CASE STUDY 1

Background

Several sites in Lake Victoria (a huge lake spanning several African countries including Uganda, Kenya and Tanzania) were examined for mercury (Hg) contamination in two fish species, the Nile Tilapia (Oreochromis niloticus) and Nile Perch (Lates niloticus). These fish are part of a big fishery, but contamination levels sometimes exceed consumption limits set by the World Health Organization. The authors were studying whether or not these two species differed in the concentrations of mercury in their tissues and whether or not their tissue mercury concentrations differed among sites.

Questions

  1. Contrast the trends in the plots for the two species. Among all sites, which species shows a clearer trend for biomagnification of mercury with increasing body weight? What to you suspect causes this difference?
  1. Compare the concentrations of mercury in L. niloticus (Nile Perch) of the same weight from Thurston Bay (closed circles) and Uganda (open circles). What factors might explain the consistent difference in mercury concentration between these two fish populations?
  1. Which fish would you eat, if you had a choice? What would be the ramifications for your health if you continued to eat fish with mercury concentrations above the WHO limit? At what age are humans most susceptible to the effects of mercury poisoning?
  1. What happens to the mercury when these fish die?

Extra questions:

  1. Examine the plot for O. niloticus (Nile Tilapia) and discuss why you think the overall trend among all sites is different than the pattern for each site. What misleading conclusions would a resource manager make by creating consumption criteria for Thurston Bay from the complete dataset rather than one specific to that site?
  2. Would you expect that as an individual fish grows larger, the rate of bioaccumulation would remain the same, increase or decrease? (Hint: remember “rate” is the pace of change over time, not the concentration). What controls the shape of the relationship between size and the rate of bioaccumulation?

Campbell, L.M., J.S. Balirwa, D.G. Dixon and R.E. Hecky. 2004. Biomagnification of mercury in fish from Thruston Bay, Napoleon Gulf, Lake Victoria (East Africa. African J Aquatic Science 29(1): 91-96