Evaluation of the effects of toxic chemicals in Great Lakes cormorants: has causality been established?

-Toxic contaminants have influenced Double-crested Cormorant (Phalacrocorax auritus) reproduction on the Great Lakes for at least three decades. Four recent studies of contaminants in cormorants of the Great Lakes region are reviewed for their conformation with epidemiological criteria used to infer cause and effect linkages (CELs). Growth of the population was interrupted by DDT which led to egg-shell thinning and reduced reproduction which hid other developmental problems that were most likely caused by planar PCB, dioxin and furan contaminants. Three studies investigated embryo viability and deformities in the post-DDT era. A study of the effects of planar contaminants measured by the H4IIE bioassay technique on egg viability wasjudged to be strong demonstration of contaminant effects on the cormorant population. Two studies of bill defects and the co-occurrence of embryonic abnormalities with specific toxic PCB and dioxin congeners as cormorant teratogens were found to have weaknesses, but were consistent with the paradigm that planar chlorinated hydrocarbons (PHHs), especially the non-ortho-substituted PCB congeners, are now the most important toxic chemical problem for cormorants in the Great Lakes. The rapid uptake of PHHs and great interspecific variations of sensitivity to effects of these chemical between Larids and cormorants have been observed. These biochemical differences and high cormorant fecundity help account for the rapid recovery of cormorants on the Great Lakes. PHHs have relatively trivial impacts on cormorant populations, but do cause significant damage to individuals in a dose-dependant manner. Congener-specific and bioassay techniques support the development of a new toxic chemical paradigm with large implications to policy, management and water quality criteria used to make regulatory decisions. Cormorants may have actually benefited from exposure to toxic contaminants in their competition with large gulls (Larus spp.) on the Great Lakes in the post-DDT era.

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