Effect of aerially-applied Malathion on juvenile brown and white shrimp Penaeus aztecus and P. setiferus.

During summer 1969, the effect of standard aerial application of malathion used in mosquito control (85.7 g/ha) on juvenile brown and white shrimp was examined under field conditions in a saline marsh near West Galveston Bay. Water samples contained high concentrations of malathion (2.0-3.2 ppb) immediately after application, with a progressive reduction afterwards. Shrimp at treated stations exhibited mortalities ranging from 14% to 18%. A comparison of the means with a paired t test indicated no significant difference between brown shrimp mortalities and white shrimp mortalities. Malathion concentrations in live shrimp taken from the test sites were consistently higher (0.5-3.2 ppm) than concentrations measured in dead shrimp removed during the same period (0.5-2.1 ppm). No deaths attributable to malathion were observed at the control stations, and no traces of malathion were observed in water and shrimp samples removed from the control stations. Malathion was the most likely contributor to the shrimp deaths at the treated sites.