Diet Composition of Pilot Whales 

 Globicephala sp. and Common Dolphins 

 Delphinus delphis in the Mid-Atlantic 

 Bight during Spring 1989 



William J. Overholtz 



Gordon T. Waring 



Woods Hole Laboratory, Northeast Fisheries Science Center 



National Marine Fisheries Service, NOAA, Woods Hole, Massachusetts 02543 



Several species of marine mammals 

 are found in large numbers along 

 the eastern United States and are 

 seasonally abundant in the Mid-At- 

 lantic Bight (Kenny et al. 1983, 

 Payne et al. 1984). Concentrations 

 of pelagic fishes also occur in the 

 Mid-Atlantic Bight area during 

 winter and spring and are preyed 

 upon by marine mammals, seabirds, 

 and piscivorous fishes (Overholtz et 

 al. 1991). Pilot whales Globicephala 

 sp. and common dolphins Delphinus 

 delphis are two of the more abun- 

 dant marine mammals found in the 

 area and potentially account for 

 30% of the total consumption of 

 pelagic fishes by all cetaceans in the 

 region (Overholtz et al. 1991). De- 

 tailed analyses of diet composition, 

 specific rations, and size and age 

 composition of prey consumed by 

 marine mammals are sparse, since 

 few healthy, recently dead animals 

 are available for analysis. Most in- 

 formation on the diet composition 

 of marine mammals found in the 

 northwest Atlantic is from stand- 

 ings, data from commercial whaling 

 operations in Canada in the 1950s 

 (Sergeant 1962), or incidental kills 

 in commercial fishing operations. 

 Pilot whales were thought to feed 

 extensively on short-finned squid 

 Illex illecebrosus and secondarily on 

 Atlantic cod Gadus morhua (Ser- 

 geant 1962, Mercer 1975), but more 

 recently have been shown to feed on 

 several other pelagic fishes and 

 squids in the northwest Atlantic. In 



the area off New England and the 

 Mid-Atlantic, pilot whales coincide 

 with short and long-finned squid as 

 well as Atlantic mackerel and butter- 

 fish Peprilus triacanthus (Smith et 

 al. 1990). Pilot whales and common 

 dolphins captured in foreign fishing 

 operations during the 1980s were 

 feeding on Atlantic mackerel and 

 long-finned squid during winter and 

 spring in the Mid-Atlantic region 

 (Waring et al. 1990). 



Studies quantifying fish consump- 

 tion in the pelagic ecosystem off the 

 eastern United States have sug- 

 gested that marine mammals may 

 be important predators (Kenny et 

 al. 1983, Overholtz et al. 1991). 

 Large numbers of marine mammals 

 may reside in this region, but the 

 impact of these predators on prey 

 resources is difficult to assess since 

 so few studies have documented the 

 seasonal prey, ration size, and other 

 dynamics of mammalian diets. 



The objectives of this study were 

 to describe prey types and quan- 

 tities found in pilot whales and com- 

 mon dolphins, investigate the age 

 distribution of their fish prey, and 

 to assess the trophic role of these 

 two mammals in the region. These 

 data are necessary for understand- 

 ing the sources and magnitudes of 

 predation on fish resources at the 

 age-specific level, a critical compo- 

 nent of ecosystem modeling. 



Manuscript accepted 9 August 1991. 

 Fishery Bulletin, U.S. 89:723-728 (1991). 



Methods 



Stomachs were collected from 5 

 pilot whales and 4 common dolphins 

 that were taken incidental to fishing 

 operations conducted in the Mid- At- 

 lantic region of the eastern United 

 States. Data were collected by U.S. 

 observers on foreign vessels in the 

 offshore Atlantic mackerel trawl 

 fishery. Animals were obtained on 

 five separate occasions from 19 

 March 1989 to 17 April 1989 in the 

 Hudson Canyon area (Fig. 1). Sam- 

 pling involved morphometric mea- 

 surements of total length, girth, 

 various fin and flipper measure- 

 ments, determination of sex of the 

 mammals, and excision of the stom- 

 ach which was frozen for later 

 analysis. 



Contents of the thawed stomachs 

 were sorted to the lowest taxonomic 

 group possible and wet weights 

 were obtained to the nearest gram. 

 Individual fish were weighed separ- 

 ately, while squid were weighed as 

 a group. Mantle length of squid was 

 measured to the nearest mm (Lange 

 and Johnson 1981). Fork length was 

 recorded for mackerel to the near- 

 est mm. Skull shape and otoliths 

 were used to identify the Atlantic 

 mackerel which were in various 

 stages of digestion. Long-finned 

 squid were identified by examining 

 mantles, pens, and beaks. Mackerel 

 were aged by mounting whole oto- 

 liths in resin and counting growth 

 layers at 60 x magnification (Dery 

 1988). 



In some cases mackerel were 

 fragmented usually at the caudal 

 peduncle. Length of these speci- 

 mens was estimated by realigning 

 fragments or by visual reconstruc- 

 tion of the missing pieces through 

 observations of fresher specimens 

 along with a general knowledge of 

 relative body proportions in the 

 caudal region. Measurement error 

 based on this method is unlikely to 

 be greater than 10-20 mm (3-6% of 

 total body length), and is accurate 

 enough for estimating the relative 



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