Effects Of Dietary Magnesium Levels On The Growth And Physiology Response Of Japanese Seabass (Lateolabrax Japonicus) Cultured In Freshwater And Seawater: A Comparative Study

A series of studies were conducted to investigated the effects of magnesium （MgSO₄） atdifferent levels in diets on the growth performance, activities of metabolic enzymes, contents ofserum ions and levels of osmotic pressure in Japanese seabass （Lateolabrax japonicus）. Thedietary magnesium requirements of Japanese seabass cultured in freshwater and seawater weredetermined, and the difference in physiology response between Japanese seabass reared infreshwater and seawater. The results are presented as follows:1. Effects of different dietary magnesium levels on the growth performance, digestive enzymaticactivities, ions’ contents and serum biochemical indexes of Japanese seabass cultured infreshwater and seawater:The Japanese seabass with an average weight of10.5±0.4g were randomly distributed into6treatments with3replicates in each treatment, and were fed the diets at magnesium levels of395,566,712,1037,1558and1980mg/kg in freshwater or seawater for56d. The growthperformance in freshwater in fish fed with1558mg/kg magnesium were significantly higherthan those in other groups （P<0.05）, while no significant effects of dietary magnesium on thegrowth performance cultured in seawater were observed （P>0.05）. The contents of body proteinand lipid of Japanese seabass in freshwater increased with the increase of dietary magnesiumlevel to1558mg/kg, and then decreased （P<0.05）, while the body protein and lipid contents forJapanese seabass fed diet unsupplemented MgSO₄were higher than that fed diet supplementedMgSO₄in seawater. The levels of serum low-density lipoprotein and triglyceride, and activitiesof glutathione-S transferase （GST）, glutathione peroxidase （GSH-PX） and digestive enzymes infish cultured in freshwater and seawater were affected significantly by the level of dietarymagnesium （P<0.05）. The CAT activities and Mg²⁺in the liver of fish cultured in freshwater, andthe contents of MDA and Ca²⁺in the liver of fish cultured in seawater were affected significantlyby dietary magnesium （P<0.05）. There were significant differences in the Ca²⁺content in fishbody, Mg²⁺, Na⁺and Cu²⁺contents in vertebra and the serum Na⁺and Cl^-contents for Japaneseseabass cultured in freshwater, and the Na⁺contents in fish body and serum Na⁺and Ca²⁺contents for Japanese seabass cultured in seawater （P<0.05）. Fish in freshwater with1037mg/kgMg in diet had the highest values of Mg²⁺, Na⁺and Zn²⁺in vertebra （P<0.05）. Fish in freshwater with1980mg/kg Mg had the highest values of serum Na⁺and Cl^-contents （P<0.05）. The bodyCa²⁺, K⁺and bone Cu²⁺contents, the Ca²⁺and K⁺contents, and serum Ca²⁺, Na⁺, K⁺and Cl^-contents for Japanese seabass cultured in freshwater were lower than those of fish cultured inseawater （P<0.05）. The vertebra Mg²⁺contents and body Mg²⁺, Na⁺and Zn²⁺contents infishcultured in freshwater were higher than those of fish cultured in seawater. The results indicatedthat Japanese seabass could absorb sufficient magnesium from seawater （magnesium level:1285mg/L） to meet requirement by itself, therefore it is not essential for magnesium supplymentationin diet of Japanese seabass in seawater. Quadratic cruve analysis based on weigt gain indicatedthat the optimal level of magnesium in diet for Japanese seabass in freshwater was1084mg/kg.Dietary magnesium promoted vertebra mineralization, and is helpful for balancing ions of bodyfor Japanese seabass fed in freshwater. However, magnesium supplementation in diets mightdisturbe balance of ions of body through restraining Na⁺and Cl^-output from body of Japaneseseabass in seawater.2. Effects of acute salinity stress on the osmotic pressure and serum ions of Japanese seabasscultured in freshwater and seawater（1） Japanese seabass （initial weight,30±0.5g） were randomly distributed into6groupsreared in freshwater and seawater, and were fed with6experimental diets at different magnesiumlevels （413,592,729,1042,1577and1991mg/kg） for50d. At the end of the feeding trial, thefish in freshwater were transported into seawater while the fish in seawater were transported intofreshwater. Gills and blood were collected at0（before stress）,1,3,6,12and24h. The bodymoisture, serum osmolality, cortisol and glucose, the structure of gills, the activity ofNa⁺/K⁺-ATPase and Ca²⁺/Mg²⁺-ATPase were significantly affected by the dietary magnesium（P<0.05）. But this effect on body moisture and activity of Ca²⁺/Mg²⁺-ATPase were not observedin Japanese seabass in seawter （P>0.05）. The serum osmolality levels and glucose concentrationof fish in freshwtaer increased, but the body moisture of fish in seawater decreased with thestress time extension （P>0.05）. The fluctuations of serum osmolality, body moisture and gillATPase activity of fish in freshwater at high-magnesium diets （1042,1577and1991mg/kg） wassmaller than that of fish fed the diets at low-magnesium diets （413,592and729mg/kg）. Incontrast, there was no singificant difference among the osmoregulation of fish fed with diets atmagnesium levels in seawater （P>0.05）. The effects of dietary magnesium on the osmoregulationof Japanese seabass in freshwater were greater than those for fish in seawater, and fish fed thediet at high-magnesium level in freshwater exhibited better abilitiy of osmotic pressureadjustment than those fed diets at low-magnesium level.（2） The contents of serum Na⁺and Mg²⁺significantly increased with the increase inmagnesium levels in diets for the fish both in freshwater and seawater （P<0.05）, however, the serum Ca²⁺of fish in seawater decreased with the increase in dietary magnesium （P<0.05）.Under the acute salinity stress conditions, the contents of serum Na⁺, Cl^-, K⁺, Ca²⁺and Mg²⁺fluctuated during the stress period, serum Na⁺and Mg²⁺of fish in freshwater were significantlyaffected by dietary magnesium （P<0.05）, while no differences on the serum Cl^-, K⁺and Ca²⁺were observed （P>0.05）. The dietary magnesium levels had significant effects on the content ofserum Na⁺, Cl^-and Mg²⁺of fish in seawater （P<0.05）. The fish fed the diet at high-magnesiumlevels （1042,1577and1991mg/kg） had advantages over the fish in freshwater in regulation ofserum ions, which implied dietary magnesium levels could affect the osmoregulation of Japaneseseabass through affecting the content of serum ions.