Chemical Pollution and Marine Mammals

Between 1990 and 2008, a close collaboration between the Centre for Environment, Fisheries and Aquaculture Science (Cefas) and the UK Cetacean Strandings Investigation Programme (CSIP) has generated one of the largest time-series datasets on chemical contaminants in a marine mammal species the UK-stranded harbour porpoise (Phocoena phocoena). This dataset shows that some organochlorine pesticides like summed DDTs (n=368) and dieldrin (n=429), have gradually declined over time since 1990. In contrast, summed 25 individual chlorobiphenyl levels (sum25CBs) (n=540) have remained relatively stable in UK-stranded harbour porpoises since the mid-1990s, and levels in many individual porpoises often exceed all known or proposed toxicity thresholds for marine mammals. Brominated flame retardants including summed brominated diphenylethers (sumBDEs) (n=415) and hexabromocyclododecane (HBCD) (n=223) emerged in the 1990s with periods of increasing blubber concentrations in UK-stranded harbour porpoises towards the late 1990s-early 2000s, before mean levels peaked and subsequently declined. Brominated diphenylethers trends were linked to anticipated and actual EU-wide bans (e.g. penta-mix PBDE). Using relatively large sample sizes, sum25CBs levels are statistically linked here to susceptibility to fatal infectious diseases (n=182) in case-control epidemiological studies using “healthy” acute trauma cases as controls (n=276). Although using much smaller sample sizes, mean sum25CBs levels in UK-stranded bottlenose dolphins (Tursiops truncatus)(n=15) and killer whales (Orcinus orca)(n=5) were up to one order of magnitude higher than mean levels found in UK-stranded harbour porpoises. Such high PCB exposure poses a significant (but very cryptic) threat to the conservation status of at least some bottlenose dolphin and killer whale populations/ecotypes in the NE Atlantic region. INTRODUCTION Marine mammals are exposed to a variety of anthropogenic contaminants mainly through their diet. This group of chemicals includes the organohalogenated compounds (such as the polychlorinated biphenyls PCBs), the dichlorodiphenyltrichloroethanes (DDTs), chlordane, toxaphene, the cyclodienes (such as aldrin and dieldrin), and polychlorinated terphenyls (PCTs), polybrominated biphenyls (PBBs) and polybrominated diphenyl ethers (PBDEs). Such compounds are typically highly lipophilic and hydrophobic, and bioaccumulate sometimes to high concentrations in lipid-rich tissues like marine mammal blubber (Tanabe et al., 1994). They can have many different isomers and congeners, and comprise hundreds of different chemical formulations, which may have different behaviours and toxicities. They are chemically very stable and persistent, many compounds being resistant to metabolic degradation. Top predators are at particular risk from biomagnification of persistent organic pollutants (POPs) due to the greater abundance, resistance to environmental degradation, and known toxicity of these compounds. In marine and terrestrial mammals these