Fish Population Dynamic and Stock Assessment (2+1)

2.2.3. Construction of life table

Construction of life table is a simple method, and it keeps the tracks of births, deaths and reproductive output of a population.

Three methods are used for construction of life table. They are

(i) Cohort life table

The cohort life table follows a group of same aged individuals from birth throughout their lives. This life table may work out for semelparous species, i.e. showing one reproductive event in their life time. This life table could be worked out easily by counting all individuals of a population during its life span.

(ii) Static life table

The static life table could be worked out for those organisms having complex life histories. This life table instead of working on a single cohort, it compares the population sizes of different cohorts of all life spans at a single point of time. The life table could be worked out with the assumptions that the proportion of individuals to each age class does not change and the size of the population nearly consistent.

(iii) Life table using mortality data collected from a specified time period with a stable age distribution

Both cohort and static life tables will be of less use to tropical fishes which shows year class variations and the pattern of recruitment is also not constant, as most of the species are prolific breeders. In such cases, the life tables could be worked out using mortality records.