Bull. Jpn. Soc. Fish. Oceanogr. 76(3), Page 115-122, 2012   Hybrid method for estimating the number of components in a finite mixture of normal distribution from length frequency data to age composition Hiroshi Shono1 † 1 Fisheries Biology and Oceanography Division, Faculty of Fisheries, Kagoshima University, 4-50-20 Shimoarata, Kagoshima, Kagoshima 890-0056, Japan †e-mail: shono@fish.kagoshima-u.ac.jp To discuss the problem of estimating age composition from length-frequency data in a finite normal mixture distribution, I introduce a hybrid method for estimating the number of components in the full model without limitation and a structured model with relationship among unknown parameters. The computational procedure was expressed in four steps and statistical interpretation/significance of the hybrid method suggested by Eguchi and Yoshioka in 2001. The hybrid method was applied to the well-known length-frequency data for yellow sea bream, Dentex hypselosomus. The computer simulation experiment estimated the number of components in the finite normal mixture model whose probability density function resembles a log-normal distribution. Results showed that the selection performance of the hybrid method for choosing the true model was superior to traditional methods such as Akaike’s information criterion (AIC) and/or Baysian information criterion (BIC). Key words: age composition, length-frequency data, finite normal mixture distribution, hybrid method, number of components estimation, growth curve