Differentiation between Inshore and Offshore Spotted Dolphins in the Eastern Tropical Pacific Ocean

Abstract

Multivariate analyses were performed on 36 morphometric and meristic skull measurements taken on 613 spotted dolphins (Stenella attenuata) from the eastern tropical Pacific Ocean. Corrections were made to account for sexual dimorphism. Findings extend previous results which indicate sharp demarcation between two stocks: a larger, more robust coastal or inshore form; and a smaller, more terete pelagic or offshore stock. Sample means from 5° latitude-longitude blocks were projected onto principal component I and showed separation of four inshore from 19 offshore blocks, primarily on the basis of size involving most morphometric characters. A second component with high loadings on tooth counts summarized differences of three southern inshore blocks (adjacent to Central America) from a fourth, more northerly block located at the Gulf of California. Cluster analyses provided a similar summary of interblock associations. Principal components and cluster analysis based on individual specimens were also used to identify seven individuals with questionable affinity. The remaining 606 specimens (575 offshore, 31 inshore) were used to generate a discriminant axis and associated classification functions that correctly allocated all specimens. The best character for distinguishing between offshore and inshore specimens is Tooth Width. Classificatory information is provided to enable other researchers to assign unknown specimens.