en تخصصي Special پژوهشي Research <span style="font-size:11pt"><span style="font-family:Calibri,sans-serif"><span lang="EN" style="font-size:12.0pt"><span style="font-family:&quot;Times New Roman&quot;,serif">The Pacific white shrimp (<i>Litopenaeus vannamei</i>) is a major species in global aquaculture, but its production is constrained by bacterial diseases such as Acute Hepatopancreatic Necrosis Disease (AHPND). This field study evaluated the effects of different water preparation and management systems on microbial dynamics, water quality, disease severity, and production performance in commercial shrimp farms along the Persian Gulf coast of Iran. Five farming systems were compared: traditional reservoir filtration (T1), mechanical drum filtration (T2), lined ponds without reservoir (T3), earthen ponds (T4), and small lined ponds with intermediate storage and chlorination (T5). The results showed significant differences among treatments. T2 and T5 exhibited lower Vibrio and total heterotrophic bacterial loads, improved dissolved oxygen levels, and reduced ammonia and nitrite concentrations. In contrast, T4 showed the highest bacterial densities, poorest water quality, and lowest shrimp survival (25%), whereas T5 achieved the highest survival rate (80%). PCR analysis confirmed the presence of pirA and pirB toxin genes of Vibrio parahaemolyticus across all systems; however, disease severity varied depending on environmental conditions. These findings indicate that pathogen presence alone does not determine AHPND outcomes, and that microbial control and environmental stability are critical determinants of disease severity. Integrated water treatment strategies, including mechanical filtration, sedimentation&ndash;disinfection reservoirs, and lined pond systems, were associated with reduced microbial loads and improved production performance. Overall, effective water preparation and biosecure management are essential for mitigating AHPND and enhancing sustainability in intensive shrimp farming systems.</span></span></span></span> Litopenaeus vannamei, AHPND, water treatment systems, microbial dynamics, shrimp aquaculture, biosecurity. 23 37 http://injvr.com/browse.php?a_code=A-10-25-4&slc_lang=en&sid=1 M.Kh. Pazir dr.pazir@gmail.com 10031947532846001612 10031947532846001612 Yes Shrimp Research Center, Iranian Fisheries Sciences Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Bushehr, 75169-89177, Iran. A. Eftekhari a_eftekhari88@yahoo.com 10031947532846001613 10031947532846001613 No Aquaculture Veterinary Practitioner and Shrimp Farmer, Sheif Aquaculture Zone, Bushehr Province, Iran. S. Pourmozaffarur sajjad5550@gmail.com 10031947532846001614 10031947532846001614 No Persian Gulf Mollusks Research Station, Persian Gulf and Oman Sea Ecology Research Center, Iranian Fisheries Sciences Research Institute, Agricultural Research Education and Extension Organization (AREEO), Bandar-e- Lengeh, 75145, Iran. S. Kakoolaki 10031947532846001615 10031947532846001615 No Iranian Fisheries Sciences Research Institute, Agricultural Research, Education and Extension Organization, Tehran, 1588733111, Iran