Awareness of the threat of environmental contaminants to «_ marine mammals is widespread. High concentrations of . certain compounds in the tissues of these animals have been associated with organ anomalies and impaired reproduction and immune function, as shown by large die-offs among seal and cetacean species. This has prompted alertness about the impact of pollution and stimulated research into the relationship between observed effects and pollutants. However, a clean cause and effect relationship between the residue levels of contaminants and the observed effects has been demonstrated in only a few studies. In the absence of evidence, this might elicit a serious backlash because concerns expressed are easily interpreted as fear mongering. This could lead to inertia in taking appropriate management measures, which is undesirable from a conservation as well as an environmental management perspective.
The main reasons for the lack of proof of the impact of pollution on marine mammals are the difficulty or impossibility of experimenting in laboratory conditions with these animals and the frequent occurrence of confounding factors that hamper the establishment of cause-effect relationships. Examples of these factors are the fact that pollution always occurs as a mixture of a large number of chemical compounds, the lack of data on biological variables influencing tissue levels, quality of samples usually analyzed, the limited information on pathology and occurrence of disease in the specimens studied, the absence of reliable population data, and the lack of information on the influence of other detrimental factors, such as the impact of fisheries and of other human-related sources of disturbance.
I. Substances of Concern
In general, the concept of pollution incorporates many different substances to which marine mammals are exposed and might affect their health adversely. These include chemical compounds, oil pollution-derived substances, marine debris, sewage-related pathogens, excessive amounts of nutrients causing environmental changes, and radionuclides. The influence of oil and petroleum-derived compounds, such as polycyclic aromatic compounds, of marine debris, of sewage-related pathogens, and of nutrients-related changes, such as the occurrence of biotoxins, has not been the subject of focused research in marine mammalogy. As a consequence, data on these pollutants, either as concentrations in tissue of the affected marine mammals or as effects on them, are extremely limited. This article therefore only addresses pollution caused by chemical substances.
Traditionally, most laboratories tended to routinely analyze organohalogenated compounds such as DDT, DDE, DDD, polychlorinated biphenyls (PCBs), lindane, dieldrin, endrin, hexachlorobenzene (HCB), heptachloro-epoxide (HEPOX), and mirex, and trace elements such as mercury, lead, selenium, and cadmium. Some laboratories, able to use more sophisticated equipment, have also analyzed polychlorinated dibenzo-dioxins (PCDDs) and polychlorinated dibenzofuranes (PCDFs). Such a narrow approach brings the risk of overlooking the impact of other, poorly known compounds. However, the monitoring of all known synthetic organic chemicals and their metabolites currently in use would require analysis of about 300,000 compounds. Therefore, criteria have to be developed to identify priority compounds on which to focus monitoring. Criteria for the identification of these compounds should include the level of production and release into the environment, bioaccumulation potential, and toxicity. Examples of “novel” compounds that fall into the category of priority compounds are organotins, polybrominated biphenyls (PBBs), and polybrominated diphenyl ethers (PBDEs).
Because research funds are limited, another issue to be addressed is the choice between monitoring pollutant concentrations or investigating their effects. The latter option is in our view clearly preferable. If an effect is observed, more focused research for the responsible compounds can follow.
Taking into account the two elements discussed earlier and without ignoring the potential impact of other compounds, it is currently accepted that a list of compounds of highest priority should include all organohalogens usually referred to as persistent organic pollutants (POPs), particularly including PCBs, DDTs, PCDDs, HCB, dieldrin, endrin, mirex, PCDFs, PBBs, PBDEs, polycyclic aromatic hydrocarbons (PAHs) and phenols, and metals, particularly including their organic forms such as methyl-mercury and organotins.
II. Pollution from an Environmental Perspective
Pollution is only one of the many environmental factors that influence the health status of marine mammals. The assessment of the impact of pollution on marine mammals has therefore to be undertaken in a holistic perspective, considering also the potential of pollutants to interfere with their ability to recuperate from stress caused by other environmental forces. As an example, PCBs could cause immune suppression in a given seal population without directly leading to an increased mortality. However, if such a population is exposed to an introduced virus, the extent of a resulting epidemic is likely to be much aggravated.
Marine ecosystems are complex, and environmental forces operating on populations are often multifactorial and produce synergistic or cumulative effects. Therefore, it will be complicated to attribute a given effect to a single factor. To illustrate the complexity of unraveling the impact of pollution, we discuss here some of the environmental factors, natural or anthropogenic, that influence the resilience of marine mammals to pollution.
A. Prey Depletion
Natural environmental variations such as redistribution of planktonic organisms may bring changes in distribution, abundance, or recruitment of the species that constitute the food of marine mammals. However, depletion of prey may often also be caused by overfishing by commercial fisheries. Depending on the extent of the depletion, marine mammals may respond to the reduced supply of prey either by switching to other species, or by temporarily moving to another area. Frequently, however, they undergo an impoverishment of their body condition and their recruitment rates become lower. The resilience of animals in such populations/stocks is affected negatively, potentially increasing the detrimental impact of pollutants.
