Fish Population Dynamic and Stock Assessment (2+1)

2.2.6 Cohort Dynamics

A cohort is a batch of fish of all of approximately the same age and belonging to the same stock.  In the cohort of a stock, at birth the cohort has age zero.  From age O to Tr, the cohort is in the “Prerecruitment phase”.  The symbol N(t) is used to designate the number of survivors from a cohort attaining age ‘t’.  The age is usually expressed in years.  The symbol ‘R’ is used to designate the recruitment. ‘N(Tr)’ is the “number of recruits” to the fishery.

‘Tr’ is the minimum age at which the fish can enter the fishery ie., becomes liable to encounter with fishing gears.

‘Tc’ is called the age at first capture which marks the beginning of the exploited phase.  ‘Tc’ is dependent on the mesh size.

The following figure illustrates the basic features of cohort dynamics.

Length based cohort analysis is available in computer programs namely “LCOHOR” in the LFSA package of microcomputer programs developed by Sparre, 1987.

The FISAT (Gayanilo et al., 1995) contain routines for a length based analysis similar to LCOHOR.

 

In the above illustration, due to mortality there is a continuous decrease in the number of survivors.  In the prerecruitment phase (age ‘O’ to Tr), there is only ‘M’ and after age ‘Tr’, the fish may be caught if suitable small meshed gear is used.  At age ‘Tc’, the fish get caught with the mesh size actually in use.