Invaders of Lake Superior
You may have heard about the risks when consuming fish and seafood due to mercury concentrations in their tissues, but one fish that you may not have considered is the invasive sea lamprey (Petromyzon marinus). Present in the great lakes since the 1940s, sea lamprey are parasitic jaw-less fish that have a single reproductive phase before death. Larvae hatch from the lamprey eggs and burr
ow into the bottom sediment of their spawning stream where they filter feed for 3-7 years before returning to the lake or sea in a parasitic phase lasting 1-2 years. Once they have reached sexual maturity, the lamprey return to streams and rivers to spawn and their life cycle is complete.
A Great Lakes Indian Fish & Wildlife Comission (GLIFWC) administrative report by environmental biologist Sara Moses provides new detailed information about mercury in Lamprey through their life stages and a first estimate of how much mercury they are capable of moving between Lake Superior and the streams around it.
The Results are Shocking
The study, conducted by the GLIFWC with the University of Wisconsin, measured changes in mercury concentrations throughout the three life stages–eggs, larvae and adults–of lamprey sampled from tributaries leading to Lake Superior in Michigan and Wisconsin. The mercury levels were high at every life stage.
For example, adult lamprey had ten times the mercury levels found in trout, the lamprey’s preferred food source. Mercury concentrations may be so high in adult lamprey because they are parasitic, attaching to a host and feeding off their blood and bodily fluids for 1-2 years before spawning. They prefer large trout which are more likely to have higher mercury levels and which have been shown to carry more mercury in their blood than in other tissues.
Adult lamprey had the highest mean mercury levels of 3.01μg/g, followed by eggs at 0.942μg/g, and larvae with the lowest mean of 0.455μg/g. For context, Canadian and US guidelines recommend that fish exceeding 0.5μg/g and 0.3μg/g, respectively, are not suitable for human consumption.
What this means for the Great Lakes ecosystem
Although lamprey may be considered a delicacy elsewhere, consumption of these creatures is rare in the Lake Superior area. The greater risk is to wildlife who feed on the lamprey and their larvae and eggs. The mercury concentrations measured by this study in all eggs and adult lamprey as well as in most larval lamprey exceeded the upper threshold for mercury concentration criteria for fish-eating wildlife set by the U.S. Environmental Protection Agency (EPA).
After spawning, adult lamprey die at the spawning location and mercury is released from their bodies, mainly as methylmercury. As a result, they contribute significantly to the transportation of mercury between the lake and surrounding rivers in the Superior Basin. Sara Moses suggests that based on 2015 spawning lamprey population estimates of ~80,000 and the mean lamprey mass and mercury concentrations determined in the GLIFWC study, the spawning migration of adult lamprey transports 49.1g of like-derived mercury to the spawning streams annually.
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