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Differences in Diet of Atlantic Bluefin Tuna

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16 8 Abstract–The stomachs of 819 Atlan­ Differences in diet of Atlantic bluefin tuna tic bluefin tuna (Thunnus thynnus) sampled from 1988 to 1992 were ana­ (Thunnus thynnus) at five seasonal feeding grounds lyzed to compare dietary differences among five feeding grounds on the on the New England continental shelf* New England continental shelf (Jef­ freys Ledge, Stellwagen Bank, Cape Bradford C. Chase Cod Bay, Great South Channel, and South of Martha’s Vineyard) where a Massachusetts Division of Marine Fisheries majority of the U.S. Atlantic commer­ 30 Emerson Avenue cial catch occurs. Spatial variation in Gloucester, Massachusetts 01930 prey was expected to be a primary E-mail address: [email protected] influence on bluefin tuna distribution during seasonal feeding migrations. Sand lance (Ammodytes spp.), Atlantic herring (Clupea harengus), Atlantic mackerel (Scomber scombrus), squid (Cephalopoda), and bluefish (Pomato­ Atlantic bluefin tuna (Thunnus thyn- England continental shelf region, and mus saltatrix) were the top prey in terms of frequency of occurrence and nus) are widely distributed throughout as a baseline for bioenergetic analyses. percent prey weight for all areas com­ the Atlantic Ocean and have attracted Information on the feeding habits of bined. Prey composition was uncorre­ valuable commercial and recreational this economically valuable species and lated between study areas, with the fisheries in the western North Atlantic apex predator in the western North exception of a significant association during the latter half of the twentieth Atlantic Ocean is limited, and nearly between Stellwagen Bank and Great century. The western North Atlantic absent for the seasonal feeding grounds South Channel, where sand lance and population is considered overfished by where most U.S. Atlantic commercial Atlantic herring occurred most fre­ the International Commission for the catches occur. quently. Mean stomach-contents bio­ Conservation of Atlantic Tunas (NMFS, Previous food habit studies have mass varied significantly for all study 1999). Bluefin tuna are the largest shown that western North Atlantic areas, except for Great South Channel and Cape Cod Bay. Jeffreys Ledge had scombrid species, and the largest tele- bluefin tuna are opportunistic feeders the highest mean stomach-contents bio­ ost occurring in the Gulf of Maine (Big- on a wide variety of finfish, cephalo­ mass (2.0 kg) among the four Gulf of elow and Schroeder, 1953). Bluefin tuna pods, and crustaceans (Crane, 1936; Maine areas and Cape Cod Bay had migrate to coastal waters off New Eng- Krumholz, 1959; Dragovich, 1970; Ma- the lowest (0.4 kg). Diet at four of land during warmer months, feeding son, 1976; Holliday, 1978; Eggleston the five areas was dominated by one on local concentrations of prey. This and Bochenek, 1990; Matthews et al.1). or two small pelagic prey and several migration supports a major component Pinkas (1971) reported similar results other pelagic prey made minor contri­ of the U.S. Atlantic commercial fishery for Pacific bluefin tuna (Thunnus thyn­ butions. In contrast, half of the prey for bluefin tuna; from 1978 to 1992 New nus orientalis). These bluefin tuna food species found in the Cape Cod Bay diet England accounted for between 73% habit studies either reported on small were demersal species, including the frequent occurrence of the sessile fig and 98% of annual commercial land- numbers of samples or were qualitative sponge (Suberites ficus). Prey size selec­ ings (Chase, 1992; and NMFS, 1995). studies on samples distributed over a tion was consistent over a wide range of Substantial annual variation has been broad geographic range. Previous bluefin length. Age 2–4 sand lance and seen in the harvest locations on the studies have not evaluated size Atlantic herring and age 0–1 squid and New England continental shelf (Chase, relationships between bluefin tuna Atlantic mackerel were common prey 1992). Spatial variation in prey popu- and their prey and the amount of for all sizes of bluefin tuna. This is the lations is suspected to be the primary food consumed. The present study tar- first study