INVESTIGATION OF CAUSES OF MASS MORTALITIES AMONG TILAPIA FRIES

Abstract

Recently, aquaculture in Egypt has faced multiple records of mass mortality resulting in high economic losses. The mass mortality among tilapia fries is mostly attributed to bacterial pathogens either with or without other pathogens. Aquaculture, in particular, has played a major role over the past decades by increasing the production of fish efficiently and at an affordable cost. It currently accounts for more than half of worldwide fish production. Six thousand tilapia fries (Oreochromis niloticus) were collected from Kafr Sheikh hatcheries with average body weight 0.65 g and average length 0.5 cm and transported in clean plastic bags to the laboratory of Aquatic animal's medicine and management, faculty of Veterinary Medicine, Cairo University for bacteriological examination. Water samples were collected in clean, sterile glass bottles for water analysis. Water samples were used to analyze BOD, COD, NH3, NH4, NO2, Fe, Cu, Cd, and Pb in the central laboratory for water analysis, Faculty of Agriculture, Cairo University, following international standard methods of water and wastewater examination. In Kafr El Sheikh, fish farming relies on earthen ponds supplied by runoff water from agricultural fields. Four water samples taken over the year revealed high levels of ammonia in samples 1 (Spring) and 2 (Summer), reflecting the impact of varying fertilizing cycles on the quality of wastewater from the field. The high degree of biochemical contamination is indicated by the respective BOD and COD scores for these samples. Under complete aseptic conditions, swab from grinded pooling samples of fries was streaked on Nutrient agar, Todd Hewitt agar, tryptone soya agar, blood agar, and pseudomonas agar media after external sterilization by 70% ethyl alcohol then incubation at 28°C for 24-48 hr. A single colony from isolate was picked up, re-streaked again on the aforementioned media, and re-incubated at the same conditions for purification of bacterial isolates. When pure colonies have been identified by gram stain, a loopful of each pure culture was streaked onto trypticase soya agar m