■178- 



is not on a unit stock. In addition, environmental studies indicate that 

 perturbations caused by environmental fluctuations may mask changes in 

 apparent abundance caused by fishing effort. The fishing effort has 

 increased in recent years, due to increases in the size of the fleet. This 

 has apparently not depleted the resource because some of the greatest total 

 catches have been made during the last few years. This indicates that 

 there is still no apparent relationship between skipjack tuna stock 

 abundance and fishing effort. 



III.B.3. Results of Yield-per-Recruit Analysis 



The estimated relationships among size-at-entry, fishing 

 effort, and yield-per-recruit are shown in Figure 8. The top panel is 

 based upon age-specific fishing mortality rates estimated from length- 

 frequency data obtained during 1967 to 1969, the middle panel is based upon 

 rates estimated from data obtained during 1971 to 1973, and the lower panel 

 is based upon rates estimated from data obtained during 1975 to 1977. 



Increasing the size-at-entry would not appreciably 

 change the yield-per-recruit at fishing effort levels observed during the 

 years in Question (F multiplier = 1.0). The yield-per-recruit remains 

 between 0.75 and 1.00 pounds for the 1975 to 1977 period. At higher 

 fishing mortality levels, an increase in size-at-entry would decrease the 

 yield-per-recruit. 



Increasing effort at current size-at-entry would 

 increase the yield-per-recruit to the fishery. An increase in effort which 

 would double the instantaneous fishing mortality would result in an 

 increase in yield-per-recruit of approximately ZSk Increased effort would 

 result in an increased yield-per-recruit even at higher sizes-at-entry for 

 both the 1971 to 1973 and 1975 to 1977 periods. 



In general, the yields are highest with a size-at-entry 

 of about 35 cm (about 1.7 pounds) and with fishing effort considerably 

 greater than has so far been the case in the eastern Pacific Ocean. 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, even when the 

 fish are only 35 cm long and are presumably growing rapidly. 



III.B.4. Results of Spawner/Recruitment Analysis 



No spawner/recruitment analyses are available for central 

 and eastern Pacific skipjack tuna. 



Possible environmental effects on year-class recruitment 

 of skipjack caught in the eastern Pacific have been investigated by the 

 lATTC in an attempt to explain some of the variation in the catches. 

 Significant correlations have been found between indices of skipjack 

 abundance in the eastern Pacific and sea- surface temperatures in the 

 spawning areas of the central tropical Pacific (180° to 130° W longitude) 



