Sharks were considered as top level predators in most marine ecosystems and had a key role because they controled the community structure and the ecosystem process. 700.000 to 850.000 tons of sharks were captured worldwide and this quantity is probably underestimated. Accidental capture in industrial fishing, on purse seine or on longliner, the total shark captured was badly known and underestimated because the carcass were discharged dead or alive after practicing shark fining. In additional, landings were not mentioned on logbooks and the number of observatory program was very limited. These situations may distort stock evaluation and conduct to a lack of accurate biological data and reliable statistics. The blue and the silky shark, taken into account in this study are the main accidental bycatch of the industrial fisheries. Sharks in general, supported badly the overexploitation and the knowledge on their biology, their ecology, and the demographic structures were cruelly far away from the knowledge which deserves such indispensable functional group in the oceanic ecosystems. This research consists of determining the various preys and the evolution of the diet of these two species over its lifetime through the analysis of the stable isotopes 13C and 15N in tissues and vertebras. The life history of these populations will be determined (growth-age relation, size at 1st sexual maturity, fertility and number of embryos). Then, intra and interspecific comparison of the life history traits and the feeding ecology was realized. Associated with a demographic analysis of the populations, they will allow estimating the current abundance of these two species.