Development of standard toxicity tests for tropical aquatic environments: 1. The use of tilapia, Oreochromis niloticus (L.), as toxicity test organism for freshwater habitats

Abstract

Development of toxicity tests that are more representative of tropical conditions is of primary importance in the light of the need for biological standards for assessing the quality of our freshwater bodies. The Department of Environment and Natural Resources–Environmental Management Bureau (DENR–EMB) does not have policies and standards for water quality which include biological parameters except for microbiological tests used for water potability. In response to this need, we explored the potential of using Oreochromis niloticus, popularly known as “tilapia”, as a test organism for toxicity tests designed for freshwater habitats. The study focused on the following: (a) handling and acclimation procedures; (b) determination of a reference toxicant and; (c) application of the tilapia toxicity test to selected chemicals. Both range-finding and definitive test results indicate the suitability of copper sulfate (CuSO₄·5H₂O) as a reference toxicant for the tilapia toxicity test. Application of the test to selected chemicals such as the synthetic hormone 17α-ethinylestradiol (EE2), and three commercial detergents labeled Brands B, T, and V (brand names withheld for purposes of confidentiality, national market share by volume approx. 10%, 17% and <1%, respectively) have shown its sensitivity in detecting toxicity. Average 96–h LC₅₀ for EE2 was 408.50 ± 77.5 ng/L, while values obtained for the three detergents were all within the same order of magnitude, classified under moderate toxicity. Their rank order of toxicity is as follows: Brand B (12.04 ± 1.22 mg/L) > Brand T (19.08 ± 5.93 mg/L) > Brand V (41.88 ± 10.81 mg/L). Results of the study demonstrate the excellent potential of the tilapia toxicity test as a tool for the monitoring of freshwater habitats and its inclusion as a biological parameter in the DENR–EMB Water Quality Standards for purposes of compliance and policy formulation.