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Fishery Bulletin 100(2) 



ported higher daily consumption rates for pen-held bluefin 

 tuna, an average of 28 kg per day during single feedings 

 to satiation, and 40 kg per day during multiple feedings. 

 Data from my study implied that naturally feeding bluefin 

 tuna do not often reach these consumption rates; only five 

 of 568 stomachs had prey contents over 10 kg. Regurgi- 

 tation is a major confounding factor for stomach-contents 

 biomass data and presents a serious challenge for calcula- 

 tions of daily rations and bioenergetic modeling of bluefin 

 tuna feeding habits. The anatomy of the alimentary canal, 

 combined with the stress of capture, appears to cause a 

 high rate of regurgitation. 



Cape Cod Bay 



The bluefin tuna prey composition from Cape Cod Bay was 

 unlike any prey composition previously reported for the west 

 Atlantic, in that no mdividual prey item dominated nor was 

 encountered at a high frequency of occun-ence or percentage 

 of stomach-contents biomass. In previous west Atlantic blue- 

 fin tuna food-habit studies, prey composition was dominated 

 by a single, pelagic, schooling prey species (followed by a 

 few secondary species of much less importance), and Pinkas 

 ( 1971 1 reported similar results for Pacific bluefin tuna. Find- 

 ing fig sponge as the highest ranked prey item in the diet of 

 bluefin tuna in Cape Cod Bay was surprising; sessile prey 

 organisms had not been found in previous bluefin tuna food- 

 habit studies. Occurrence of fig sponge, locally known as 

 "monkey dung," in the stomachs of bluefin tuna is commonly 

 known to Cape Cod Bay fishermen who have questioned 

 whether it is natural prey or whether it is accidentally swal- 

 lowed during capture. 1 found the sponge at various stages 

 of digestion; well-digested pieces as well as fresh pieces were 

 found in the same stomach. The differential stages of diges- 

 tion implies that fig sponge was ingested either as selected 

 prey of unknown attraction and nutritional value or as inci- 

 dental catch to a targeted bottom dweller 



Bottom foraging in the relatively shallow Cape Cod Bay 

 (average depth: 25 m) was evident because eight of the 

 16 prey items identified were benthic or demersal species. 

 Vertical excursions to the bottom in shallow regions were 

 not unexpected, yet bottom foraging has not been report- 

 ed for west Atlantic bluefin tuna. Bottom feeding was evi- 

 dent for most locations in this study, including foraging 

 on small crabs (Cancer spp. ) and finger sponge at Great 

 South Channel where depths approached 100 m. In Cape 

 Cod Bay, bottom foraging may not be the optimal feeding 

 strategy for bluefin tuna and may simply be a response to 

 low food availability, as suggested by the low prey biomass 

 and higher proportion of naturally empty stomachs. 



Prey size 



To date, prey size in bluefin tuna food habit studies in the 

 western North Atlantic has received little attention. Only 

 Matthews et al.M presented prey size data, a mean value 

 based on 38 bluefin tuna stomachs. Young et al. (1997) 

 found no relationship between prey size and size (40-192 

 cm) of juvenile southern Atlantic bluefin tuna (Thunnus 

 maccoyii). I found only minor evidence of a predator-prey 



size relationship for bluefin tuna in the Gulf of Maine. 

 A positive correlation was found between sand lance and 

 bluefin length, although regional differences in the lengths 

 of both species appears to bias the correlation. Smaller 

 sand lance and bluefin tuna were found at Great South 

 Channel than at Stellwagen Bank. A positive interaction 

 between bluefin tuna and prey sizes was indicated by the 

 consumption of the largest prey ( >40 cm ) by large bluefin 

 tuna (>230 cm). Prey this large were not common in stom- 

 achs and were probably limited by mouth and esophagus 

 gape. Otherwise, the sizes of prey were consistent across 

 the range of bluefin tuna sampled in the Gulf of Maine. 



The findings on prey size and numbers (per stomach) 

 provide evidence of three selective foraging strategies used 

 by bluefin tuna in the Gulf of Maine. Feeding on indi- 

 vidual, fast-swimming pelagic prey was evident from con- 

 sumed bluefish and Atlantic mackerel that are abundant 

 pelagic species in the Gulf of Maine, but which occurred 

 much less frequently in stomach contents and with few in- 

 dividuals per stomach. Ram feeding (swimming through 

 a dense school prey with mouth open) of small prey may 

 have resulted in high average number of sand lance found 

 in stomachs and contributed to the similar prey sizes in 

 most sizes of bluefin tuna. Bluefin tuna in Cape Cod Bay 

 displayed a different foraging behavior, selecting larger, 

 individual demersal prey. The variation in prey compo- 

 sition and different feeding strategies is consistent with 

 previous descriptions of bluefin tuna as an opportunistic 

 predator However, the dominance of sand lance and At- 

 lantic herring in the Gulf of Maine diet suggests a depen- 

 dence on these species as an optimal energy source. 



Trophic influences on bluefin tuna distribution 



Changes in biomass and spatial availability of forage pop- 

 ulations may affect the distribution of bluefin tuna on 

 the New England continental shelf Major changes in the 

 prey community of the Gulf of Maine have occurred in 

 recent decades. After Atlantic mackerel and Atlantic her- 

 ring stocks off New England were severely overharvested 

 in the 1960s and 1970s (NOAA, 1998), sand lance popula- 

 tions increased, presumably as a result of decreased pre- 

 dation and competition for food (Sherman et al., 1981). 

 Atlantic herring and Atlantic mackerel stocks off New 

 England increased steadily during the 1980s and 1990s 

 ( NEFSC, 1998). By the mid-1990s, the U.S. Atlantic coastal 

 spawning stock biomass for these species increased to the 

 highest levels on record (NEFSC, 1998). 



Commercial bluefin tuna catches have increased in areas 

 where Atlantic herring abundance has increased (western 

 Gulf of Maine and Great South Channel) and diminished 

 at traditional areas south of the Gulf of Maine (Chase, 

 1992). The northward shift in bluefin tuna distribution on 

 the New England continental shelf may be influenced by 

 improved foraging opportunities on Atlantic herring in the 

 Gulf of Maine. I suspect that the timing of bluefin tuna 

 migi-ations to the New England continental shelf are as- 

 sociated with seasonal spawning and feeding aggregations 

 of Atlantic herring. Sand lance populations appear to be 

 an important influence on bluefin tuna feeding migrations. 



