Study of Benthic Fauna in the Discharge Lagoon of a Shrimp Farm With Special Emphasis on the Polychaetes

A study of benthic fauna, with special emphasis on polychaetes, was made in the discharge lagoon of an experimental shrimp farm and a pond of the farm. The discharge lagoon was divided into three sections and stocked with shrimp at low density (6 Pl/m2). Neither formulated feed nor fertilization was used in these sections. The control pond was stocked with shrimp at high density (32 Pl/m2). The pond was fertilized and fed with a commercial shrimp diet. Enclosures of 4 m² were made in the sections and control ponds with plastic nets to exclude shrimp. Water quality parameters (temperature, salinity, dissolved oxygen, pH and organic matter) and the nutrients (N-NO2, N-NO3, TAN and P-PO4), were recorded during the study. Abundance and composition of benthic fauna were recorded biweekly in sections and pond. Significant differences in nutrients and organic matter were found among control pond and the discharge sections, with the highest values in the pond. The main benthic groups collected during the study were Chironomidae insect larvae, polychaete worms (Polydora socialis), gammarid crustaceans and gasteropods (Cerithidea sp). Total abundance of benthic organisms was greater in the discharge sections (5657 to 6202) compared with the control pond (5336). The abundance of polychaetes showed the same tendency (2318 to 2681 versus 1697). Total benthic abundance was always lower in sections and pond than in the respective enclosures (6882 to 7606 organisms). The same pattern was found for polychaetes abundance (3386 to 4006 organisms). Temporal variation of polychaetes abundance was mainly related to shrimp predation as confirmed by the more abrupt abundance decline in pond as compared to sections and in pond and sections as compared to the respective enclosures.