1) The dynamics of the saury population which occurs in the Pacific Ocean off northern Japan in autumn was studied by analysing the catch and effort statistics of the saury lift net fishery and length composition data of the catch.
2) Taking into consideration such population conditions as the different patterns of the dynamics among size categories, yearly change of body length, growth within the fishing season, and appearance and disappearance of the offshore fishing ground, the abundance index was calculated in terms of number by size category and fishing ground.
3) It was verified that the abundance index of the large sized fish in number by fishing ground was more steadily and rapidly decreasing than that of other size categories, but the tendency was quite variable by year.
4) Regression of the total coefficient of decrease of the available population on the effective overall fishing intensity was examined, but no positive correlation was found except in the large sized fish. The values of q and M were also estimated from the abundance index in number for every 10-day period, but unrealistic nagative values were often obtained for each size category.
5) Emigration and immigration of the fish shoals were considered to affect seriously the abundance index in number by size category and fishing ground, and the total coefficient of decrease; therefore, a new model on the dynamics incorporating the shift of fishing ground and oceanographic information must be developed in the future analysis.