IOSR Journal Of Environmental Science, Toxicology And Food Technology (IOSR-JESTFT) e-ISSN: 2319-2402,p- ISSN: 2319-2399. Volume 6, Issue 6 (Nov. - Dec. 2013), PP 22-27 www.iosrjournals.org www.iosrjournals.org 22 | Page Effects of Salinity Variations on Acute Toxicity of Copper Sulphate against Oreochromis niloticus Fingerlings in Laboratory Bioassays Osuala, Fidelia I. 1 Bawa- Allah, Kafilat A. 2 1, 2 (Department of zoology, university of Lagos, Akoka-Yaba, Lagos, Nigeria) Abstract: The study was carried out to investigate the salinity tolerance and relative acute toxicity of copper sulphate (CuSO 4 ) under fresh water in varying salinity level, against Oreochromis niloticus fingerlings. A total of 280 fingerlings of O. niloticus (mean weight; 20 ± 4 g; 4-6 weeks old) were used as test animals in this study. The test compound used was copper as CuSO 4 .5H 2 O analar grade (molecular weight 249.68 g). The dose response data were analyzed by probit analysis. The results showed that on the basis of daily mortality assessment, O. niloticus were unable to survive in water with salinity above 22ppt within a 24hr period, but the fish survived well at lower salinities below 12ppt. Based on 96hr LC 50 values, toxicity of CuSO 4 against the fingerlings increased with increase or decrease in salinity except at 12ppt where the toxicity was minimal. The significance of this study is in setting ecologically sound, safe limits that are relevant for the prevailing two hydrological seasons in the tropics for the discharge of heavy metals into lagoons and the advantages of breeding O. niloticus under typical brackish water conditions, instead of freshwater. Keywords: Acute toxicity, Fingerlings, Copper sulphate, Oreochromis niloticus, Salinity I. Introduction All over the world including West Africa, there are lagoons off the coast lines that are characterized by brackish water at salinity between 1 and 35 parts per thousand and usually known to be productive, being rich in flora and fauna. The Lagos lagoon is the largest among four others found along the West African coast (1). This lagoon covers about 700km on the western coast of Nigeria (6 0 -26 1 N; 3 0 -23 1 -4 0 20 1 E). The Lagos lagoon is an abode for different types of organisms (2). Studies carried out in the middle of the last century showed that Lagos lagoon and others in West Africa had much wider variety of pelagic and benthic organisms than are indicated by studies carried out in the last two decades (3, 4). The authors (3, 5) suggested that pollution of lagoon ecosystems by industrial wastes is one of the major reasons for the observed reduction in diversity and population densities of pelagic, benthic animals and plant species. These industrial wastes are complex admixtures of several classes of pollutants such as hydrocarbons, pesticides and heavy metals (6). The exact type and amount depends on the type of industry, technology adopted, raw materials used and waste treatment facilities. Heavy metals are among the most common constituents of industrial effluents, hence this serves as a major source of these pollutants into the ecosystems. Heavy metals are elements with density greater than 5gcm 3 and atomic number greater than that of calcium. They are important sources of pollution, toxic at relatively low concentrations, in most cases nondegradable and persistent in the environment long after the source of emission has been removed (7). Thus heavy metals can bioaccumulate in one or several compartments across foods webs as shown by several scientific observations (8, 9, 10). Heavy metals are highly toxic to living organisms and their non degradable characteristic creates a highly significant environmental problem as the concentrations accumulate over time, whereas the toxicity persists in the ecosystems. Copper is one of the most common heavy metals detected in the Lagos lagoon (3) and although it is useful in living systems, it becomes toxic at high enough concentrations. It is toxic to amphibians, benthic invertebrates, fishes found in the aquatic environment (11, 12). In brackish water ecosystems, the major physicochemical parameter that varies widely in response to the two hydrological seasons (Wet and Dry) which characterize tropical regions is salinity. Due to this characteristic the lagoon is usually inhabited mainly by animals that can tolerate a wide range of salinity changes (euryhaline species). Variation of salinity in lagoon ecosystems in response to the two seasons influenced heavy metal toxicity and therefore the damage potential of metals in the field to biota (3) The implications of this are that the extent of biological damage caused by pollutants may vary with salinity, which changes from dry season to wet season. More research is therefore justifiable along these lines, in which the responses of animal species typical of the Lagos lagoon, to heavy metals and other priority pollutants will be investigated under varying salinity conditions typical of local lagoon ecosystems.