Multiplicative catch-at-age analysis of Scad ( Trachurus trachurus L.) from Western Iberian waters

Shepherd and Nicholson (1986) developed the multiplicative model as a parsimonious alternative method to virtual population analysis (VPA), which allows for the possibility of sampling error in the catch-at-age data. The solution must be constrained by specifying the trend in one of the fitted effects on the basis of independent data. This model was fitted to the international catch in numbers at age data for Scad from ICES Divisions VIIIc and IXa, with the yearclass effect matched to the October/November survey yearclass effect as an appropriate constraint. The resulting estimated age effect and year effect were analysed, and the same procedure was followed for each of the fleets. A catch forecast for 1987 and 1988 was performed using assumed yearclass strengths and “status quo” fishing mortality.