Age structure estimation from size data for hirtail in the nortern East China Sea.

The estimation of age structure for hadrtail Trichiurus haumera (Forskal) inhabiting the north-ern East China Sea has been tried using size-frequency data over the years. Since most of size-frequency distributions of samples are not clearly polymodal, an improved least-squares method for analysing such data was used. The method does not require initial values of fish number for each age group for starting computation and only requires a possible range for mean size and standard deviation, which can be determined according to biological information. The result of this study shows that the dominant age group in the catches of hairtail is age 1. The effectiveness of this method was compared with the linear approximation and the Marquardt's methods pub-lished by other authors. It is verified that this method could be applicable to non-polymodal frequency distribution for which the last two methods may provide rather poor results.