Morphological discrimination between the dorsal-fin spines of the blue marlin Makaira nigricans: Reducing biases in age and growth estimations

A first step to reduce biases in the age and growth estimations of fish species is to consistently collect and analyze the particular calcified structure that is most useful for the species. For the female blue marlin Makaira nigricans, the fourth dorsal-fin spine was collected from sport fleet landings at Cabo San Lucas, Baja California Sur, Mexico. Preliminary analysis indicated that, during collections in the field, the fifth spine was sometimes inadvertently collected, making it necessary to corroborate the anatomical position of each structure before performing aging studies. In this study, we searched for a general pattern of morphological differences to discriminate between the fourth and fifth dorsal-fin spines. The morphology of these structures was characterized based on coordinates and linear measurements between homologous landmarks located in the basal zone of the spines and the measurements were analyzed using three different multivariate statistical methods. A sharp morphological distinction was identified between the fourth and fifth spine for each of the three methods which were in agreement for 98% of classification cases. The distances measured in the upper (D2) and lower (Dl) sides of the basal zone and their ratio (D2/D1) can be used as discriminating morphological characteristics for both spines. A D2/D1 ratio of less than 1.47 corresponds to the fourth dorsal-fin spine. This characterization will be useful for ensuring that only the correct structure is used in age and growth studies of the blue marlin.