WATANABE and LO: PRODUCTION AND MORTALITY OF SAURY 



10 



20 



30 



AGE 



40 50 



MDAYS 



FiGiRE 4. — Mortality cur\-e of young Pacific saurj' calculated 

 from fish production at ages calculated from the NIT data of 

 15 years (1972-86). Data points of the smallest two size 

 classes were not used for the computation. Po is the lar\'al 

 production at hatching, a is the daily IMR, and n is the 

 number of tow samples. 



Mortality Cunes for Individual Spawning 

 Years 



To assess the mortality of young saurj' for 

 individual spawning years, we used all the tow 

 data collected around the clock, after the correc- 

 tion of catch data by the size- and time-specific 

 retention rates (Table 1). The con-ection pro- 

 cedures are the same for all the spawning years. 

 To illustrate the data assemblage and computa- 

 tion, the 1979 catch data set for 11 size classes 

 was given (Table 3). Again, the data points of the 

 fii'st two size classes were not used in the estima- 

 tion procedure because of the apparent down- 

 ward bias. The parameter estimates and the 

 mortahty curves are summarized in Table 4, and 

 the mortahty curves are presented in Figure 5. 



The larval production at hatching (P,,) fluctu- 

 ated by more than 30-fold ft'om the maximum 

 5.176 larvae/tow day in 1978 to the minimum 

 0.154 in 1985 spawning yeai*. Daily IMR has also 

 ranged from 0.115 down to 0.041, with daily mor- 

 tahty rates of 10.9'7f and 4.07f , respectively. 



Year-to-year fluctuations of these two pa- 

 rameters (Po and IMR) and the total catch of 

 saurv' in Japan (Statistics and Information De- 

 partment, Japan 1987) are presented in Figure 

 6. The total catch of saury can be an index of the 

 stock size, though possibly including effects of 

 economic factors such as fish price. The fishing 

 season of saury opens in the fh*st half of August 

 and closes in the middle of December. Since the 

 sauiy becomes adult in 9-10 months and 2 jt old 

 fish were not found in the northwestern Pacific 



Table 3. — Daily production (catch'tow/day) at age (day) of Pacific saury in 1979 spawning year (September 



1978-August 1979). 



'Fish knob length after preservation. 



^Flsh knob length after capture (before preservation). 



^Not used for the computation. 



607 



