LESTER ET AL.: PARASITES OK SKllMACK TUNA 



Table 2 —Average numbers of parasites per fish in all skipjack tuna (878) from the areas listed in Table 1, unadjusted for 

 length. The last column gives the correlation coefficient (r) for length against log (parasite number + 1) for Pacific tropical fish. 



No. 



Parasites 



B 



H 



I 



K 



M N 



the fish, and their longevity was doubtful. 

 Philometra sp. (No. 7) was found predominantly in 

 developed ovaries, which were present in less than 

 half of the fish sampled. The data were used for com- 

 paring school-school variability only. 



Seven parasites were considered "temporary". 

 They appeared to be short-lived or easily lost from 

 the fish. The caligoid copepods (No. 1, primarily 

 Caligits productus in the tropics and C. bonito in 

 temperate waters) were not permanently attached 

 and probably moved from fish to fish (Kabata 1981). 

 Syncoelium filiferum (No. 6) was common on the 

 gills in New South Wales and New Zealand samples 

 (I, J, and K), but was not recovered from anywhere 

 in the tropics. It is common on fish endemic to New 

 Zealand (D. Blair^). It was considered possibly a 

 temperate short-lived parasite, at least on skipjack 

 tuna, and this was verified by the school integrity 

 study and by conventional tagging data (see later). 



Some hemiurids are known to be readily lost from 

 the gut of other species of fish (Table 3). Margolis 

 and Boyce (1969) observed that over half the Leci- 

 thaster gibbosus were lost from salmon fingerlings 



*D. Blair, Deparlment of Zoology, University of Canterbury, 

 Christx;hurch, New Zealand, pers. commun. September 1984. 



within 3 wk of bringing the fish into captivity. We 

 found Dinurus euthynni (No. 15) in all tropical sam- 

 ples from the central and western Pacific but not in 

 the temperate samples I, J, and K. As it showed 

 strong school associations and as the didymozoid 

 data described later showed that New Zealand fish 

 had a recent origin in the tropics, D. euthynni was 

 evidently a short-lived tropical parasite that was lost 

 as the fish migrated south. This also appeared to be 

 true for Hirudinella ventricosa (No. 17) and possibly 

 for two relatively rare gut-lumen digeneans, Terges- 

 tia laticollis (No. 21) and Rhipidocotyle sp. (No. 22). 



In other fish, adult acanthocephalans may be short 

 lived (Table 3). Moller (1976) found that over half the 

 Echinorhynchus gadi in three species of fish were 

 lost within 2 wk of the fish being brought into capti- 

 vity. In our data, Raorhynchus terebra (No. 18) was 

 present in reduced numbers in I, J, and K, sug- 

 gesting it was lost in southern waters. All these 

 parasites then were labelled "temporary". 



Didymozoid digeneans were considered "semi- 

 permanent" parasites. In other fish, some didymo- 

 zoids, or at least the remains of them, are believed to 

 stay in the tissues for the life of the fish. Others, in- 

 cluding some species found in the gonads or gills, are 

 lost annually (Table 3). In general, therefore, skip- 



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