SOURCES OF VARIATION IN COUNTS OF GROWTH INCREMENTS IN VERTEBRAE FROMGUMMY SHARK, MUSTELUS-ANTARCTICUS, AND SCHOOL SHARK, GALEORHINUS-GALEUS - IMPLICATIONS FOR AGE-DETERMINATION

Sources of variation in counts of vertebral growth increments from gum my shark, Mustelus antarcticus, and school shark, Galeorhinus galeus, were examined in vertebrae taken from the cervical, thoracic, and prec audal regions of the vertebral columns. Vertebrae from school shark we re more difficult to read than those from gummy shark. The number of i ncrements an experienced reader counted on the external surface of ali zarin-stained whole vertebral centra was consistent with the number of major hypermineralized increments counted on microradiographs of vert ebral sections. Differences between increment counts obtained by four different readers were significant. Increment counts by experienced re aders were more precise and less biased between repeated readings. Inc rement counts from vertebrae sampled in the same region of the vertebr al columns were not significantly different. Increment counts from ver tebrae sampled in different regions were significantly different and w ere highest for vertebrae from the thoracic region. Differences in inc rement counts between regions of the vertebral column may distort von Bertalanffy growth curves and therefore affect the reliability of fish eries stock assessments based on age-structured population models.