The bacterial flora occurring in brackish pond water, sediment, gills and intestine of healthy tilapia cultured in Saudi Arabia were estimated both quantitatively and qualitatively, and the isolates were identified to genus or species level. Total viable count of bacteria ranged from 1.4±1.5×103 to 8.6±2.7×103 cfu ml-1 ; 1.2±3.1×106 to 7.3±1.1×107 cfu g-1; 8.7±1.9×105 to 2.1±0.9×106 cfu g-1; and 2.8±2.4×107 to 1.0±1.6×108 cfu g-1 in the pond water, sediment, gills and intestine of brackish water tilapia, respectively. In total, 19 bacterial species were identified. The bacteria were predominantly Gram-negative rods (87%). Pond water and sediment bacteria influenced the bacterial composition of gills and intestine of tilapia. In contrast to gill bacteria, more diversification was observed in intestinal bacteria. The predominant (prevalence N10%) bacterial species were Vibrio parahaemolyticus, Vibrio carchariae, Vibrio alginolyticus, Chryseomonas sp., Vibrio vulnificus, and Streptococcus sp. in all the populations with the exception of the sediment population where Streptococcus sp. was replaced by Shewanella putrefaciens. Vibrio spp. (58% of the total isolates) dominated the total bacterial population.

Introduction

In a world where wild capture fisheries are becoming increasingly depleted, tilapia offer the possibility of commercial and home-grown protein sources because of their superior culture facilities. World tilapia aquaculture production doubled between 1986 and 1992, and followed Chinese carps and salmonids in total farm production (FAO, 1994). The more common species in brackish water ponds are mullets, tilapia and catfish, although the detailed economic viability of brackish water pond tilapia culture has yet to be determined. The omnivorous, fast growing tasty tilapia appear to be well equipped to survive in brackish water along the Gulf Coast (Peterson, 2002). Tilapia can tolerate, grow and even reproduce in saline waters, although this capacity is somewhat offset under high salinity conditions. A range of 10–20 ppt is optimal for growth (Suresh and Kwei Lin, 1992), although some tilapia strains can be grown well has in brackish water (Romana-Eguia and Eguia, 1999). In Saudi Arabia, tilapia culture has increased in fresh water, particularly because of its fast growth and the fact that it can be easily produced in many confined water bodies throughout the country. However, in brackish water bodies tilapia are not cultured commercially though they are available in such water. Since modern aquacultural practices are
quite new in Saudi Arabia, basic information on the bacterial populations and types associated with cultured fish species are scarce. Nevertheless, this data is required for the development of preventive measures to safeguard against infectious agents which could cause disease and financial losses. Aquatic microorganisms not only influence the water quality but are also known to be closely associated with the physiological status of the fish, disease and postharvest quality. Almost no information is available in Saudi Arabia on the bacteriology of brackish water bodies though aquatic animals take a large number of bacteria into their gut and gills from water, sediment and food. The intestinal microflora may be involved in the spread of fecal contaminants (Al-Harbi, 2003). Bowen (1976) reported the contribution of bacteria to the diet of fishes. SyvokienJ and MickJnienJ (1998) established that in the brackish water environment viable counts of the bacteria were many times lower than those in the digestive system of fish indicating that the digestive tract provides favourable ecological niches for these organisms. Fish with abundant and diverse microflora have considerable opportunities to adapt to changing nutritional substrates and to assimilate food better, and thus enhance their adaptive possibilities.

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