WATANABE and LO: PRODUCTION AND MORTALITY OF SAURY 



tember to December as the early spawning 

 season (Fig. 3). We calculated catch/tow values 

 of 4 seasons separately in every size- and time- 

 group by summing up the number of fish and net 

 tows of 15 years and then taking average catch/ 

 tow values of 4 seasons. These values, average 

 catch/tow values of the year, were used for cal- 

 culation of the size- and time-specific retention 

 rates mentioned above. 



Mortality Models and Computations 



The mortality model of the young saury from 

 hatching (age 0) to 52 days old (57.5 mm pre- 

 served KnL) was based on the data of catch/ 



tow/day (daily production) at ages. Two types of 

 mortality curves were fitted to both the com- 

 bined data collected in the NIT period for 1972- 

 86 and the data set from 14 individual spawning 

 years. One mortality curve is based on an age- 

 dependent instantaneous mortality rate (IMR), 

 Zi(t) = 3/^ and the other based on the age-inde- 

 pendent constant IMR, ZoU) = a, where t is the 

 fish age in days and a and (3 are coefficients of 

 IMR. The age-dependent IMR was considered 

 because of the possibility that early larvae may 

 suffer higher mortality than the older ones, as in 

 the case of northern anchovy larvae along the 

 Cahfornia coast (Lo 1985, 1986). We constructed 

 both mortahty curves for saury larvae and juve- 



MAINspawningseason 



^-FEB 



A MAR 



2.0 



1.0 



OFF 



— JUL 



-•>• AUG 



SIZE CLASSES (mm 



Figure 3. — Division of the year into four spawTiing seasons based on the number of Pacific 

 sauries captured by a net tow (catch/tow) by months. 



605 



