Biometric relationship between bone dimensions and body size are presented for non-native gibel carp and native black sea roach in Iznik Lake, Turkey. Regressions of the data were highly significant, with coefficients variation of the determination > 89% in most of cases. Non-linear and linear functions provided the best fit for black sea roach and gibel carp, respectively. The maximum values of relative errors of different structure’s measures for black sea roach were considerably higher than those of in gibel carp. Such information facilitates the assessment of the potential role of these fish species in the diet of piscivorous fauna.