In a recent post, I described how the popularity of loons and the willingness of many folks to pontificate about them without solid data or thoughtful scientific analysis makes loons unique. I tried to point out that this practice can be harmful, if we make misleading statements about loon conservation at a time when some loon populations are declining.
Now, let me give an example. Millions of state, federal, and private dollars have been spent in recent decades in attempting to measure the effect of methylmercury (the toxic form of mercury) on loons. Without a doubt, more funds have been spent analyzing mercury impacts on loons than on any other aspect of loon biology. What have we learned from this body of work? The major take-homes are that: 1) mercury certainly can affect behavior and survival of adult loons and chicks if it occurs in a high enough concentration in their tissues, 2) potentially harmful mercury levels do occur in loons living on small, acidic lakes — which are also lakes that have negative effects on loons that have nothing to do with mercury, 3) harmful concentrations of mercury, however, do not occur in most breeding loons, and 4) there is no evidence that mercury negatively impacts loon populations despite attempts to detect impacts over three decades and in numerous breeding populations across the species range.*
The situation is dramatically different with lead. Careful analysis of loon carcasses in New England has shown us that lead is quite deadly and affects a great many loons. (The featured photo above shows a deadly lead sinker in the stomach of a loon that died a few days ago in Wisconsin. Photo by Wild Instincts.) In a 2017 study, Grade et al. determined that a whopping 48.6% of the loons they examined had been killed by lead sinkers and lures. The authors estimated that this mortality rate had reduced the New Hampshire loon population by 43%.
The contrast between mercury and lead is stark. Mercury affects loon survival and breeding success slightly in some populations. Lead has been shown to cause half of all loon deaths in one state and to make an enormous dent in the loon population of that state. Now we wish we had more data from other states and provinces so that we could assess lead’s impact there. But we already know that lead is far more consequential than mercury.
The contrast between these two toxins goes further. Mercury exposure is pushed to high levels mainly through burning of fossil fuels like coal and oil, incineration of waste, and mining. So reducing loons’ exposure to mercury requires a long-term effort to reduce atmospheric mercury levels over a large geographic area and across many industries. (That effort is underway and will no doubt produce benefits to health of humans and wildlife.) In contrast, loons are exposed to lead almost exclusively through our use of lead sinkers and fishing lures. The remedy for lead-related loon deaths is simply to implement use of lead-free fishing tackle in lakes where loons breed. (Steel, tin, and tungsten are common alternatives). Indeed, limited lead bans are now in place across New England. One hopes that common-sense lead bans will follow in the Upper Midwest and elsewhere. The point is that we can much more easily limit the exposure of loons to highly-damaging lead than to much-less-harmful mercury.
Here is the problem. Despite the lack of evidence that mercury affects loons in nature, mercury has become the “go-to” environmental toxin mentioned by many loon researchers. Mercury has become such a prevalent scapegoat in grants, reports, and even published papers that many of us are not keeping its limited impact on loons in proper perspective. A clear-headed, candid, objective review by a loon researcher with a strong background in mercury toxicity would do wonders for loon conservation. At a time when studies have just reported long-term declines in two loon populations, those of us trying to conserve loons would do well to focus our attention on the real enemy.
* — One study, to my knowledge, has shown that loons most exposed to mercury in the Adirondacks of New York exhibit lower population growth than those with lower mercury levels (Schoch et al, 2014: Waterbirds 37 (Special Publication 1): 133-146). This paper, however, shows a small effect and one limited to a small minority of Adirondack loons.