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Fishery Bulletin 89(4), 1991 



74 



72 



70 



Figure 1 



Mid-Atlantic region of the northeastern United States with 

 locations in the Hudson Canyon area (north lat., west long.) 

 of incidental captures of pilot whales (triangles) and common 

 dolphins (circles) along the 100-m contour (dotted line) dur- 

 ing March-April 1989. 



size distribution of the prey. Partially digested mack- 

 erel were also weighed individually when possible, 

 resulting in a minimum estimate of individual weight. 

 The food habits data were summarized to show diet 

 characteristics of each animal. The age composition 

 data for mackerel from stomach contents were sum- 

 marized by percent number-at-age and compared with 

 year-class size information from virtual population 

 analysis (VPA) (unpubl. data) to discern if diet composi- 

 tion of the mammals was similar to age composition 

 in the stock. To facilitate this comparison, a food selec- 

 tion index was calculated as 



Li 



n 



Pi 



(1) 



where ri is the proportion of prey i in the stomach, and 

 pi is the proportion of prey i in the environment 

 (Strauss 1979). This index ranges from - 1 to + 1 with 

 indicating no predator preference (Strauss 1979). 



In addition, the theoretical daily energy budget of 

 each individual animal was compared with the esti- 

 mated caloric value of the food in the stomach. Indi- 

 vidual weights of pilot whales and common dolphins 

 were obtained from length-weight equations (Lockyer 

 1991, J. Mead, Smithson. Inst., Wash. DC, pers. com- 

 mun. 1989). Individual daily caloric requirements 

 (Kcal/day) were estimated from a relationship between 

 resting metabolism and body weight (Lockyer 1981), 

 with a correction for assimilation and activity (Hinga 

 1979, Kenny et. al. 1983). The energy values of each 

 animal's stomach contents were estimated by convert- 

 ing prey weight to caloric content (Kcal) by using con- 

 version rates of 1.43 Kcal/g for Atlantic mackerel and 

 1.34 Kcal/g for long-finned squid (Steimle and Terra- 

 nova 1985). 



Results 



Pilot whale and common dolphin fed on Atlantic mack- 

 erel and long-finned squid during March and April of 

 1989 (Table 1). A few otoliths of hake Merluccius sp. 

 were found in two pilot whales and one common 

 dolphin. The condition of prey items in both species 

 ranged from slight to well digested, but could be iden- 

 tified to species. In contrast to previous years (War- 

 ing et al. 1990), none of the stomachs contained fresh 

 material, although all the mackerel in pilot whale no. 1 

 were only slightly digested (Table 1). This suggests that 

 all the animals had fed prior to being captured in the 

 trawl. 



The pilot whales that were examined were between 

 400 and 471cm, both male and female (Table 1). Total 

 weight of stomach contents in pilot whales were in the 

 range 620-7200 g and averaged 2555 g (Table 1). Atlan- 

 tic mackerel averaged 71% of the wet weight of stom- 

 ach contents, and long-finned squid comprised the 

 remaining (Table 1). Atlantic mackerel dominated the 

 diets of individual pilot whales in terms of percent 

 weight, except pilot whale no. 2 which fed exclusively 

 on long-finned squid (Table 1). The estimated mean 

 length of mackerel were in the range 355-460 mm, with 

 mean weights of 332-426 g, and modal age of 7 years. 

 (Table 1). Mean lengths of long-finned squid were 

 88- 146 mm and weights 32-93 g, although both of these 

 measurements are probably underestimates since all 

 squid were at least partially digested. 



The four common dolphins were also medium-sized 

 individuals of 170-213 cm (Waring et al. 1990) of both 

 sexes (Table 1). They fed primarily on mackerel, al- 

 though the remains of long-finned squid were present 

 in the stomachs of two individuals (Table 1). It was im- 

 possible to measure any mackerel found in the stom- 

 achs since they were well digested, and they were 



