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



169 



42 N 



Materials and methods 



Study area 



Five fishery areas (Jeffreys Ledge, Stelhva- 

 gen Bank, Cape Cod Bay, Great South Chan- 

 nel, and South of Martha's Vineyard IFig. 11) 

 were selected for their traditional associa- 

 tion with the bluefin fishery and because geo- 

 graphic and bathymetric differences among 

 areas could result in distinct prey communi- 

 ties. The northernmost area, Jeffreys Ledge, 

 is a major bathymetric feature in the Gulf 

 of Maine and is important for commercial 

 fisheries from northern Massachusetts, New 

 Hampshire, and southern Maine. Stellwagen 

 Bank is a distinct bathymetric ridge located 

 on the eastern boundary of Massachusetts 

 Bay (DOC^) that provides close access for 

 ports from Gloucester to Cape Cod. Cape 

 Cod Bay is a large, relatively shallow Gulf 

 of Maine bay that is semi-enclosed on three 

 sides by the land mass of Cape Cod and 

 the south shore of Massachusetts. The two 

 southern areas. Great South Channel and 

 the area south of Martha's Vineyard, are 

 larger regions with less distinct bathymetric 

 features and are separated by Nantucket 

 Shoals. The Great South Channel covers a 

 wide nearshore region running east of Cha- 

 tham and Nantucket and is bordered by the 

 slopes of Nantucket Shoals on the west. The 

 area south of Martha's Vineyard is located 

 on the continental shelf off southern New 

 England and is distinguished from the other 

 areas by its warmer water, predominance of 

 smaller bluefin tuna (<50 kg), and the sea- 

 sonal occurrence of other large pelagic fish, 

 such as marlins and tropical tunas. 



Commercial catch records and trawl sur- 

 vey indices of abundance indicate that At- 

 lantic herring (Clupea harengus) and Atlan- 

 tic mackerel (Scomber scombrus) are the principal pelagic 

 fish species for all these study areas (Clark and Brown, 

 1976; and NOAA, 1998). Atlantic herring occur in lower 

 relative abundance in the region south of New England 

 than in northern regions. Jeffreys Ledge and Great South 

 Channel are primary spawning locations for Atlantic her- 

 ring. Catch records and trawl survey indices indicate that 

 the following prey species have been abundant in the 

 study areas: silver hake (Merluccius bilineahs), butterfish 

 (Peprilus triacanthus), northern shortfin squid (lllex illece- 

 brosus), and longfin inshore squid (Loligo pealeii). 



70°W 



2 DOC (Department of Commerce). 1991. Stellwagen Bank Na- 

 tional Marine Sanctuary, Draft environmental impact statement/ 

 management plan. U.S. Dep. Commerce (DOC», Nat. Oceanic 

 Atmos. Adm., Sanctuaries and Reserves Division, Washington, 

 D.C., 238 p. 



Figure 1 



Map of the five bluefin tuna feeding grounds used as study areas ( * ) July- 

 October 1988-92. Depths are given in fathoms. 



Sample collection and analysis 



Bluefin stomach samples were collected primarily from 

 commercial landings at ports in Massachusetts during 

 the 1988-92 seasons (July-October). Sportfishing tour- 

 naments were a secondary source of stomach samples. 

 Handgear landings (rod and reel, handline, and harpoon) 

 were primarily collected in Gloucester and Cape Cod. 

 Purse-seine landings were also a large source of samples, 

 and were collected in Gloucester, New Bedford, and Cape 

 Cod. A majority of stomach samples was collected during 

 1989 and 1990. A reduced number of samples after 1990 

 was influenced by the increasing practice of gutting blue- 

 fin at sea to sustain a quality product for the sashimi 

 export market. 



Most samples were collected at the docks and process- 

 ing locations of commercial tuna buyers. The vessel cap- 

 tain or tuna buyer was interviewed for location of catch. 



