Fish scale identification: From individual to species-specific shape variability

Abstract

Background. Species identification from recovered fish scales, e.g., from the sediment or stomach contents, has proven to be problematic for clupeid species. To ensure reliable species discrimination within Clupeidae, we attempted to reveal inter- and intra-specific, as well as, the intra-individual variability of scale shape in two sympatric clupeid species the European pilchard, Sardina pilchardus (Walbaum, 1792) and the round sardinella, Sardinella aurita Valenciennes, 1847. Our aim was to test, whether the landmark-based geometric morphometric method is reliable for species identification from mixed samples. Materials and methods. The specimens of European pilchard and round sardinella were collected between May and October 2014 from the Gulf of Ambracia, Greece. The scale samples were taken from ten body areas (marked by letters A–J) of 487 specimens in total. A multiple-step analysis was conducted on randomly selected blind samples. The differences between body areas were assessed with canonical variate analysis (CVA) and discriminant function analysis (DFA) to compute generalized Mahalanobis distances (D) and discriminant functions (T2), respectively. Results. The CVA based on the total scale samples showed that in sardinella, all body areas differed significantly from each other (D = 2.54 ± 0.79, P < 0.001) except for one (area I vs. J: D = 22.36, P = 0.085). In pilchard, two body area pairs represented non-significant group pairs (area D vs. G: D = 1.03, P = 0.12; and area F vs. G: D = 1.21, P = 0.06), whereas all other areas were significantly different (D = 2.43 ± 0.88, P = 0.003). At the last step of the blind sampling procedure, all scales were correctly classified by species with an average discrimination rate as high as 96.3%. Conclusion. In contrast to scale identification based merely on morphology, the geometric morphometric method is capable of detecting differences in scale shape from mixed samples that are unattainable with the former approach. Therefore, a more reliable and effective species identification can be accomplished from recovered scales, especially when discriminating species within the same family or genus.