B. Habitat Disturbance
Habitat may be disturbed by a wide range of human activities, including recreation, construction works, and many others. For pinnipeds, sea otters (Enhydra lutris), and some coastal cetaceans, the physical alteration of the litoral, including the mere presence of humans and their associated infrastructure, may be a significant detrimental factor. Noise pollution is a particular source of concern because many marine mammals rely on sound emission and detection for finding their prey, communicating, and navigating. Activities producing noise-related disturbance include shipping, boating, military maneuvers, seismic testing, and oil and gas drilling.
Natural factors influence the incidence of disease. For example, a shift in distribution of prey species may lead to an increased parasite infestation rate likely to affect the resilience of populations to pollutants. Although the incidence of infectious disease in marine mammals is poorly known in general terms, morbillivirus epizootics that have recently affected pinnipeds as well as cetaceans have elicited extensive research on the effects of viral diseases on marine mammal populations.
Deadly bacterial diseases are generally considered to be secondary to other conditions such as viral disease, parasitic infection, or trauma. However, like some pollutants, bacteria can also interfere with reproduction, as was demonstrated by the finding of Brucella organisms in porpoises and dolphins.
In marine mammals, similarly to other better studied groups of vertebrates, disease and the toxic effect of pollutants are of ten interrelated. This relationship is discussed in more detail in the next section, although it should be mentioned here that diseases can affect metabolic systems and, consequently, alter physiological functions. Chronically diseased females, for example, usually have a poor reproductive performance, as do females affected by some pollutants.
D. Overall Environmental Changes: Global Warming, Ozone Depletion
Albeit the potential effect of global changes on marine mammals has been little investigated and its consequences are considered less imminent than those caused by other factors, this matter certainly deserves concern. It is predicted that the global rise in temperature will alter marine communities and their productivity, cause a sea level rise, reduce ice cover, and modify rainfall and water current systems. The consequences for marine mammals are unclear, but undoubtedly those alterations will affect their behavior and distribution. An increased incidence of epizootics among pinnipeds is also postulated, as higher densities as a result of increased haul-out behavior will result in a higher transmission rate of infectious agents.
Despite the longer term character of these threats, changes in the distribution and behavior of marine mammals caused by climate variation should be monitored to detect potential relationships at an early stage.
The examples just mentioned show clearly that studying the impact of pollution on marine mammals requires a multidisci-plinary approach. Therefore, we advocate assessing pollution impact not in an isolated way, but always in relation to other environmental factors.
III. Factors in Assessing Pollution Impact
Two sources of information may warn that pollution might affect a given population: high tissue pollutant concentrations in the members of that population and changes in the biological parameters of the population such as physiological condition and changes in reproductive or mortality rates. The latter are often derived from population monitoring and/or pathological investigations. However, a number of biological factors and inconsistencies in the sampling and analytical procedures seriously hamper the establishment of such relationships, sometimes even leading to spurious interpretations of environmental data.
A. Biological Factors Affecting Variability in Pollutant Levels
Some persistent chemicals are bioaccumulative and their concentrations in living organisms undergo a progressive amplification through food chains, a process called biomagnifica-tion. However, the increase at each trophic level is usually considerably higher than the 10-fold increase predicted by ecological models. Biomagnification, defined as the ratio of concentrations of a compound in the predator to its prey, can be altered significantly, and often much increased, by a number of variables, such as the route of exposure, the physical and chemical properties of the compound, the metabolic capacity of the predator, and its physiological constitution.
Diet composition is a key factor determining resultant tissue concentrations. Because baleen whales feed on planktonic crustaceans and are thus situated lower in the food web, their tissue organochlorine (OC) concentrations are almost invariably lower than those in the top-predator toothed whales living in the same ecosystem.
Within a population of the same species, OC levels can also differ because of variation in diet. For example, juvenile pinnipeds often exploit different food resources than adults, and in many species of cetaceans and pinnipeds, adult males prey on different species than adult females. In some marine mammals, differences may even be associated with reproductive status: the diet of lactating females of some dolphin species is different from pregnant or resting females. Also, the geographical region where food is consumed is critical: during most of the year, male sperm whales (Phtjseter macrocephalus) occupy different geographical regions than females and, as a consequence, their pollutant profiles are quite dissimilar. Differences in diet are also assumed to have an influence on the tissue concentrations of PAHs in marine mammals. Levels of these compounds in marine mammals are generally low, although they tend to be higher in cephalopod-eating marine mammals than in those relying on fish. The explanation appears to be that the ability of fish to metabolize PAHs is better than that of cephalopods.
