HOFER, S.C.*; DRACKETT, M.; WATTS, S.A.: Low temperature tolerance of GMT^TM tilapia,Oreochromis niloticus.

Genetically male tilapia (GMT^TM) are produced by crossing XX female and YY male broodstock. They exhibit hybrid vigor with increased growth rates and are favored for aquaculture in warm climates. In the US tilapia fingerlings are cultured at hatcheries and are reared at 28 to 30 C. In southern regions fingerlings can be transferred to outdoor ponds for growout in the spring. Temperature tolerance dictates the duration of the valuable outdoor growing season. We examined the low temperature tolerance limit by exposing GMT^TM fingerlings acutely to low temperatures. Fingerlings (ca. 5 g) were reared at 28 C for several weeks and exposed (n = 90) to reduced temperatures for 24 hr and then placed back into 28 C water for 72 hr. Survivorship was recorded after the initial exposure (24 hr) and at 24, 48, and 72 hr post-exposure. Greater than 98% of the fingerlings survived the initial exposure to 20 C and 18 C and the subsequent recovery period. Fish exposed to 16 C exhibited a 97% survival rate after the initial exposure and a 90% survival rate after the 24 hr recovery period. Fish exposed to 14 C exhibited an 88% survival rate after the initial exposure and a 76% survival rate after the 24 hr recovery period. No fish survived exposure to 12 C. The significant difference in survivorship between the fish exposed to 14 C and 12 C suggests an LT50 of approximately 13 C. To maximize production we suggest that fingerlings be transferred to ponds with temperatures no lower than 18 C prior to the summer growing season.