PARASITES OF SKIPJACK TUNA, KATSUWONUS PELAMIS: FISHERY IMPLICATIONS

Abstract

The numbers of 26 types of parasites were counted in 878 fish, of which all but 3 we.e from 14 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 using 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 of Ten­ !(J!'U{(J';(J coryphaena.e, a larval tapeworm, was positively correlated to fish size in the tropics. In New Zealand, however, fish over 55 em carried about the same number of T. corypha.enae as fish 45 to 55 em, SUI!;l.;csting 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 but 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.