Publication Abstract

An hybrid age and length structured model for the assessment of changes of effort, mesh and/or landing sizes in multiple-species, multiple-fleet fisheries

B. Mesnil and J.G. Shepherd

A hybrid model combining the features of age-based and length-based models is proposed for assessing technical measures applicable to multiple-species and multiple-fleet fisheries. The model is primarily structured in age to enable medium-term predictions to be carried out, and employs the usual catch equation of analytical models. The length-distributions-at-age observed in catches are used to distribute the fishing mortalities by length component of each age for each species and fleet, and to calculate the contribution of each length group to the relevant catches in weight. These contributions are converted to landings by application of sorting selection ogives, and can also be expressed in value. Changes in minimum mesh sizes in each fleet can easily be modelled by changing the fishing mortalities-at-length in each age in accordance with the simulated gear-selection ogive. Changes in effort and in minimum landing sizes can also be assessed. Basically, the model does not require additional data compared with more conventional age-based models, provided that the length compositions of the catches-at-age are preserved when the age-length key is applied, and the parameters can be estimated using virtual population analyses (VPA). Alternative length-to-age conversion methods are presented for stocks which are not routinely sampled for age compositions, but which are important resource components for some fleets. The different estimates produced are used in an example which evaluates these methods and illustrates the possibilities of the hybrid model.

Reference:

J.G. Shepherd and M.D. Nicholson, 1991. An hybrid age and length structured model for the assessment of changes of effort, mesh and/or landing sizes in multiple-species, multiple-fleet fisheries. ICES Journal du Conseil, 47: 284-294.