Scientists are determining the lifespan of the world’s largest fish with the help of nuclear weapons tests from the 1950s and 60s.
While researchers have made a lot of progress understanding the behavioural patterns of the endangered whale shark, there are some glaring holes when it comes to basic details about this species, including how long individuals can live. They’ve now gotten some help from a surprising source: radioactive carbon that humans injected into the global carbon cycle during weapons testing, which shows up in the vertebrae of whale sharks. The team used this “bomb radiocarbon” in order to age whale shark samples.
“The use of bomb radiocarbon assays as age validation for long-lived fishes has been increasingly applied to both fishes and sharks, and we have now applied this, for the first time, to the vertebrae of whale sharks,” Joyce Ong, the study’s first author from Rutgers University, told Gizmodo in an email.
The researchers acquired whale shark vertebrae samples used in a previous study. These samples were taken from a Taiwanese whale shark fishery before it closed in 2007, as well as from a dead stranded whale shark in Pakistan. First, they photographed the vertebrae under a microscope and counted the number of bands in a cross section of bone, kind of like counting tree rings. But this method alone can’t calculate the age of the sharks, since it’s unclear how quickly the rings grow.
That’s where the radioactive bomb carbon comes in. Countries, including the United States and the USSR, detonated nuclear bombs in the atmosphere during the 1950s and 60s, leading to a spike in the radioactive isotope of carbon called carbon-14 in the global carbon cycle. This carbon deposits itself into the food chain and can show up in animal tissues. Scientists can then use it as a sort of time stamp.
The researchers analysed samples from two of the whale sharks’ vertebrae using a technique called accelerator mass spectrometry (AMS) to calculate the abundance of the isotope carbon-14. AMS essentially involves taking bits of a sample and accelerating them in a particle accelerator to isolate and calculate its carbon-14 composition. They also need to compare it to the composition of something they knew the age of—they chose carbon-14 dissolved in seawater.
The analysis confirmed that one of the Taiwanese sharks’ three innermost growth bands formed in 1972. Meanwhile, the bands in the shark from Pakistan seemed to correspond with one-year increments in the bomb carbon analysis, allowing the researchers to date the shark at 50 years old—the oldest whale shark on record. They published their results in the journal Frontiers in Marine Science.
“If you know the age and size of a fish, you can calculate a growth rate. This is a critical parameter for management, because it tells you how resilient a species is to harvest and how fast it can recover if threats such as overfishing reduce population sizes,” Mark Meekan, study author and principle research scientist in fish biology at the Australian Institute of Marine Science, told Gizmodo in an email. “For whale sharks, it looks like they grow very slowly to these large sizes. Sharks may not become mature until they are around 30 years old. This means that whale shark populations are unlikely to bounce back quickly if numbers are reduced.”
Bomb carbon ageing has its limitations. It works best in long-lived species whose bodies can actually record the environment’s changing carbon-14 levels. Additionally, the mixing of carbon-14 into deep waters can complicate the analysis, Meekan explained. Thankfully, whale sharks spend a lot of their time near the ocean surface.
Weapons testing was shameful, and its legacy remains in the environment—but at least some good has come out of it.