Canadian Marine Mammal Rescue Network

Mercury bioaccumulation in fish and its implications for top predators: Humans

Chelsea Lim (1), Mike Belanger(2), Carin Wittnich(1,2)

1 Department of Ecology & Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario, Canada
2 Oceanographic Environmental Research Society, 12 Burton Avenue, Barrie, Ontario, Canada, L4N 2R2

The oceans have long since been a dumping ground for the world’s toxins and recently some of these have been suspected of leaking into our food chain through the consumption of seafood. The types of health problems this poses for humans and other mammals continues to be debated.  One of these toxins is mercury, which has been shown to be detrimental to the neurological development of mammalian fetuses. Mercury concentrations in fish are variable and can be attributed to many factors including trophic level, diet and the age of the fish. This study focused on species of fish commonly eaten in North America such as tuna and salmon as well as fish known to have high levels of mercury such as shark, swordfish and tilefish.

Fish samples were collected from supermarkets, specialty fish markets and from ethnic areas (such as Kensington market and Chinatown) within the downtown Toronto area and were tested for mercury. Samples were then analyzed using isotope dilution-inductively coupled plasma mass spectrometry (ID-ICP/MS).  Results specific to the Toronto area were compared to the mercury levels published by the U.S. FDA in 2006 and by Health Canada in 2007. The levels of mercury in species that are government regulated (such as salmon) were compared to species that are not regulated (such as swordfish, tuna and shark) using 1-way ANOVA and Tukey’s post hoc tests. 

The data shows the highest levels of mercury in swordfish (1.3ppm) and shark (1.1ppm). These values were over 100 times the value found in salmon (0.01ppm). Tuna had levels of mercury close to the Canadian action level of 0.5ppm. The study also found great variability in the mercury concentrations within a species, particularly the species with high levels of mercury such as tuna, swordfish and shark.

This finding indicates that certain “at-risk” populations such as fecund females, should avoid eating these high mercury species altogether, even if the average mercury content of the species is below the 0.5ppm action level.  Despite government action to curb mercury emissions, mercury is still being released into our environment by both natural and anthropogenic sources.  Therefore, unless drastic measures are taken to stop mercury emissions, mercury levels in fish will continue to rise and higher order species that consume mercury contaminated fish will continue to accumulate dangerous amounts of mercury that may be detrimental to their health.

Presented as a poster 2007-08 Ecology & Evolutionary Biology Poster Day, April 15th, 2008