Age Validation Studies on the Spines of the Spurdog (Squalus Acanthias) Using Tetracycline

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Abstract
Elasmobranchs, unlike teleosts, do not possess otoliths or scales suitable for age determination. Various skeletal structures of elasmobranchs have been used in an attempt to find a reliable ageing technique. These include the centra of Raja fusca (Ishiyama, 1951). R. eglanteria (Daiber, 1960), R. erinacea (Richards, Merriman & Calhoun, 1963) and Cetorhinus maximus (Parker & Stott, 1965). Yokota (1951, 1952) has described annual growth increments on the claspers of various elasmobranch species. Stevens (1975) made vertebral ring counts in the blue shark, Prionace glauca L. More recently Jones & Geen (1977a,b), using X-ray spectrometric analysis, have examined the vertebral circuli of dogfish found in British Columbia waters. Methods of age determination, using the spines of the spurdog (Squalus acanthias L.) have been described by Kaganovskaia (1933, 1937), Bonham et al. (1949) and Aasen (1961). All these authors assumed that the more or less regular series of dark bands or ‘rings’, extending from a white pigment-free base of the enamel to near the tip, were annual. Holden & Meadows (1962), on the basis of the internal and external features of the spines, concluded that the dark bands did represent annual growth increments. In particular, they argued that the dark bands were laid down during the winter when growth would be slowest, whereas the faster growth in the summer was represented by the light bands. Holden & Meadows (1962) identified several conditions, e.g. birth, double, false and spawning rings – that made interpretation of the ring structure and subsequent age determination difficult.

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