Nuclear Testing During the Cold War Reveals True Whale Shark Age
Radiocarbon dating from cold-war era nuclear bomb testing has been used to positively identify the ages and growth rate of whale shark specimens, according to a new paper published in the online journal Frontiers in Marine Science.
Carbon-14 (C14) is a naturally occurring radioactive isotope of carbon which combines with oxygen in the atmosphere to create a radioactive form of carbon dioxide (CO2). Like regular CO2, it dissolves into the ocean, is absorbed by plants during photosynthesis and transferred to animals which consume the plants. Once the plant or animal dies and can no longer absorb radioactive CO2, the amount of C14 in the tissue undergoes radioactive decay, reducing levels of the isotope over time.
Radiocarbon dating measures the amount of carbon-14 present in the tissues. Because we know the rate at which C14 decays – its 'half-life' – scientists can accurately date a tissue sample based on the amount of C14 left within it – the less C14 present, the older the sample. During the 1950s and '60s, the testing of nuclear armaments was so prolific that it almost doubled the amount of C14 in the atmosphere. Measurements of atmospheric radiation taken throughout the years of testing can thus be used to add an extra element of accuracy into radiocarbon dating models.
The age of a fish can be determined from its skeleton due to the presence of banding within the structure of its bones – similar to tree-rings in which a new layer of material is formed each successive year during periods of growth. In sharks, which are elasmobranchs and have bones made of pliable cartilage rather than the rigid structures of most other species of fish, the banding is measured in the animal's vertebrae, the sections comprising its backbone.
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The use of bomb radiocarbon dating to age sharks is not new, with a 2017 report suggesting that the age range of many shark species has been vastly underestimated. It is known that some sharks are very long-lived - the Greenland shark, for example, has been recorded at 392 years of age, with a 120-year margin of error. Whale sharks and other pelagic nomads, however, present a particular problem for scientists in that so little is known about their behaviour once they head out into the open ocean that tagging and tracing a particular shark for a long enough period of time to determine their growth rate is virtually impossible.
The new research reports that there has only been one prior study attempted to 'validate an ageing method for whale sharks', which involved a juvenile whale shark being held in captivity in an aquarium. The animal was fed with oxytetracycline (OTC) which 'dyes' the banding deposited as the shark's vertebrae grow. According to the report, 'when the animal died two years later, two growth bands were observed following the OTC marker.'
For the new study, the team of scientists analysed 20 samples of whale shark vertebrae, nineteen of which had been preserved from sharks which had been taken by a Taiwanese fishery between 2001-2006, and one from a dead whale shark that had been discovered trapped in abandoned fishing gear in Pakistan. Two of the vertebrae were further divided into samples to be utilised for bomb radiocarbon age validation, one from the 10m long female which was found in Pakistan in 2012 and a 9.9m male landed in Taiwan in 2005.
Banding in the sampled vertebrae suggested that the sharks were, respectively, 50 and 35 years of age at the time of death, a measurement validated by the bomb radiocarbon dating. The results indicate that banding present in shark vertebrae are an accurate method of determining a whale shark's true age, and confirm that whale sharks are a long-lived species. Fully-grown sharks have been observed at over 18m in length, and so a 50-year-old animal measuring just 10m in length indicates that it is many years away from reaching the end of its natural lifespan.
The results are important for whale shark conservation. Although much research into the species has been undertaken in recent years, and they have become a popular tourist attraction, whale sharks are still in decline worldwide, and have been classed as 'endangered' on the IUCN Red List. Species longevity has a notable effect on conservation status, as the length of time it takes to reach sexual maturity and subsequently reproduce can have a devastating impact if the animals are unnecessarily culled.
As the report's conclusion states: 'Our estimates of slower growth and greater observed longevity have important implications for conservation of whale sharks. Underestimation of longevity and overestimation of growth is a serious concern for management strategies for fisheries, because it has led to population crashes due to overharvesting. We are hopeful that the demographic data we have provided in this study will help to improve the accuracy of population models and hence, better inform management and conservation efforts for this iconic species.'
The full paper 'Annual Bands in Vertebrae Validated by Bomb Radiocarbon Assays Provide Estimates of Age and Growth of Whale Sharks' is published in Frontiers in Marine Science under a Creative Commons license and can be found at www.frontiersin.org/articles/10.3389/fmars.2020.00188/full