392 



Fishery Bulletin 88(2). 1990 



Figure 2 



Mean combined displacement volume of identified and unidentified 

 teleost remains from stomachs of bluefin tuna collected from three 

 different areas off Virginia. Numbers within each bar indicate number 

 of specimens sampled; vertical lines indicate + 1 SE. 



or "Fish hook and S.E. Lumps" areas (SNK: EWER 

 0.05) (Fig. 2). 



Digenetic trematodes Hirudinella ventricosa were 

 found in 8 (11.1%) of the stomachs and averaged 10 

 mm in length and 2-3 mm in width. The worms were 

 never attached to the lining of the stomach and were 

 typically found at the posterior end. The number of 

 worms per stomach ranged from 1 to 2 with a mean 

 of 1.14 H. ventricosa per individual stomach. The possi- 

 ble effects of area landed on the number of trematodes 

 occurring in the stomachs were not evaluated because 

 of the relatively low rate of parasitism. 



Discussion 



Diet 



This study indicates that school bluefin tuna, captured 

 off the Virginia coast, feed predominantly on the sand 

 lance. Mason (1976) was the first to report sand lance 

 as a prey item of school bluefin tuna caught off the U.S. 

 East Coast. He also reported sand lance to be the most 

 important prey of school bluefin tuna caught off 

 Virginia, but for fish taken north of Virginia, mackerel 

 (Scomber spp.) replaced sand lance as the dominant 

 prey. Holliday (1978) also found the sand lance to be 

 the predominant food item for bluefin tuna captured 

 by trolling along the U.S. East Coast. The IRI of sand 

 lance in this study (IRI = 5583) is very similar to that 

 reported by Holliday (1978) (/ie/ = 4896). 



Sand lance form dense schools over New England 

 and mid- Atlantic Continental Shelf areas. They occur 



throughout the water column during daylight hours, 

 and are available to tuna predation at various depths 

 (Meyer et al. 1979, Auster and Stewart 1986). Tuna 

 predation on sand lance may affect the populations of 

 this important forage species off Virginia. Reproduc- 

 ing populations of sand lance, as indicated by egg and 

 larvae counts, exist on the Virginia shelf (Norcross 

 et al. 1961); hence, the Virginia coast is an important 

 habitat to the species. The sand lance serves as an im- 

 portant link between secondary producers and higher 

 trophic-level fish and mammals in marine food chains 

 (Bigelow and Schroeder 1953); thus, extensive preda- 

 tion by tuna could affect marine mammal populations. 

 A cause-and-effect relationship may exist between low 

 mackerel and herring stocks (resulting from heavy 

 fishing mortality) and the observed population explo- 

 sion of sand lance larvae in the mid- to late 1970s 

 (Sherman et al. 1981); thus, tuna predation on sand 

 lance could be beneficial to the return of mackerel and 

 herring stock abundance. 



The Atlantic brief squid was the second most impor- 

 tant item consumed by school bluefin tuna examined 

 in our study. Mason (1976) found two squid in the 20 

 fish he examined from Virginia waters. Holliday (1978) 

 also reported similar species of cephalopods in stomach 

 contents of the bluefin tuna taken off the U.S. East 

 Coast. Krumholz (1959), working near the Bahamas, 

 reported salps as the second most important food item. 

 In the western North Atlantic, Dragovich (1970) noted 

 molluscs (mainly cephalopods) as second in trophic 

 importance. Similarly, Matthews et al. (1977) also 

 reported cephalopods, pteropods, and heteropods as 

 being the most frequent invertebrate forage group 

 after fishes. For California bluefin tuna, the second 

 most important food item was the California market 

 squid Loligo opalescens or the pelagic swimming crab 

 Pleuroncodes planipes, depending upon the area of cap- 

 ture (Pinkas 1971). 



The butterfish, lined seahorse, and scrawled filefish 

 were found in very few stomachs, being rare con- 

 tributors to the diet of bluefin tuna in this study. These 

 prey species demonstrate considerable diversity in their 

 foraging locations, including nearsurface, mesopelagic, 

 and demersal habitats. It is possible that the butter- 

 fish, lined seahorse, and scrawled filefish are associated 

 with drifting Sargassum weed; thus, tuna may feed in 

 part around drifting Sargassvm. communities. Other 

 miscellaneous items found in the stomachs were salps, 

 the isopod Idotea spp., a cigarette wrapper, and 

 Sargassum weed. Holliday (1978) also reported the 

 occurrence of Idotea spp. in stomachs of bluefin tuna 

 caught trolling near Sargassum communities. He 

 hypothesized that the isopod and Sargassum- weed 

 were accidently ingested by the tuna while pursuing 

 other prey. 



