Stock assessment of any fish species requires estimates of growth rates, maximum age, cohort structure and age of maturity, all of which rely on accurate estimates of age. Typically in bony fishes, structures such as scales, fin rays or otoliths (ear bones) can be used in the determination of age. Each year, calcium is deposited in these bony structures in a systematic manner allowing for the counting of annulli. However, elasmobranchs have cartilaginous skeletons and therefore lack bony structures with systematic calcium depositing. As a result, for many elasmobranch species there is no method for estimating age, and the important life history parameters can not be estimated. The lack of age and growth information has been a limiting factor in the development of shark and skate management plans.
The exception is the spiny dogfish (Squalus suckleyi) in which the second dorsal spine has been used for age determination since the 1930s. Systematic calcium deposits in the enamel covering the dorsal spine form visible bands or ridges. This method of estimating the age of dogfish has been extensively studied by many researchers worldwide, and has been validated using OTC-injected (oxytetracycline) fish which were tagged, released and later recaptured up to 20 years later (1,2). The OTC forms a mark on the spine which is visible under ultraviolet light; knowing the date of OTC injection and the date of recapture allows researchers to determine the expected number of annuli that should have formed on the spine subsequent to the OTC mark.
Deposits of calcium phosphate have been found in many components of the cartilaginous skeleton of sharks and skates. In some species, systematic calcium deposits in various vertebral components are being used to estimate age. Vertebral centra are being used to estimate age for a number of Atlantic shark and skate species on the Atlantic coast, and the method has been validated for a few species (see the Canadian Atlantic Shark Research Laboratory for more information).
Big skate (Raja binoculata) and longnose skate (Raja rhina) have been aged successfully using vertebral centra (3). Thin sections of the centra were examined and found to exhibit banding thought to represent annuli.
For some sharks, the vertebral centra are too poorly calcified to provide age information. In British Columbia, the sixgill shark (Hexanchus griseus) is an important bycatch species in commercial fisheries. However, it is a deep-water species with poorly calcified vertebral centra. However, another component of the vertebrae, the neural arch (right), exhibits systematic banding consistent with expected annuli, and can therefore be used to estimate age (4).
We are currently investigating the applicability of vertebral centra and neural arches to age determination in other Pacific elasmobranch species. Blue sharks tagged in 2007 were injected with OTC, so that any recaptures may be used to develop a validated aging method for this species.