principal diet; herring, cod, squid, and mackerel for the gray seal and 

 herring, squid, alewife, smelt, and mackerel for the harbor seal. Field 

 studies conducted by Richardson (1973b) in Maine suggest that seals forage for 

 food, often diving and surfacing in local areas for periods of time. Maine 

 seals probably feed on all flatfish species, sculpins, and schooling fishes. 

 Crustaceans comprise an insignificant fraction of the seal's diet (Richardson 

 1973b). Alewives are a major food item of seals utilizing the upper estuaries 

 in late spring. Herring also appears to be a preferred prey species and may 

 determine local movements and distribution of seals. Richardson (1973b) 

 calculated the hypothetical predation by seals on finfish stocks. Using 

 estimates of seal populations from his surveys (6000 sighted, assumed 7500 

 maximum) , daily food intake of 6% of body weight for 300 days (Sergeant 1973 

 and Spaulding 1964) and a mean weight by year class and life table from Bigg 

 (1969), Richardson (1973b) calculated that Maine seals consume about 18 

 million pounds of finfish annually. In comparison to Maine's commercial 

 fishery, seals would appear to consume the equivalent of 14% of the average 

 number of pounds of fish landed annually in Maine from 1967 to 1976. Since 

 the total abundance of fish stocks remains unknown, it is not possible to 

 determine whether seals are in serious competition with people for some fish 

 species . 



FACTORS AFFECTING DISTRIBUTION AND ABUNDANCE 



The major abiotic factors that influence the distribution and abundance of 

 marine mammals are water temperature, currents, and physiography but 

 supporting data are scarce. The following factors are better known and will 

 be discussed in terms of their influence on populations of these mammals: 

 food availability, disease and parasites, predation, hunting, pollutants, and 

 habitat disturbance or alteration. 



Food Availability 



One of the major biotic factors controlling the distribution and abundance of 

 marine mammals is food availability. Seasonal distribution of squid, 

 schooling fishes, and zooplankton may determine local populations of marine 

 mammals. Aggregations of marine mammals at offshore banks have been observed. 

 Evidence exists of a drastic change taking place in the summer distribution of 

 humpback whales on feeding grounds in Canadian waters (Gaskin et al. 1979). 

 Inshore movements of humpbacks from Grand Banks and associated offshore 

 shallows may be in part due to overfishing of capelin stocks there. The 

 humpbacks may be moving inshore in search of alternative food supplies (e.g., 

 herring) . Considering the species composition of the major food items of 

 Maine's whales, porpoises, and seals (table 13-6), there is the potential that 

 overfishing of certain commercial fish species could impose limits on many 

 marine mammal populations. 



Disease and Parasites 



Marine mammals fall victim to a full complement of afflictions, knowledge of 

 which is quite limited because most observations are based on captive animals 

 and must be extrapolated to animals in the wild. There is no evidence that 

 disease and parasites severely impair individuals in the wild. Documented 

 viral infections are rare (e.g., seal pox and viral hepatitis) but bacterial 

 disease is common and is reported to be the single leading cause of death in 



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