| In the current context of climate change,salinity levels in some regions are increasing considerably and this phenomenon is expected to accelerate in the coming decades.For aquatic species,increasing salinity levels are likely to have a direct impact on metabolic functioning and consequently on the performance of(growth,reproduction,survival).Metabolic traits such as resting metabolic rate(RMR),maximum metabolic rate(MMR)and aerobic scope(AS)parameters are three fundamental physiological variables that constitute the floor and ceiling of metabolism in living organisms.Therefore,these parameters can provide information on the effects of salinity in freshwater fish.In the normal state freshwater fish,require stable internal ionic content and composition to maintain homeostasis,which results in an energetic cost of osmoregulation(i.e.,an increase in SMR)that can be significant at salinities where there are large concentration gradients between the internal and external environment.In addition,in a salt stress situation the osmotic potential of water in the gills could be reduced,and this compromises gas exchange,which is often seen as a reduction in MMR.However,it is some observations that motivated this study and raises questions such as:are fish that have a resting and maximum metabolic rate not more tolerant to salinity stress?How can salinity affect the physiological state of freshwater fish(example:grass carp)?Details of the research content and key findings are as follows.1.Fifty-four herbivorous grass carp(Ctenopharyngodon idella)were subjected to progressive salinity(1 g·l-1 per day until all fish died)to detect potential physiological correlation between salinity tolerance and other parameters such as RMR,MMR,AS,ventilation frequency(VF)and ingestion rate.Before salinity treatment,RMR,MMR,AS,and FAS were measured individually and the mean values found were 143.785±25.40,767.532±179.21,623.748±173.15,and 5.441±1.39 mg O2·h-1·kg-0.75,respectively.RMR was positively correlated with MMR(r2=0.093,P=0.025)while it was negatively correlated with FAS(r2=0.18,P=0.0016).MMR was positively correlated with FAS(r2=0.53,P<0.0001).In contrast,no significant correlation was observed between salinity tolerance and RMR(r2=0.00061,P=0.86),MMR(r2=0.0074,P=0.54),and FAS(r2=0.0060,P=0.53).A gradual decrease and then increase in ventilation frequency with the increase in salinity was also observed.However,starting at 12.93 g·l-1,an exponential increase in VF reaching nearly 41.96 times·min-1 was observed.At the same time,the ingestion rate was significantly affected by salinity,especially towards the end of the experiment,where the fish did not consume nearly 95%of the distributed food.2.Sixty grass carp(15.3±2.4 g)were randomly assigned to six 100-liter tanks filled with dechlorinated tap water(n=10)with salinities of 0,0.375,0.75,1.5,3,and 6 g·l-1for 15 days.Salinity was gradually increased by 0.375 g·l-1 per day until the desired concentration for each group was reached.At the extreme concentrations of salinity 0 g·l-1 and 6 g·l-1,the RMR was lower with 47.24±2.05 and 32.58±2.55 mg O2·h-1·kg-1,respectively.Therefore,salinity significantly affected fish RMR(F=7.33,P<0.001).In contrast to RMR,MMR hardly changed during this experiment the mean value recorded were 276.91±14.60,274.34±20.78,282.45±15.65,296.62±14.76,323.89±16.41,312.28±17.50 mg O2·h-1·kg-0.75 at 0,0.375,0.75,1.5,3,6 g·l-1,respectively.The AS at0 g·l-1 was 229.66±15.09 mg O2·h-1·kg-1 after it significantly decreased to 0.75 g·l-1(P<0.005).It then increased from 1.5 g·l-1,AS=238.78±14.14 mg O2·h-1·kg-0.75 to 6 g·l-1AS=279.71±6.81 mg O2·h-1·kg-0.75.The FAS was 5.98±0.43 at 0 g·l-1 then gradually increased with salt concentration until it was extinguished 10.05±0.90 at 6 g·l-1.The activity of the enzyme was practically high in all three organs studied,but it was much more intense in the gills with 145.86(U·g-1)and the digestive tract with 109.11(U·g-1)than in the kidneys 61.68(U·g-1)of the experimental fish.Statistical analysis reveals that salinity to actually affect NAK activities of gill(F=6.42,P<0.001)and digestive tract(F=8.95,P<0.001).Regarding the variation of plasma ion concentration during this experiment.We found that only the concentration of Na+and Ca2+was not affected by salinity.On the other hand,salinity significantly affects the plasma concentration of ions such as Mg2+(F=2.96,P<0.05);Cl-(F=4.15,P<0.001)and k+(F=3.89 P<0.005).The results suggest that salinity affects both ventilation and food ingestion.As salinity increase,the grass carp gradually cease eating,and increase ventilation frequency,and finally dies.Although grass carp is a stenohaline fish species,its tolerance to salinity is relatively acceptable.Therefore,the grass carp can be considered in brackish water aquaculture.A better metabolic capacity may not necessarily allow a better salinity tolerance of the grass carp,which suggests that it is impossible to use those metabolic capacity parameters for predicting salinity tolerance of this species.However,the larger body size allows the better salinity,indicating that body size could be a predictor of salinity tolerance.The present study contributes to our understanding of the effect of salinity on the variation of certain physiological parameters of a grass carp.The indicators studied during this experiment would be useful for the domestication of freshwater fish in brackish or saline environments in the regions where is shortage of fresh water.The results showed that the VF of grass carp increased to its maximum when the external environment became isotonic with respect to the internal environment.When the peak was reached,the VF gradually decreased as the salinity concentration increased,which could be due to the fact that the fish tend to limit the contact of salty water to gill epithelium.It can be explained as that in the stressful conditions,the fish need more energy,but less contact with water in order to minimize losses of ions.The FR of fish was not influenced by the relatively low salinity at the beginning of the gradual salinity increase.However,the FR regressed as the salinity increases.FR decreased as salinity increase from 0 to 0.75 g·l-1.It suggests that the increase in salinity led to a decrease in food intake.It suggests that the metabolic energy required for digestion and assimilation of food may not be important in isosmotic environments.Most previous studies have shown that salinity affects the energetics of fish.Our results also showed that RMR of the grass carp tended to increase as salinity increase to 0.75 g·l-1,however,decrease as salinity increased further.It indicates that the fish give up energy consumption for osmoregulation when salinity is too high.There was almost no variation in MMR during this experiment,which was different from our expectations as acclimatization to salinity could be an osmo-respiratory compromise.The constant MMR in our results suggests that the gills permeability of the grass carp may not be changed by salinity,which allows the grass carp perform important physiological and behavior activities,e.g.swimming and digestion.It is a very useful tool to assess the energy capacity of a fish facing with environmental stress.In the present study,different from the RMR,both AS and FAS decreased to intermediate so-called isotonic concentrations and then elevated to extreme concentrations,which could be due to the higher RMR at the intermediate concentrations. |
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