INTEGRITY OF SCHOOLS OF SKIPJACK TUNA, 



KATSUWONUS PELAMIS, IN THE EASTERN PACIFIC OCEAN, 



AS DETERMINED FROM TAGGING DATA 



William H. Bayliff' 



ABSTRACT 



Little information concerning the integrity of schools is available for any species of fish. In this study 

 the integrity of schools of skipjack tuna, Katsuwonus pelamis, was analyzed with data for returns of 

 tagged fish which had been in the same schools when originally tagged. Two methods, the first using 

 Chi-square contingency tests and the second using binomial homogeneity tests, were employed. From 

 the results obtained with the first method it appears that after 1 month at liberty the tagged and un- 

 tagged fish were randomly mixed with one another in some cases and after 3 to 5 months at liberty 

 they were randomly mixed with one another in nearly all cases. The results obtained with the second 

 method indicate somewhat less rapid mixing of the tagged and untagged fish. 



Schooling occurs in many species of fish, and many 

 studies have been made of the reasons for school- 

 ing and the behavior of the fish in the schools (e.g., 

 Parr 1927; Shaw 1970; Pitcher 1986). Almost 

 nothing has been written, however, about the integ- 

 rity of schools over extended periods of time. 



Parr (1927) stated that "apparently permanent" 

 schools are formed by pelagic fishes such as mack- 

 erel, sprat, and herring, and Sharp (1978) reported 

 that, "From the genetic sample data for the east- 

 ern Pacific yellowfin [{Thunnus albacares)] and the 

 Pacific-wide skipjack [{Katsuwonus pelamis)] there 

 is evidence for a cohesiveness of related fishes in 

 schools .... What is observed is that where more 

 than one very rare allele (overall expected occur- 

 rence <.01) is encountered in a large sample, the 

 individuals exhibiting the rare alleles are often the 

 same length or within 1 cm of each other. This is 

 highly unlikely unless they are related." 



On the other hand, Helfman (1981) reported "ag- 

 gregations [ of freshwater fish] that disbanded dur- 

 ing twilight," and Moyle and Cech (1982) stated that 

 "most schools break up at night." Observers on 

 fishing vessels and aircraft have reported that the 

 schools of tunas frequently break up and reform. 

 Scott and Flittner (1972), for example, stated that 

 "the relatively large nighttime schools [of bluefin 

 tuna, Thunnus thynnus,] break down into several 

 smaller foraging schools and begin their search for 



ilnter-American Tropical Tuna Commission, P.O. Box 271, La 

 Jolla, CA 92038. 



food .... The gradual increase in school size during 

 the daylight hours may be due to regrouping of the 

 smaller schools through random encounters." Such 

 observations might lead to the conclusion that there 

 is considerable mixing of fish from different schools 

 and that fish of the same species and same approx- 

 imate size in the same areas would mix thoroughly 

 with one another within a period of a few days or 

 weeks. 



Anonymous (1960) stated that "Tag returns from 

 individual schools [of skipjack tuna] suggest that, 

 normally, the skipjack in Hawaiian waters remain 

 within a school for one month or less, then at least 

 some of the school break off, move into new areas, 

 and regroup with other fish or schools. From the 

 releases off Hilo and Mexico, however, it is evident 

 that there are situations, possibly environmentally 

 conditioned, where the schools remain intact ... for 

 at least 2 or 3 months." Lester et al. (1985) studied 

 the occurrence of various parasites in skipjack tuna 

 of the same and different schools and concluded 

 that "school half-life is likely to be in terms of at 

 least weeks rather than days." They stated, how- 

 ever, that their data did "not support the hypothesis 

 [of Sharp (1978)] that fish stay in the same school 

 for life." 



Examination of data for fish tagged and released 

 at the same location and time shows that some have 

 been recaptured weeks or months later in the same 

 purse seine set or baitboat stop, and others have 

 been recaptured weeks or months later on the same 

 date in widely separated locations (Hunter et al. 



Manuscript accepted July 1988. 



FISHERY BULLETIN: VOL. 86. NO. 4, 1988. 



631 



