■180- 



approximately 1-1/2 years earlier; related meteorological variables such as 

 barometric pressure differences and wind speeds are also important factors. 

 These investigations indicate that approximately 47°^ of the variation in 

 the annual catch rate (numbers of fish per standardized day's fishing) of 

 12- and 24-month old skipjack in the eastern Pacific may be explained by 

 the wind-mixing index in the spawning areas of the central Pacific 

 approximately 1-1/2 years earlier. 



III.B.5. Results of Other Analyses/Simulations 



Estimates of the general magnitude of the potential yield 

 of skipjack in the Pacific are available. These estimates follow less 

 rigorous techniques and give widely different results. Following the 

 method of Beverton and Holt, Rothschild estimated the potential yield of 

 skipjack from the central Pacific. Using 70,000 mt as an average steady- 

 state yield in the eastern Pacific and assuming a sojourn time of 2 months 

 in the fishing areas of the eastern Pacific, he estimated the potential 

 yield in the central Pacific to be 5 to 17 times greater than the yield in 

 the central Pacific, or 350,000 mt to 1,190,000 mt. Assuming a sojourn 

 time of 6 months, he estimated the yield to be 140,000 mt to 420,000 mt. 

 However, Silliman, using a population simulation method, estimated a 

 potential yield of 180,000 to 275,000 mt for the eastern Pacific fishery 

 and the unexploited areas of the central and eastern Pacific. 



The Fisheries Agency of Japan has estimated the 

 potential yield for the entire Pacific to be from 800,000 to over 1,000,000 

 mt. They assumed the skipjack spawning stock was about twice that of other 

 tunas, based on a 1.7 to 1.8 ratio of skipjack larvae to other larvae 

 collected. They then extrapolated the catch of other tunas at that time to 

 obtain estimates of the potential skipjack tuna yield from the entire 

 Pacific Ocean. 



III.C. Current Evaluation of Stocks and the Fishery 



A relationship between stock abundance and total effort could not 

 be detected, and the potential production of skipjack tuna stocks in the 

 eastern Pacific has not been satisfactorily established. In the eastern 

 Pacific skipjack fishery, the yields per recruit generally are highest with 

 a size-at-entry of 35 cm (about 1.7 pounds) and at fishing effort levels 

 considerably greater than has so far been the case. This is because the 

 losses to the total weight of a cohort of fish by natural mortality and 

 emigration exceed the gains to.it by growth. The lATTC indicates that 

 neither the general production models nor the age- structured models 

 examined so far indicate any need for the management of skipjack. 



