This paper attempts to derive a reasonable method for simply estimating the size of an exploited population which migrates in a certain direction, under the assumption that emigrant number in an area is equal to the immigrant number in its adjacent areas. In deriving the method, it was assumed that the emigration rate was the same in the considered areas and that the natural mortality rate was negligibly small as compared with the emigration rate. An application of the derived method gave about 0.1 during 5 days as an estimate of the emigration rate of the swimming crab Portunus trituberculatus population along the Buzen District, Fukuoka Pref., in 1978 and 1979. The present method can not be used for the populations that immigrate continuously. © 1982, The Japanese Society of Fisheries Science. All rights reserved.

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