Measuring changes in the direction and magnitude of size-selective mortality in a commercial fish population

A 28-year time series (1971-1998) of backcalculated length-at-age was used to investigate changes in the direction and magnitude of size-selective mortality of prerecruit and adult Atlantic cod (Gadus morhua) in the southern Gulf of St. Lawrence, Canada. Size selection changed from favouring fast growth in the 1970s to favouring slow growth in the late 1980s and 1990s. There was an intervening period of disruptive selection where fast and slow growth was favoured while intermediate growth rates were selected against. The intensity of size selection declined at the end of the study period following the closure of the commercial fishery. These different forms of selection (positive directional, negative directional, and disruptive selection) can all be accounted for by the sharply dome-shaped curve of fishing mortality against length observed in the fishery.