Tissue levels of metals also appear to be related to the feeding habits and region of exposure. Cadmium, copper, and zinc levels are higher in cetaceans that feed primarily on squid than in those feeding on fish. This is attributed to the ability of squid to retain these elements selectively. Intraspecific differences in tissue metal concentrations have also been linked to segregation in feeding areas; the levels of lead in kidney and muscle tissue of long-finned pilot whales (Globicephala melas) and white beaked dolphins (Lagenorhynchus albirostris) occurring during summer in the same areas are much different because they segregate geographically—and feed—during the winter.
C. Age and Sex
The tissue concentration of a pollutant in a marine mammal is a function of the difference between the intake rate and the metabolization and excretion rates. OCs have been found to correlate positively with age; levels are relative low in young animals, increase until a certain age, and then either continue to increase or reach a plateau level or decrease. The leveling off or decreasing phase is different for males and females, as is addressed later on. Factors that influence the age-related pattern of accumulation of organochlorines are detoxification ability and the feeding rate. The capacity for detoxification is low in young animals and improves with age; thus the initial increase during the juvenile stage is slowed down by improved metabolization and excretion rates. The resulting leveling off of tissue concentrations is enhanced further by reduced feeding rates in adults.
Superimposed on these is the effect of reproduction in females. OCs, as most lipophilic compounds, cross the placenta and reach the fetus, although not all chemicals do it at equal rates. For example, the lower chlorine-substituted (lower weight) congeners of PCBs are transported more easily than higher chlorinated ones. In addition to placental transport, OCs are also transferred from mother to offspring through milk. Higher chlorinated OCs are transferred less efficiently from the lipid tissue of the mother to her milk and hence to the suckling calf or pup. This process obviously does not start until the females reach sexual maturity and become pregnant for the first time. Therefore, the first pregnancy marks the start of the leveling off or decrease phase in females. There are differences among species and compounds. Moreover, this reproductive discharge in females is not uniform and depends on the characteristics of the reproductive cycle of the species and the physicochemical properties of the compound. The transfer during lactation is much higher than that occurring through deposition in the tissues of the calf or pup during pregnancy. In cetaceans, the discharge of PCBs, expressed as percentage transferred in relation to maternal tissue load, ranges from 5 to 96% during lactation and from 4 to 6% during pregnancy. In pinnipeds, the ranges are 23-81 and 1-10%, respectively. Not surprisingly, the length of the lactation period significantly influences the proportion of the OCs’ load transferred to the offspring. It has been estimated that this proportion ranged from 3 to 27% in fin whales (Balaenoptera physalus), with a lactation period of around 7-8 months, whereas it was around 80% in bottlenose dolphins (Tursiops truncatus) and 72-91% in striped dolphins (Stenella coertileoalba), two species in which lactation lasts about 14 months. Irrespective of the amount transferred, the reproductive discharge results in lower levels of lipophilic pollutants in reproductively active females as compared to males of the same age. However, there are some exceptions to the general rule. In Antarctic minke whales (Balaenoptera bonaerensis), levels of PCBs and DDT were found to be higher in immature males than in mature males as a result of a shift in diet caused by adult migration to less polluted areas. In the North Atlantic, adult female sperm whales are more polluted than males of comparable age because they feed on more polluted species and are distributed year-round in regions where pollutant loads are higher.
Age-related variation in tissue concentrations of trace elements is less homogeneous. Mercury, cadmium, selenium, and lead increase with age, somewhat more steeply in females compared to males. There is no clear leveling off for any element except for lead, in which a slower increase has been observed at an older age. Because these elements are not lipophilic, reproductive transfer does not affect their loads in females. It has been suggested that the higher levels of those elements found in females compared to males may be related to differences in metabolic pathways linked to hormone cycles.
Information on other trace elements is scarce. Copper and zinc show no increase with age. In fact, concentrations in newborns are higher than in adults, which is attributed to an age-related decrease in absorption and retention of these essential elements.
D. Nutritive Condition
Nutritive condition affects the volume of fat in the body and its lipid composition. In some cetaceans and pinnipeds, blubber lipid richness may decrease from 90% in a female near term to 30-35% in females just having weaned their offspring.
Although less impressive, males also show changes in blubber layer thickness during the reproductive season. Apart from this reproduction-related change, seasonal variation may also be significant. Variation in blubber layer thickness is lower in toothed whales compared to baleen whales. In some pinnipeds, independently of the reproduction-related changes, blubber layer thickness can vary by as much as 50% (taking the maximum thickness as a reference). This variation has implications for the dynamics of lipophilic contaminants. Because lipids are mobilized more readily from the blubber than lipophilic pollutants, lipid metabolization typically results in an increase in the residue levels. However, it has been found that the increase is less than a kinetic concentrative model would predict. It has been suggested that the more polar fraction of the pollutants is mobilized more readily through the enhanced metabolization and excretory capacity stimulated by a rise in tissue pollutant concentrations subsequent to lipid metabolization.
