Disentangling effects of size-selective mortality, density, and temperature on length-at-age of Atlantic cod (Gadus morhua) in the southern Gulf of St. Lawrence.

Dramatic changes have occurred in length-at-age of Atlantic cod in the southern Gulf of St. Lawrence over the past 30 yr. We assessed the relative contributions of variation in size-selective mortality, density, and temperature to these changes. We examined size-selective mortality using a 28-yr time series of backcalculated length-at-age. Striking changes in size selection occurred over the time series, from directional selection favouring large size in the early cohorts, through disruptive selection in intermediate cohorts, to directional selection favouring small size in late cohorts. We simultaneously tested effects of these changes in size selection and effects of density and temperature using a modified von Bertalanffy growth model in which each annual growth increment depended on indices of temperature, population density and size selection. The model was fit to observed population mean lengths-at-age for ages 5-11 yr. The backcalculated length at age 4 yr was included as a covariate in the model to account for variation in growth at earlier ages. The strongest effect on mean length-at-age was variation in size-selective mortality, followed by a negative effect of population density, and a weak positive effect of cod ambient temperature.