Cold Shock Susceptibility of Blue Tilapia from the Susquehanna River, Pennsylvania

A population of blue tilapia Oreochromis aureus, a native species of Africa and southern Asia, became established in the Susquehanna River, Pennsylvania, after escaping from the Brunner Island Aquaculture Facility. It appears that blue tilapias are able to survive low water temperatures during winter by congregating in thermal effluents. The objective of this study was to determine the susceptibility of the Susquehanna River population to cold shock, and the conditions necessary to eliminate these fish from the thermal effluent at Pennsylvania Power and Light’s Brunner Island Steam Electric Station. Blue tilapias were acclimated to 20° and 15°C and then exposed to sublethal and lethal temperatures under three patterns of temperature reduction-rapid cold shock or one of two rates of gradual temperature decrease (l°C/h or 3°C/h) to lethal temperatures each test lasting 96 h and involving 20 fish per test group. Among fish acclimated to 20°C and immediately exposed to test temperatures, none died at the control (acclimation) temperature and at 14°C, 65% died at 11°C, and all died after 30 min at 8° and 5°C.A similar pattern occurred for fish acclimated to 15°C: 5% died at the control temperature, none died at 11°C, 95% died at 8°C, and all died at 5° and 3°C after 30 min. Exposure to gradually decreasing temperatures increased resistance times at test temperatures in all cases. Among fish acclimated to 20'C and cooled by 3°C/h, 100% mortality occurred within 14 h at 8°C and within 8 h at 5°C. Among fish acclimated to 15°C and cooled by l°C/h, 95% mortality occurred after 96 h at 8°C and 100% mortality occurred within 20 h at 5°C. Among fish acclimated to 15“C and cooled by 3°C/h, 100% mortality occurred within 12 h at 5°C and within 6 h at 3°C. Among fish acclimated to 15°C and cooled by l°C/h, 100% mortality occurred within 16 h at 5°C and within 14 h at 3°C. We recommend that the temperature be lowered to 5°C for at least 16 h to kill blue tilapias in the thermal effluent at Pennsylvania Power and Light’s Brunner Island Steam Electric Station.