It is unclear to what extent changes in nutritive condition affect tissue concentrations of nonlipophilic compounds. Changes in mass and composition of tissues where chemicals (e.g., heavy metals in liver and kidney) are likely to accumulate influence the dynamics of these pollutants, but data on these processes in marine mammals are lacking.
Body growth in young animals also influences tissue level of pollutants. In both pinnipeds and cetaceans it has been found that dilution of contaminants occurs in the early stages of growth due to the rapid deposition of blubber and the amassing of liver and kidney tissue. Calculations of tissue concentrations on a lipid basis instead of a fresh weight basis can partially account for such variation, but it does not account for variation in the qualitative composition of the lipid fraction, which is also likely to affect the retention ability of the tissue.
E. Body Size
The influence of body size on variation in the accumulation pattern of pollutants is somewhat complex. Generally, elimination rates of xenobiotic compounds per unit of bodyweight are related inversely to body weight, a trend that also holds for the activity of detoxifying enzymes. Both would tend to favor accumulation of higher pollutant levels in larger animals. Contrary to that effect, the metabolic rate is inversely correlated to body size. Because metabolic rate is correlated with pollutant intake, a higher pollutant accumulation can be expected in smaller species. The influence of metabolic rate has been found to outweigh the countereffect of elimination and detoxifying activities. The concentration factor in a marine mammal is largely dependent of its daily rate of food consumption—inversely related to body size—and the mean concentration of pollutant in its prey. Small animals therefore carry generally higher loads of pollutants relative to their body weight than larger animals.
Variation in body size is more dramatic in cetaceans than in pinnipeds. Some dolphin and porpoise species weigh, when adult, about 30-40 kg, whereas the larger whales can weigh more than 150,000 kg. The range in adult pinnipeds varies from 50 to 4000 kg. An example of variation in pollutant levels between two species of different size is that of two krill-eating Atlantic baleen whale species, in which differences in tissue pollutant levels were explained by differences in body mass. It has been proposed that in species sharing the same waters, the effect of body mass on tissue concentration outweighs that of the small differences in diet or other biological traits.
F. Body Composition
The distribution pattern of pollutants in the body of an animal depends largely on the physical and chemical properties of the substances involved. For example, much work has been carried out to investigate die influence of die position of H atoms on die biphenyl ring in all PCBs, which largely determines the possibilities for their metabolization by marine mammals.
Because lipophilic pollutants accumulate in fatty tissue, about 70-95% of lipophilic pollutants end up in the blubber, which in marine mammals is the largest fat compartment. The chemical composition of the blubber also influences pollutant concentrations. In species with thick blubber, pollutants are stratified in the different layers and significant differences may be found between inner and outer strata. Therefore, the whole blubber layer must be sampled to obtain a representative picture of the individual’s load. Mercury, cadmium, zinc, and other heavy metals accumulate mostly in the liver and kidney, and lead accumulates predominantly in bone tissue.
G. Analysis and Sampling
One of the major handicaps in assessing temporal and spatial trends of contaminants in marine mammals is the poor comparability of data. This holds partly for heavy metals, but it is definitely critical for analyses of OCs. The analytical techniques used, and their accuracy, have changed considerably over time and also varv between different laboratories. This greatly hinders comparison between studies undertaken by different laboratories or time periods. Significant improvement in standardizing procedures has been achieved in the last decade through intercalibration exercises. Quality assurance and quality control are of utmost importance, but this also holds for the sampling procedures. To avoid contamination by the packaging material, clean glass or aluminium foil should be used to preserve samples for OCs analyses, and plastic bags should be used to preserve samples for heavy metal analyses. Each sample should be accompanied by the appropriate biological data and, if possible, also with a detailed pathological examination to reveal the incidence of alterations in reproductive biology, early development, and occurrence of diseases. Detailed field and laboratory protocols taking these considerations into account have to be developed before embarking on any ecotoxicological study.
IV. Impacts of Pollution on Marine Mammals
Numerous studies have suggested that exposure to pollutants has an impact on marine mammal populations, mainly on reproduction and mortality. However, in most of these studies, the existence of confounding factors prevents reaching conclusive results and only a few have actually succeeded in demonstrating such a relationship.
The effects of pollution, either observed or suggested, can be grouped conveniently under three categories: impaired reproduction, indirect mortality, and direct mortality.
A. Impaired Reproduction
OCs, particularly PCBs, have been demonstrated to be responsible for impaired reproduction in the harbor seal (Phoca vitulina). This conclusion was reached by means of a feeding experiment in which 12 female harbor seals were fed diets low in OCs and 12 females received a diet high in OCs, particularly PCBs and DDE. The conclusion was that reproductive success was significantly lower in the more polluted diet group: 4 pups were born instead of 10 born in the control group. The latter figure is similar to what is normally found in free-ranging harbor seals. In addition, the analysis for estradiol-17(3 and progesterone in blood samples from these seals revealed that reproductive failure occurred at the implantation stage, as such failure was accompanied by low levels of oestradiol-17p. A plausible explanation of this effect is that PCBs impaired the enzymatic metabolism, lowering the circulating levels of estradiol, which in turn led to imperfect endometrial receptivity and prevented successful implementation of the blastocyst.
