By Stuart Dunlop, Conservation Manager on Cousine Island, Seychelles
Have you ever wondered how long fish live for? I don’t mean the goldfish you have swimming around in your tank at home. I’m referring to the fish you see when you scuba dive or snorkel. Whether it be a wrasse, parrotfish or even a shark, all fish have different live history strategies. Some grow slowly and live to an old age, while others grow very quickly and live much shorter lives. Long-lived species can take many years to reach maturity, which means many years until they become reproductive.
It is these attributes that determine whether a species is vulnerable to fishing or not. For example, a slow growing species that takes many years to reach maturity faces a greater risk than a species that matures early and begins producing offspring at a young age. Knowledge of fish growth is therefore necessary to develop management and conservation plans for exploited species.
Although size may be an indicator of age, this is not always the case. Two different species can have similar ages, but have very different sizes. A small seabream known as a blacktail Diplodus capensis can live for 21 years, but attains a maximum length of only 40 cm, while many sharks of a similar age would be considerably larger. Within one species, it is much easier to estimate age from length, but even then some individuals grow faster than others, similar to humans.
By now you might be wondering how does one then actually know how old a fish is or how long they can live for? For animals that are visible on land, age can easily be determined over time as they can be regularly encountered and monitored. However, when it comes to fish in the big blue, it is quite difficult to accurately age them as they are often only seen for as long as the air in your scuba cylinder lasts!
Methods of ageing fish
Fortunately, scientists have developed several methods to accurately find out the age of various fish species. All of these methods involve looking for structures which increase in size incrementally with age. The process is similar to calculating the age of a tree by counting the growth rings in a cross-section of the trunk. The most commonly used method in fish involves counting the growth rings on the otoliths or ear stones. These are are calcium carbonate structures that grow in the inner-ear of most vertebrates.
Tagging is another important method used by scientists to study fish growth. Tagging involves the insertion of a uniquely-coded plastic dart tag into the fish’s flank after capture. At this time, the length of the fish is recorded along with the date and place of capture. Tagged fish are then released back into the wild. If re-caught at a later stage, the length, date and place are recorded once again. In this way, scientists can track the movement of fish, but also their growth over time.
Tagging is therefore an important tool that can be used to verify the age of a fish after it has been calculated by counting growth rings. For example, the oldest galjoen Dichistius capensis (South Africa’s national fish), which has been aged using otoliths, was 21 years old. Tagging has shown that the longest time free for a galjoen was 14 years between tagging and recapture. Accounting for the age at tagging, this fish would have been at least 18 years old, thereby providing strong evidence that the original age estimates from otoliths were likely to be correct.
The world’s largest seabream, the red steenbras Petrus rupestris, was aged using sectioned otoliths, and estimated to be 33 years old. The longest time free for a tagged red steenbras was 22 years. Based on its estimated age at tagging (750 mm Fork Length = 10 years), this fish would have been 32 years old, again providing strong evidence that the original age estimates were in the right ball park.
Ageing sharks and rays
Sharks and rays don’t have otoliths. So, scientists have found other ways to determine their ages. The most common method involves looking at growth layers deposited in calcified parts of their vertebra or fin spines. The ragged-tooth shark Carcharias taurus, also known a sand tiger or grey nurse shark, is estimated to live to a maximum age of 32-38 years old. This age was verified by tagging data, when a mature female that was tagged in 1988 was re-caught in 2011, after 22.6 years at liberty. Considering it takes 9-10 years to reach maturity, this female would have been at least 31 years old.
Of all the fish that have been aged to date, the Greenland shark, which is a large cold-water species found in the North Atlantic and Arctic Oceans, is the most interesting. Its maximum recorded age has been estimated to be between 300 and 500 years old and it is considered to have one of the longest known lifespans of all vertebrate species found on earth! Do they possibly hold the key to longevity?
On your next underwater adventure, take a breath or two longer to appreciate the size and age of the various fish that you encounter. Who knows, you could be the youngster in the crowd!