392 



Fishery Bulletin 105(3) 



Grade C or worse 



Grade C+ 



0- 80' 



Grade B 



I -O June -iV-.Iiily — C— Aug -O- Sept 



Figure 1 



The predicted probabilities of a northern bluefin tuna (Thunnus thynnus) being in grade j 

 in month m and year y, ;r^(m,v) for fat and oil grades of C or worse, and for grades C+, B-, 

 and B. The most dramatic shifts occurred in the C+ fish which made up between 68% and 

 75% of the catch in 2004 as opposed to 10-20 % in the early 1990s. Good quality (grade B) 

 fish comprised <1% of the total commercial catch in 2004. 



models, and Akaike's information criterion (AIC) for 

 non-nested models. 



Results 



Our analysis identified significant declines in the somatic 

 condition (fat and oil content and fish shape) of northern 

 bluefin tuna in the Gulf of Maine. For fat and oil content, 

 the effect of month, year, and the month-year interaction 

 were each significant (all P<0.0001; X4=429, X4=1088, 

 and X4=29.95, respectively). Fish landed in the month 

 of June arrived in poorer condition than they did in the 

 early 1990s (Fig. 1). For example, the probability of land- 

 ing a poor quality fish (grade C or worse) in June 1991 

 was 30% compared with 70% in 2004. Similar, but more 

 striking results were observed in the C+ category during 

 August and September (Fig. 1). In 1991, the probability 

 of landing a fish in the C+ category was 16% and 9% for 

 August and September, respectively. In 2004, the prob- 

 ability increased to 68% and 76% in the C+ category for 

 August and September, respectively. Good quality fish, 

 such as B or better now comprise <1% of the commercial 

 catch at this particular co-operative (Fig. 1). 



For fish shape, the effect of month, year, and the 

 month-year interaction were each significant (all 



P<0.0001; x~3 =207, xl =388, and xl =47.18, respec- 

 tively). Results for shape were similar to fat and oil 

 content, likely because shape and fat and oil content 

 are closely related (fat fish should have a more rotund 

 shape). The probability of landing a fish that received 

 a B- (lesser quality) grade for shape increased by 30%, 

 32%, 43%, and 28% from 1991 to 2004 for the months of 

 July, August, September, and October, respectively (Fig. 

 2). For the same time period, fish with a good shape (B 

 or better) decreased by 18%, 22%, 47%, and 25% for 

 the months of July, August, September, and October, 

 respectively (Fig. 2). 



Discussion 



Our multinomial regression analysis demonstrated 

 highly significant declines in the fat and oil content 

 and shape of northern bluefin tuna landed in the Gulf 

 of Maine over the last 14 years, corroborating the obser- 

 vations of fishermen. Northern bluefin tuna arrive in 

 leaner condition and are not increasing their fat stores 

 on the feeding grounds as they did in the early 1990s. 

 This was particularly true in late summer and early 

 fall, when fish usually fatten and become more rotund 

 (Butler'). 



