170 



Fishery Bulletin 100(2) 



The cui-ved fork length (CFL) and weight of the catch 

 were recorded from the National Marine Fisheries Ser- 

 vice (NMFS) tuna logbook. Stomachs were removed by 

 cutting the esophagus above the pylorus and were stored 

 on ice until analysis later that day or were frozen for anal- 

 ysis at a later date. Stomach samples were typically re- 

 moved from bluefin tuna the day of capture, except those 

 from purse-seine landings, where the catch was often sub- 

 merged in ice for one or two days prior to landing. 



Stomach contents were identified to the lowest possible 

 taxon and weights and counts were made of individual 

 prey species. Wet weights of prey were measured with 

 Homs tubular scales (±5 g) after contents were rinsed 

 through a standard testing sieve (2.00-mm mesh). Prey 

 counts were made only when all individual prey items 

 could be identified and counted for a given stomach sam- 

 ple. Because bluefin tuna consume relatively large prey 

 items and swallow prey whole, species identification was 

 possible for nearly all contents. Skeletal remains were 

 compared with the skeletons of known specimens to assist 

 with identification. Fish contents that could not be identi- 

 fied were categorized as "unidentified fish." Otoliths, squid 

 beaks, and skeletal traces less than .5 g were rounded up 

 to a minimum weight of 5 g. 



Stomach-contents data were analyzed by frequency of 

 occurrence, percent composition by number, and percent 

 composition by weight for each prey item (Hyslop 1980; 

 Bowen 1986). Frequency of occurrence can indicate prey 

 composition and availability, and number and weight per- 

 centages can represent the quantity a prey item contrib- 

 utes to a diet. "Stomach-contents biomass" refers to all 

 prey in stomach contents, and "prey weight" refers to the 

 weight of individual prey species. 



Stomach samples were assigned a status of "empty," 

 "chum," "chum and prey," or "prey only." Chum refers to cut 

 pieces of bait fish that fishermen use to attract bluefin. Both 

 empty and chum stomach samples were eliminated from 

 further data analysis. For "chum and prey" stomachs, chum 

 weight was eliminated from the analysis, and prey weight 

 was included because chum and natural prey were easily 

 distinguished in bluefin tuna stomachs. Wlien the status of 

 contents, or the veracity of catch location could not be re- 

 solved, the samples were eliminated from the analysis. 



Statistical analysis 



Stomach-contents biomass data were analyzed to deter- 

 mine differences among the five fishery areas. Prey weight 

 data were tested for normality (Shapiro and Wilk, 1965) 

 and equality of variance (BMDP. 1990). Stomach-contents 

 biomass data were transformed to natural logarithms and 

 tested for area differences by using the Brown-Forsythe 

 test for unequal variances and the Welch test for painvise 

 comparisons of areas (BMDP, 1990). 



The species composition of stomach contents were test- 

 ed among areas with the Spearman rank correlation test, 

 under the null hypothesis of no association between spe- 

 cies composition and feeding area. The twelve most com- 

 mon prey species were ranked according to frequency of 

 occurrence for each location. The Spearman rank correla- 



tion test was applied by pairwise ranking for all locations, 

 excluding missing cases. 



Bluefin tuna size was compared with prey length and 

 stomach-contents biomass data by using the Pearson prod- 

 uct moment correlation coefficient (Sokal and Rohlf, 1981) 

 to test for significant associations between prey and pred- 

 ator length, and to correlate bluefin tuna weight and stom- 

 ach-contents biomass for all samples from the Gulf of 

 Maine. The relationship between bluefin tuna size and 

 food consumed was also evaluated by comparing the ratio 

 of stomach-contents biomass and tuna weight (% kg wet 

 weight of prey biomass/kg wet weight of tuna) to tuna 

 length (cui-ved fork length) (Young et al., 1997). The area 

 south of Martha's Vineyard area was excluded from tests 

 on size relationships because of the limited sample num- 

 bers of juvenile bluefin tuna collected there. 



Results 



A total of 819 bluefin tuna stomachs were analyzed during 

 1988-92 (Table 1) and 568 contained prey; empty stom- 

 achs (206) and "chum only" samples (45) were eliminated 

 from further analysis. Approximately equal quantities of 

 the samples with prey came from purse-seine landings 

 (273) and from rod and reel and handline landings (264). 

 The fishing method used for the hook-and-line landings 

 was recorded for 242 samples and was evenly divided 

 between chumming (where chum was used as bait) (123) 

 and trolling (119). Size composition of sampled bluefin 

 tuna was similar for the four Gulf of Maine study areas; 

 a large majority of fish were large, mature adults, esti- 

 mated to be age 10 and older (Mather and Schuck, 1960). 

 In contrast, nearly all fish sampled from the area south of 

 Martha's Vineyard were small juveniles, ages 2-6. Tuna 

 sampled from Cape Cod Bay had the largest average size 

 <251 cm CFL, and 273 kg). " 



Prey composition 



All areas combined Stomach contents comprised at least 

 21 species of teleosts, two species of elasmobranchs, and 

 at least nine species of invertebrates (Table 2). Stomach- 

 contents biomass in terms of taxonomic composition was 

 dominated by Osteichthyes (Fig. 2A). Of the invertebrates, 

 only squid ("squid" refers to two species, Loligo pealei and 

 lllex illecebrosus) were a consistent component of prey 

 composition. Although squid accounted for only about 2'7c 

 of the stomach-contents biomass. it was the second most 

 common prey, occurring in a third of all stomach samples. 

 The only other common invertebrate was the fig sponge 

 iStiberites ficus) found in Cape Cod Bay. Despite the large 

 diversity of prey items, few species made major contri- 

 butions to overall prey composition. Sand lance (Amino- 

 dytes ssp). squid, Atlantic herring, Atlantic mackerel, and 

 bluefish {Pomatomus saltatrix) exceeded all other prey in 

 terms of frequency of occurrence and accounted as a group 

 for 88% of total stomach-contents biomass (Fig. 2B). In 

 the Gulf of Maine areas, sand lance, and Atlantic herring 

 were the major prey in the diet of bluefin tuna during 



