It's sort of like counting the rings on a tree....except that we don't know how many rings a shark adds to its vertebrae every year. So, we use an injection of oxytetracycline (OTC), a chemical that will be absorbed by the tissue as it calcifies, to place a mark on the edges of the vertebrae of the sharks. The vertebrae of shark grow in diameter as they age and the calcifying tissue forms rings, kind of like rings in a tree. We don't know exactly how many rings are formed during each year, however. The band of OTC can be visualized as a fluorescent ring in the vertebrae that identifies the time of the injection.

Here's the catch (pardon the pun). The shark has to be caught for us to figure it out. (That happens, actually -- fishermen are pretty good at helping us out, because they want to know these answers, too.) So, if the shark is recaptured and the vertebrae returned to us, we can count the rings in their vertebrae that have formed since the time we injected the shark. That way we can determine how many rings represent each year's growth. Once we have determined how many rings are formed in each vertebra every year, we can then go back and determine the age of any other sharks for which we have vertebral samples. In order to understand the productivity of these species, it is important to understand how fast they grow, how long they live, and at what age they become sexually mature.