Temporal changes in the body shape of the Pacific thread herring Opisthonema libertate on the Western Baja California Sur coast

Abstract

Over the past few decades, analyzing body shape changes and their application to population dynamics has allowed the quantitative and qualitative evaluation of morphological changes associated with phenotypic expression. However, the physiological conditions of organisms need to be considered in the analyses of body shape variation. We address changes in the body shape of the Pacific thread herring Opisthonema libertate (Günther, 1867) on the western coast of Baja California Sur during an annual cycle using landmark data and geometric morphometric methods. Pacific thread herrings were collected from commercial landings of small pelagic fish in Bahía Magdalena. Morphometric analyses were based on the multivariate comparison of 22 reference points that characterize fish body shape. All comparisons in the multivariate analysis of variance revealed significant differences between the monthly means (a priori groups), as indicated by the ten significant CV axes. Furthermore, the pattern of body shape variation was consistent with reproductive changes in Pacific thread herring. The most helpful shape variation for distinguishing among the groups was in the dorsal and ventral profiles of the truncal region of the body shape. These results support the hypothesis that fish physiological condition is a temporal factor that helps us discriminate intra-population units based on body shape variation and is a potential bias in inter-population comparisons.