methylmercury to other tissues, for example, the brain. Katona and coworkers 

 (1977) discuss research supporting the identification of the biochemical 

 mechanism for mercury de-methylation and storage, perhaps a highly efficient 

 selenium "trap." A one-to-one molar ratio of mercury to selenium has been 

 observed in marine mammal liver tissue and the selenium may aid in binding the 

 mercury to protein molecules (via sulphur bonds), thus preventing the 

 transport of methylmercury. 



Documentation of the physiological effects of metal poisoning in marine 

 mammals is scarce. Ingestion of large quantities of methlymercury has caused 

 severe lesions and damage in harp seals (Tessaro and Ronald 1976). Freeman 

 and coworkers (1975) reported that methylmercury, arsenic, cadmium, and 

 selenium altered the "in vitro" biosynthesis of steroid hormones in gray 

 seals. Methylmercury altered the biosynthesis of steroid hormones in an "in 

 vivo" study of harp seals. This could have serious effects on mineral and 

 water regulation, carbohydrate metabolism, and reproduction in contaminated 

 seals . 



Petroleum . According to Katona and coworkers (1977) data on the effects 

 of oil contaminants on marine mammals are scarce. Nothing of certainty is 

 known about oil effects on cetaceans. Since all whales surface frequently 

 they are potentially in danger of being exposed to surface oil slicks. Whales 

 that are primarily surface feeders, such as the right whale, sei whale, and 

 (on occasion) the humpback and finback whales could be particularly 

 susceptible to surface oil slicks. It is not known whether these animals 

 would actively avoid oil slicks. The available evidence indicates that 

 petroleum hydrocarbons are not biologically magnified through the food chain. 

 Limited studies of oil effects on seal populations reveal either no 

 significant deleterious effects or inconclusive results (Katona et al. 1977). 

 It is safe to presume that the impact of oil pollution will be most severe in 

 populations that are already suffering from poor health or environmental 

 stress, for example, climatic extremes, high density habitat, strong 

 competition for food and space, and demands of reproduction (Geraci and Smith 

 1976). 



Habitat Disturbance 



Habitat disturbance and changes that could influence the abundance and 

 distribution of marine mammals are not well documented. Katona (1977) 

 provides some possible causes and effects. Urbanization and its associated 

 activities (boat traffic and pollution) are detrimental to the occurrence and 

 number of cetaceans in coastal waters. It is difficult, though, to separate 

 the effects of increased shipping from those associated with deteriorating 

 water quality. Areas near port cities tend to have fewer cetaceans than do 

 nearby undeveloped waters, probably due to human activity. Seals are 

 apparently intolerant of human activities at least during the pupping and 

 breeding season. Harbor seals in Maine compete for use of ledges and islands 

 in areas that are valuable to commercial fishing. Fishing efforts and 

 development of coastal shoreline and island property may render certain 

 whelping sites unsuitable. Of the small gray seal breeding colony at Grand 

 Manan, New Brunswick, Canada, Mansfield and Beck (1977) doubt that pup 

 production there will ever build up from a present level of about 15 pups per 

 year to its former level of 200 pups per year, because lobster fishing 

 activity is high during the breeding season. Increased use of islands in 



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