Interesting look into the mind of researchers with an agenda.
As onthewater102 pointed out, they have combined "suspected" with actual lead tackle deaths, and throw that figure around as the total for actual lead deaths. If the study were a study and not an attempt to sell an agenda, the "suspected" numbers would be added to the unknown, not those that are provably fishing tackle lead related.
Even the layout of the chart is misleading. Charts quoting percentage should show the scale on a 100% basis. but it doesn't look as dramatic when you only fill a small portion of the scale, so they left off the empty side.
To be honest the thing that stood out to me as the oddest, is why do loons in a fresh water environment attack each other (intraspecfic trauma) but none in a saltwater environment do.
In looking at previous work by the group at Tufts (Northeastern Naturalist 16(2):177-182. 2009 ), they scored "suspected lead toxicosis" as: 1) sick/dead loons found with lead in their digestive tract (but not analyzed for lead in the liver to confirm poisoning), 2) sick/dead loons with lead in their digestive tract and showing clinical signs of lead poisoning (but not analyzed for lead in the liver) and 3) sick/dead loons with lead in their digestive tract that had liver lead levels in the 2-6 ppm range (6 ppm is considered toxicosis). For me the conclusion that the total amount confirmed and suspected was presented is acceptable practice because they presented the data individually so that you can see what was absolutely confirmed and what was highly suspected based on a number of characters. Another way to say it is that they show the data individually based on confirmed and suspected but put their best estimate of accurate poisoning out there as 44%.
Either way, as has been said 25% (or 44%) of loon mortality is a lot of loons with toxic levels of lead in them. I am always surprised that this is so common.
As for the question why loons in freshwater have higher intraspecific mortality... It is because loons breed on freshwater lakes and not saltwater. The combative territorial interactions that injure them occur where they breed and claim territory, not where they winter on the ocean.