Methods of Age Determination using Length Frequency Method

Basic Principle Length frequency distribution tend to group themselves around a central value called mode and the progression of modes through successive intervals of time indicates the pattern of growth.

Collection and processing of Length Frequency Data Samples for length frequency should be collected at random from the commercial catches as soon as they are landed. Sampling should be done before sorting into various market size groups. Length data should be collected separately for different gears and for different mesh types of the same gear. Sexes should be treated separately. Total length or fork length or standard length should be taken depending on the convenience. The data should be recorded in mm or cm in a primary register.

It is better to take the length frequency data for a period of two years (24 months). Individual lengths are grouped into appropriate size classes which should not normally exceed twenty five. The weekly data may be pooled on a monthly basis. The data thus pooled must be drawn in the form of frequency polygons or histograms for each month. Progression of modes is traced through successive months.

Integrated method (Pauly, 1983) A growth curve is drawn with a curved ruler directly upon the length frequency samples sequentially arranged in time. This method is based on the following tenets Growth in fishes is at first rapid, then decreases smoothly and for the population as a whole is best approximated by a long continuous curve rather than by several short straight segments. A single, smooth growth curve to represent the average growth of the fishes of a given stock.

The growth patterns repeat themselves from year to year The intervals on the time axis proportional to the time elapsed between the sampling dates. The original data must be plotted at least twice or more along the time axis, which allows for longer, stabilized growth curves to be drawn and all relevant age groups should be included in one single line. The various growth curves should have the same shape, and vary only as to their origin.

The scale of the ordinate (length) should start at zero, thus allowing to identify the approximate spawning periods. The more peaks a curve connects the more likely to depict the actual growth of a population. The modal lengths corresponding to various ages (starting from an arbitrary age) can be read off the curve at regular time intervals, and may be then used to determine the growth parameters.