The European Sea Bass (Dicentrarchus labrax), a euryhaline marine teleost, is increasingly the most important marine species in European aquaculture, owing to increased farming of the marine fish, especially in the Mediterranean region. Farmers in the Mediterranean region are rearing the fish in sea cages and land-based systems, which greatly augment ammonia buildup. Upon this background, this experimental study set out to establish the adaptability of the fish to High Environmental Ammonia (HEA), at different ambient salinities. European sea bass acclimated to different salinity treatments (32ppt, 20ppt, 10ppt and 2.5ppt), and fed at up to 2% of body weight, were subjected to High Environmental ammonia (20 mgL- 1)in different experimental tanks. The effects of this exposure and adaptability of the fish to the conditions of the experiment were observed and recorded at 0 h (control) 12h, 48h, 84h and 180h intervals. Measures of the effects of exposure of fish to HEA at different salinities included ammonia excretion rate, plasma ammonia accumulation, liver and muscle glycogen, liver and muscle lipid and protein content respectively, plasma Na+, K+ and Cl- levels, as well as changes in gill NA+/K+-ATPase and H+-ATPase activities respectively. Blood was drawn and fish were dissected to obtain liver, white muscle, and gill tissues after every interval to be used in serological and chemical analysis. High salinity environments have been shown to augment ammonia toxicity because it facilitates increased concentration of the NH3 moiety, which is solely responsible for ammonia toxicity, in aqueous ammonia. As such, these changes, and even loss of function, were expected to be more severe in fish exposed to higher salinities and in chronic exposure to HEA. As was expected, chronic exposure of fish to HEA significantly affected the plasma osmolality, eliciting changes in Na+, K+ and Cl- concentrations. Additionally, the rate of ammonia excretion markedly increased with prolonged exposure, although there was a sudden increase following acute exposure to HEA, regardless of salinity. The depletion of energy reserves in form of glycogen, lipids and proteins also increased with chronic exposure of D. labrax to HEA, salinity notwithstanding. There was a decreasing trend in both muscle and liver glycogen, lipid and protein levels. However, the depletion in muscles occurred more acutely than was observed in the liver with significant reductions being observed as early as 12 h after exposure. In general, lower energy stores were recorded at chronic exposures to HEA regardless of salinity. For instance, muscle glycogen ranged between 24.46 mg/g wet weight; 32 ppt; 180 h, and 63.53 mg/g wet weight; 10 ppt; control. The liver glycogen levels were markedly higher but exhibited a significant decreasing trend ranging between 110.48 mg/g wet weight; 2.5 ppt; 180 h, and 355.54 mg/g wet weight; 10 ppt; control. The same trend was observed with both liver (2.07mg/g wet weight; 10ppt, 84hrs - 5.09 mg/g wet weight; 32ppt, control) and muscle (0.311 mg/g wet weight; 2.5 ppt, 84 h - 1.22 mg/g wet weight; 20 ppt, control) lipid content, and liver (269.14 mg/g wet weight; 20 ppt, 180 h - 576.05 mg/g wet weight; 32 ppt, control) and muscle (229.86 mg/g wet weight; 10 ppt, 180h - 668.57 mg/g wet weight; 2.5 ppt, control) protein levels. These findings indicate that it was chronic exposure to HEA and not necessarily salinity that had adverse effects on the physiology of the fish. However, salinity significantly (p<0.05 or p<0.01) affected osmolality of D. labrax since more acute changes in osmolality were observed in European Seabass acclimatized to brackish and estuarine water (10 and 20 ppt) salinities. At 20 ppt, a significant effect on osmolality of D. labrax was observed among the control group. At 10 ppt, there was a significant (p<0.05) alteration in osmolality in fish exposed for 48 h, as compared to all other salinity treatments and exposure periods. These shows that brackish water salinities significantly affected the osmolality of D. labrax. The effect of HEA on osmolality was only observed in more chronic exposure periods (i.e. 48h, 84h, and 180h). The same trend was observed with Na+, K+, and Cl- levels in plasma whereby chronic (i.e. 48h, 84h, and 180h) exposure of fish to HEA triggered a significant decreasing trend in these parameters. |
