Larval Production and Mortality of Pacific Saury, Cololabis 

 sairUy in the Northwestern Pacific Ocean^ 



Yoshiro Watanabe and Nancy C. H. Lo 



ABSTRACT: Because quantitative samples of 

 saury eggs are difficult to obtain owing to their 

 adhesive nature, reproductive level of the Pacific 

 saury, Cololabis saira, has been estimated from the 

 standing stock of the lan'ae and juveniles in Japan. 

 Mortality curves were constructed and daily larval 

 production at hatching was estimated for combined 

 data from 1972 to 1986 and for consecutive individ- 

 ual years within this period. Durations of size 

 classes were estimated from a growth curve and 

 used to calculate fish production at age. Because 

 number of fish captured by a net tow (area = 401.3 

 m~) showed a diel cycle due to fish behaviors, such 

 as net frame evasion and diel vertical movement, 

 the average mortality curve for 15 years was based 

 on data collected at night. We used an exponential 

 decay model to describe the mortality of saury lar- 

 vae and juveniles. The daily instantaneous mortal- 

 ity rate was 0.078, and the larval production at 

 hatching was 1.255 larva/tow/day. Mortality curves 

 for the individual years were based on data col- 

 lected throughout the day and night after correc- 

 tion by size- and time-specific retention rates. Daily 

 lar>al production at hatching fluctuated among in- 

 dividual years ranging from 0.154 to 5.176. Daily 

 instantaneous mortality rates positively correlated 

 with larval production at hatching, which might 

 indicate the presence of a density-dependent pro- 

 cess in larval mortality. 



Together with the Japanese sardine, Sardinops 

 melanosticta, and the mackerels, Scomber 

 japonicus and S. australasicus, the Pacific 

 saury, Cololabis saira (Brevoort), is one of the 

 most important offshore pelagic fishes in Japan. 

 Although total catch of the saury in Japan 

 (210,000 t [metric tons] in 1984) was much small- 

 er than that of the sardine (4.180,000 t) or the 

 mackerels (810,000 t) (Statistics and Information 



'Contribution No. 432 from Tohoku Regional Fisheries 

 Research Laboratory, Fisheries Agency, Shiogama, Miyagi, 

 Japan. 



Yoshiro Watanabe, Tohoku Regional Fisheries Research 

 Laboratory. Fisheries Agency, .3-27-5 Shinhama, Shiogama. 

 Miyagi 985, Japan. 



Nancy C. H. Lo, Southwest Fisheries Center La Jolla Lab- 

 oratory, National Marine Fisheries Service, NOAA, P.O. 

 Box 271, La Jolla, CA 92038, U.S.A. 



Department, Japan 1986), more than 95% of the 

 catch is destined for human consumption (Japan 

 Saury Fishery Association 1985). In sharp con- 

 trast, the sardine (81%) and the mackerel (40%) 

 catches (Fisheries Agency 1985) are procesed for 

 animal foods. 



Saury fishermen in Japan e.xpect to have a 

 rehable fishing forecast provided by the Tohoku 

 Regional Fisheries Research Laboratory, Fish- 

 eries Agency of Japan, every summer before the 

 beginning of the fishing season. The forecast in- 

 cludes the expected catch in the coming season 

 and the potential fishing gi'ounds. The expected 

 catch depends on fish stock size, and the location 

 and size of the fishing gi-ounds are a function of 

 fish migration in relation to oceanographic condi- 

 tions. Fish stock size is determined by both 

 reproductive level and mortality rates of devel- 

 opmental stages from postspawning through re- 

 cruitment. Since the saury grows rapidly and 

 becomes an adult within one year (Watanabe et 

 al. 1988), larval production and mortality rate in 

 young stages are believed to be more or less 

 directly related to recruitment and catchable 

 stock size. Matsumiya and Tanaka (1978) sug- 

 gested from their intensive study of population 

 dynamics of the northwestern Pacific saury that 

 the fluctuations of population size are seriously 

 affected by reproductive success. 



An egg survey may be the best method for 

 estimating the reproduction level of pelagic 

 fishes. However, it is difficult to conduct a quan- 

 titative egg survey of the saury by towing plank- 

 ton nets because the eggs attach by filaments to 

 floating objects such as drifting kelp. We there- 

 fore have been conducting larval and juvenile 

 surveys to estimate the reproductive level. We 

 use catch/tow (number of fish/net tow) values of 

 several size classes as abundance indices. The 

 year-to-year changes of the indices may reflect 

 the fluctuation of reproductive level. 



The apparent number of larvae in a size class 

 is influenced by the duration of growth through 

 that size class. If gi'owth is slow, the duration 

 will be extended; conversely, if growth is fast, 

 the duration will be short. The duration of 

 growth through a size class thus defines the 



Manuscript accepted November 1988. 

 Fishery Bulletin, U.S. 78:601-613. 



601 



