Chase: Differencesin diet of Thunnus thynnus at seasonal feeding grounds off New England 



171 



the study period. Twenty-one bluefin tuna stomach sam- 

 ples were collected outside of the five study areas; mostly 

 at two inshore locations north of Gloucester and south of 

 Stellwagen Bank. 



Jeffreys Ledge Atlantic herring were the dominant prey 

 in the 123 stomach samples from Jeffreys Ledge (Table 3). 

 The frequency of occurrence for Atlantic herring ( 74*7^ ) was 

 the second highest and the percentage of stomach-contents 

 biomass iST^'i ) was the highest for any individual prey item 

 among areas. Squid were in nearly half of all samples, 

 but comprised less than 2% of stomach-contents biomass. 

 Atlantic mackerel were the third most common prey for this 

 region {32"r occurrence). All other prey species occurred inci- 

 dentally. Menhaden iBrevoortia tyrannus) and pollock (Pol- 

 lachius virens) were unique to samples from Jeffreys Ledge. 

 Mean stomach-contents biomass (-2.0 kg) was the highest 

 among study areas and few empty stomachs were found. 



Stellwagen Bank As at Jeffreys Ledge, a single species 

 dominated the stomach-contentss from Stellwagen Bank. 

 Sand lance were found in nearly 80% of 93 stomachs and 

 accounted for nearly 70'^^ of the stomach-contents biomass 

 (Table 3). Atlantic herring, squid, spiny dogfish (Squalits 

 acanthias), Atlantic mackerel, and bluefish tuna were sec- 

 ondary prey items, with frequencies of occurrence ranging 

 from 9% to 14%. All other prey species found in stomach 

 contents from Stellwagen Bank occurred incidentally, and 

 there were no species from this area that were unique to 

 the overall studv area. 



dicate a dominant pelagic prey but did include the common 

 occuiTence of demersal prey. Six prey species were only 

 found in Cape Cod Bay. Squid occurred most frequently but 

 accounted for only 2% of stomach-contents biomass biomass 

 (Table 3). The fig sponge was the top prey in terms of per- 

 centage by weight (27%). The diet of bluefin tuna caught in 

 Cape Cod Bay displayed the most diversity among study 

 areas. A total of 16 prey species were identified, of which 

 eight were demersal species. Three species of flounder 

 were identified, representing 9% of the stomach-contents 

 biomass. The occurrence of bluefish tuna as prey in Cape 

 Cod Bay (25%) was the highest among study areas. The 

 amount of food in Cape Cod Bay stomach samples was 

 the lowest for the four Gulf of Maine locations; 60% of the 

 stomachs collected from this area were empty. 



Great South Channel A large number of stomach sam- 

 ples with prey ( 183 ) were collected from the Great South 

 Channel during 1989-91. The most abundant prey was 

 sand lance, occurring in 62% of the stomach samples 

 and accounting for 28% of the stomach-contents biomass. 

 Atlantic herring was also important, with a frequency 

 of occurrence of 27% and stomach-contents biomass of 

 48%. As with Stellwagen Bank and Jeffreys Ledge, squid 

 was an important secondary prey item, and bluefish and 

 Atlantic mackerel were secondary prey of lesser impor- 

 tance. Four species were unique to Great South Channel 

 samples: shrimp i Panda! us spp.), finger sponge (Haliclona 

 oculata), silverstripe halfbeak [Hyporhaniphus unifascia- 

 tus), and Atlantic cod (Gadus morhua). 



Cape Cod Bay Unlike diet composition for the other 

 study areas, diet composition for Cape Cod Bay did not in- 



South of Martha's Vineyard Only 48 stomachs with prey 

 were analyzed from the area south of Martha's Vineyard. 



