PREVALENCE AND EFFECTS OF INFECTION OF THE DORSAL AORTA 



IN YELLOWFIN TUNA, THUNNUS ALBACARES, BY THE 



LARVAL CESTODE, DASYRHYNCHUS TALISMANI 



Richard W. Brill,' Robert Bourke,' James A. Brock,^ and Murray D. Dailey^ 



ABSTRACT 



Approximately 60% of small (<3 kg) yellowfin tuna, Thunnus albacares, caught near the Hawaiian 

 Islands carry the plerocercoid (larval) stage of the cestode (tapeworm), Dasyrhynchus talismani, in 

 their anterior dorsal aortas. Because the worms and the resultant host inflammation appear to 

 occlude the vessel almost totally, we assumed that the parasite could increase natural mortality rates. 

 Tuna could be limited in their ability to capture prey and therefore should show evidence of long- or 

 short-term food deprivation. We measured body weight, fork length, liver weight, heart weight and, 

 in fish captured from one school, RNAVDNA ratios (a measure of short-term growth rate), and otolith 

 weight (a measure of long-term growth rate) from parasitized and unparasitized fish. We found no 

 significant differences between infected and uninfected fish nor any evidence of starvation in infected 

 fish. How small yellowfin tuna remain apparently unaffected by the parasitic occlusion of their dorsal 

 aorta remains to be demonstrated. 



We also examined changes in incidence of infection in small yellowfin tuna caught between Febru- 

 ary 1985 and March 1986 as well as the prevalence in large (>45 kg) fish. Large yellowfin tuna were 

 rarely parasitized (5.2'7f ) in the dorsal aorta, but showed a high rate (>80%) of infection within other 

 major arteries. The prevalence in small fish varied dramatically with season, dropping suddenly from 

 66% in June-July 1985 to 11% in August-September 1985. Unparasitized fish caught during August- 

 September 1985 showed significantly higher condition factors, relative heart weights, and relative 

 liver weights than did unparasitized fish caught at other times of the year. We hypothesize that the 

 sudden decrease in prevalence was due to influx of a separate group of small yellowfin tuna into the 

 Hawaiian fishery. We believe that this parasite may therefore serve as a marker for tracing the 

 movements of small yellowfin tuna into and out of specific fisheries or areas. 



During a series of experiments that involved 

 catheterization of the anterior dorsal aorta of 

 small ( 1-3 kg) yellowfin tuna, Thunnus albacares, 

 we discovered that approximately 60% of the ex- 

 perimental fish had this blood vessel infected 

 with parasites. The parasites were white, round 

 (2-4 mm in diameter), often more than 4 cm long, 

 and usually folded repeatedly. As a result of the 

 parasites and the tissue inflammation that devel- 

 ops as part of host immune response, the lumen of 

 the infected aortas appeared almost totally oc- 

 cluded. Because all the blood to the internal or- 

 gans and swimming muscles must flow past this 

 occlusion, we assumed the parasite could be a 

 major factor contributing to the natural mortality 

 of small yellowfin tuna. 



The first demonstration of a dorsal aorta para- 



1 Southwest Fisheries Center Honolulu Laboratory, National 

 Marine Fisheries Service, NOAA, Honolulu, HI 96822-2396. 



2Aquaculture Development Program, Department of Land 

 and Natural Resources, State of Hawaii, Honolulu, HI 96819. 



3Southern California Ocean Studies Consortium, California 

 State University, Long Beach, Long Beach, CA 90840. 



Manuscript accepted July 1987 



FISHERY BULLETIN: VOL 8.5. NO 4. 1987. 



site in yellowfin tuna was by Kishinouye (1923), 

 who stated, "Often a species of nematod [sic] is 

 found in the dorsal aorta of Neothunnus macrop- 

 terus [now Thunnus albacares]; the parasite 

 causes the tissues of the canal to become thick 

 and tough, giving it at the same time a yellowish 

 tint." Other investigators described intravascular 

 parasites from the branchial vessels and arteries 

 serving the stomach, liver, spleen, pyloric cae- 

 cum, and gall bladder of this species (Baudin Lau- 

 rencin 1971). The parasites have been described 

 simply as the plerocercoids (larval cestodes) 

 (Chen and Yang 1973), identified to the family 

 Dasyrhynchidae (Ward 1962), or as the species 

 Dasyrhynchus talismani (Baudin Laurencin 

 1971). 



Intravascular infection by plerocercoids has 

 been reported from yellowfin tuna caught in the 

 western Pacific (Chen and Yang 1973), eastern 

 Atlantic (Baudin Laurencin 1971), Gulf of Mexico 

 (Ward 1962), and now central Pacific (this re- 

 port). Infection rates have been reported to be as 

 high as 100% (Baudin Laurencin 1971). If all re- 



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