For the first time, a scientist has been able to discern the age of a marlin by measuring radiocarbon levels stemming from nuclear bomb detonations in the mid-1950s.
The 1,245-pound, 12-foot blue marlin caught in waters near Honolulu in 2009 was a peculiar find because most large marlins are caught around Kona, said Allen Andrews, a fisheries research biologist with the National Oceanic and Atmospheric Administration Fisheries division.
And even if you’re lucky enough to catch such a huge fish, it’s a cumbersome feat to bring it ashore.
Marvin Bethune, Mike Hennessy and Nate Varnadoe caught this giant blue marlin 21 miles south of Honolulu.
“Not only was this unusually large specimen made available to science, but we were able to get the fish’s ear bones, which takes a lot of work to do,” said Andrews, who has validated the age, growth and longevity of marine organisms for more than 20 years.
Commonly known as “ear bones” or “ear stones,” otoliths are similar to the inner ear in humans and function as a balancing structure.
Like a tree’s rings, otoliths grow incrementally in layers and can help determine age in fish.
But otoliths are much harder to analyze than tree rings — the structures are deeply nested in the brain and are difficult to remove without breaking them, Andrews said.
Jeff Sampaga, a biological science technician with NOAA for 40 years, is particularly adept at removing otoliths, Andrews said.
The otoliths were collected before Andrews’ arrival at NOAA, but remained untouched until last January, when Andrews decided to try to date them as a going-away present for his boss.
The largest otolith found in the marlin weighed in at just 8 milligrams — less than half the weight of a grain of white rice, Andrews said. It was so tiny, he had to use tweezers to mount the otolith in resin and study it under a microscope.
Even though most nuclear bomb testing happened in the 1950s and 1960s, radioactive signals in the ocean peaked in the 1970s and 1980s.
The otoliths are pictured next to a dime for scale.
Otoliths can still be penetrated by radioactive signals floating in the water, he said. The signals are harmless to humans, but still leave what’s known as a radiocarbon signature in organisms such as sea turtles and corals with a skeletal structure conserved over time.
Based on his prior work analyzing radioactive signatures in coral, Andrews was able to figure out that the massive marlin was just 20 years old. Since few marlins grow larger than the one found, Andrews said 20 years is likely near the maximum age for the species.
His findings were recently published in the Canadian Journal of Fishers and Aquatic Sciences.
Next, Andrews said scientists can look at bigger and smaller fish to see whether his findings match up with markings in their otoliths.
“The tricky part is getting someone that wants to deal with the 60-pound (fish) head,” he said.
This otolith from an onaga fish is about the size of a fingernail. Its “growth zones” can be seen above.
Plenty Of Marlin In The Sea
The findings are good news for Hawaii’s fishing community, Andrews said. Because blue marlins grow large quickly, the species can recover from fishing faster.
NOAA’s website lists the Pacific blue marlin as being “above target population levels,” meaning there’s plenty of them in the sea. The fishing status of the blue marlin is at the “recommended level.”
“U.S. wild-caught Pacific blue marlin is a smart seafood choice because it is sustainably managed and responsibly harvested under U.S. regulations,” NOAA says on its site.
Blue marlin are not overfished, according to a 2013 report cited by NOAA, but it recommended fishing levels remain the same. Populations of the fish have declined since the 1970s, but have been stable and even slightly increased since the mid-2000s, according to the report.