NOTE Overholtz and Waring: Diet composition of Globicephala sp. and Delphmus delphis in mid-Atlantic Bight 



727 



cannot be used on these data since current methodology 

 is not available to calculate the variance of stock size 

 estimates based on the VPA. Mackerel school by size- 

 age groups, and the older fish tend to be distributed 

 farther offshore than younger fish (Sette 1950). The 

 observed feeding pattern may reflect the absence of 

 smaller, younger fish offshore and low abundance of 

 older, larger fish (Table 2). 



The age distribution of mackerel in the diet indicates 

 that these smaller odontocetes are probably feeding on 

 the older age groups of mackerel during the winter- 

 spring season (Table 2). This is in contrast to fish 

 predators such as Atlantic cod and spiny dogfish 

 Squalus acanthias that feed on younger age groups of 

 mackerel (Overholtz et. al. 1991). Smaller mouth size 

 of the predatory fish probably limits their prey size. 

 Marine mammals can feed on larger prey and are in 

 direct competition with fisheries targeting these older, 

 larger fishes (Overholtz et. al. 1991). 



Energy budget calculations, when compared with 

 caloric values of stomach contents, indicate that both 

 pilot whales and common dolphins may need to feed 

 several times per day to obtain their needed ration 

 (Table 3). Although the analysis may be biased low and 

 therefore somewhat conservative, it does suggest that 

 the observed stomach contents may represent only a 

 percentage of estimated daily requirement (Table 3). 

 Therefore, stomach data from these cetaceans may 

 underestimate the actual impact on prey resources. 



The role of marine mammals has not been adequate- 

 ly described in many ecosystems since many popula- 

 tions are remote and the autopsy of fresh animals is 

 infrequent. In the northwest Atlantic, most informa- 

 tion on the diet composition of cetaceans has come from 

 stranded incidentally captured animals, and commer- 

 cial harvests (Sergeant 1962, Mercer 1975, Sergeant 

 et al. 1980) or observations of codistributed mammals 

 and prey (Overholtz and Nicolas 1979, Payne et al. 

 1986, Mayo et al. 1988, Smith et al. 1990). In the future, 

 collection of more data on feeding behavior, prey 

 species consumed, and overlapping distributions of 

 mammals and prey on a seasonal basis, as well as a 

 thorough sampling of incidental captures, will be 

 necessary if the trophic role of marine mammals is to 

 be determined. An intensive survey of the entire mam- 

 mal population during this winter-spring period and in 

 other seasons, along with some observations of activ- 

 ity around fishing vessels, would help determine how 

 important the mackerel fishery is in influencing the 

 behavior of smaller cetaceans. 



Acknowledgments 



We thank the foreign fishery observers for their 

 diligent work in collecting stomach samples that we 

 analyzed. We also want to thank N. Shepherd for 

 ageing the fish from this study. Thorough reviews of 

 initial drafts of this manuscript by J. Burnett and 

 G. Shepherd, and advice from D.Hayes on the use of 

 the food selection index, are much appreciated. 



Citations 



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