BIOMASS PER RECRUIT MODEL

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Presentation transcript:

BIOMASS PER RECRUIT MODEL

Introduction The model of Beverton and Holt, yield per recruit model could be used to determine annual average biomass of survivors as a function of F. The average biomass is related to CPUE. The equation is YR = F x / R The formula used to calculate B/R is the same as in equation - 1 (see chapter on Analytic models of fish stocks) divided by the F. The average biomass B/R thus obtained is considered to be the biomass of exploited part of the cohort i.e. the biomass of fish of age Tc or older.

Mean age and size in the yield When Z is constant from time of recruitment () and age at first capture (), mean age and mean length in the annual yield could be estimated with the following equation. = + Mean length in annual yield, where is the mean age in yield Similarly mean length in the annual yield is = L[ ] S = exp [-K * ( - )] = 1 -

Mean age and size in the yield conted...... or can be replaced by any age from which the fish have a constant mortality, so as to give mean length in that part of population. Mean weight in annual yield = Z * [ – + - ] The three parameters , and and exploited biomass along with CPUE will decrease by increasing Z i.e. with effort.

Mean age and size in the yield conted...... In the unexploited fishery, the decrease may be faster for low values of F. In all the three parameters ( , and ) forms a common input as is determined by mesh size. When the mesh size is large, the mean age and size will be higher.

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