Guessing a fish’s age may not be as easy as scientists once thought.
That’s according to a new study by a researcher at James Cook University in Australia who found that in as much as one-third of previous studies involving sharks and rays, evidence was found of consistent age underestimation, with results being off the mark by between five and 34 years. The mistake could have repercussions not just for growth modelling of these animals but for the fishing industry, as gauging a fish’s age is important to determining stock productivity.
While there are a number of methods used to tell a fish’s age, the typical approach for sharks and rays (chondrichthyan fishes) involves counting the growth rings on thin sections of the animal’s vertebrae or spine. But research is showing that this method can lead to bad guesses, as after a certain age, these growth rings either cease to form or are difficult to accurately distinguish, meaning that what looks like a 30-year-old fish could really be 45, a big difference, says the new study’s author, Dr. Alastair Varley Harry of the Centre for Sustainable Tropical Fisheries and Aquaculture at James Cook University in Townsville, Australia.
“Questions arose over methods of ageing sharks after it was found that grey nurse sharks can live up to 40-years-old, double the length of time first thought, and the age of New Zealand porbeagle sharks had been underestimated by an average of 22 years,” says Dr. Harry, whose research is now published in the journal Fish and Fisheries.
More accurate than calcium ring counting, however, is bomb carbon dating, which has been in use since the early 2000s. As a result of nuclear testing in the late 1950s, the oceans leave a recognizable signature of radioactive carbon isotopes which is identifiable in the calcified tissues of fish. Looking at 59 previous age validation studies involving sharks or rays, Dr. Harry found that age was underestimated in about 30 per cent of the populations studied, with an average underestimation of 18 years.
“These characteristics suggest age underestimation is likely a systemic issue associated with the current methods and structures used for ageing,” writes Dr. Harry. “The outcomes of this study highlight the ongoing difficulties of ageing sharks and rays using calcified structures, particularly the validation of growth zones in older individuals.”
The new revelations could impact more than just taxonomic and biological descriptions of sharks and rays. It is known that shorter-lived species are typically more productive within their more brief life spans, thus, being longer-lived could mean that sharks and rays —already under threat— are more vulnerable to over-fishing. “The much greater longevity found for many species may indicate they have a lower natural mortality and in turn lower productivity and resilience to fishing than currently thought,” says Dr. Harry in a press release.
Currently, about one-quarter of shark and ray species are threatened by overfishing. Shark research is thin when it comes to characterizing their lifespan. As a wide average, most sharks are thought to live for between ten and 20 years, but others like the tiger shark could potentially live to 40 years or older, according to some accounts, while the great white shark’s lifespan is still anyone’s guess — some claim the species can live to up to 100 years.
Even knowledge of habitats for these aquatic species can still be thin. Blue sharks have been found in the Gulf of St. Lawrence, for example, but scientists are unclear on population estimates in the region. Video of a blue shark recently shot off the north shore of Prince Edward Island recently caused a stir among the public, but local fishermen say that while blue sharks are not an uncommon sight, they are more of a nuisance for the tuna fishing industry than a threat to humans.