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



725 



Table 1 



Summary of stomach contents of 5 pilot whales and 4 common dolphins collected in March-April 1989 off the northeastern United 

 States. AM = Atlantic mackerel, LS = long-finned squid. 



1 Only lengths of measurable prey are included here. 



2 Includes weight of all stomach contents for this species. 



3 Specimens 3 and 4 had long-finned squid pens in the stomach from a previous meal. 



4 Atlantic mackerel found in the stomachs could not be measured or weighed individually due to the state of digestion. 



identified by otoliths. The estimated mean weight of 

 mackerel was 1 13-403 g, and modal age was 7 years 

 (Table 1). 



The age distribution of mackerel was similar in pilot 

 whales and common dolphins, with age groups 4-8 

 representing the bulk of the prey items. Age 7 was 

 the dominant age group in the diet of both predators 

 comprising 71% and 56% of the mackerel eaten by pilot 

 whales and common dolphins, respectively (Table 2). 

 This age group is from the very large 1982 year-class 

 of mackerel (Table 2). Values from the food selection 

 index comparing percent in stomachs with the stock, 

 indicate that both predators may have concentrated on 

 mackerel from age group 7. Positive index values were 

 also obtained for ages 5-8 for common dolphins 

 (Table 2). 



A comparison of theoretical daily energy budgets of 

 individual animals with the caloric content observed in 

 stomach contents revealed that, in most cases, the diets 

 represented only a small fraction of the daily require- 

 ment (Table 3). The amount of observed prey in pilot 

 whale stomachs would supply 1.8-28% of the daily 

 theoretical requirement, and 6-44% in common 

 dolphins. 



Discussion 



Pilot whales and common dolphins captured during 

 fishing operations for mackerel in 1989 appeared to 

 feed primarily on mackerel and long-finned squid, 

 although three specimens contained otoliths of hake 

 Merluccius sp. Waring et al. (1990) reported on the 

 diet of 33 common dolphins collected from 1986 to 1988 

 during the same time period and found similar prey 

 items, including the presence of a few hake otoliths. 

 Hake may either be consumed infrequently or the 

 otoliths are entering the stomachs through other prey 

 such as long-finned squid. 



Capture location of the two mammal species was 

 localized (Fig. 1), but captures extended over a 30-day 

 period. The data may represent a normal pattern for 

 winter-spring feeding or an attraction to fishing activ- 

 ity. This study suggests that the captured mammals 

 were not entirely dependent on fishing operations for 

 prey, since long-finned squid were present in stomachs 

 but not in catches by commercial vessels. 



Results from the food selection index analysis sug- 

 gest a slight positive selection for ages 5,6, and 8 by 

 common dolphins, and moderate selection for age 7 by 

 both cetaceans (Table 2). Statistical tests are possible 

 with this index (Ready et al. 1985); however, they 



