Studies On Nutrient Requirement Of Selenium And Magnesium In Japanese Seabass(Lateolabrax Japonicus) During Middle Growth Stage

Feeding trials were conducted to investigate the nutritional physiology ofselenium and magnesium for the Japanese seabass，Lateolabrax japonicus．Thedietary requirements of selenium and magnesium for Japanese seabass wereconducted in seawater floating net cages (1.5m×1.5m×2m).During the experiment,water temperature was from25to30℃and salinity from28to30‰. The dissolvedoxygen content was7.5–8mg/L.Results of the present study are presented asfollows：1. Nutrient requirement of selenium in Japanese seabass （Lateolabraxjaponicus）during middle growth stageA10-week feeding trial was conducted to evaluate the effects of dietary seleniumon the growth，physiological status and body composition of Japanese seabass(initialaverage weight214.5±1.0g)．Six semi-purified diets were formulated to contain0.12mg/kg、0.36mg/kg、0.42mg/kg、0.6mg/kg、0.78mg/kg and1.10mg/kg Se, suppliedas NaSeO3·H2O．Six diets were fed twice daily(05：30and17：00) to triplicategroups(18fish/group) in seawater floating net cages (1.5m×1.5m×2m). Thewaterborne Se concentrations were monitored during the feeding period，and was notdetectable．Survival and feed efficiency of seabass was not significantly influencedwith increasing dietary Se level（P>0.05）.The results showed that Japanese seabassfed the diet with0.60mg/kg(diet) Se level obtained the highest weight gain, whole body crude protein content among the various groups (P<0.05). Hepatopancreas andwhole body selenium contents were positively related to the dietary Se level. Bothserum and heaptopancreas glutathione peroxidase activities increased with increasingdietary Se level, and peaked at0.60mg/kg (P<0.05). When dietary Se increased to0.78mg/kg, serum glutathione peroxidase activity decreased significantly (P<0.05).Both serum and hepatopancreas GST activities initially decreased and then increasedwith increasing dietary Se level. Serum GST activities in0.78mg/kg dietary seleniumgroup was significantly higher than that in0.12mg/kg group (P<0.05). Serum GRactivities was significantly affected by dietary selenium level. Serum GR activities in0.78mg/kg dietary selenium group was significantly higher than that in0.12mg/kggroup (P<0.05). Serum SOD activities increased with increasing dietary Se level up to0.60mg/kg,thereafter, declined (P<0.05).Level of0.78mg/kg dietary seleniumsignificantly decreased serum SOD activities compared with0.12mg/kg group(P<0.05).The malonaldehyde content initially decreased and then increased withincreasing dietary Se level. Japanese seabass fed the0.60mg/kg diet had the lowestmalonaldehyde content, and differed from that of the control and1.0mg/kg groups(P<0.05). The results indicated that dietary selenium level of0.60mg/kg can promotegrowth and anti-oxidative ability of Japanese seabass. Quadrate broken line analysiswith SGR showed that a level of0.63mg/kg selenium in diet could meet the optimalgrowth of Japanese seabass.2. Nutrient requirement of magnesium in Japanese seabass （Lateolabraxjaponicus）during middle growth stageA10-week feeding trial was conducted to evaluate the effects of dietarymagnesium on the growth，physiological status and body composition of Japaneseseabass(initial average weight204.5±1.0g)．Six semi-purified diets were formulatedto contain184mg/kg、278mg/kg、487mg/kg、929mg/kg、1299mg/kg and1737mg/kgMg,supplied as MgS04)．Six purified diets were fed twice daily(05：30and17：00) to triplicate groups(18fish/group) in seawater floating net cages (1.5m×1.5m×2m). Thewaterborne Mg concentrations were monitored during the feeding period，and was930mg/L.Survival of seabass was not significantly influenced with increasing dietaryMg level（P>0.05）. Specific growth rate(SGR) and weight gain (WGR）wassignificantly decreased with increasing dietary Mg supplement（P<0.05）.Japaneseseabass fed the1737mg/kg Mg diet had the lowest feed efficiency. Both serum andhepatopancreas glutathione peroxidase activities initially increased and then decreasedwith increasing dietary Mg level. Serum GST activities were significantly influencedwith increasing dietary Mg level（P<0.05）. Serum SOD activities was significantlydifferent between278mg/kg and1737mg/kg diet group (P <0.05). Liver SOD activitywas not significantly affected by dietary magnesium level (P>0.05). Themalonaldehyde content initially decreased and then increased with increasing dietaryMg level. Japanese seabass fed the278mg/kg Mg diet had the lowest malonaldehydecontent. Dietary Mg level had no significant effect on Mg and Ca concentration inscale, vertebrae and whole body（P>0.05）.Based on these results, we believe thatwhen the magnesium content of the basal diet high184mg/kg, water magnesiumcontent of930mg/kg, there is no need to add magnesium in Japanese seabass diet. Ifadding too much magnesium, it will inhibit the growth of Japanese seabass.