Skipjack Taken in Hawaiian Waters 



The Hawaiian skipjack fishery uses pole-and- 

 line gear and live bait. The average annual catch 

 is about 10 million pounds, fluctuating in recent 

 years between 7 and 14 million pounds. The bulk 

 of the catch (usually about 70 percent by weight) 

 is taken during June, July, and August, and gen- 

 erally consists of large numbers of "season- 

 size fish" {>than 60 cm.). "Season-size fish" 

 also occur sporadically during the winter but 

 are not nearly as abundant as during the summer 

 "season." The magnitude of each year's catch 

 has been predicted for several years (Seckel and 

 Waldron, 1960). This prediction is based on 

 changes in sea-surface temperature that occur 

 in February and March and, in essence, foretells 

 the magnitude of the catch during the "season" 

 in June, July, and August. Several problems 

 arise in interpreting the nature of the fluctua- 

 tions in the Hawaii catch. A consideration of 

 these problems generates several hypotheses 

 pertinent to the origin of those skipjack taken by 

 the Hawaiian fishery. 



Skipjack spawn in the Hawaiian Zone so it is 

 likely that at least some of the skipjack caught 

 in the Hawaiian Zone fishery are generated in 

 this Hawaiian Zone. Immunogenetic studies have 

 shown that those skipjack caught in the Hawaiian 

 Zone fishery are comprised of two subpopula- 

 tions. On this basis I postulate that at least one 

 of these two subpopulations has its origin in the 

 Hawaiian Zone. The other subpopulations may 

 or may not originate in the Hawaiian Zone, but in 

 any event the two subpopulations must maintain 

 a degree of Isolation at spawning time. 



A wide variety of isolating mechanisms that 

 tend to reduce interbreeding between populations 

 of many organisms are described by various 

 authors (see, for example, the concise resume 

 by AUee, Emerson, Park. Park, and Schmidt, 

 1949: p. 606). Some possible isolating mecha- 

 nisms that could maintain immunogenetic dis- 

 tinctness between two groups of skipjack are 

 distance between spawning units, differences in 

 the time of spawning between breeding units, 

 island association of spawning, island association 

 of relatively high larval survival, length segre- 

 gation at spawning time, and the association of 

 breeding groups with particular water types. 

 Both distance and time of spawning could operate 

 as isolating mechanisms between the Hawaiian 

 and Marquesas Zones because these Zones are 

 at the extremes of the north-south spawning 

 range; spawning in the Hawaiian Zone opcurs 

 predominantly during the northern summer 

 whereas spawning in the Marquesas takes place 

 predominantly during the northern winter. These 



mechanisms also probably operate between the 

 Hawaiian and Equatorial Zones. Included in the 

 concept of distance as an isolating mechanism is 

 the apparent island-association of spawning or 

 high larval survival, suggested by what appears 

 to be relatively high densities of skipjack around 

 major island groups. 



Another possible isolating mechanism is length 

 segregation of skipjack schools (Brock, 1954: 

 p. 98-99). Perhaps at spawning time small dif- 

 ferences in length, on the average, may tend to 

 segregate schools of skipjack. Any differences 

 in length between the two populations would act 

 as isolating mechanisms. Since present sam- 

 pling techniques do not capture skipjack that are 

 spawningor are in an imminent spawning condi- 

 tion this hypothesis cannot be tested. One final 

 type of isolating mechanism involves the hypoth- 

 esis that subpopulations of skipjack are associ- 

 ated with water types as indicated by tempera- 

 ture and salinity at the sea surface (Sprague, 

 1963). 



Another characteristic of the skipjack taken 

 in the Hawaiian Zone that may indicate a non- 

 Hawaiian originfor some of the fish taken in the 

 Hawaiian fishery is the roughly equal numbers 

 of fish in the 45- and 70-cm. modal group. Catch- 

 es representative of the full range of sizes for 

 any population should have considerably more 

 fish in the smaller size groups. Several expla- 

 nations can be offered for the relatively few 

 small fish in the Hawaiian catch. Small fish 

 might not be caught in the Hawaiian pole-and- 

 line fishery; however, there are no data that 

 describe this selectivity. A second possibility 

 is that the large size group of fish contains sev-- 

 eral age groups indistinguishable with respect to 

 size. This may be partially true, but 1 believe 

 its extent is insufficient to explain the equal 

 numbers of large and small fish. A third type of 

 selectivity may cause mature fish to be unavail- 

 able to pole-and-line gear during the spawning 

 season. The unavailability of imminent spawners 

 already has been pointed out. If the proportion 

 of larger fish in which spawning is imminent is 

 smaller than the proportion of smaller fish in 

 which spawning is imminent, then the numbers 

 of larger fish in the catch will be greater than 

 would be expected if there were no differences 

 in maturation between large and small fish. No 

 data are available on the extent to which immi- 

 nent spawning affects the representativeness of 

 skipjack samples. 



Other mechanisms that might produce rela- 

 tively large numbers of large fish are the possi- 

 bility of a disproportionate increase in the density 

 of large fish in the immediate vicinity of the 



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