Hawaiian Islands (i.e., the fishery area) or that 

 equal numbers of large and small fish are pro- 

 duced by an emigration of small fish rather than 

 by an immigration of large fish. The latter pos- 

 sibility is contradicted, however, by the appar- 

 ently tremendous influx of large fish into the 

 Hawaiian fishery during the summer. If we as- 

 sumed negligible fishery selectivity and minimal 

 effects of maturity-associated size-specific 

 availability, then the samples indicate there are 

 roughly equal numbers of large and small fish in 

 Hawaiian waters during the summer. This prob- 

 ably is caused by an immigration of large fish 

 that did not originate in the Hawaiian Zone. 



Evidence from subpopulation studies and the 

 distribution of skipjack lengths then admit the 

 hypothesis that one of the subpopulations enter- 

 ing the Hawaiian fishery did not originate in the 

 Hawaiian Zone and that this subpopulation is com- 

 prised at least, in part, of large numbers of 

 season-size skipjack. As previously noted a 

 prediction has been available for forecasting the 

 magnitude of the annual catch of skipjack in the 

 Hawaiian fishery. A large annual catch is an 

 index of a high apparent abundance of season- 

 size fish. I have shown that certain years with 

 relatively large catches, and thus a high apparent 

 abundance of seasonfish, were characterized by 

 a reduction in size of the season-size fish. Thus, 

 in years of relatively poor catches the proportion 

 of fish over 60 cm. consists mostly of fish over 

 70 cm. long (fig. 6). In years of relatively large 

 catches, the season-size fish are mostly between 

 60 and 70 cm. long and their proportion in the 

 total catch is apparently independent of the pro- 

 portion of fish over 70 cm. Therefore, since the 

 magnitude of the catch could be predicted and 

 the size distribution is related to the magnitude, 

 the size distribution also could be estimated in 

 the sense that in years of high catch the size of 

 the season-size fish would be slightly reduced. 



These predictions imply that the season catch 

 of skipjack is controlled by events that are asso- 

 ciated with the warming of the sea surface in the 

 vicinity of Oahu several months before the onset 

 of peak fishing. By this reasoning it would seem 

 that the biomass and size distribution of prere- 

 cruits to the Hawaiian fishery are oriented in 

 space and that this orientation is controlled by 

 the absolute position and rate of movement of 

 either the isotherms or properties related to the 

 isotherms. 



On the other hand, a study in the degree of 

 variation in the relation between time of warm- 

 ing and catch indicated that a large component of 

 variability in total catch might not be attributable 

 to the sea-surface time of warming index. There- 



fore, the unexplained variation is due to events 

 other than the time of warming index which is an 

 evident part of the temporal change in sea-sur- 

 face temperature and occurs a few months prior 

 to eachfishing season in the vicinity of Oahu. A 

 possible component of the heretofore unexplained 

 variation is the strength of the various year 

 classes that enter the skipjack fishery. The 

 formation of large or small year classes of the 

 large skip jack taken in the Hawaii an fisheryprob- 

 ably takes place 2 or 3 years prior to the recruit- 

 ment of this large size group and therefore seems 

 independent of events, such as the time of warm- 

 ing, that occur a few months before the fishing 

 season. The hypothesis that year class strength 

 influences the summer apparent abundance of 

 large skipjack is strengthened by the observation 

 that a small-size component of the large-size 

 group becomes particularly evident in years of 

 relatively high catches. It is postulated that this 

 small-size component represents an unusually 

 strong year class. If year class strengths are 

 important components of variability in skipjack 

 catches, then it is also likely that slight reduc- 

 tions in average size of the skipjack during years 

 of high apparent abundance may also reflect den- 

 sity dependent growth that is typical of large 

 year classes for many species of fish. 



Additional inferences pertinent to the origin of 

 skipjack taken in the Hawaiian Zone are based 

 on size distribution and growth data from the 

 Hawaiian fishery. Growth of skipjack tuna based 

 on modal progressions within size distributions 

 of Hawaiian samples is often less, on a month- 

 to-month basis, than that estimated from tag 

 returns (Rothschild^/). lnfact,for some months 

 modes remain the same or decrease in length. 

 These observations suggest that, as in the east- 

 ern Pacific, the fishery is constantly being re- 

 plenished withfish of smaller or equivalent size 

 while at the same time larger fish leave the 

 fishery area. During some years there is a 

 U-shaped distribution of lengths with a minimum 

 in the summer. This could be interpreted as a 

 removal of the larger fish of the small size group 

 during the summer fishery. A second possibility 

 is that smaller fish enter the fis.iery initially, 

 then emigration is reduced, the ascending right- 

 hand limb of the U-shaped distribution reflecting 

 growth of those that are residual. Thus it is 

 apparent that all exploited subpopulations of skip- 

 jack exhibit less than expected increases in 

 length. It is hypothesized that this characteristic 

 is due to certain sizes of fish moving through 

 the fishery. 



Rothschild, Brian J. Manuscript. Estimates 

 of skipjack tuna ( Rats uw onus pelamis ) growth in 

 the Hawaiian Islands. Bureau ot Commercial Fish- 

 eries Biological Laboratory, Honolulu. 



16 



