Especial vulnerability of killer whales
The work, presented by Paul Jepson from the Institute of Zoology, Zoological Society of London (UK) and Rune Dietz from Aarhus University (Denmark) during the recent Working Group on Biological Effects of Contaminants (WGBEC) meeting, detailed the detrimental consequences of high concentrations of persistent and toxic pollutants in killer whales and other toothed whales in the North Atlantic as well as polar bears in the Arctic. The uptake and storage of such contaminants from the marine environment – bioaccumulation and increasing concentrations up the food chain (biomagnification) – means very high concentrations of toxicants (contaminants) in the tissues of these creatures, and subsequent negative impacts on their immune, endocrine, and reproductive functions.
Marine mammals, especially killer whales, are particularly vulnerable to bioaccumulation of persistent contaminants, as they have long life spans, are top predators feeding at a high trophic levels, and typically have low ability to metabolically eliminate and excrete contaminants. The marine environment allows easy transport of infectious diseases through the animals that carry them, which is of particular interest given increasing evidence that pollutant exposure can suppress immune function and exacerbate the effects of lethal viral outbreaks.
Models to strengthen conservation
WGBEC discussed the need for evaluating the effects of pollutant exposure on killer whales' immune function for data with which to build a population model of the effects on this species. Aarhus University is currently studying whale blood to isolate immune cells, testing for responses that represent important aspects of innate and acquired immunity. A cocktail of contaminants drawn from killer whale blubber has been used in experiments with the individual immune cells.
These trials will offer the most realistic exposure scenario possible as well as give insight into how environmental toxicants impact the health of marine mammals. All this output will then feed into the model, enabling scientists to estimate how current and future contaminant levels might affect population growth as well as the risk of viral epidemics. Such information is crucial for ongoing conservation efforts for killer whales, which continue to have extremely elevated body burdens of persistent organic pollutants (POPs).
Severe health impacts by PCBs
Although other toxicants contribute, the group of manufactured chemicals known as polychlorinated biphenyls (PCBs) are presumably the most important toxicants leading to reproductive failures in, for example, killer whales. Despite a global PCB ban more than 35 years ago leading to a decline in their environmental prescene, high concentrations of the chemicals are still present in many top predators, especially those in the North Atlantic and Arctic. These loads are high enough to cause severe population-level reproductive failure, immuno-suppression, carcinogenic effects, and other severe health impacts. This is leading to ongoing population declines and potentially to extinctions of subpopulations, particularly for killer whales, with the species now close to extinction within the industrialized regions of Europe.
Such population declines of both these whales and other cetaceans has recently been linked to reproductive impairment caused by PCBs. This is the case in the Northeast Atlantic and the Mediterranean Sea, where highly PCB-exposed killer whales and bottlenose dolphins have small or declining populations associated with very low rates of reproduction. Since most adult female dolphins can only normally produce a single calf every two to four years, any further PCB-induced suppression of fertility can and will have catastrophic consequences on population viability. In a recent review of the PCB threat to marine apex predators (including seabirds), the bioaccumulation and biomagnification of PCBs was considered a significant threat not just to killer whales but also to bottlenose dolphins, false killer whales, polar bears, river dolphins and porpoises, and numerous apex predator shark species.
Risk assessment needed urgently
The IUCN Red List of Threatened Species does not state concern for killer whales due to a deficiency in data, mainly because there is scientific uncertainty around whether they constitute one or more species. Nonetheless, an international risk assessment is urgently needed to determine physiological effect thresholds of PCB exposures that will allow us to identify population impacts.
Worldwide collaborative efforts are crucial for identifying populations at risk of extinction and those that could maintain this iconic species. Killer whales are excellent marine sentinel species, indicating that not one nation can address the persistent threat that is environmental PCB pollution. WGBEC believe the choice for international PCB mitigation is both timely and urgent.
Image by Rune Dietz