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).
continue
to next section
|