Elevated OC concentrations have been associated with reproductive impairment in gray seals (Halichoeriis grypus) and ringed seals (Pusa hispida) in the Baltic Sea and in California sea lions (Zalophus californiantis). Female Baltic gray and ringed seals exhibited uterine occlusions and stenosis, leading to partial or complete sterility; concentrations of OCs were higher in affected animals than in normally reproducing females. It has been proposed that pregnancy was interrupted by PCBs (or PCB-metabolites), followed by the development of pathological disorders. Epidemiological studies on the involved populations strongly support the hypothesis that PCBs or their metabolites, i.e., methyl sulfones, are responsible for the observed reproductive impairment. This has been apparently confirmed by the fact that the incidence of pathological conditions in younger but mature age classes decreased. OC levels in seals as well as other Baltic biota declined sharply between 1970 and 1980. However, unequivocal evidence for a cause-effect relationship has not been provided, although this stage of proof is probably as far as one can get with the constraints of this type of field research.
The case of the California sea lion is even more complex. Initially, still births and premature pupping were attributed to high OC (PCBs and DDE) concentrations. Later studies demonstrated that pathogenic disease agents could also have been responsible. These confounding factors prevented reaching a clear-cut conclusion on the causative role of pollution.
The proof for reproductive disorders in cetaceans caused by specific pollutants is even weaker than for pinnipeds. Impaired reproductive performance caused by PCBs has been suggested in beluga whales (Delphinapterus leucas) in the St. Lawrence River. In 2 out of 120 examined belugas, hermaphroditism was observed. However, the pathological studies were not conclusive, and the lack of sound population data which with to compare the observed findings made it impossible to reach a conclusion on the actual role of pollutants on such abnormalities.
Low levels of testosterone were associated with high levels of PCBs and DDE in Dall’s porpoises (Phocoenoides dalli). However, the biological significance and underlying mechanism are unclear because both variables are age related; further studies are needed to clarify the potential involvement of pollutants.
Abnormal testes, i.e., transformed epididymal and testicular tissue, were observed in North Pacific minke whales (Balaenoptera acutorostrata). A possible relation with high levels of OCs has been suggested, but not proved.
Numerous pathological disorders, including skull lesions (paradenitis, osteoporosis, exostosis), cortical adenomas, hyperkeratosis, nail malformations, uterine stenosis and occlusions, uterine tumors (leiomyomas), and colonic ulcers, have been observed in Baltic gray and ringed seals and, to a lesser extent, in harbor seals. Pathological and epidemiological investigations revealed that the observed symptoms were part of a disease complex called hyperadrenocorticism, a disease syndrome associated with high levels of PCBs and DDT and their metabolites. Contrary to reproductive impairment, it is not possible to evaluate conclusively which of these substances elicit a response in seals because of crossed or synergistic effects. As in the case of reproductive disorders, the prevalence of uterine lesions, adrenocortical hyperplasia, and skull bone lesions was found to decrease following a decline of DDT and PCBs in Baltic biota. Conversely, however, the incidence of uterine leiomyoma in Baltic seals has not changed to date. Of even more concern is the increasing incidence during recent years of colonic ulcers in young Baltic gray seals, indicating an increasingly compromised immune system in these animals. DDT tissue levels in these animals decreased strongly between 1969 and 1997, annually by 11-12%. but PCB levels decreased during the same period at a much lower pace, only 2^1% annually. This may suggest a role of PCBs and/or their metabolites in the observed pathologies, although the potential effect of novel, unknown compounds cannot be excluded.
Some studies have shown direct evidence of the immuno-toxicity of OCs. Reduced immune responses were correlated with high levels of PCBs and DDT in in vitro immune function assays with peripheral blood lymphocytes from free-ranging bottlenose dolphins. In an experiment with captive harbor seals, in vitro and in vivo immune function tests showed lower immune function related to higher dietary concentrations of OCs. While these two studies show that OCs adversely interfere with immune function, the toxicological and biological significance unfortunately remains unclear.
It has been suggested that lowered immunocompetence induced by contaminants aggravated the die-offs of bottlenose dolphins in the Gulf of Mexico (1990, 1991, 1993) and on the east coast of the United States (1987-1988), striped dolphins in the Mediterranean Sea (1990-1992), harbor seals in the North Sea (1988), Baikal seals (Pusa sibirica) in Lake Baikal (1987-1988), and Caspian seals (P. caspica) in the Caspian Sea (2000). In most cases the mortalities were ultimately caused by a morbillivirus infection, but exposure to high levels of OCs was proposed to have played a key role by facilitating viral transmission and increasing the susceptibility of individuals to the disease. However, it has been difficult to conclude on the etiology of these mortalities. Different studies have tried to establish links between die-offs and pollution. In the case of the striped dolphin morbillivirus epizootic, animals killed by the disease carried significantly higher PCB concentrations than survivors. This finding could be explained by (1) immune suppression caused by PCBs, leading to higher mortality of the more polluted individuals; (2) mobilization of pollutants stored in depot tissues thinned by the disease; or (3) changes in physiological functions of the affected individuals, leading to increased PCB concentrations.
