The presence of the pacific whiteleg shrimp (Litopenaeus vannamei, Boone, 1931) in the wild in Thailand

Pacific whiteleg shrimp (Litopenaeus vannamei, Boone, 1931) was first introduced to Thailand for aquaculture in the late 1990s as an alternative to a native shrimp species, black tiger prawn (Penaeus monodon, Fabricius, 1798). We documented the presence of L. vannamei in the Bangpakong river system, an important watershed in eastern Thailand with a high density of shrimp farms. This paper is a part of a larger study to evaluate potential ecological consequences of introduced L. vannamei. During January-November 2005 (three sampling periods), we sampled wild marine shrimp with commercial shrimp nets (6 m wide X 5 m deep X 25 m long with 2.5 cm mesh) at four sites within the Bangpakong estuary. Results indicated that L. vannamei were present at least once at all sampling sites during the study. Proportion of L. vannamei relative to all Penaeid shrimp per net in the Bangpakong estuary was 0.005 +/- 0.0016 (January-March 2005), 0.0005 +/- 0.00021 (June 2005), and 0.061 +/- 0.0035 (SSeptember-November 2005). Litopenaeus vannamei were present in 30%, 16% and 100% of nets used for the three consecutive sampling periods. The mean sizes of L. vannamei captured were 22.+/- 0.75,25.1 +/- 0.07, and 22.0 +/- 0.29 mm postorbital carapace length (POCL) and 85.6 +/- 2.66, 105.5 +/- 6.13, and 85.8 +/- 1.04 mm body length. The CL and body sizes were significantly larger in samples collected in June 2005 than the two other periods. Increasing frequencies of occurrence of L. vannamei in the Bangpakong estuary call for the determination of sources (escapes versus a self-sustaining population) and mitigation.