"The life span of sharks can be determined by keeping them in captivity, by tagging, or, more commonly, by counting growth rings in their vertebrae or spines and estimating the age. Only a few species of sharks have been kept in captivity for long periods of time. When sharks have been captured young and have lived in captivity for prolonged periods, a rough idea of their potential life span can be obtained. Although keeping sharks in captivity for many years is a difficult feat, some sharks have lived for decades in captivity, indicating that they have long life spans. Unfortunately, most aquaria do not keep accurate records of the life span of their captive sharks, and valuable data on the longevity of sharks is lost."