NOTES Eggleston and Bochenek: Stomach content analysis of juvenile Thunnus thynnus off Virginia 



393 



Parasites 



The digenetic trematode Hirudimlla ventricosa occur- 

 red in 11% of the stomachs examined in this study. 

 Mason (1976) also found an annulated hemiurid trema- 

 tode in 2% of the bluefin tuna stomachs he examined 

 from the western Atlantic Ocean. The trematodes in 

 Mason's (1976) study were found in both empty stom- 

 achs and stomachs which contained food. Crane (1936) 

 reported Distoma-Vike worms in 25% of giant bluefin 

 tuna stomachs he examined off Maine. Hirudinella 

 ventricosa (= marina) occurred in 9% of school bluefin 

 tuna and 48% of giant bluefin tuna stomachs collected 

 from North Carolina to Massachusetts (Holliday 1978). 

 Giant trematodes of the genus HinidineUa frequent- 

 ly parasitize scombroid fishes (Nigrelli and Stunkard 

 1947, Nakamura and Yuen 1961, Watertor 1973, 

 Manooch and Hogarth 1983). Adult parasites typical- 

 ly attach to the stomach lining and remain near the site 

 of attachment throughout this life-stage (Manooch and 

 Hogarth 1983). These digenetic endoparasites have 

 complicated life cycles involving an alternation of 

 generations and hosts; however, the life cycle oi Hiru- 

 dinella spp. is still unknown (Manooch and Hogarth 

 1983). 



Attempts to evaluate the incidence of parasitism by 

 size and sex of the host and by geographical area of 

 collection have demonstrated mixed results (Nakamura 

 and Yuen 1961, Manooch and Hogarth 1983). Naka- 

 mura and Yuen (1961) examined the occurrence of the 

 parasite H. ventricosa ( = marina) in the stomachs of 

 skipjack tuna Euthynnus pelamis collected from 

 Hawaii and off the Marquesas. They concluded that 

 significant differences in the occurrence of trematodes 

 collected from these two areas were attributable to 

 time (year of collection) rather than area. Manooch and 

 Hogarth (1983) reported distinct differences in the in- 

 cidence of parasitism by H. ventricosa between wahoo 

 Acanthocybium solanderi from the coast of Florida- 

 South Florida and wahoo from the rest of the south- 

 eastern Atlantic. They suggested that this difference 

 may reflect two subpopulations of wahoo along the 

 southeastern U.S. coast: a northern population char- 

 acterized by high incidence of trematodes, and a south- 

 ern population with a much lower incidence. 



Watertor (1973) examined 258 bluefin tuna captured 

 off the East Coast of the United States Oat. 35-40°N; 

 long. 65-75°W) and off the northeast coast of South 

 America (lat. 0-18°N; long. 50-82°W). Of these, 51 

 were infected with H. ventricosa ( = marina), nearly 

 twice the infection rate noted in this study. There are 

 two possible explanations for this difference. First, the 

 parasites described by Watertor (1973) were pooled 

 from both the eastern U.S. and northeastern South 



American samples. Inclusion of a South American 

 group, with possibly a higher prevalence of parasitism, 

 similar in nature to that described for wahoo by 

 Manooch and Hogarth (1983), might have biased the 

 values reported by Watertor (1973). Secondly, Water- 

 tor (1973) did not report the overall size ranges of blue- 

 fin tuna used in his study. Inclusion of giant bluefin 

 tima, with a higher prevalence of parasitism (see Crane 

 1936) may also contribute to apparent differences in 

 levels of infection. 



Area effects 



Environmental factors such as temperature and ocean- 

 ographic frontal zones have been shown to markedly 

 influence the distribution, abundance and catchability 

 of tunas (Murphy 1959, Uda 1973, Laurs and Lynn 

 1977, Rockford 1981, Sund et al. 1981, Laurs et al. 

 1984). Murphy (1959) suggested that the aggregation 

 of albacore Thunnus alalunga in clear water on the 

 oceanic side of fronts in nearshore areas may reflect 

 an inability to efficiently capture large, mobile prey in 

 turbid coastal waters. This same mechanism may help 

 to explain the higher combined displacement volumes 

 of sand lance and unidentified teleost remains in the 

 stomachs of tuna taken from the "21 Mile Hill" com- 

 pared with the "Hot Dog" or "Fishook and S.E. 

 Lumps" areas (Fig. 2). Turbidity associated with ef- 

 fluent from Chesapeake Bay might have reduced the 

 ability of bluefin tuna to detect mobile forage such as 

 the sand lance and other teleosts. The effluent from 

 the Chesapeake Bay appears in shelf waters as a lens 

 of freshened water (with high concentrations of bay 

 water constituents) extending offshore and towards the 

 south as a part of the general shelf circulation (Ruzecki 

 1981). The three areas in question are directly offshore 

 of the Chesapeake Bay mouth (Fig. 1). Differences in 

 the diet of bluefin tuna have also been attributed to 

 depth of capture, availabOity and type of food in a given 

 area, time of day or year, spawning, atmospheric con- 

 ditions, physiological conditions of predator fish, size 

 of prey, and size of the bluefin tuna (Dragovich 1970). 

 We conclude that the sand lance is the most impor- 

 tant forage of school bluefin tuna off the Virginia coast 

 and suggest that this prey species, as well as teleosts 

 in general, may become more vulnerable to tuna preda- 

 tion in areas least affected by the turbid waters of the 

 Chesapeake Bay plume. In addition, the occurrence of 

 the digenetic trematode Hirudinella ventricosa in a 

 small but significant number of bluefin tuna off Virginia 

 suggests that variation in infestation rates of this para- 

 site might provide a mechanism to help distinguish 

 among possible subpopulations of bluefin tuna occur- 

 ring in the Western Atlantic. 



