Effects Of Low Salinity Stress On Physiology And Biochemical Of Juvenile Mullet (Mugil Cephalus)

The Yangtze River estuary is one of the most important spawning ground andfeeding ground for many commercial fishes, owing to the effects of the upstream runoff,outside tide and estuary navigation channel engineering the estuary water salinitychanges sharply. As one of the important environment factors, the water salinity have acertain effect on eco-physiological indexes. Mullet are widely cultured estuaryeconomical fish, researches on effects of low salinity stress on growth, metabolism andother physiological and biochemical indexes for rich fish physiological ecology andenvironment factor and the interaction of aquatic organisms is of important theoreticalsignificance, it has important reference value to the development of estuarine fishfarming, at the same time. This paper takes trained wild juvenile mullet as the researchobject,and the mullet were cultured at water salinity20,15,10,5and0for20d,respectively, with the changes of feeding rate, the special growth rate (SGR) in bodyweight and body length, the biochemical composition of muscle, metabolism, theNa+-K+-ATPase and the MDA contents determined on the0thd,5thd,10thd,15thd,20thd.The experiment were guided by the indoor static experimental method which wascombined with molecular biology techniques, and the results shows as follow:1Effects of low salinity chronic stress on feeding, growth and biochemical compositionof juvenile mullet （Mugil cephalus）To facilitate the presentation,S20were used to stand for the experiment groupwhich the salinity were20, the same as S15, S10, S5, S0.The results revealed that thefeeding rate and the growth rate were highly salinity-dependent (P<0.05) at thebeginning of the experiment. Low salinity had an obvious inhibitory effect on thefeeding rate of juvenile mullet，the feeding rate in S0was5.67%, while S20was12.90%at the first day. Compared with the beginning, the feeding rate had a great advance at theend of the experiment (20thd). The feeding rates of juvenile mullet in S0increased by 6.39%. The special growth rate in body weight and body length were also sensitive tothe changes of salinity of water, the SGR in higher groups such as S20, S15were higherthan in lower groups (S5,S0)(P<0.05). In the beginning of this experiment, the SGR inbody weight and body length were quite different among different salinity group, theSGR were2.5%,0.61%in S0while they were3.91%,1.31%in S20(0thd). The specialgrowth rate of juvenile mullet under the same salinity increased as the experiment wenton. The gap between various salinity groups also gradually became narrow. At the endof the experiment, the SGR in lower salinity groups became very close to the highersalinity groups (P>0.05). The test results of RNA/DNA value also revealed that at firstthe juvenile mullet grew better when they lived in high salinity water, and then the gapbecame smaller among these experimental groups which were consistent with the aboverules. The biochemical composition of juvenile mullet were significantly affected bylow salinity stress (P <0.05). The testing results of juvenile mullet muscle showed thatthe mullets’ body had a highly contents of moisture which accounting for about75%ofthe body weight. And the fat and protein contents were occupied about9%and6%respectively. The content of moisture in muscle increased with decreasing salinity(P<0.05). Though the salinityhad noticeable impact on the ratios of fat, protein and ashin mullets’ muscle (P<0.05), the change regulation of fat, protein and ash were quitedifferent with the variation of moisture. The ratio that fat, protein and ash occupied inbody weight of juvenile mullet increased with the decreased salinity and the increasedtest time.2Effects of low salinity chronic stress on oxygen consumption rate and ammoniaexcretion rate of juvenile mullet （Mugil cephalus）The results indicate that both oxygen consumption rate and ammonia excretion rateof juvenile mullet are highly salinity-dependent, they became lower as the salinitydecreased, and get the lowest value at S0. From the5thday to15thday, the ammoniaexcretion rate at S0, S5, S10are extremely different （P<0.05）, but the oxygenconsumption are not （P<0.05）. There is no significant difference of the ammoniaexcretion of every group on20thday, while the oxygen consumption rate have alreadytend to a certain value （P>0.05）. Compared with oxygen consumption rate, the effectof salinity on excretion has an obvious hysteretic nature. Both oxygen consumption rate and ammonia excretion rate decrease as the experiment time extension, the main reasonwould be the change in avoirdupois of juvenile mullet. The O/N value under differentsalinity and different time varied from0to10show that juvenile mullet mainly useprotein as energy source at the experiment salinity.3Effects of chronic stress with low salinity on branchial，muscular，intestinalNa+-K+-ATPase (NKA) activities and MDA contents of juvenile mullet (Mugilcephalus)The NKA activities and MDA contents were quite different while the tissue weredifferent, from the highest to lowest were intestines, gill and muscle.The results showedthe branchial, muscular and intestinal NKA activities and MDA contents weresignificantly affected by low salinity at the beginning of this experiment. But theresponse levels to the change of water salinity were different among these tissues.Interms of the activities of NKA, branchial and intestinal NKA activities were moresusceptible to lowering salinity than muscular, but the variation of MDA contents wereexactly upside down.Juvenile mullets which were cultured in low-salinity environmentsshowed higher NKA activity. And the results revealed that the appropriate low salinitystress can activate NKA activity which can enhance their ability to adapt to low salinityenvironment. The NKA activities and MDA contents of experimental groups decreasedgradually and even stabilized at last. At the end of this experiment, the insignificantdifferences among the different salinity groups showed the juvenile mullets had a strongability to adapt to salinity changes. The time NKA activity and MDA content requiredfor stabilization were characterized by tissue indicated the effects of low salinity onNKA activities and MDA contents had clearly tissue specify and certain time sequence.