Human Health Implications

During the course of shark landing surveys conducted in Southern Belize, Lighthouse Reef Atoll and Belize City, we took tissue samples for mercury and stable isotopes analyses to assess mercury loading and an initial insights into foraging strategies of sharks. Analysis of samples was performed in collaboration with the Biodiversity Research Institute (US). Of the 172 samples of sharks analyzed, over 80% were above the US Environmental Protection Agency advisory levels of 0.3 parts per million and over 37% were above the Federal Drug Administration and World Health Administration advisory levels of 1.0 parts per million.

Fishers process a hammerhead captured in Southern Belize into fillet

Fishers processing and filleting sharks caught in Southern Belize

Although alarming, these results are not wholly unsurprising as sharks are apex predators that are known to biomagnify toxins including heavy metals such as mercury due to the many food chain linkages existing in the marine environment. Absorption of large pulses of mercury and continued ingestion above the advisory levels impacts cognitive abilities, memory, depresses serotonin levels leading to depression and aggression and can severely impact fetal development. Affected fetuses suffer permanent damage to brain cells leading to, on the least problematic scale, future problems associated with learning, weakened attention spans and anger management among others (see: http://water.epa.gov/scitech/swguidance/fishshellfish/outreach/advice_index.cfm or  http://toxics.usgs.gov/definitions/methylmercury.html or www.gotmercury.org for rapid facts). US EPA and FDA advisories suggest that pregnant women should avoid eating fish such as shark, king mackerel and not eat more than two meals of fish or shellfish of lower mercury content a week (no more than 12 ounces or two average meals) (see: http://www.cfsan.fda.gov/~dms/admehg3.html). However, these advisories are considered too high for pregnant women and young children with suggestions that 0.12ppm would be the acceptable threshold for these groups. More sampling is required to determine the breadth of the mercury contamination throughout the trophic levels and identify point sources, if these exist. With these data in hand we will be well placed to develop advisory levels on fish consumption on a species basis for Belize. Meanwhile it is advisable to not eat shark meat, a suggestion that mirrors the US EPA and FDA’s advice for the Gulf of Mexico.

Please download the PDF of our poster on the mercury levels assessed in the sharks of Belize below.

Elevated mercury in sharks of Belize_2008

Part of our results on sharks and mercury are also included in the following published paper:

Evers, D.C., R.T. Graham, C. Perkins, R. Michener, and T. Divoll.  Mercury concentrations in the goliath grouper of Belize: Is this an anthropogenic stressor of concern?  Endangered Species Research.  7:249-256