Stock structure analysis of Megalaspis cordyla (Linnaeus, 1758) along the Indian coast based on truss network analysis

Populations of Megalaspis cordyla (horse mackerel) from four areas, two each from the east (Digha and Mandapam regions in the Bay of Bengal) and west (Cochin and Mumbai regions of the Arabian Sea) coasts of the Indian peninsula, were studied using body shape morphometrics. A truss box method ( Strauss and Bookstein, 1982) was followed, and 33 distance variables were extracted from digital images of sample specimens using the software platforms tpsDig2 and PAST. The transformed truss measurements were subjected to factor analysis and classification by cross-validation of discriminant analysis. Measurements from the anterior half of the fish body showed meaningful loadings on the first factor, and those from the caudal peduncle gave high loadings on the second factor. The combination of distance variables that produced the minimum amount of misclassification consisted of variables belonging to the middle portion of the body. Our results indicated a clear separation of the Bay of Bengal and Arabian Sea populations. The Mumbai and Cochin populations exhibited obvious mixing, indicating the possible existence of a unique stock along the west coast of India. In the cross-validation of the morphometrics by discriminant analysis, the most well-defined group was the Mandapam population, with only 3.59% of the individuals being misclassified, followed by Digha, indicating limited gene flow in the Bay of Bengal populations of the horse mackerel. The strong morphometric differentiation observed between the Mandapam and Digha populations, in addition to the considerable coral reef features of the Gulf of Mannar region, suggests the existence of separate spawning stock populations of horse mackerel in these regions, which might require distinct stock assessment programs to provide effective management strategies for the east coast.