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Effects Associated with Chemical and Trace Element Contamination in Marine Mammals

The effects of chemical contaminants on marine mammals vary considerably depending upon such factors as age, sex, nutritional state, reproductive status, trophic level, pollution source, excretion, and metabolism (Gaskin, 1982, Borrell and Aguilar, 1987, Aguilar, 1982). From an ecological perspective, studying pollutants in marine mammals is important because they provide valuable information about the fate and effect of certain chemicals, although they are not particularly good as direct indicators of pollution due to limited knowledge regarding physiological processes and their low susceptibility to short-term changes (Reijnders, 1988). It has been suggested that in order to develop predictive capabilities for evaluating the effects of pollution on marine mammals, research will be needed on two fronts: the kinetics of contaminants with baseline studies of their occurrence in the environment and the physiological response with baseline studies on normal (or reference) blood chemistry (Rejinders, 1988).

In this region, important baseline oceanographic studies are now underway, as evidenced by research conducted by the MassBays Program, Environmental Protection Agency, and the Massachusetts Water Resources Authority (MWRA) aimed at describing sources, loadings, concentrations, and distributions of numerous chemical compounds. The results of this research may be useful for identifying the potential pathways of exposure to marine mammals, but continued and improved coordination between marine mammal researchers, the Sanctuary, and these programs will be needed to address the fate and effects of identified contaminant sources on resident species.

Because of their known persistence in the environment and toxic properties, much of the research regarding chemical contamination of marine mammals has focused on chlorinated organic compounds, generally referred to as organochlorines. Organochlorines (OC) in marine mammals generally show bioaccumulation proportional to trophic level and metabolic rate (Aguilar and Jover, 1982, Woodley, et al., 1991). This is evidenced by comparing residual concentrations in right whales, which are slow moving and feed low in the food chain and thus have relatively lower OC levels than for instance, harbor porpoise, which feed higher in the food chain, have a higher metabolism, and thus higher levels of OC concentrations in their tissue. Some studies have found DDT and PCBs in concentrations sufficient to represent a possible risk to cetacean populations (Aguilar and Jover, 1982)

Reproductive disorders have been associated with high levels of organochlorines (especially DDT and PCB's) in two seal populations from the Dutch Waddensee (Reijnders, 1986), but not convincingly so in separate studies of California sea lions and Canadian beluga whales (Addison, 1989). Addison attributes the inability to conclusively explain possible causative links to a number of factors. These include the complexity of PCB mixtures and the variety of effects they cause singularly or in combination, an absence of relevant biological data in the reviewed experiments, and the general state of ignorance regarding the basic biochemistry of normal marine mammal reproduction. However, enough evidence does exist from laboratory studies of other mammals (primarily mink) to expect that PCB's could effect reproductive processes especially in female marine mammals (Addison, 1989).

Recent findings from the eastern North Atlantic successfully demonstrated a causative link between chronic exposure to environmental contamination through the marine food chain and impaired immunological functions in harbor seals (Rik de L. Swart, et. al., 1994). This study utilized a controlled feeding regime where groups individually identifiable animals where some animals where fed PCB contaminated herring from the Baltic sea, while others were fed relatively uncontaminated herring from the Atlantic Ocean. Subsequent examination of immune system functions indicated significantly lower response capabilities in the animals fed the contaminated prey. These findings illustrate one potential effect of chronic exposure to sub-lethal concentrations of PCBs which may help in the design of similar experiments for species potentially at risk due to food chain transfer of contaminants in the GOM. Such species might include those feeding at higher trophic levels including humpback whales, harbor porpoise, Atlantic white-sided dolphins, harbor seals, and grey seals.

The effects of hydrocarbons on marine mammals varies according to species, type of petroleum product, and mode of contact. Preliminary studies of the effects of petroleum found no conspicuous negative effects (Geraci and St. Aubin, 1982, Geraci and St. Aubin, 1985) but information on problems associated with direct ingestion as well as long-term chronic effects from exposure remain incomplete (Winn et al., 1987). Toxicological effects include aberrations in hematological parameters, diagnostic enzymes, and in tissue structure (NRC, 1985). Short-term exposure to oil (not including ingestion) may result in eye and skin damage, but apparently has no long-term consequences, whereas the same cannot be said for long-term exposure, which in phocids, causes clinical damage, and may lead to adrenal steroid exhaustion (ibid.).

Behavioral reactions by marine mammals to the presence of petroleum in the water appear to differ depending upon the type of product encountered, species involved, and behavior immediately prior to the encounter. For example, under ideal conditions bottlenose dolphins can visually detect and avoid certain heavy grades of oil and may also be able to detect thick surface slicks (12mm or greater) of heavy oil using echolocation (Geraci and St. Aubin, 1982). However these results, which were obtained from tank tests, may not accurately account for or may be overcome by such factors as sea conditions, the desire to feed, and/or social behaviors. Other observations suggest that seals do not actively avoid oil and report feeding and swimming in oil by humpback, fin, northern right whales and Atlantic white-sided dolphins (National Research Council, 1985).

Experimental data on the functional and structural effects of oil-coated baleen were examined. Studies indicate a temporary inhibiting effect on normal function, which was restored after continuous rinsing. The possibility of reduced nutritive intake due to food sticking to the oil coated plates and of toxicity associated with increased ingestion of oil were not evaluated (Geraci and St. Aubin, 1982). In addition to its direct toxicological effects, oil may also increase mortality of preferred prey species, thus indirectly effecting marine mammals by reducing prey availability. Seals immersed in oil contaminated water demonstrated hydrocarbon absorption, but whether this was by inhalation or dermal absorption was not confirmed. Grey and harbor seal mortalities have been temporally associated with coating by oil (National Research Council, 1985).

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