Come spring, hundreds of harbor seals will haul out on the rocky ledges of Maine islands for pupping season. The silvery bodies of harbor seals sprawled in the sun have become a common sight since their numbers have increased over the last twenty years. Yet this seemingly healthy population of seals carries a heavy burden of mercury contamination.
Now, studies of seals from Mount Desert Rock are asking whether the old evolutionary idea of natural selection may play a role in the seals’ high levels of mercury. Natural selection predicts that seals would eat fish that were easier to catch. Slower fish. Fish that are weakened by disease. Fish that don’t act normal because toxic chemicals interfere with their brain signals. If too much mercury somehow makes fish more vulnerable to predation, then seals may preferentially select the more contaminated food, and become more contaminated themselves.
Dianne Kopec has studied harbor seals on the West and East coasts for close to 15 years. Now a Ph.D. student at the University of Maine, Kopec is looking at the movement of mercury from fish to harbor seals off the coast of Maine. She first looked at the seals’ diet, which consists mostly of silver hake, Acadian redfish, and Atlantic herring, though the proportion of each varies from year to year. Kopec then analyzed mercury in the general population of these fish, taking a sample from the Department of Marine Resources Inshore Trawl Survey. Redfish tend to be highest in mercury because it grows the slowest, and a redfish is much older than a silver hake of the same size — giving it more time to accumulate mercury.
Mercury magnifies up the food web from bacteria to plankton to fish and fish-eating mammals. Wildlife studies indicate that mercury levels in predators are usually one to five times greater than in their prey; Kopec found that seals from Mount Desert Rock had 140 times as much mercury as the fish they were eating. After seeing the magnification numbers, Kopec knew that that the seals had to be getting more mercury from somewhere. “There is more mercury in the seals than you would expect if they were just eating fish from the general population,” she said.
Determining the amount of mercury in the fish the seals were actually eating is more difficult, because there is not much left of a fish once it’s digested. Once inside, mercury travels through the bloodstream and is distributed among organs, muscle and proteins. Over time, animals can process mercury and eliminate it from their bodies if they are not continually exposed. But mercury in protein like a fish’s eye lenses or otoliths (ear bones) is not metabolized. This “locked in” mercury may illustrate the fish’s lifetime accrual of mercury. Fish eye lenses and otoliths are also not digested by seals.
Kopec sifted through seal scat found on Mount Desert Rock to collect remnants of redfish the seals were eating. She is now in the process of analyzing how much mercury is in the remnant otoliths and eye lenses in order to answer the question of whether the fish that seals are eating have higher mercury than fish in the general population.
Marine biologists have known for decades that seals and other marine mammals have extremely high mercury levels, although there are very few studies of how the toxic metal actually harms the animals. What is known is that contaminants like PCBs and DDT have been linked to seal die-offs, and that the levels of mercury found in Maine seals are the same as or higher than levels that are known to be toxic to mammals on land. Yet by taking on a higher burden of mercury, harbor seals may be benefiting the rest of life in the ocean.
Kopec’s next question is, “If the seals are eating the more highly contaminated fish, does that result in an overall lowering of the average mercury in the remaining fish population?” Fish pass on contamination to their eggs, and mercury exposure during development has long-term, permanent impacts on fish. If, through the process of natural selection, seals remove the more polluted fish, then the healthier fish are left in the ocean to survive and reproduce.
“It would suggest a beneficial role of natural predation on fish populations,” says Kopec. As for the seals, who may be providing this service at their own expense, their relatively high population numbers may allow for natural selection to play out once again.