I.KSTKKKTAL.: PAKASITKSOF SKIIMACK TUNA 



jack tuna didymozoids were thought to be in the fish 

 probably for at least several months. However, there 

 was some suggestion that 3 of the 10 skipjack tuna 

 didymozoids had a shorter adult life span than the 

 others. Didymozoid No. 16 was much less common in 

 New Zealand waters than in the tropics (Table 4), 

 and didymozoid Nos. 19 and 20 were also less com- 

 mon and, in addition, showed strong school associa- 

 tions (see later). These three didymozoids (possibly 

 representing four species) were omitted from the 

 analysis for Figure 3. 



The remaining four parasites (Nos. 8, 9, 10, and 

 12) were classed as "permanent". Larval cestodes 

 and nematodes, particularly those found in the 

 tissues, are generally believed to survive for several 

 years, often for the life of the fish (Table 3). They 



have been used successfully many times as fish 

 population markers (see MacKenzie 1983). In skip- 

 jack tuna, the larva of a trypanorhynch cestode, Ten- 

 tacuLaria coryphaenae, was found in the wall of the 

 body cavity and occasionally in the viscera. No 

 degenerating forms were seen, suggesting that it 

 survived for an extended period and hence could be 

 an excellent population marker, though counts were 

 not available from areas C, N, and 0. Larval 

 anisakids were found on the wall of the stomach or in 

 the mesentery. The literature suggested that they 

 should also be good long-term markers (Table 3). 

 They were counted in all areas. 



Protozoan parasites have been used successfully to 

 separate stocks of several species of fish. However, 

 none has been reported from skipjack tuna, and we 

 found none in this study. 



Table 4— Average number of didymozoids in New Zealand 

 fish (all lengths) compared with fish caught m the tropical 

 western Pacific (areas A, B. C, D, E, F, G, and L). In paren- 

 theses, log (X -I- 1) length-adjusted means for fish 40 to 57 cm 

 only. 



'Code no. from Table 2. 



^Stomach. 



^Intestine. 



Relationships Between Areas 



Analyses of individual distributions of permanent 

 and semipermanent parasites showed that the abun- 

 dances of individual parasites varied across the 

 Pacific. However, these differences were inconsis- 

 tent, the pattern established by one parasite being in 

 conflict with that of a second, and so on. 



The data from the three anisakid nematodes and 

 the seven didymozoids considered longest lived were 

 therefore analyzed using canonical variate analysis. 

 Because of the more permanent nature of these 

 parasites and the completeness with which they were 

 recorded from all areas, these data were considered 

 the most reliable for statistically assessing the 

 similarities and dissimilarities between areas. The 

 first three canonical axes accounted for 75% of the 



Figure 3. -Results of multivariate analysis using 3 

 "permanent" and 7 "semipermanent" parasites (Nos. 2, 

 3, 5, 8, 9, 10, 11, 13, and 14). Values for first two can- 

 nonical vectors plotted, and 95% confidence rings in- 

 dicated for samples of more than 24 fish. The letters 

 refer to the sampling sites indicated in Figure 1. 



349 



