An article published by Nature explains that scientists have found mercury levels in the ocean much higher than anticipated. They found the levels tripled in the upper ocean and concluded "that human activities - mostly the burning of fossil fuels, but also mining - had boosted the mercury levels in the upper 100 metres of the ocean by a factor of 3.4 since the beginning of the Industrial Revolution."
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Fish are intelligent, sentient beings and they are killed by the billions every year (hundreds of millions of tons). Humans eat them and feed them to farmed animals as well, even though cows, for example, are strict herbivores; and we see the consequence of industrial fishing on the environment and also human health given fish's high mercury, fat and cholesterol content.
Hopefully, studies like the one above help raise awareness. We can do our part by not eating them and supporting sustainable industries.