Identification of fish stock based on otolith as a natural marker: The case of Katsuwonus pelamis (Linnaeus, 1758) in the Southwest Atlantic Ocean

Morphological, morphometric, and chemical analysis of otoliths have proven to be useful tools for intraspecific comparison and identification of fish stocks. In the present study, we used otolith silhouette and morphometric analysis (n = 103) in addition to chemical signatures (n = 20) to test the hypothesis of a single stock unit of skipjack tuna (Katsuwonus pelamis) in the Southwest Atlantic Ocean (20–34°S). No significant differences were found between sexes (t-test p > 0.05), enabling analyses for all study specimens to be grouped. Cluster analysis using Fourier coefficients (30 first harmonics) differentiated three otolith morphotypes within South (28–34°S) and Southeast (20–28°S) regions of the study area. Harmonic principal component analysis using the first two significant axes showed no pattern of spatial separation by region. Among the seven shape descriptors examined, only circularity and form factor showed significant differences between the morphotypes (A and B). Microchemistry analysis (LA/ICP-MS) applied to the core portion of fish otoliths showed significant differences in Ba, Mn and Mg concentrations between the two regions and morphotypes (p < 0.05). These results support that the Katsuwonus pelamis population in the Southwest Atlantic Ocean belong to a single stock unit with a probable common area of origin. Specimens captured in the South and Southeast regions of Brazil had heterogeneous phenotypic attributes regarding otolith shape, probably determined by a temporal equilibrium selection process.