In two other studies, levels of organochlorines were related to mortality. In one study, OC levels in seals that died during the morbillivirus outbreak were compared with those in surviving seals. In the other study, OC concentrations in harbor porpoises (Phocoena phocoena) diat died from physical trauma were compared with animals known to have succumbed to an infectious disease. Both studies were inconclusive in establishing a direct cause-effect relationship between pollutants and susceptibility to disease because of the existence of confounding factors such as heterogeneous body condition between the groups compared. A follow-up study on harbor porpoises from England and Wales has been more conclusive. In this study, PCB concentrations in blubber from animals that died due to physical trauma (e.g., bycatch) were compared with those from animals that died because of an infectious disease. A significant association was demonstrated between blubber PCB concentrations and mortality due to infectious disease, suggesting a causal relationship with chronic PCB exposure. Here, again, die possibility of additive or synergistic effects of other contaminants must be considered.
Other ecotoxicological studies point toward other effects of pollutants on marine mammals. It has been proposed that OCs produced thyroid hormone and vitamin A deficiency in at least harbor seals. Thyroid hormones are important in the structural and functional development of sex organs and the brain, both intra-uterine and postnatal. A vitamin A deficiency may lead to increased susceptibility to microbial infections and retarded growth, as appeared to be indicated by the significant lower birth weights of pups bom in the more contaminated dietary group of the captive harbor seal study discussed earlier.
Another noteworthy example of impaired health status possibly caused by pollution is the case of the St. Lawrence beluga population. A range of pathological conditions have been documented in this population, particularly a high prevalence of tumors and digestive tract and mammary gland lesions. High tissue levels of OCs, lead, and mercury have been found in these animals. The establishment of a cause-effect relationship between contaminants and the observed effects in this population is hampered by the possible adverse role of other environmental factors, such as previous overhunting, high levels of noise pollution, and overall habitat destruction. Any of these factors has the potential for causing most of the observed conditions and the population’s small size and slow recovery
C. Direct Mortality
There is no record of any acute chemical poisoning event affecting marine mammals, apart from one case that affected harbor seals: a small colony had been acutely poisoned by an accidental discharge of mercury-contaminated agricultural disinfectant and several deaths occurred.
D. Endocrine-Disrupting Chemicals
Concern has been expressed about xenobiotic-induced endocrine disruption in wildlife and humans. Adverse effects of contaminants on mammalian wildlife through the modulation of endocrine systems are documented predominantly in fish-eating (aquatic) mammals. Indeed, a large number of xenobi-otics with endocrine-disrupting properties, such as OCs, have been detected in marine mammal tissue. In previous sections, reproductive and nonreproductive effects, including possible links with the functioning of the immune system, have been discussed in relation to these pollutants. Except for the reproductive toxicity in harbor seals and Baltic seals, evidence of a causal link between endocrine disruption and observed effects is weak or nonexisting. Most often neither a positive proof nor a dismissal, simply a negative endocrine-like effect, could be provided. The reasons for lack of proof are the unavailability of reliable population data, the potential interaction between the many pollutants present, the role of disease agents and other environmental factors, the lack of biomarkers to assess endocrine effects, and the little research on early development in marine mammals.
V. Species Vulnerability
The impact of pollution on marine mammals can occur throughout the entire chain from exposure, uptake, metabolism, and excretion. Concentration in prey is a determining factor. Generally, coastal species are exposed to higher environmental levels than more pelagic species, and species occurring in industrialized (including intensive agricultural) areas usually have higher pollutant levels compared to animals in less developed regions. Among marine mammals, coastal seals and dolphins usually carry the highest tissue residue levels. Superimposed on that is the preferred trophic level of feeding. In the same water mass, species feeding at lower trophic levels are exposed to lower levels of pollutants compared to species feeding higher in the food chain. This is why pollutant levels are almost always lower in baleen whales than in toothed whales. Exceptions to this pattern have been discussed earlier, e.g., for metals in species feeding on squid rather than on fish.
