A SIMPLE STRESS TEST FOR PENAEUS-VANNAMEI POSTLARVAE

The effects of age on resistance to low salinity and formalin stress w ere determined in early Penaeus vannamei postlarvae (PL) with acute st atic bioassays, Two-hour median lethal concentrations (2 h-LC50) of fo rmalin were 274, 288, 298 and 293 ppm for PL ages of 1, 2, 3 and 4 day s (PL1-PL4), respectively. After PL4, resistance to formalin increased with age to 374, 497 and 598 ppm for PL5, PL6 and PL7, respectively. For exposure to low salinities, 2 h-LC50 decreased from 16.8 ppt for b oth PL1 and PL2 to 14.3, 10.3, 8.3, 4.5. and 3.0 ppt for PL3, PL4, PL5 , PL6 and PL7, respectively. Based on salinity decreases or differenti als, 2 h-LC50 increased from 12.9 and 11.8 ppt for PL1 and PL2, respec tively, to 14.2, 18.8, 19.5, 23.3 and 24.9 ppt for PL3, PL4, FL,, FL, and FL,, respectively. A practical stress test to evaluate the hardine ss of a hatchery produced P. vannamei PL was proposed, using exposure to a single concentration of formalin or single reduced salinity as a stressor. Because resistance to formalin and reduced salinity increase d with age, different stressor concentrations are recommended for PL a ges with different sensitivities. The proposed stress tests, which are rapid, inexpensive and simple, can be used by shrimp hatcheries as a quality control procedure. (C) 1998 Elsevier Science B.V. All rights r eserved.