Parasitism, mercury contamination, and stable isotopes in fish-eating double-crested cormorants: no support for the co-ingestion hypothesis S.A. Robinson, M.R. Forbes, and C.E. Hebert Abstract: Mercury and parasitism have been positively correlated in free-ranging birds. One proposed explanation is that mercury reduces host immunity, resulting in a greater susceptibility to parasitism. However, alternative explanations should be addressed to further inform and test hypotheses about relationships between mercury and parasitism. We investigated whether total mercury and Contracaecum spp. were correlated in double-crested cormorants (Phalacrocorax auritus (Lesson, 1831)) and whether there was support for mercury and infective stages of parasites being co-ingested. For breed- ing cormorants, males had 1.5 times more total mercury in breast muscle than did females and >2 times more Contracae- cum spp. in the proventriculus and stomach region. Males responsible for the sex biases in mercury concentration were not the same males responsible for sex biases in parasitism, hence separate explanations for these patterns were required. Males foraged in more pelagic areas and at a slightly lower trophic level than did females, as determined by stable C and N isotope signatures, respectively. These sex differences in foraging and expected differential consumption of intermediate fish hosts could explain the sex bias in parasitism but not the sex bias in mercury concentration. We suggest when testing contaminant–parasite linkages that sex differences in exposure be addressed. Re ´sume ´: Il existe un relation positive entre le mercure et le parasitisme chez les oiseaux sauvages en liberte ´. Il a e ´te ´ sug- ge ´re ´ que le mercure affaiblit le syste `me immunitaire des ho ˆtes, en les rendant plus susceptibles au parasitisme. Par contre, des explications de rechangedoivent e ˆtre examine ´es afin de mieux comprendre et mettre a l’e ´preuve les hypothe `ses sur les relations entre le mercure et le parasitisme. Nous avons essaye ´ de de ´terminer s’il y a ou non une relation entre le mercure total et la pre ´sence de l’espe `ce Contracaecum chez le cormoran a ` aigrettes (Phalacrocorax auritus (Lesson, 1831)) et si le mercure et les stages contagieux des parasites sont co-inge ´re ´s. Chez les cormorans reproducteurs, les ma ˆles ont une fois et demie plus de mercure total dans les muscles pectoraux que les femelles et deux fois plus de Contracaecum spp. dans la re ´gion proventiculaire et l’estomac. Parce que les ma ˆles responsables du de ´se ´quilibre entre les sexes de la concentration de mercure ne sont pas les me ˆmes ma ˆles responsables du de ´se ´quilibre du parasitisme, il fallait trouver des explications se ´par- e ´es pour ces patrons. Les ma ˆles se nourrissent dans les aires pe ´lagiques et a ` des niveaux trophiques le ´ge `rement plus bas que les femelles d’apre `s les signatures respectives des isotopes stables de C et N. Les diffe ´rences de parasitisme relie ´es au sexe mais non celles de la concentration de mercure peuvent s’expliquer par des diffe ´rences sexuelles dans la recherche de nourriture et une consommation distincte attendre de poissons servant d’ho ˆtes interme ´diaires. Nous sugge ´rons que lorsque les liens entre les contaminants et les parasites seront e ´tudie ´s, que les diffe ´rences sexuelles soient prises en conside ´ration. Introduction Exposure to mercury is increasing worldwide because in- dustry and mining are making it more available (Weiner et al. 2003). Mercury is a natural metal in the environment that is nonessential to life; however, when converted in aquatic environments to methyl mercury (its organic form), it becomes a contaminant of particular interest because it is toxic to wildlife (Weiner et al. 2003). Methyl mercury bio- accumulates with trophic level, therefore top consumers are exposed to high concentrations (Weiner et al. 2003). Researchers have shown that degree of endoparasitism and methyl mercury burden is linked in free-ranging birds (Wayland et al. 2001; Sagerup et al. 2009). A similar posi- tive correlation was also found between endoparasitism and organochlorine levels in a natural population of glaucous gulls (Larus hyperboreus (Gunnerus, 1767)) (Sagerup et al. 2000). One hypothesis to explain such relationships is that methyl mercury and also organochlorines reduce immunity, thereby resulting in a higher degree of parasitism (e.g., Luebke et al. 1994, Boros ˇkova ´ et al. 1995). However, when immune status was measured and compared with concentra- tions of organochlorines and abundance of parasites in northern fulmars (Fulmarus glacialis (L., 1761)), there was no relationship with either (Mallory et al. 2007). There are no experimental studies on birds in their native habitat showing that increased methyl mercury levels lead to an in- Received 26 March 2009. Accepted 27 May 2009. Published on the NRC Research Press Web site at cjz.nrc.ca on 5 August 2009. S.A. Robinson 1 and M.R. Forbes. Department of Biology, Carleton University, 209 Nesbitt Building, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada. C.E. Hebert. Environment Canada, National Wildlife Research Centre, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada. 1 Corresponding author (e-mail: srobinsc@connect.carleton.ca). 740 Can. J. Zool. 87: 740–747 (2009) doi:10.1139/Z09-062 Published by NRC Research Press