As mentioned earlier, females get rid of pollutants through reproduction. Species that reach sexual maturity at a younger age are at an advantage compared to those that start reproducing at an older age. Early reproduction is also positive for the offspring. The amount of pollutants descendants receive is lower if mothers initiate reproduction activities early because they have not yet built up high tissue concentrations. Similarly, an earlier onset of sexual senescence is a disadvantage in this respect because it halts the discharging process. A protracted lactation period is clearly beneficial for reproductive females because the amount of lipophilic pollutants that they transfer is high. This obviously depends on the time they start to feed again because then the pollutant uptake will counterbalance the discharge. However, the protracted lactation period may have adverse effects on the offspring because the milk is often more polluted than the food that decendants will consume once weaned. It is unclear how this resolves at the population level, i.e., whether the benefit for the reproductive female is higher or lower than the costs for the offspring.
A factor likely to lead to differential vulnerability between species is body size. Small species generally have higher levels of pollutants relative to their body weight than those of larger body mass.
Metabolization is another operative factor in this context. The P450 enzyme system is the main physiological tool for metabolizing OCs. For example, this system can be induced by PCBs, mediated by the arylhvdrocarbon (Ah) receptor, which is found in mammals and birds. The metabolic ability, however, is not uniform among marine mammals. Overall, cetaceans have a lower metabolization capacity, as measured by pheno-barbital (PB) and metbylcholantrene (MC) types of activity. Initially, all cetaceans were thought to lack the (PB) type of enzyme. However, research has shown that several dolphin species possess at least some microsomal PB type of enzyme. Still, their PB and MC type of metabolic activity is usually lower than that of pinnipeds and terrestrial species. At a more specific level, ringed seals and harbor porpoises seem to have metabolic capacities intermediate between those of other seals and cetaceans. In conclusion, apart from the more apparent cetacean-pinniped difference in metabolic capacity, sharp differences also exist among species within any given taxa.
The critical question in this respect is, however, whether a low activity of PB-type and/or MC-type enzymes renders cetaceans more vulnerable to pollution, as has been suggested repeatedly. This may not automatically be the case. For example, PCBs can potentially elicit toxicity in at least two ways: as parent compounds (persistent congeners) and as metabolized congeners. The persistent compounds show a PB and mixed PB and MC type of toxicity associated with liver hyperproliferation, lowered levels of thyroxin and vitamin A, and a dioxin type of toxicity (MC) resulting in thymic atrophy, dermal disorders, and liver necrosis. Metabolization of parent compounds can result in at least two contrasting effects: a decreased level of dioxin type of toxicity and an increased metabolic-specific toxicity such as immunotoxicity. The resultant effect of a lower metabolization capacity therefore depends on the relative contribution of the mitigating influence of a decreased dioxin type of toxicity vs a continued PB and mixed PB/MC induction and the effect of reactive intermediates.
In this respect, attention should be drawn to the often misused concept of toxic equivalency. This concept is based on structure-activity relationships of contaminants with receptors. Tetrachlorodibenzo-dio.xins (TCDD) and PCBs have a structure that fits the Ah receptor. The degree of induction by TCDD has been correlated with their toxic effects observed in laboratory animals. Given the similarity in structure of PCB congeners, the ability of these latter compounds to induce the Ah receptor-mediated response is expressed as a ratio to the induction by TCDD. This is called the toxic equivalency factor (TEF), which has been used extensively to assess the toxicity of PCB congeners and their mixtures with DDT and PCDD. That toxicity is calculated by multiplying the TEF of each compound by its concentration, and the sum of the resulting values is considered to be the total toxic equivalent (TEQ) for the mixture of compounds found in the sample.
However, it needs to be stressed that TEFs are based on laboratory animal models. Therefore, the TEQ for a given marine mammal sample only means the effect that the mixture of compounds found in that sample would have on a laboratory animal. Because (1) large differences between species exist in the induction of P450-based enzymes, (2) the toxicities of PCB metabolites are not incorporated into the calculations, and (3) the toxicity of modes other than that of a dioxin type are disregarded, the application of TEQ to assess the toxicological risk to which a particular species is subject to is not necessarily reliable. The same holds for extrapolating TEQ between species. We would therefore emphasize that the frequently used practice of assessing whether the toxic significance of a certain value of TEQ found in a marine mammal is lower/higher than a TEQ value found in a species where effects were observed is unfounded and scientifically unsound.
Another issue that remains to be clarified is the apparent ability of marine mammals, particularly observed in species in the northern arctic regions, to tolerate high levels of some heavy metals, such as mercury, lead, and cadmium. It is known that marine mammals are able to detoxifv these metallic compounds by, for example, demethylating the highly toxic form of organic mercury into the less dangerous inorganic mercury, by the binding of metals to metallothioneins, or by the binding of selenium to mercury where inactive salts are produced. It is tempting to speculate whether the animals in those areas have evolved responses to mitigate the effects caused by the naturally occurring contaminants.
