A novel method for the estimation of age and growth in rajids using caudal thorns

Abstract

The caudal thorns of four bathyrajid species from the Falkland Islands exhibit well-defined surface ridges. The apparent periodicity of ridge deposition suggests that caudal thorns exhibiting this sculpture have potential for use as ageing devices. Caudal thorns and vertebrae were removed from selected specimens. Thorn surface band counts were compared with counts from longitudinal sections of vertebral centra. Structures were read three times by each of three readers in a double-blind study. Differences in precision were found between readers. Estimates made by the most experienced reader showed least variation. A consistent difference between thorn and centrum band counts was recorded in the males of two species, with centrum counts lower than thorn counts by one growth increment. These differences were attributed to difficulties in interpreting the centrum sections of these species. In all other groups, counts were highly correlated with no significant difference between group means. Caudal thorns represent a novel ageing structure that provides estimates with precision similar to that of more conventional vertebral ageing techniques. Centrum and thorn edge analysis, supported by evidence from a tetracycline-marked individual of a related species, confirms the nature of band formation in both structures. The use of thorns as ageing structures, where appropriate, has the potential to be of significant benefit to the resource assessment and management of skate and ray fisheries.