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



175 



al., 1989). and the youngest were age 2. There 

 were no young-of-the-year (YOY) sand lance. 

 Most sand lance from Great South Channel 

 samples were age 2. in contrast to age 3 or 

 age 4 at Stellwagen Bank (Weston et al, 1979). 

 Most squid were YOY or age 1. 



Numbers of prey species Few prey species 

 were found in large numbers in a given stom- 

 ach sample. Only sand lance, squid, Atlantic 

 herring, and Atlantic mackerel had sample 

 counts higher than 20 individual fish (Table 6). 

 Data on prey numbers are limited because prey 

 counts were made for only 208 samples. Many 

 samples with large numbers of well-digested 

 sand lance were difficult to count. From this 

 subsample, the mean number of sand lance 

 per stomach was 159, much higher than the 

 next highest mean of 19 for Atlantic herring. 

 Squid and mackerel commonly occurred as 

 prey, although typically only a few individuals 

 were found per stomach. 



Weight of prey species Sand lance and Atlantic herring 

 (combined) accounted for 75'7i of the total stomach-contents 

 biomass for all areas combined. The next highest species 

 was bluefish at T^i. Despite a high frequency of occurrence, 

 squid accounted a low percentage of overall biomass (2%) 

 and a mean stomach prey weight of only 58 g. The highest 

 mean prey weight was 1794 g for Atlantic herring. Only 

 three other prey items averaged over 500 g in stomach-con- 

 tent weight: spiny dogfish (807 g), bluefish (742 g), and sand 

 lance (661 g). With few exceptions, stomachs that were full 

 or near full, contained only sand lance or Atlantic herring. 

 Only five stomachs contained over 10 kg of prey contents: 

 three with Atlantic herring, and one each with sand lance 

 and Atlantic cod ( 16.0 kg, the largest meal obsei-ved). 



In summary, five prey items occurred in frequency and 

 mass to be considered important dietary components of 



n=520, r=0 048. P=0,276 



<i\ ****** 



100 



200 300 



Tuna weight (kg) 



400 



500 



Figure 3 



Scatterplot of bluefin tuna stomach-contents biomass and bluefin tuna 

 weight for all samples from the Gulf of Mame, 1988-92. 



the stomach contents sampled: sand lance, Atlantic her- 

 ring, squid, Atlantic mackerel, and bluefish. Of these prey 

 items, sand lance and Atlantic herring were the primary 

 contributors to the diet of bluefin tuna sampled. 



Discussion 



Comparison of this study with previous west Atlantic blue- 

 fin tuna food habit studies revealed that fish prey rep- 

 resented a large majority of the diet, and cephalopods 

 (primarily squid) were important secondary prey. Other 

 prey taxa were minor components of the diet, except for 

 Salpidae (Mason, 1976; Eggleston and Bochenek, 1990) 

 and Amphipoda (Dragovich, 1970) for juvenile bluefin 

 tuna. Comparisons with the four studies in which bluefin 



