2 Reproductive Biology of Short Mackerel Rastrelliger brachysoma (Bleeker, 1851) and Indian Mackerel R. kanagurta (Cuvier, 1816) in Thai Waters 1* 2 1 3 Tassapon Krajangdara Patcharee Puntuleng Panida Chalee and Piyawan Hussadee 1 Andaman Sea Fisheries Research and Development Center (Phuket) 2 Chumphon Marine Fisheries Research and Development Center (Chumphon) 3 Upper Gulf Marine Fisheries Research and Development Center (Samutprakarn) Abstract Reproductive biology of short mackerel, Rastrelliger brachysoma and Indian mackerel, R. kanagurta were studied in Thai Waters from October 2003 to September 2005. The data was collected from purse seiners, trawlers and gill neters, which landed at fishing ports around the Andaman Sea, the upper and middle Gulf of Thailand. The results showed the relationship between total length and body weight of R. brachysoma (TL 11-24 cm) were W = 0.0142 TL2.9402 and W = 0.0074 TL3.1670 while those of R. kanagurta (TL 12-29 cm) were W = 0.0145 TL2.9350 and W = 0.0072 TL3.1521, respectively. Both species of fish spawned all year round and spawned highly in 2 phases. R. brachysoma in the Andaman Sea spawned during November-May and July-September, while in the Gulf of Thailand they spawned during February-May and July-October and R. kanagurta in the Andaman Sea spawned during December-March and August-September, while in the Gulf of Thailand they spawned during January-March, May, July, September and November. The average size at first maturity of male and female of R. brachysoma in the Andaman Sea were 16.09 and 15.33 cm; in the Gulf of Thailand were 16.45 and 17.95 cm while those of R. kanagurta in the Andaman Sea were 17.83 and 18.92 cm; in the Gulf of Thailand were 20.07 and 17.12 cm, respectively. Sex ratio of male : female of R. brachysoma was 1 : 0.85 and 1 : 1.19, while sex ratio of male : female of R. kanagurta was 1 : 0.91 and 1 : 1.31. Key words: reproductive biology, short mackerel, Indian mackerel, Rastrelliger brachysoma, R. kanagurta, Thai Waters _______________________________________ * Corresponding author: 77 Sakdidej Road, Muang District, Phuket Province 83000 Thailand. e-mail : [email protected] .