VI. Developments in Spatial and Temporal Trends of Pollutants
Data on levels of pollutants in marine mammals are more numerous for western Europe, North America, Canada, and Japan. Limited data are available for many other countries and regions (e.g., Africa, New Zealand, India), and very little information is available for the Southern Hemisphere. As mentioned in an earlier section, the fish-eating marine mammals from die midlatitudes (industrialized and intense agricultural use) of Europe, North America, and Japan have the highest loads (see Fig. 1). Residue tissue concentrations are lowest in the upper north polar region and the Antarctic. Nearly all of the OC contamination in marine mammals in the Arctic and Antarctic has reached these areas via atmospheric transport. Levels of the more volatile OCs are higher compared to PCBs and DDT and are distributed more homogeneously. This pattern of distribution of residue concentrations in marine mammals, however, is gradually changing. Levels of OCs are declining in the midlatitude areas, whereas they are increasing in regions distant from pollution sources. The transfer of OCs released in (sub)tropical countries to the atmosphere causes global redistribution. It is predicted that in the near to midterm future the Arctic and, to some extent, the Antarctic will become the major sinks for OCs.
Temporal trends of contaminants in marine mammals have been relatively little investigated because of the lack of long time series of samples and lack of comparability of the analytical results.
For PCBs, DDT, mercury, lead, and cadmium, some data on tissue concentrations in marine mammals from certain areas are available. In most heavily industrialized and agricultural regions, the production and use of DDT and PCBs was halted in the early 1970s. From the mid-1970s onward, levels of DDT and PCBs in marine mammal tissues decreased. The decline in DDT levels was stronger than that of PCBs. In pinnipeds the decline was 80-90% for DDTs and 60% for PCBs. The difference is most likely due to less stringent control measures for PCBs; large quantities of these compounds have remained in use in many applications. The overall time trend for PCB and DDT levels in marine mammals is that concentrations have decreased since the mid-1970s. The decrease in DDT levels has continued thereafter. However, PCB levels in some areas leveled off at the end of the 1970s/early 1980s. Figure 2 shows the compartmentation of the global budget of all produced PCBs bv industry. Given the fact that only 1% of all the PCBs produced has reached the oceans, that 35% are still in use, 30% are accumulated in dump sites, and that the fate of the other 34% is unknown, it is expected that the observed leveling off of the decrease in marine mammals will not be followed by a strong reduction in the near future. Trends for heavy metal pollution are less apparent. In general, it is accepted that between the mid-1970s and mid-1990s there was no clear trend for mercury and cadmium in pinnipeds from the Canadian Arctic and Greenland. On the contrary, levels of mercury and lead in pinnipeds from the Wadden Sea have decreased considerably.
VII. A Fundamental Approach to Address Pollution Impact on Marine Mammals
It is clear that a considerable amount of fundamental research is needed before it will be possible to adequately address the impact of pollutants on marine mammals. Realizing this situation, the International Whaling Commission (IWC), through its scientific committee, has developed a comprehensive program to investigate pollutant cause-effect relationships in cetaceans: “Pollution 2000 + .” At a later stage, the International Council for the Exploration of the Sea, through its Marine Mammal Habitat Working Group, developed a similar research program, this time focusing on pinnipeds.
Figure 2 Compartmentation of global budget of produced PCBs (kilotons).
The ultimate objective of pollution studies as related to marine mammal management is to determine a predictive model to link tissue pollutant levels with effects at the population level. This is obviously a longer term goal. It is realized that if any progress is to be made within a reasonable time frame, a multidisciplinary, multinational focused program of research is required that concentrates on species and areas where there is most chance of success.
The mentioned programs earlier focused on PCBs because these chemicals can be used as model compounds of OC pollution. Moreover, PCBs are found at extremely high tissue levels in cetaceans, their effect on mammals are well known, and substantial information is available on their patterns of variation, geographical distribution, and tissue kinetics. The focal species in the IWC program are common bottlenose dolphin and harbor porpoise because both species occur in waters extending over a gradient of pollution and are likely to provide reasonable sample sizes.
In this program, two short-term objectives were established: (1) to select and examine a number of biomarkers of exposure to and/or effects of PCBs and try to determine whether a predictive and quantitative relationship with PCB levels in certain tissues exists and (2) to validate/calibrate sampling and analytical techniques to address such questions for cetaceans, specifically: determine changes in concentrations of variables with postmortem times and examine relationships between concentrations of variables obtained from biopsy sampling with those of concentrations in other tissues that can only be obtained from fresh carcasses.
Because the ultimate aim of the program is to look at potential effects of pollutants at the population level, it was considered necessary to test and develop techniques to feasibly collect data from large numbers of free-living animals. Because biopsy techniques allow such a type of sampling, an initial step in the project has been to calibrate information obtained from biopsy sampling with that collected from dead animals. It was also considered similarly important to ascertain the influence of postmortem time on levels of contaminants and on indicators of exposure and effect. This calibration is needed to ensure that collected samples are representative of actual pollutant loads.
It is clear that Pollution 2000+ is a core program to address some fundamental questions. It does not imply that other research on pollutants and marine mammals is not important. On the contrary, its value is enhanced by cooperation with existing studies and as a context for the development of new programs.