Fish Population Dynamic and Stock Assessment

7.1.4. Derivation of the model

At birth the cohort of a fish has age zero. The cohort grows and attains Tr, which is called as ‘age at recruitment.’ From ‘O’ age to Tr, the stock is in the pre-recruitment phase. From age Tr to Tc (age at first capture), the cohort is not experiencing any fishing mortality. In these lengths, the fish escapes through the meshes if they enter the gear. From age O to Tc, the cohort experiences only natural mortality and this is assumed to be constant through the entire life span of the cohort. At age Tc, the fish start to be caught with the mesh size actually in use and from age Tc onwards, the fish experiences fishing mortality. The fishing mortality is also assumed to be constant throughout the life span of the cohort.