A simple stress test for Penaeus vannamei postlarvae

The effects of age on resistance to low salinity and formalin stress were determined in early Penaeus vannamei postlarvae (PL) with acute static bioassays, Two-hour median lethal concentrations (2 h-LC50) of formalin were 274, 288, 298 and 293 ppm for PL ages of 1, 2, 3 and 4 days (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 both 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 differentials, 2 h-LC50 increased from 12.9 and 11.8 ppt for PL1 and PL2, respectively, 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 hardiness 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 increased with age, different stressor concentrations are recommended for PL ages 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 reserved.