food-producing sector growing at an annual rate of almost 10% per year since 1970 (Subasinghe, 2005). Shrimp farming constitutes an important source of revenue and employment in many developing countries in Asia and Latin America. However, rapid expansion of this industry has spawned various technical, ecological, economical and social ordeals. A multitude of factors has contributed to the health problems currently faced by the industry. Between 1987 and 1994, the combined economic losses from shrimp diseases in 11 countries were estimated at US $ 3019 million. In India, the loss during 1994 was estimated at US $ 125 million (Israngkura and Sae-Hae, 2002). Therefore, the sustainability of shrimp culture industry largely depends on disease control and health status of the shrimp. Outbreaks of vibriosis and low survival rates in hatchery and grow-out ponds (Lightner, 1988; Lightner, 1993; Brock and Lightner, 1990) have become a severe barrier to the development of shrimp aquaculture. Vibrios are among the most abundant cultivable microbes in aquaticenvironments (Heidelberg et al., 2002) and usually constitute a part of normal micro flora in farmed and wild penaeid shrimps. Opportunistic vibrios may turn pathogenic when shrimps are stressed due to deteriorated environmental conditions like sub-optimal or unstable environment, high stocking densities and inadequate management. When such stresses prevail, a large increase in the number and composition of Vibrio spp.in the culture environment is expected (Sung et al., 1999). Despite the opportunistic nature of most pathogenic vibrios, findings by Costa et al. (1998) suggest that some diseases occurring in penaeid shrimps have been caused by Vibrio spp., which behave more like true pathogens rather than opportunists. Pathogenic vibrios usually invade the host through the hepatopancreas, the common target organ of most bacterial pathogens of shrimps (Chen et al., 1992; Frelier et al., 1992). They then proliferate and colonize the host digestive tract becoming pathogenic.
Studies on vibrios in shrimp ponds have focused on their isolation, characterization and identification (Gomez-Gil et al.,1998; Lavillo-Pitogo et al.,1998; Leaño et al., 1998; Sung et al., 1999; Abraham and Palaniappan, 2004). A comparison of vibrios in water, sediment and shrimp samples from multiple shrimp ponds from east and west coast of India has been made by Gopal et al (2005). However, the changes in the abundance and composition of Vibrio spp. in shrimp pond components during the entire crop cycle from post larvae (PL) till the harvestable size has not been studied, particularly from modified extensive ponds. Elevated organic loading from shrimp farming activities can result in an increase in the abundance of heterotrophic bacteria (Zaccone et al., 2005) as they are active degraders of organic matter. Generic composition of heterotrophic isolates in an aquaculture pond has shown the predominance of Vibrio spp., (Ganesh et al., 2010). A comprehensive knowledge of the changes in the abundance and species composition of vibrios would therefore be inevitable for a chosen set of environmental conditions in aquaculture ponds. This approach would not only help understanding the dynamics but also plan strategies for disease containment and increasing shrimp yield. Therefore, during a 135 day production cycle of Penaeus monodon we aimed to (i) monitor changes in physico-chemical parameters along with the abundance of total heterotrophic bacteria (THB), total Vibrio like organisms (TVLO; potential pathogens) and luminescentbacteria(LB; confirmed pathogens) in different pond components (rearing water and sediment, pond water and sediment and shrimp hepatopancreas) (ii) examine the relationship between environmental variables and abundance of vibrios and (iii) assess changes in the composition of the Vibrio community in different components of shrimp pond. Results obtained during the study revealed prevalence of a healthy environment within the culture ponds with no outbreak of pathogenic vibrios during the culture period. The Vibrio diversity within the shrimp hepatopancreas was similar to the aquatic system with no detection of pathogenic LB towards the end of the culture. Thus, this study enabled us to establish a framework within which the modified extensive shrimp ponds operated optimally leading to a sustainable yield.
1 Results
1.1 Physico-chemical characteristics
Results of physico-chemical characteristics analyzed in the pond water and sediment during the culture of P. monodon have been tabulated in Table 1 a. Between ponds, no significant differences in physico-chemical parameters were observed. Hence, the values from two ponds were pooled (n = 6) to obtain an average (± SD). A gradual increase in the temperature and salinity of the pond water through the culture period was noted. Temperature of pond water increased from 22.2 ± 0.4 at the start of the cycle reaching to a maximum of 32.5 ± 0.4 at the end of 135 days. Similarly, salinity increased from 8.7 ± 0.2 at 0-doc to 34.0 ± 0.2 at the end of the cycle. Near neutral pH (7.3 ± 0.2 to 8.0 ± 0.1) was recorded in the pond water whereas the pH of sediment in the ponds was more acidic with little variation through the culture period (6.2 ± 0.2 to 6.7 ± 0.2). Eh values in the pond water ranged between 27.90 ± 0.19 and 142.77 ± 3.25 whereas in the pond sediments the range was relatively lower varying from 26.78 ± 2.57 to 127.73 ± 1.60. Dissolved oxygen generally showed a decreasing trend as the culture period progressed. The lowest dissolved oxygen content of 4.4 ± 0.3 mg L-1 was recorded at 120-doc. POC in the pond (Table 1 a) and creek water (Table 1 b) followed a similar trend with a steady increase in concentration until 60-doc to reach about 25.2 ± 8.4 g C m-3. Thereafter, POC in the pond and creek varied with doc. The organic matter content in the pond sediments increased with the progress of the culture period. A maximum of 58.1 ± 10.6 mg g-1 was recorded on 120th day of observation.


Table 1 Average values of physico-chemical characteristics in (a) rearing water and sediment (± SD; n=6), (b) Source water (creek) (± SD) during the cultivation of Penaeus monodon


In the water/sediment sourced from the nearby creek, the trend in the variation of temperature, salinity, pH, Eh, dissolved oxygen and sediment organic matter were similar to those observed in the culture ponds (Table 1 b).
1.2 Bacterial counts
No significant difference in THB, TVLO and LB abundance was found between ponds. Hence, the bacterial counts from the two ponds were pooled to obtain average values (n = 6). Fortnightly variations in counts of THB, TVLO and LB in different shrimp pond components (source water and sediment, rearing water and sediment and shrimp hepatopancreas) are depicted in Figures 1 a-e.


Figures 1 a-e Fortnightly changes in the abundance of total heterotrophic bacteria (THB), total Vibrio like organisms (TVLO) and luminescent bacteria (LB) in creek water (a), creek sediment (b), pond water (c), pond sediment (d) and shrimp hepatopancreas (e) during the cultivation of P. monodon. Vertical bars denote standard deviation


Counts of total heterotrophic bacteria (THB) in the pond water did not vary much and remained at 103 CFU mL-1 throughout the culture period. In the pond sediments, their abundance varied from 104 CFU g
International Journal of Marine Science
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