Toxicidade do efluente de uma fazenda de cultivo de camarão marinho Litopanaeus vannamei e do metabissulfito de sódio em juvenis de Mysidopsis juniae.

Abstract

Shrimp represents the most important aquaculture product in many countries. However there are considerable ecological costs associated with this activity and the introduction of effluents into receiving water without any treatment have attracted considerable attention. These effluents contained organic matter, nitrate, nitrite, phosphates and other substances that could be considered as potent contaminants, such as the sodium metabisulphite used to prevent the melanosis during harvesting phase. The aim of the present work was to evaluate the toxicity of water samples collected at the supplying and draining composite facilities from a Litopenaeus vannamei shrimp farm using the acute toxicity test with Mysidopsis juniae, and also to evaluated sodi um metabisulphite toxicity using the same assay. The ecotoxiclogical analyses were performed in accordance to standardized methods (CETESB L5.251 from may, 1992). It was conducted 8 water collections on april, june, july, november and december 2005, and january and february, 2006 at both the supplying and draining composite facilities. The tested concentrations from these effluents were 6.25, 12.5, 25, 50 and 100%. Despites the evaluation of sodium metabisulp hite toxicity itself, the reduction of this toxicity by aeration in the presence and absence of Ca(OH)2 during 24 hours was also assessed at sodium metabisulphite concentrations of 10, 30, 100, 300 and 1000 mg/L. The results showed measurable toxicity only in one sample from the supplying composite (LC 50 of 82.49% on 02/16/06), while for the draining composite four among eight tested samples were considered toxic with LC 50 ranging from less than 6.25% on 04/14/05 to 100% on 01/17/06. A water sample collected at the harvesting tank containing sodium metabisulphite led all tested organism to death immediately after exposition at all tested concentrations, and it was not possible to determine its LC 50 (less than 6.25%). The LC 50 for sodium metabisulphite fresh solution was 38.2 ± 4.7mg/L. After 24 hours, the toxicity was not altered both in the absence or in the presence of Ca(OH) 2, and the obtained LC 50 were 36.8 ± 5.6 mg/L and 44.4 ± 3.2. mg/L, respectively. When the metabisulphite solution was aerated, the toxicity was reduced by three times (LC 50 of 150.7 ± 8.5 mg/L). The addition of Ca(OH) 2 in the presence of aeration e liminated the toxicity in three out of five experiments, and in the remaining two experiments, the toxicity was significantly reduced, and the obtained LC 50 were 209.8 and 669.4 mg/L. Thus, present data showed that the toxicity in the draining composite was increased, what suggested a contaminants load from the shrimp farm. However this contamination is quite variable and occasional. On the other hand, M. juniae was very sensitive to sodium metabisulphite when compared to other described crustaceans, and moreover the addition of Ca(OH) 2 to sodium metabisulphite solution in the presence of aeration for 24 hours partially removed its toxicity.