“How old are you?” is not a straightforward question for sharks

By January 9, 2018

When we are asked the question “How old are you?” we can usually answer with a degree of accuracy. Things are somewhat more complicated for fish and very much more for sharks – and not just because they can’t literally tell us how old they are.

The ageing of fish is believed to originate from speculations made by the scientist Aristotle in 340 B.C. He noted, in the Historica Animalium, that fish age may be determined by examination of the size and hardness of fish scales. However, it was not until the invention of the lenses and the microscope in 1650s, that scientists were able to identify growths rings in the scales that were similar to those found in trees. By 1924, these rings, known as annuli, were found in otoliths (ear bones), vertebrae, fin rays, and other hard structures. It was also noted that there were variations in the amount of growth over the year with the majority of growth occurring during the summer months.

Sharks and their relatives, rays and chimeras, do not have a bony skeleton. Their skeleton is made of cartilage which is lighter and more flexible. Their otoliths, or staticonia, are similar in size to sand grains and their scales (known as dermal denticles) do not grow continuously. Some species have hard spines that contain rings suitable for ageing, but this is not the case for many elasmobranchs. Vertebrae are used to age the species without spines and this is only possible after the animal is dead.

A thin section of shark vertebrae under the microscope showing the banding pattern (NOAA Fisheries)

Today ageing fish and shark species is a vital part of fisheries management. Ages are important data that are used for assessing the health of a population of a species (also known as a stock). The age of maturity and growth rates may tell us how vulnerable a species is to being over harvested.

Before we can confidently use age estimates for a species, the rate of growth ring deposition must be validated. This is frequently achieved by injection of fluorescent dyes (e.g. oxytetracycline, calcein) into a tagged animal prior to release. The dye incorporates into the age structures so that when the animal is recaptured, the point in time when the animal was tagged will be visible. Thus the amount of growth deposited since the tagging event can be confirmed.

Another method of age validation uses the levels of the radio-isotope of carbon known as  C14 in the growth zones of the vertebrae to match it against references with known timelines such as  coral reef cores. This isotope is a direct result of nuclear testing during the 1950s and has hence earned the title “bomb carbon dating”. However, a recent review of both these methods has suggested that the underestimation of shark ages may be more widespread than previously thought. Growth bands on aged structures may actually only be reliable during the early years of life. This new evidence will be important to future ageing studies and the resultant impact to the life history and conservation status of sharks and rays.

Rock saw used to section otoliths and vertebrae

Thanks to the ongoing collaboration with Sea Leucas, a small start-up in the field of fish ageing, MarAlliance now has ready access to two age/production saws for taking sections of shark vertebrae and fish spines and otoliths. We have begun to process our collections and look forward to working with other researchers along the MesoAmerican Reef to age their animals.