238 Environmental Toxicology and Chemistry, Vol. 24, No. 1, pp. 238–248, 2005  2005 SETAC Printed in the USA 0730-7268/05 $12.00 + .00 RELATIONSHIPS BETWEEN POLYCHLORINATED BIPHENYLS AND HEALTH STATUS IN HARBOR PORPOISES (PHOCOENA PHOCOENA) STRANDED IN THE UNITED KINGDOM PAUL D. JEPSON,*² P ETER M. BENNETT,² R OBERT DEAVILLE,² C OLIN R. ALLCHIN,‡ JOHN R. BAKER,§ and ROBIN J. LAW‡ ²Institute of Zoology, Zoological Society of London, Regent’s Park, London NW1 4RY, United Kingdom ‡Centre for Environment, Fisheries, and Aquaculture Science, Burnham Laboratory, Remembrance Avenue, Burnham-on-Crouch, Essex CM0 8HA, United Kingdom §Department of Veterinary Pathology, University of Liverpool, Leahurst, Chester High Road, Neston, South Wirral CH64 7TE, United Kingdom ( Received 4 December 2003; Accepted 30 June 2004) Abstract—To investigate possible relationships between polychlorinated biphenyl (PCB) exposure and infectious disease mortality in harbor porpoises (Phocoena phocoena) in United Kingdom waters, summed blubber concentrations of 25 chlorobiphenyl congeners (25CB) in healthy harbor porpoises that died of acute physical trauma (mainly by-catch; n = 175) were compared with 25CB values in animals that died of infectious disease (n = 82). The infectious disease group had significantly greater 25CB values (mean, 27.6 mg/kg lipid) than the physical trauma group (mean, 13.6 mg/kg lipid; p  0.001). This association occurred independently of other potentially confounding variables, including age, sex, two indices of nutritional status, season, region, and year found. Total blubber PCB levels (as Aroclor 1254) were also calculated, enabling direct comparison with a proposed threshold for adverse health effects (including immunosuppression) in marine mammals of 17 mg/kg lipid. In porpoises with total PCB levels greater than 17 mg/kg lipid (n = 154), total PCB levels were significantly higher in the infectious disease group compared to the physical trauma group (p  0.001). This association was no longer significant in porpoises with total PCB levels of less than 17 mg/kg lipid (n = 103; p  0.55). These findings are consistent with a causal (immunotoxic) relationship between PCB exposure and infectious disease mortality, and they provide a framework for future quantitative risk-assessment analyses of porpoise populations of known size and PCB exposure. Keywords—Polychlorinated biphenyls Harbor porpoise Immunosuppression Infectious disease Ecotoxicology INTRODUCTION A number of persistent marine pollutants may pose a sig- nificant and global threat to the health status and viability of marine mammal populations, with persistent organochlorine compounds such as polychlorinated biphenyls (PCBs) gener- ally posing the greatest concern [1–3]. Marine mammals ap- pear to be particularly vulnerable to high-level bioaccumula- tion of these contaminants because of their high trophic level, long life span, and limited capacity for metabolism and ex- cretion of compounds such as p,p'-DDT and PCBs [2,4]. The ability of small cetaceans in particular to metabolize PCBs appears to be extremely low compared with that of birds and terrestrial mammals [2], and the ability to metabolize PCBs may be even lower in harbor porpoises (Phocoena phocoena) than in many other small cetaceans [5]. The harbor porpoise is the most abundant cetacean in north- ern European waters [6], and it is the most frequently stranded cetacean in United Kingdom waters [7,8]. Clear evidence in- dicates a decline in the density of harbor porpoise populations in the English Channel and southern North Sea [6,9]. A range of possible causes have been proposed, including increased mortality because of entanglement in fishing gear (by-catch) and habitat degradation because of increased shipping activity, noise, changes in prey availability, and pollution [9]. Harbor * To whom correspondence may be addressed (paul.jepson@ioz.ac.uk). porpoises in the eastern North Atlantic may be especially vul- nerable to the effects of environmental pollutants, because they feed at high trophic levels in heavily polluted coastal waters [10]. A number of studies have demonstrated high levels of contaminants in the tissues of harbor porpoises from the Baltic Sea [11], the North Sea [12], Scandinavia [13], and United Kingdom waters [14–17]. Among harbor porpoises in United Kingdom waters, no associations were found between PCB concentrations in blub- ber and adrenocortical hyperplasia in stranded or by-caught individuals during 1990 and 1991 [14] or between blubber PCB levels and mortality because of infectious disease in an- imals stranded or by-caught in England and Wales between 1989 and 1992 [15]. However, later studies of stranded harbor porpoises in the United Kingdom utilizing larger datasets found statistically significant associations between elevated blubber levels of PCBs [16], and hepatic mercury and mercury to selenium ratios [17], in animals dying of infectious disease compared to a control group of healthy animals dying of acute physical trauma. The lack of experimental data on cause–effect relationships between contaminants and their toxic effects in cetaceans has led to considerable difficulty in interpreting the biological sig- nificance of levels of contaminant exposure or the direction of causality in associations between contaminant exposure and disease. Recently, experimental data regarding aquatic mam- mals have been collated to derive dose–response relationships