This paper proposes an empirical model for the recruitment of the scallop stock, Pecten maximus (L.), in Saint-Brieuc Bay (English Channel, France). This stock shows important fluctuations in recruitment. The use of traditional models for fish population dynamics based upon an equilibrium state were inadéquate. This model includes three independent variables: spawning biomass, effects of fishing effort on the unexploited year classes, and temperature. Correlations between these expressions of independent parameters and recruitment are systematically analysed. The choice of the final model relies on minimization of the total variance unexplained portion. Introduction of fishing pattern into the model is realized by testing fishing effort effects upon two seasons of the unexploited cohort life (0 and 1-age groups). This procedure shows a more significant effect of fishing effort upon the 0-age group. The expressions of temperature are first tested on the seasonal anomalies defined by différences between monthly or bi-weekly temperature periods and the mean value for the same period. Seasonal anomalies do not give a significant explanation of recruitment, and other temperature expressions are defined. They are called thermal residues and are defïned as seasonal anomalies weighted by anomalies of the corresponding year. Thermal residues are used as algebraie or absolute values for monthly and bi-weekly periods. Absolute values of residues for bi-weekly periods give results that fit best. Only bi-weekly periods with a significant contribution to the recruitment explanation are retained. Détection of these critical values is carried out using Fisher-Snedecor test.
This procédure leads to a multiplicative combination of the spawning biomass, the 0-age group mortality due to fishing effort and exponential functions of absolute values for the thermal residues. Two critical periods are detected by the statistical operation: the first fortnight of March and the second of July. Correspondence between the two critical periods and stages of the reproduction cycle (start of sexual maturation, first spawning and larval development) is discussed. Analysis of absolute values shows scallops are adapted to mean thermal conditions. Temperature appears to be a regulating factor of sexual maturation and spawning. The exponential multiplicative functions for the spawning biomass translates the interannual fluctuations of the population fecundity.