There is a simple method based on lenght frequencies.

You will need FISAT II, a free software downloadable fron internet.

Here you will find a comprehensive explanation:

http://www.fao.org/docrep/w5449e/w5449e00.htm

There are generally 3 methods to estimate growth of aquatic animals, such as (i) hard part studies (e.g., otolith, scales, vertebrae), (ii) mark recapture method and (iii) length frequency distribution analysis. However, in case of aquatic animals having no aging characters (e.g., shell fish), the length frequency distribution is used.

Rahman, could you please expantiate on the use of the length frequency distribution and its possible application to non-shell fish?

Do you know of decomposition of multimodal distribution? There is a R package (http://www.r-project.org) (MIXDIST) (downloadable here http://www.math.mcmaster.ca/peter/mix/mix.html) that does it very well and that should help you getting the number of cohort from a length frequency distribution, assuming a gaussian distribution in length for each cohort.

Dear Mario, I'm not too conversant with the R-package, I use statistica, spss and minitab for most of my analyses; but i'll appreciate a bit of theoretical insight into the use of length frequency distribution to estimate fish age before attempting the in-depth statistical aspects. Thanks alot

The principle is easy to understand and relatively easy to apply. One length frequency distribution is composed of a sum of several distributions. Each one has its own probability density function that can be linked to the mean size at age with the variance (Gaussian distribution). When using MIXDIST, you can also chose different distribution laws like log-normal, binomial, Poisson, etc. So you need to do size classes and determine the potential number of group in your distribution. Then you run the MIXDIST and you get a potential decomposition for your distribution. This way you can evaluate the overlap in size between cohort (year classes). You will most probably need to give many try in order to better fit to the growth (with deviance) of your individuals. If you have good growth data then you should have a good idea of the strength of each year class in your sample and a good idea of the age of an individual given a certain size and this with probability of being age 1, age 2, age 3, or more. Good luck.

There is a simple method based on lenght frequencies.

You will need FISAT II, a free software downloadable fron internet.

Here you will find a comprehensive explanation:

http://www.fao.org/docrep/w5449e/w5449e00.htm

Dear Chukwuka

Here are some literature on the use of the length frequency distribution and its possible applications in studies on fish aging.

1. http://www.ku.edu.np/kuset/aej/bibhuti.pdf

2. http://www.iccat.int/Documents/CVSP/CV060_2007/no_4/CV060041160.pdf

3. http://alaska.fws.gov/fisheries/fish/Data_Series/d_1998_5.pdf

4. http://www.osti.gov/bridge/servlets/purl/7312272-GydXln/7312272.pdf

Best

Dola

for length frequency analysis, with particular focus on fish, see FAO FISAT and SEDA programs: I have been working with the use of eye lenses on the estimation of age, you can check my paper RG

Thanks Javier, Dola and Erick for your contributions, the materials were really helpful. Thanks again