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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