Lethal and sub-lethal effects of salinity on osmotic challenge
In this study lethal and sub-lethal effect of salini-ty on the osmotic change was determined by ob-serving variations of body weight of exposed fish to different salinity concentrations (Figure 5). At 12ppt, a gradual and slow decrease of weight was noticed up to 12h of exposure and within 24h body weight got back to its original state. On the other hand, at lethal concentrations of a sudden decrease of average weight (body fluid reduced up to 19%) of fish were recorded at the beginning hours of exposure. Then the weight decreased gradually and got back to its original state (Fig-ure 5a, b).
The stress response of a freshwater Java barb to different salinity concentrations was evaluated using the levels of biochemical and hematologi-cal parameters of blood and behavior as indica-tors. Aquaculture urges for more accurate infor-mation on stress control, in order to be assured of good health status of fish, especially those trans-ferred to a new environment. Environmental sa-linity fluctuation can act a stressor and leads to give information on behavioral and physiological responses.
Behavioral endpoints serve as valuable tools to distinguish and evaluate effects of exposure to environmental stressors, and fish behavioral al-terations can provide important indices for eco-system assessment (Kane et al., 2005). Through-out the study period fish exhibited low behavioral stress responses when exposed to sub-lethal con-centration of salinity (12ppt). Impatience or agi-tated activeness or erratic movements were ob-served when exposed to lethal concentrations (14ppt and 16ppt). From the beginning up to 3 hours of the lethal salinity exposure fish showed frequent movement from surface to bottom, ag-gression and sometime showed jumping activity. In addition, fish expressed highly increased oper-cular movements accompanied by excessive se-cretion of mucus which indicating respiratory distress of fish. Lawson and Anetekhai (2011) and Aysel and Ayhan (2010) also noted similar behavioral stress responses after exposing fish in higher salinities.
The osmotic gradient between the plasma and the environment is reduced in freshwater fishes when exposed to increase salinity. Kilambi and Zdinak (1980) stated decreasing survival rates with in-creasing salinity is characteristics of freshwater stenohaline fish, and it has been postulated that this is due to increasing osmotic pressures at