How Nuclear Bombs Helped Scientists Find the Age of this Gigantic Marlin Fish

Subsurface image of a blue marlin pictured with the mushroom cloud of nuclear test "Mike" during Operation Ivy in 1952. This was a 10 Mt nuclear test that provided part of the change in radiocarbon that was recorded in the otolith of the grander blue marlin. The inset figure (upper left) shows how radiocarbon changed in the waters of the Hawaiian Archipelago over the last 60 years—shown are coral records ranging from Kona, Hawaii Island to Kure Atoll. The alignment of radiocarbon from the earliest otolith growth with the declining side of the coral records led to a birth year of 1989 and an age of 20 years. Courtesy of Allen Andrews, NOAA

Off the coast of Hawaii in 2009, two fishermen caught an enormous grander blue marlin weighing in at 1,245 lbs and measuring 12.2 feet long. Conservationists wondered, how long does it take for these animals to reach this size? It turned out that the answer lies in tiny bones in the marlin's ears, according to a new report.

Although the animal was huge, certain ear bones (called "otoliths") are, in this animal, each the size of half a grain of rice. What's special about otoliths, as opposed to other types of bones, is that the cells in otoliths don't get slowly re-absorbed and replaced. Instead, the inside of the otolith is the same as it was the day the fish was born, and it slowly grows more bone around it, like the rings of a tree.

ograph of the grander blue marlin (Makaira nigricans) at time of capture 21 miles south of Honolulu, Hawaiʻi on September 1, 2009. The fisherman Marvin Bethune, Captain Mike Hennessy, and crew member Nate Varnadoe of the F/V Maggie Joe out of Honolulu landed the 1,245 pound fish and provided a rare opportunity to conduct specialized life history research. NOAA Fisheries

"This ear bone is actually more like a stone," Allen Andrews, a research fisheries biologist at the National Oceanic and Atmospheric Administration (NOAA) told Newsweek. "It grows like a mineral. As the fish swims around in its environment from when it's born to when it's caught, it actually records a history of its environment."

Jeff Sampaga, a biological science technician with NOAA fisheries, removed the otoliths from the enormous marlin head. Then Andrews analyzed them and found radiocarbon signatures in certain layers of the otolith. He knew that in the 1950s and 1960s, nuclear bomb testing took place all over the world and left an impact on the ocean, and that he could use that impact as a marker of time.

"It's this long slow rise," Andrews said of the mark left on marine habitats by the testing. "It takes about 10 years to reach the ocean." He explained that the radioactivity went up into the atmosphere, circulated around the planet, and then returned to the ocean, leaving in fish radiocarbon signatures that slowly decrease in intensity over time.

By examining the otoliths and understanding the amount of radioactivity into which the fish was born, Andrews found that this individual marlin was only 20 years old when it died. Marlins may live up to 20 or 30. Although some larger marlins have been recorded, this one appears to have achieved its adult size, meaning that marlins must grow rapidly and achieve immense sizes in just 20 short years.

The small ear stones (otoliths) used to age the fish with the edge of a U.S. dime for scale. NOAA Fisheries / Allen Andrews

Andrews recently published his research on this marlin and the technique he used to age it in the Canadian Journal of Fisheries and Aquatic Sciences. This study is the first time that a bomb has been used to confirm the age of a marlin, he says, and it's good news for fisheries. The fact that marlins grow so fast means that a population of marlins can replace their largest members more quickly than we had thought because they can grow to such immense sizes in two decades.