Parasitism, mercury contamination, and stable isotopes in fish-eating double-crested cormorants: no support for the co-ingestion hypothesis

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 breeding cormorants, males had 1.5 times more total mercury in breast muscle than did females and >2 times more Contracaecum 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.