Abstract

Heavy metal pollution tends to be persistent, have long lasting effects in the environment, and bioaccumulates up the food chain to affect the top predators. Relevant to wildlife conservation efforts, there are conflicting reports as to the effects of pollution by heavy metals such as mercury on species particularly pertinent to zoological parks and aquaria. Information on the total amount of mercury (µg/g wet weight) in livers of various marine mammals such as pinnipeds (pinnipeds: harp Phoca groenlandica; harbor Phoca vitulina; northern fur Callorhinus ursinus; ringed Phoca hispida; Antarctic fur Arctocephalus gazella), cetaceans (dolphins: striped Stenella coeruleoalba; Risso’s Grampus griseus; bottlenose Tursiops truncatus; white beaked Lagenorhynchus albirostris), and polar bears (Ursus maritimus) from different geographic locations around the world spanning over 35 years are reviewed.

Depending on the geographic location, species, age, or sex, mercury levels vary from very low (0.05 µg/g seals in Greenland) to very high values (700 µg/g dolphins in the Mediterranean). However, across all areas, the mercury levels within species have either increased or at best remained unchanged over time. For example, in Greenland seals, mercury increased from 0.05 µg/g (1978–1987) to 7 µg/g (1994–1995). Mercury levels in seal pups from the east coast of Canada (1972–1978) increased three-fold over a 6-year period. Mean mercury levels in seals world-wide showed a dramatic increase to 25 µg/g by 1994. Dolphins in the waters around the United Kingdom have values which rose 12-fold from 1989–1998 (120 µg/g). In the Mediterranean, even in 1972, dolphins had high mercury levels (up to 700 µg/g) and these continued to increase over time so that by 1996 levels reached up to 1000 µg/g. Many are showing levels which are well above the toxic level reported in dolphins of 50 µg/g. Mean mercury levels in polar bears from Greenland have risen 2.5 times from 1983–1994, with some regions showing values as high as 26 µg/g.

In summary, since 1972 the amount of mercury in the livers of seals found in the Canadian Arctic, east coast of Canada and from Greenland has either remained stable or dramatically increased. Values in dolphins are high and have continued to increase, and levels in both seals and dolphins show geographic variability. Mercury levels in polar bears from Greenland have doubled. Geographically, levels of pollutants appear to vary considerably depending upon several factors: animal species, feeding behaviors, and bioavailability. Symptoms of mercury toxicity include renal failure, toxic hepatitis, changes in gonadal and adrenal steroid synthesis, and severe neurologic dysfunction. Additionally, stress is known to release stored mercury, and procedures such as tagging, sampling, and handling could potentially exacerbate symptoms. High levels of mercury could negatively impact the well-being of both captive and wild animals; and in pregnant females, could produce offspring that are neurologically impaired or developmentally challenged who cannot survive under natural conditions. This could adversely affect breeding programs in aquaria and zoos, leading to unsuccessful reintroduction of many threatened species and even the extinction of populations of marine animals.