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Otoliths are calcified structures and the information contained within their chemistry or shape can be used to infer life history events, migration patterns, and stock structure of a fish population. Understanding how otolith chemistry is affected by temperature, salinity, interactive effects of abiotic factors, ontogeny, physiology, etc. is essential for the reconstruction of the environment that affected the fish. Otolith shape is also affected by environmental conditions in addition to the genotype. The applications of otolith chemistry and shape for stock discrimination have increased in recent years because of the advancements in analytical methods and the related software. The stock identification methods sometimes provide variable results but if we use complementary approach the information generated could be more reliable which can be used to prepare effective management and conservation strategies. It appears warranted to generate more information on the factors influencing otolith chemistry and shape especially when two or more factors exert synergetic influence. Therefore, the objectives of this review paper were to provide comprehensive information on various factors influencing the otolith chemistry and shape, and the utility of otolith chemistry and shape for fish stock discrimination with an emphasis towards the research areas needing additional studies.

Anadromous fishes like hilsa shad are found near the delta region of Ganges River which are exotic and carry economic potential. The river offers a tidal environment connecting with marine and freshwater ecosystem providing nutrient-rich seasonal water for their growth and development. The complex migratory pattern of these fishes is a major concern in fisheries research. The present study is based on a large collection of isotopic data from otolith specimens of varying sizes covering population of varying age. Here we examined their stable isotopic signature and relate it with their age and locations in the stream length. Otolith δ18O showed significant relationships with fish sizes (length and weight) and distance from the river mouth and can provide a new tool to distinguish their habitats. Small size individuals show affinity for freshwater, while bigger individuals originates from Bay area. The ANOVA and discriminant function analysis (DFA) is used to check whether the fish inhabiting upstream and downstream can be categorized by their δ18O values and to define the composition of the end member. Further, a numerical model of two component mixing is introduced to prescribe the integrated time interval, equated with their size to define specific habitat. The significant variations in otolith δ18O values are found between upstream and downstream of the Hooghly River, and the DFA showed the overall 81% of individuals were assigned to the upstream or downstream of the Diamond Harbour, while the remaining individuals of 1–2 years age represent mobile population, probably participating in the successful spawning and feeding activities. This method allowed understanding the migration tendency of adult individual (fraction of time spend in the freshwater) and clearly identified the habitat based on isotopic ratios in otolith specimens. This study will serve as template for understanding the aquaculture potential and scientific management policies for hilsa shad in the West Bengal region.

Heavy metals have an immense impact on aquatic ecosystems, and their toxic effects are transferred to the inhabiting organisms. Experiments were conducted to investigate the health of snow trout Schizothorax esocinus inhabiting Dal Lake. Heavy metals (Cd > Ni > Cu > Cr) were found to accumulate in the major immune organs of the fish (head kidney, liver, spleen, thymus) which led to change in the overall physiology. The head kidney, liver, and spleen of a fish contain high amount of these metals. The least accumulation of these metals was found in the blood, whereas Cd and Ni were completely absent in the integument. Hepatic marker enzymes (aspartate aminotransferase (AST), alanine aminotransferase (ALT), and alkaline phosphatase (ALP)) were normal, while the renal marker enzyme creatine kinase showed marked difference in its value. The cortisol level was normal, while immunoglobulin M showed elevated level representing active immunity. At a cellular level, the histopathology of immune organs showed marked damage. Metallothionein ( MT ) and glutathione peroxidase ( GPX ) genes showed variable expression pattern in the immune organs with the head kidney showing the highest expression of both the genes, and blood showed the least. We observed that the aquatic plants ( Nelumbo nucifera and Trapa natans ) inhabiting the lake played an important role in phytoremediation. An integrated approach involving biochemical, hematological, genotoxic, and histopathological studies can provide a valuable information to understand fish adaptive patterns and monitor water quality. Graphical Abstract

Fish age estimation is essential for stock assessment, population dynamics, and fisheries management. The presently reported study was conducted to evaluate the relation between otolith weight and observed age in Labeo bata (Hamilton, 1822) inhabiting the Ganga River. A total of 187 L. bata specimens were collected from Narora site of the Ganga River. Linear regression analysis was applied between otolith weight and observed age to predict the age of fish from the otolith weight. The relation between otolith weight and observed age was significant for L. bata. The otolith weight increased with the fish age. Discriminant function analysis revealed 96.3% of the predicted ages were correctly classified to their observed ages. Thus, it can be inferred that the relation between otolith weight and fish age can provide an additional method of age estimation in L. bata.

This article reports enhancement in the specific capacitance of Mg-doped tin oxide nanoparticles (NPs) synthesized through a simple and low-cost solvothermal method. The synthesized NPs were investigated through cyclic voltammetry, galvanostatic charge/discharge and electrostatic impedance spectroscopy studies. The specific capacitance, as determined through cyclic voltammetry and galvanostatic charge/discharge characteristics, respectively, was found to be 523 and 55 F g −1 for undoped samples. Whereas it exhibited an increase with Mg doping and was found to be 981 and 134 F g −1 , respectively. Also, according to the electrochemical impedance spectroscopy, the enhancement of ion diffusion was found to be the major cause for enhancement in capacitive performance of Mg-doped NPs.