Sea Ice

Narwhal Tusks Indicate Changing Mercury Levels in Arctic Waters

Source: Dietz, Rune, Jean-Pierre Desforges, Frank F. Rigét, Aurore Aubail, Eva Garde, Per Ambus, Robert Drimmie, Mads Peter Heide-Jørgensen, and Christian Sonne. “Analysis of narwhal tusks reveals lifelong feeding ecology and mercury exposure.” Current Biology 31, no. 9, 2012–2019. DOI: https://doi.org/10.1016/j.cub.2021.02.018 

Swimming amongst the sea ice in the Canadian Arctic are real life sea unicorns called narwhals. But instead of a horn, their long, spiraling tusk is a canine tooth that protrudes from the upper left of the male narwhal’s mouth (females usually lack tusks). For the first time, scientists have used this elongated tooth to analyze mercury levels in the animal’s diet and to understand how their diet is evolving in response to climate change. Their results were published in Current Biology in March. 

Narwhals gliding through Arctic waters. (Image credit: By AE Brehm – http://www.gutenberg.org/files/24008/24008-h/24008-h.htm Public Domain, https://commons.wikimedia.org/w/index.php?curid=27474156)

Unicorns of the Sea 

First described by Carl Linnaeus in 1758, narwhals live exclusively in the frigid waters of Baffin Bay, west of Greenland. Their name is derived from the Norse words for corpse (nar) and whale (hval), referring to the whale’s sickly grey color that resembles that of a drowned sailor. One of the deepest diving marine mammals, narwhals plunge to depths of up to 1,800 meters and spend much of their time below 800 meters. They depend on sea ice for breeding grounds, food, and shelter from predators like killer whales. 

Narwhal tusks, which can be more than 8 feet long, grow like trees—each year a new layer forms and, because the tusks are connected to the whale via blood vessels, the growth “ring” preserves information about their physiology and diet from that year. The whales can live to be fifty years old, so the tusks give scientists a look at conditions in the Arctic over a long period of time, unlike other analyses, which only examine single time points. 

Two of the greatest threats to narwhals and other top Arctic predators are mercury exposure and declining sea ice due to climate change. Baffin Bay’s summer sea ice coverage decreased by 11.4% every decade since 1968, altering the ocean’s salinity, increasing pollution levels, and forcing ice-dependent species like narwhals to search the open ocean for food. One pollutant that has been found to increase with melting sea ice is mercury. A potent neuro- and reproductive toxin, mercury accumulates in its consumers as it moves up the food chain in a process called biomagnification, making top predators particularly vulnerable to mercury poisoning. (To learn more about biomagnification and its effects on marine environments, check out these other oceanbites posts!) 

A Look Back in Time 

To find out how mercury levels in the Arctic have changed in the last half-century, scientists turned to the narwhal tusks. They bought ten tusks from Inuit subsistence hunters who have hunted narwhal for millennia and continue to do so (sustainably) today. After cutting the tusks lengthwise, the researchers counted the rings in the tusks to determine the age of the whales. The oldest sample belonged to a 50-year-old narwhal and dated back to 1962. The mercury analyses revealed that mercury intake varied greatly across the narwhals’ lifespans.  

What caused this lifelong variability in mercury intake for the whales? One source could be the narwhals’ diets since the toxin mainly enters the whales through consumption of mercury-laden prey. The scientists uncovered clues to the animals’ diets by analyzing levels of distinct forms of carbon and nitrogen in the tusks, called stable isotopes. This analysis is based on the idea that “you are what you eat”: different foods have differing amounts of these specific forms of carbon and nitrogen, so by looking at the amount of each in the layers, they can determine what the narwhal was eating from year to year. Narwhal prey that live in the open ocean generally contain less of the stable carbon isotope than ice-dependent prey. After analyzing the tusk layers, scientists saw a decline in the amount of the stable isotope of carbon in the whales’ diets over their lifespans, leading them to believe that narwhals adapted to hunting open ocean prey, away from sea ice—a direct effect of climate change and melting polar ice. 

Prior to 1990, when sea ice was more plentiful, narwhals relied on ice-dependent prey like Greenland halibut and Arctic cod. During this period, the narwhals’ mercury intake was higher since these prey are generally larger and higher in the food chain. But after 1990, their mercury intake leveled off as sea ice rapidly declined and the narwhals adapted to eating open ocean prey like capelin and armhook squid that are lower in the food chain (and thus lower in mercury). However, after 2000, the mercury concentrations in the tusk steeply increased, despite the narwhals still consuming open ocean prey; scientists attribute this increase to declining sea ice coverage and human sources of mercury like fossil fuel combustion (see figure). 

A) Narwhals swimming in the ice-covered waters of Baffin Bay. B) Narwhal tusks cut lengthwise for analyses. The inset shows the dentine layers that were used to age the animals and to analyze changes over time. C) Changes in mercury levels and stable carbon and nitrogen isotope amounts in tusks over time charted with changes in sea ice cover. The main prey of narwhals over time is represented above the chart. Sympagic means the prey is dependent on ice, and pelagic means the prey lives in the open ocean. (Image courtesy of Rune Dietz.)

A Changing Arctic 

Because narwhals lack the ability to rid themselves of harmful substances by producing hair, teeth (other than the tusk), or ear plugs, they are thought to be one of the most sensitive animals to pollution in the Arctic. The results of this study confirm that pollution in the Arctic is increasing and that narwhals could be harmed in the future, despite their current status as a “species of least concern”.  

As the Arctic rapidly evolves amid climate change, its inhabitants must adapt to new conditions. Fortunately, narwhals show flexibility in their diet that could be the key to their survival, provided they can withstand increased mercury exposure. Now that scientists have shown the importance of narwhal tusks in determining Arctic conditions over long periods of time, they hope to sample narwhal tusks from museums around the world that will provide an even broader glimpse of changes to polar environments. 

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