Marine Mammals as Sentinels in Ecological Risk Assessment

Abstract

As high trophic level organisms in the marine environment, fish-eating seals, dolphins and whales are often exposed to very high levels of fat-soluble environmental contaminants. Assessing the sources, levels and patterns of contaminants found in the tissues of marine mammals, and the biological effects of these contaminants on individuals, is essential to determining any population- or species-level impacts. While the number of contaminants to which marine mammals are exposed is staggering, designing strategies to assess the effects of complex mixtures represent a challenging yet vital part of an understanding of the “real world”. At present, an accumulated “weight of evidence” suggests that ambient levels of lipophilic contaminants have adversely affected aspects of reproduction, immune function and endocrine function in marine mammals inhabiting a number of industrial coastal regions. This body of evidence is drawn from a combination of (1) epidemiological or descriptive studies of effects observed in free-ranging populations of marine mammals inhabiting contaminated areas; (2) mechanistic, cause-and-effect, laboratory rodent studies, using single- or multiple- chemical exposures in acute or chronic designs; (3) semi-field or captive studies of marine mammals fed fish from contaminated areas; and (4) laboratory studies where rodent species are used as surrogates for marine mammals, and are exposed to extracts of complex contaminant mixtures found in fish. While these approaches have been used to delineate the effects of historically introduced environmental contaminants such as PCBs and DDT on wildlife, they may serve to identify the ecological risks presented by (1) the continued leaking of stored, discontinued, chemical supplies that have not yet been destroyed (e.g., PCBs); (2) new chemicals that may have lipophilic or persistent characteristics similar to those found currently in marine mammals; and (3) diet selection as a source of contaminants for humans, since certain human groups share the same food chain with marine mammals. Contaminant mixtures to which marine mammals are exposed differ greatly from the original industrial mixtures as a result of differing rates of accumulation and capacities to metabolize certain chemical types by the various trophic levels (e.g., invertebrates, fish, and marine mammals) of the food chain. Marine mammals ultimately provide information on the chemicals which present the greatest risk to consumers at the top of the food chain, something that cannot be adequately described or predicted in laboratory models.