recent research<\/a>, which has found that the sinking carcasses of fish from near-surface waters deliver toxic mercury pollution to the most remote and inaccessible parts of the world’s oceans.<\/p>\nMercury is a globally distributed neurotoxic pollutant that can accumulate in marine fish to levels that are harmful for consumption by humans and other animals, with the effects of mercury poisoning on humans including damage to the central nervous system, the heart, and the immune system. Whilst mercury is a naturally occurring element, huge quantities of it are also emitted into the atmosphere each year from human (i.e. anthropogenic) activities including the burning of fossil fuels and from uses in products or industrial processes. This enters the oceans via rainfall, the runoff from rivers and estuaries, and even from the deposits of windblown dust. Understanding exactly how much mercury is in the oceans and how it is distributed is essential to determine how seafood is likely to be affected. It was widely thought that anthropogenic mercury was mainly restricted to the upper 1,000 meters of the oceans, but new research has now found evidence of this pollution in some of the ocean\u2019s deepest and remotest locations \u2013 deep-sea trenches at depths of up to 11,000 metres.<\/p>\n
Previous research suggested that this mercury arrives at the bottom of these deep-sea trenches via marine snow, i.e., the microscopic particles of sinking organic matter (including faecal material and dead plankton) that is constantly falling from the upper layers of the oceans. However, this new research suggests that a more likely explanation for how mercury arrives in these trenches is via the sinking carcasses of fish that feed in the upper ocean. Mercury has several different isotopes, and the ratio of these different isotopes provides a unique chemical signature, or fingerprint, that can be used to identify and compare environmental samples from various locations. The researchers began by sampling amphipods (a type of crustacean) and snailfish (also known as sea snails) from the Mariana Trench in the western Pacific Ocean and the Kermadec Trench in the south Pacific Ocean, at depths of several thousand metres. They then measured the chemical fingerprints of the mercury in these samples and observed that they matched those of the mercury found in a wide range of fish species in the central Pacific Ocean at depths of approximately 500 m (i.e. much closer to the surface). The researchers in this study also found that the isotopic composition of the mercury found in marine snow, does not match the chemical signature of mercury in the trench organisms, lending further credence to the theory that most of the mercury in the deep-sea trenches was transported there in the carcasses of fish that feed in near-surface waters. This research provides yet another example of the extent to which human activities are impacting even the most remote marine ecosystems on Earth.<\/p>\n
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