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Mercury and Other Heavy Metals
The Renaissance physician Paracelsus is famously credited with the concept that "the dose determines the poison". One wonders whether he would have felt the same way had he known about bioaccumulation. Bioaccumulation is an inconvenient aspect of some heavy metals, where they concentrate in the body, sometimes to highly toxic levels.
Heavy metals have many different definitions, but generally speaking, are metals with relatively high atomic weights, atomic numbers, or densities. Many of the earliest-discovered metals are heavy metals, such as lead, mercury, gold, silver, and so on. Some are highly poisonous, like mercury and arsenic, while others are harmless, like gold and silver.
While "safe limits" have been published for different substances, it's a bit misleading for those that bioaccumulate, like mercury. If a toxic substance concentrates, then there really is no "safe" limit. One of the problems, of course, is that you may know about one source but have no idea of another, so it becomes difficult to accurately gauge our actual intake.
Four heavy metals, in particular, are worth thinking about in discussions about food safety and salmon: Mercury, Arsenic, Zinc, and Copper. We focus on mercury here now, and shall explore the others soon...
Mercury is the most frightening of heavy metals. It is highly toxic to living things, from bacteria to humans. It concentrates in living tissues, and as such, biomagnifies up the food chain. And we know that mercury is in salmon from the salmon industry's own flesh-testing results [1, 2]. It's in other fish too, but the point is that this product is assertively marketed as healthy without reference to its very unhealthy side.
It is unclear where the mercury in farmed salmon comes from, that is, how it enters the fish. It could be coming from its feed or from the water it is grown in. Most likely, it comes from both.
Mercury in salmon feed
Salmon feed contains about 50% soy, 20% fish, 15% chicken, 10% wheat, and 5% vitamins and pigments . Mercury could conceivably be coming from any of these ingredients, particularly the fish and chicken products where the offal and bones already rich in mercury simply concentrate with each cycling where frames and scraps become food for the next [4, 5]. In this way, the aquaculture industry could be literally mimicking the biomagnification process that so threatens us in nature.
Mercury in the water
It is also possible that mercury could be entering salmon through contamination of the water they are farmed in. Two of the primary growing regions for farmed salmon in Tasmania are Storm Bay and Macquarie Harbour. Both of these have shocking legacies of heavy metal pollution [6, 7]. An experiment on freshwater fish found that direct absorption from the water accounted for about 15% of the mercury uptake in fish muscle . The extent to which this holds true for salmon appears to be untested, but toxic absorption through the gills is a frightening proposition.
Salmon farming also adds every day to the toxic load: uneaten feed and poo are a conduit for mercury to cycle back into the sea... and accumulating on the sediments right where the fish are being raised [5, 9].
Another source of mercury in the salmon may be salps, a type of jellyfish-like marine invertebrate stimulated to bloom by the excess nutrients (poo and uneaten food) from salmon farming. Mercury in seawater is uptaken by phytoplankton [10, 11]. Salps gorge on phytoplankton to power their phenomenal individual and population growth rate [12, 13], and they store mercury in the process . Salmon eat salps opportunistically, sometimes voraciously [15, 16], and without doubt, would readily absorb their mercury.
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