Ageing of sharks

The conservation, management and rational utilisation of shark stocks are receiving increasing global attention. In a local context it is essential to have accurate information on the biology and life history of the various species being exploited in the shark nets to understand the impact of its netting program on these shark populations. Knowledge of shark age and growth is fundamental to these processes.

Unlike most other marine animals, sharks cannot be aged by counting growth rings in the teeth, scales or ear bones (otoliths). The vertebrae in the spinal column, however, can be used for this purpose. To this effect the KZNSB started collecting vertebral samples in 1985 and currently possesses over 2800 vertebrae from over 14 different species.

The principle of ageing sharks using their vertebrae resembles that of ageing trees, where growth rings in the trunk are counted. The visibility of the growth rings can be enhanced using several methods, which include various staining techniques or x-radiography.

An important aspect of shark ageing work is to determine how many growth rings are deposited per year in a shark's vertebrae. Therefore the KZNSB started to inject sharks caught alive in the nets with the widely used antibiotic, oxytetracycline (OTC). It is deposited in the vertebrae of the shark as a fluorescent marker. The number of growth rings deposited after the marker can be related to the time at liberty. The KZNSB has injected over 820 sharks, either caught in the nets or through opportunistic tagging of anglers' catches.

The only injected white shark ever to be recaptured (world-wide!) was tagged by an angler in 1994. It weighed 46 kg and measured 140 cm PCL*. After 2.5 years at liberty it was captured in the KZNSB nets at Amanzimtoti and weighed 150 kg and was 209 cm PCL*.

*PCL=Precaudal length, measured from the tip of the snout to the start of the tail.