A modified decalcification technique for enhancing growth bands in deep-coned vertebrae of elasmobranchs

Abstract

A technique for enhancing growth bands on vertebrae was assayed using 477 centra from blackmouth catsharks, Galeus melastomus. Specimens were caught at depths between 200 and 800 meters off the southern Portuguese coast (Algarve) during an IPIMAR research cruise in August 1994. Total lengths ranged from 12 to 71 cm. Standard vertebral ageing techniques, such as silver nitrate impregnation, haematoxylin staining, grinding and shading failed to produce clearly contrasting bands. A method solely based on vertebra decalcification with nitric acid (5%) yielded the most satisfactory results. This technique provided clear band differentiation and was used by two independent readers. The analysis of differences in band counts between readers was based on: percent of disagreements, percent of disagreement by length group, index of average percent error and Wilcoxon's signed ranks test. The majority of differences corresponded to only one band of disagreement and disagreements have been found to increase with length, which stresses the need for a larger sampling effort on the larger and older individuals. The index of average percent error was estimated as 4.6% and no statistical differences were detected between readers' results.