390 



Abstract — The northern bluefin tuna 

 (Thunnus thynnus) is a highly mobile 

 apex predator in the Gulf of Maine. 

 Despite current stock assessments 

 that indicate historically high abun- 

 dance of its main prey, Atlantic her- 

 ring (Clupea harengus), commercial 

 fishermen have observed declines 

 in the somatic condition of northern 

 bluefin tuna during the last decade. 

 We examined this claim by review- 

 ing detailed logbooks of northern 

 bluefin tuna condition from a local 

 fishermen's co-operative and applying 

 multinomial regression, a robust tool 

 for exploring how a categorical vari- 

 able may be related to other variables 

 of interest. The data set contained 

 >3082 observations of condition (fat 

 and oil content and fish shape) from 

 fish landed between 1991 and 2004. 

 Energy from stored lipids is used for 

 migration and reproduction; therefore 

 a reduction in energy acquisition on 

 bluefin tuna feeding grounds could 

 diminish allocations to growth and 

 gamete production and have detri- 

 mental consequences for rebuilding 

 the western Atlantic population. 

 A decline in northern bluefin tuna 

 somatic condition could indicate 

 substantial changes in the bottom- 

 up transfer of energy in the Gulf of 

 Maine, shifts in their reproductive or 

 migratory patterns, impacts of fishing 

 pressure, or synergistic effects from 

 multiple causes. 



Decline in condition of northern bluefin tuna 

 (Thunnus thynnus) in the Gulf of Maine 



Walter J. Golet (contact author)^ 

 Andrew B. Cooper^ 

 Robert Campbell^ 

 Molly Lutcavage' 



Email address for W. J. Golet; wgolet@cisunix.unh.edu 



' Large Pelaglcs Research Center, Room 177 

 Spaulding Hall, College Rd, 

 University of New Hampshire 

 Durham, New Hampshire 03824 



^ Yankee Fisherman's Co-op 

 Seabrook, New Hampshire 03874 



Manuscript submitted 23 June 2006 to 

 the Scientific Editor's Office. 



Manuscript approved for publication 



16 January 2007 by the Scientific Editor. 



Fish. Bull. 105:390-395 (207) 



The Gulf of Maine is a highly produc- 

 tive region of the northwest Atlantic 

 where substantial aggregations of 

 forage fish attract northern bluefin 

 tuna {Thunnus thynnus) and other top 

 predators, including sharks, marine 

 mammals, and seabirds. Atlantic 

 herring (Clupea harengus), Atlan- 

 tic mackerel (Scomber scomber), and 

 sand lance (Ammodytes dubius) form 

 a major part of this prey base (Big- 

 elow and Schroeder, 1953). As sea- 

 sonal migrants, northern bluefin tuna 

 arrive in the Gulf of Maine in limited 

 numbers in late May and early June 

 (Bigelow and Schroeder, 1953; Mather 

 et al., 1995). Fish landed in the Gulf 

 of Maine during June and early July 

 are typically lean and have little to 

 no perigonadal or body fat reserves 

 (Estrada et al., 2005; Goldstein et al., 

 in press). Presumably, energy stores of 

 mature northern bluefin tuna entering 

 the Gulf of Maine have been exhausted 

 after spawning, a time when shifts in 

 fat and energy stores are pronounced 

 (Mourente et al., 2001) and mesenteric 

 lipid stores are used for gametogen- 

 esis and for subsequent migration to 

 feeding grounds. 



Given its size, speed, and wide 

 thermal tolerance, the northern 

 bluefin tuna is a formidable preda- 

 tor, capable of exploiting diverse 

 prey species. Having visceral retes 

 that warm the stomach (Carey et al., 

 1971) and digestive enzymes with 



fast turnover rates, northern bluefin 

 tuna can rapidly process prey (Ste- 

 vens and McLeese, 1984). During the 

 1950s and 1970s, individual north- 

 ern bluefin tuna gained 7% (Rivas, 

 1955) and 10% (Butler^), respectively, 

 of their body mass per month on the 

 northwest Atlantic feeding grounds. 

 The majority of this mass gain was 

 reflected in the accumulation of in- 

 tramusculature and perigonadal fat 

 stores, which presumably provide the 

 necessary reserves for migration to 

 the spawning grounds and subsequent 

 gamete production following their de- 

 parture from the Gulf of Maine in 

 late September to mid November 

 (Lutcavage and Kraus, 1995; Wilson 

 et al., 2005). 



Recent observations by fishermen, 

 brokers, and co-operative managers 

 have identified two declining trends 

 in the Gulf of Maine commercial 

 northern bluefin tuna fishery. First, 

 there appears to be fewer mature fish 

 now than in the last decade. Hun- 

 dreds of surface schools were detected 



' Butler, M. J. A. 1974. Prince Edward 

 Island bluefin tuna research program 

 1974. Prince Edward Island marine fish- 

 eries and training center and depart- 

 ment of tourism, parks, and conservation, 

 Prince Edward Island, 65 p. Agricul- 

 ture, Fisheries and Aquaculture P. O. 

 Box 2000, Charlottetown, Prince Edward 

 Island, Canada C1A7N8. 



