Low salinity induced challenges in the hardy fish Heteropneustes fossilis; future prospective of aquaculture in near coastal zones

Abstract

The groundwater in coastal and abiding zones is experiencing alarmingly growing levels of salinity that might be small in absolute terms. Several reasons may contribute for retrogression of groundwater quality and the most concerning issue is they can critically distress the inhabitants. This notion encouraged us to comprehensively investigate the salinity tolerance range and effects of sub-lethal salinity on various attributes such as the growth pattern (morphometric and feed intake), haematology, histo-architecture and generation of oxidative stress in different tissues of Heteropneustes fossilis, a coastal zone hardy fish that has high market demand. The fish were exposed to sub-lethal salinity i.e. 3, 6 and 9 ppt for 45 days against a control set (tap water, ~0 ppt) under constantly monitored pH, temperature, dissolved oxygen and alkalinity. The fish can tolerate a narrow salinity range although correlation and discriminant function analyses among the computed parameters indicate that sub-lethal low salinity strongly modulates the fish physiology. Decline in the body weight, feed intake, specific growth rate and changes in morphometry coupled with histological alterations in different tissues were observed with the increase in level of sub-lethal salinity. The significant alterations in the measured parameters substantiated an overall decline in growth of the fish at 9 ppt. Significant loss in haemoglobin content, RBC and eosinophil counts accompanied by a rise in neutrophil count was also observed at higher salinities. The lipid peroxide level under higher salinities was comparatively higher than the control. Disruption in the typical histo-architecture in its muscle, gill and liver tissues was observed at higher salinities, possibly due to the generation of oxidative stress and/or osmotic imbalances in the respective tissues. In conclusion, the H. fossilis can satisfactorily grow in low salinities up to 6 ppt and critically beyond the range till 9 ppt. Therefore, the study of compromised physiology due to the toxic effects of altered salinity under changing climatic conditions and anthropogenic activities in the inhabitants especially in lower vertebrate hardy fish models is a major concern in aquaculture prospect. The development of this study on the transcriptional and translational level could provide access to unsolved problems related to aquaculture. Further it also suggests that a) it is indispensable to maintain optimal freshwater quality in coastal and allied zone globally in coming decades, b) introduction of saline tolerant hardy fish species and development of appropriate aquaculture practices based upon the altered physiology in the concerned regions.