FISHERY BULLETIN: VOL. 85, NO. 4 



purchase of intermediate-sized yellowfin tuna 

 specifically for this study (yellowfin tuna in- 

 tended for market cannot be necropsied and then 

 sold) was prohibitively expensive. 



Infection of yellowfin tuna appears to vary sea- 

 sonally. Figure 1 shows changes in prevalence of 

 dorsal aorta infection in small (<3 kg) yellowfin 

 tuna captured between February 1985 through 

 March 1986. Prevalence remained stable for ap- 

 proximately 6 months during the winter through 

 early summer. Then in late summer of 1985, 

 prevalence dropped dramatically from 66 to 11%. 

 Beginning in October 1985, prevalence increased 

 steadily, reaching 39% in February-March 1986, 

 the last months for which data are available. 



NFESTATION OF YELLOWFIN TUNA - 

 BIMONTHLY INTERVALS 



70 

 53 

 56 

 49 

 42 

 35 

 28 

 21 

 14 

 07 

 



FEB-MAR JUN-JUL OCT-NOV FEB-MAR 

 APR-MAY AUG-SEP DEC-JAN 



Figure 1. — Prevalence (percent infection) of the dorsal aorta by 

 the plerocercoid stage of the cestode Dasyrhynchus talismani in 

 small (0.3-3 kg) yellowfin tuna caught near the Hawaiian Is- 

 lands from February 1985 to March 1986. 



Pathology 



In infected fish, the anterior dorsal aorta was 

 partially to nearly completely occluded by a para- 

 sitic embolus which contained one to several 

 larval cestodes. Figure 2a shows this vessel in a 

 moderately infected yellowfin tuna. Parasites 

 and a small amount of host inflammation are ev- 

 ident in the anterior end (to the right). A normal 

 portion of vessel, with a smooth wall, is seen to 

 the left. Figure 2b shows the anterior dorsal aorta 

 from a very heavily infected fish. 



Histological examination revealed that para- 

 sitic emboli were primarily composed of larval 

 cestodes, mononuclear cells with eosinophilic 

 granules (presumed to be eosinophils), epitheloid 

 cells (histocytes), fibroblasts, and collagen fibers 

 (Fig. 3). Larval cestodes within the dorsal aorta 



were associated with a chronic severe endarteritis 

 and, to a lesser extent, mesoarteritis. In heavily 

 infected fish, collapsed channels were commonly 

 found within emboli. Undoubtedly these channels 

 expanded in life with increases in intraluminal 

 blood pressure to allow blood to flow through the 

 vessel. Necrotic worms were also seen, suggesting 

 that the host's defense system was at least par- 

 tially capable of killing the larvae located in the 

 dorsal aorta. 



Infection of other arteries was usually by one, 

 or at most two, larger parasites which were never 

 seen to be folded. These parasites had a bulbous 

 anterior end which always pointed downstream. 

 No evidence of host immune response was ob- 

 served when the parasites were in vessels other 

 than the dorsal aorta. 



Effect of Infection on Measures 

 of Physiological Fitness 



Mean (±1 SD) relative condition factor and 

 mean {±1 SD) relative organ weight data are 

 given in Table 1 for the fish sampled between 

 May 1985 and September 1986. Table 2 lists the 

 same parameters plus mean relative otolith 

 weight, and mean RNA/DNA ratio for fish sam- 

 pled from the single school captured on 15 Janu- 

 ary 1986. Means were compared with Student's 

 two-tailed T test, with P = 0.05 taken as the min- 

 imum level for statistical significance. 



Examination of data from all the fish caught 

 May 1985-September 1986 reveals statistically 

 significant differences in relative liver and heart 

 weights for parasitized and unparasitized fish. On 

 the average, parasitized fish, have 15% smaller 

 livers, and 9% smaller hearts than unparasitized 

 fish. Relative condition factors were not signifi- 

 cantly different. When data from fish coming 

 from the single school are examined, there are no 

 statistically significant differences in these three 

 parameters, relative otolith weight, or in RNA/ 



Table 1 . — Relative condition factor, relative liver weight, and rela- 

 tive heart weight, of yellowfin tuna sampled between May 1985 and 

 September 1986. 



Fish 



Mean (±SD) 



relative 



condition factor 



Mean (±SD) 



relative liver 



weight 



Mean (±SD) 



relative heart 



weight 



Uninfected 1.00 (±0.10) 

 N = 109 



1.02 (±0.22) 

 W = 82 



1.01 (±0.18) 

 W = 88 



Infected 0.976 (±0.057) 0.867 (±0.098)* 0.919 (±0.085)* 



A/ = 35 N = 22 A/ = 32 



"Uninfected and infected groups different at P = 0.05 level. 



770 