to compare diet composition influence on these annual aggregations geted regions of the New England gi­ of western Atlantic bluefin tuna among of bluefin tuna. Adaptations in the cir- ant tuna (>140 kg) fishery to quan­ discrete feeding grounds during their culatory system of the bluefin tuna titatively analyze stomach contents seasonal migration to the New Eng­ land continental shelf and to evaluate allow these fish to retain metabolic of bluefin tuna among discrete sea- predator-prey size relationships. Previ­ heat, facilitating the regulation of body sonal feeding grounds and to investi­ ous studies have not found a common temperature (Carey and Teal, 1969) gate predator-prey size relationships occurrence of demersal species or a pre- and assisting in the efficient transfer for this species in the Gulf of Maine. dominance of Atlantic herring in the of energy from consumed prey to fast diet of bluefin tuna. growth rates and large body size. These * Contribution 3 of the Massachusetts Divi­ adaptive features also allow bluefin sion of Marine Fisheries, Gloucester, MA tuna to make extensive migrations 01930. into cold-temperate waters in search 1 Matthews, F. D., D. M. Damkaer, L. W. of prey. Diet information is necessary Knapp, and B. B. Collette. 1977. Food of western North Atlantic tunas (Thun­ to improve the understanding of sea- nus) and lancetfish (Alepisaurus). Nat. Manuscript accepted 21 September 2001. sonal movements of bluefin tuna and Oceanic Atmos. Admin. Tech. Rep. NMFS Fish. Bull. 100:168–180 (2002). predator-prey relationships in the New SSRF-706, Washington, D.C., 19 p. Chase: Differencesin diet of Thunnus thynnus at seasonal feeding grounds off New England 16 9 Materials and methods 70°W Study area Five fishery areas (Jeffreys Ledge, Stellwa­ gen Bank, Cape Cod Bay, Great South Chan­ nel, and South of Martha’s Vineyard [Fig. 1]) 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 Gulf of Maine 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 42°N Bay (DOC2) 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 Great the south shore of Massachusetts. The two South Channel 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, Figure 1 such as marlins and tropical tunas. Map of the five bluefin tuna feeding grounds used as study areas (★) July– Commercial catch records and trawl sur­ October 1988–92. Depths are given in fathoms. vey indices of abundance indicate that At­ lantic herring (Clupea harengus) and Atlan- tic mackerel (Scomber scombrus) are the principal pelagic Sample collection and analysis fish species for all these study areas (Clark and Brown, 1976; and NOAA, 1998). Atlantic herring occur in lower Bluefin stomach samples were collected primarily from relative abundance in the region south of New England commercial landings at ports in Massachusetts during than in northern regions. Jeffreys Ledge and Great South the 1988–92 seasons (July–October). Sportfishing tour­ Channel are primary spawning locations for Atlantic her- naments were a secondary source of stomach samples. ring. Catch records and trawl survey indices indicate that Handgear landings (rod and reel, handline, and harpoon) the following prey species have been abundant in the were primarily collected in Gloucester and Cape Cod. study areas: silver hake (Merluccius bilinearis), butterfish Purse-seine landings were also a large source of samples, (Peprilus triacanthus), northern shortfin squid (Illex illece­ and were collected in Gloucester, New Bedford, and Cape brosus), and longfin inshore squid (Loligo pealeii). 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 2 DOC (Department of Commerce). 1991. Stellwagen Bank Na­ export market. tional Marine Sanctuary, Draft environmental impact statement/ management plan. U.S. Dep. Commerce (DOC), Nat. Oceanic Most samples were collected at the docks and process­ Atmos. Adm., Sanctuaries and Reserves Division, Washington, ing locations of commercial tuna buyers. The vessel cap­ D.C., 238 p. tain or tuna buyer was interviewed for location of catch. 17 0 Fishery Bulletin 100(2) The curved fork length (CFL) and weight of the catch tion test was applied by pairwise ranking for all locations, were recorded from the National Marine Fisheries Ser­
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