Many collapsed fish populations have failed to recover after a decade or more with little fishing. This may reflect evolutionary change in response to the highly selective mortality imposed by fisheries. Recent experimental work has demonstrated a rapid genetic change in growth rate in response to size-selective harvesting of laboratory fish populations. Here, we use a 30-year time-series of back-calculated lengths-at-age to test for a genetic response to size-selective mortality in the wild in a heavily exploited population of Atlantic cod (Gadus morhua). Controlling for the effects of density- and temperature-dependent growth, the change in mean length of 4-year-old cod between offspring and their parental cohorts was positively correlated with the estimated selection differential experienced by the parental cohorts between this age and spawning. This result supports the hypothesis that there have been genetic changes in growth in this population in response to size-selective fishing. Such changes may account for the continued small size-at-age in this population despite good conditions for growth and little fishing for over a decade. This study highlights the need for management regimes that take into account the evolutionary consequences of fishing.

In common with many fish stocks in the North Sea, cod are heavily exploited with as much as 60% of the fishable stock being removed annually. The International Council for the Exploration of the Sea (ICES), which advises fishery managers on the state of fish stocks in the northeast Atlantic, has recommended that exploitation rates be reduced considerably and immediately, in order to prevent further stock decline. We examine the most recent ICES assessment of the North Sea cod stock to see if the present exploitation regimen is sustainable. Our analysis suggests that not only is the present exploitation rate unsustainable, but that even regimens close to the maximum sustainable yield may be potentially prone to risk. There is a need for swift and effective action to protect the stock and avoid the problems of the much publicized collapse of cod stocks off the coast of Atlantic Canada

Excessive and unsustainable fishing mortality was the predominant factor in the depletion of Northwest Atlantic cod (Gadus morhua) stocks. However, despite imposition of severe catch restrictions for over a decade, stocks have mostly failed to recover at predicted rates. A number of explanations have been considered. Our analysis of demographic characteristics of 12 of these stocks indicates that recent productivity over the northern portion of the range is much lower than 20 years previous when several stocks recovered from less severe declines. Main contributing factors are, in rank order, increased natural mortality, decreased body growth, and in a few cases, reduced recruitment rates. Continued fishing in directed and bycatch fisheries is also an important factor. Under current conditions, we estimate negative or very low (<2% per year) average growth rates in eight stocks. If fishing ceases, growth rates of >5% would be expected in six stocks, with >10% in four of these. Although productivity is low, we conclude that fishing mortality is further delaying recovery.

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.

The relative importance of size-selective mortality, density-dependent growth, and temperature on growth of a commercial fish population was investigated using an integrated statistical analysis. Two indices of size-selective mortality were determined using otolith backcalculations. One index measured the direct effect on population mean growth increments in the year of the growth increment. The second index measured the cumulative effect on the growth potential of a cohort. Indices of population density, occupied temperature, and bottom temperature were developed from annual synoptic research vessel surveys of the population. We simultaneously tested effects of these factors using a modified von Bertalanffy growth model. The strongest effect was variation in size-selective mortality, followed by a negative effect of population density and a weak positive effect of occupied temperature. Effects of bottom temperature conditions were not significant. Failure to simultaneously consider alternative mechanisms, especially size-selective mortality, can lead to incorrect conclusions about the role of environmental factors in determining growth of fishes.