PARASITES OF SKIPJACK TUNA, KATSUWONUS PELAMIS: 



FISHERY IMPLICATIONS 



R. J. G. Lester,' A. Barnes,^ and G. Habib^ 



ABSTRACT 



The numbers of 26 types of parasites were counted in 878 fish, of which all but 3 were from 1 4 areas in the 

 Pacific. Data from the 22 most reliable parasites gave no evidence of discrete stocks of skipjack tuna in the 

 Pacific, either when analyzed singly or when usinjj combinations of parasites in multivariate analyses. New 

 Zealand fish carried many tropical parasites, particularly didymozoids, in numbers similar to fish caught in 

 the tropics, indicating;: that the bulk of these fish had recently migrated from the tropics. The number oiTen- 

 tirriilnria nnyphaenac. a larval tapeworm, was positively correlated to fish size in the tropics. In New 

 Zealand, however, fish over 5.5 cm carried about the same number of T. coryphaenxw as fish 45 to 55 cm, 

 suggesting they had left the tropics when they were 45 to 55 cm and had not returned. 



Analysis of the numbers of parasites from particular schools suggested that school members stayed 

 together for several weeks i)ut not for life. 



The use of parasites to delineate stocks for manage- 

 ment purposes is a well-established technique. For a 

 comprehensive review of the many examples see 

 MacKenzie (1983). 



The skipjack tuna, Katsuwonus pelamis, is one of 

 the most valuable fishery resources of the central 

 and western Pacific. At least 50 species of parasites 

 have been reported from it. The distribution of only 

 one, the hemiuroid digenean Hirudinella ventricosa, 

 has previously been investigated. In the Atlantic, 

 Watertor (1973) found it in 7% of skipjack tuna off 

 West Africa, 40% off Brazil, and < 1% off Florida. In 

 the Pacific, Nakamura and Yuen (1961) found it in 

 21% of skipjack tuna off the Marquesas and 34% of 

 fish from Hawaii. Sindermann (1961) pointed out 

 that analyzing the distributions of combinations of 

 parasites may provide more information than the ex- 

 amination of individual species. That, in general, has 

 been our approach here. 



In addition, school-school variation in parasite 

 numbers was studied to determine how long schools 

 stayed together, and secondarily to evaluate the 

 degree of permanence of the parasites. 



MATERIALS AND METHODS 



Of the 878 fish dissected, 386 were collected by the 

 Hatsutori Mam on charter to the South Pacific Com- 



'Department of Parasitology, University of Queensland, St. 

 Lucia, Brisbane, Australia 4067. 



^Faculty of Science, University of Queensland, St. Lucia, 

 Brisbane, Australia 4067. 



^Fisheries Research Division, New Zealand; present address: 

 Southpac Fisheries Consultants, P.O. Box 7230, Auckland 1, New 

 Zealand. 



mission (SPC), 246 by the New Zealand Ministry of 

 Agriculture and Fisheries (NZ), and the remainder 

 by other governments and fishing companies (see 

 Acknowledgments). Fish were obtained from 15 

 areas (Fig. 1, Table 1). 



Gills and viscera were frozen and flown to Bris- 

 bane for dissection. The SPC and NZ fisheries offi- 

 cers sampled 5 fish/school from a maximum of 3 

 schools/d. Commercial companies were unable to 

 sample from individual schools and usually supplied 

 the head and the anterior ventral body, removed 

 from frozen fish by a single slanting cut using a band 

 saw. Fork length, if not supplied, was calculated 



Table 1.— Sources of fish dissected. 



Manuscript accepted November 1984. 

 FISHERY BULLETIN; VOL. 83, NO. 3, 1985. 



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