Long‐whiskered catfish, Sperata aor (Hamilton 1822), is commercially important in food, ornamental and sport fisheries. The fish is mainly caught from the wild populations because its aquaculture practices are not commercialised. Inland fishery in the Ganga basin is mostly unorganised; hence, no published report is available on the trend of S. aor production from the selected habitat. In India, S. aor has been categorised vulnerable mainly due to natural and anthropogenic threats. Otolith chemistry shows variation with changing physico‐chemical conditions of the fish habitat. Therefore, the present study was conducted with the objective to analyse spatio‐temporal variations in water chemistry in relation to environmental factors; relationship between water and otolith chemistry; and spatio‐temporal variations in otolith chemistry to discriminate the stocks of S. aor inhabiting the River Ganga. Most of the element: Ca ratios in water samples did not show significant correlations with environmental factors, viz. temperature and conductivity. Only few element: Ca concentrations in otoliths were positively correlated to their corresponding ratios in the ambient water. In the selected study area, the S. aor populations were discriminated into four stocks possibly because of heterogeneous water chemistry at the sampling sites, and physical barriers. In the present study, otolith chemistry showed relatively low temporal variability as compared to spatial variability; thus, the classification accuracy of individuals to their original populations remained consistent over the selected time period. The findings could be useful in devising scientifically sound management strategies and/or any conservation plans for the vulnerable S. aor populations inhabiting the River Ganga.

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.

High genetic variation is an important requirement for long term survival of wild populations through adaptations to changing environmental conditions. High levels of variability and the potential to isolate large number of loci make microsatellites the marker of choice to study intraspecific genetic variation in fish. Seventy Sperata aor samples each from four sites along the river Ganga were successfully cross amplified at six loci. All the selected loci were highly polymorphic showing a slight heterozygosity excess. The mean observed and expected heterozygosities across all investigated populations were 0.971 and 0.913, respectively. F ST statistics, Bayesian model-based clustering analysis and analysis of molecular variance (AMOVA) revealed three different genetic stocks of S. aor (Narora-Kanpur, Varanasi and Bhagalpur), showing moderate genetic differentiation among them (total F ST = 0.069). Genetic differentiation was significantly correlated with the distance among stocks. The populations of S. aor are currently stable in the river Ganga as evident from high levels of genetic variability and no inbreeding. However, a precautionary approach to fishery management and conservation of S. aor should be undertaken at priority in view of the anthropogenic as well as natural threats to the fishes of river Ganga.

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Long-term isolation of populations and interbreeding can lead to morphometric variations among fish populations. This study was conducted with the objective to delineate stock structure of Sperata aor on the basis of morphometric characters, using truss network of the fish body. In total, 407 S. aor samples were collected from the four sampling locations of the Ganga River, viz. Narora, Kanpur, Varanasi and Bhagalpur. In total, 28 truss distance measurements were created by joining 13 morphometric landmarks on the fish body. MANCOVA showed significant (P<0.001) morphological variance among the sampling locations for the target fish species. Univariate ANOVA showed significant (P<0.001) differences in each of the morphometric measurements among the fish from different sampling locations. Wilk’s lambda test of canonical discriminant function analysis showed significant (P<0.001) differences in morphometric measurements of the fish from all sampling locations. Discriminant function analysis using Jackknife (leave-one-out) cross-validation classification showed 87.5% correct classification of the individuals into their original populations. Mantel test showed an overall good correlation between the genetic and morphometric datasets in the selected fish species. The results of this study can be employed in formulating stock-specific management strategies for S. aor from River Ganga.