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.

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Data taken from the literature were used to develop and compare predictors of fish biomass and yield in lakes. Two new indices, total phosphorus concentration and macrobenthos biomass/mean depth, were the best univariate predictors offish yield (r2 = 0.84 and r2 = 0.48, respectively) and biomass (r2 = 0.75 and r2 = 0.83, respectively) for four different data sets. Both new indices were stronger predictors of fish yield when compared to the morphoedaphic index, total dissolved solids, or mean depth for the same data set. The relatively constant relationship between fish biomass and macrobenthos biomass/mean depth implies a near-constant energy transfer from the benthos to the fish regardless of the number of fish species present.Key words: biomass, yield, fish, macrobenthos, phosphorus, depth, dissolved solids, morphoedaphic index, lakes

We used data taken from the literature to develop and compare several estimators of crustacean zooplankton biomass (49 lakes) and profundal macrobenthos biomass (38 lakes). Both mean zooplankton biomass (r2 = 0.72, P < 0.001) and mean profundal macrobenthos biomass (r2 = 0.48, P < 0.001) correlated better with mean total phosphorus concentration than with Secchi depth, mean depth, maximum depth, or lake surface area. Mean total phosphorus concentration was also superior to mean chlorophyll a concentration (r2 = 0.57, P < 0.001) as an estimator of zooplankton biomass, but data were insufficient to evaluate chlorophyll a concentration as an estimator of macrobenthos biomass. Inclusion of maximum depth as a variable in a multiple regression resulted in a slight but significant (P < 0.030) improvement in the zooplankton–total phosphorus relationship (R2 = 0.75, P < 0.001). Inclusion of lake surface area as a variable in a multiple regression significantly (P < 0.001) improved the predictive power of the profundal macrobenthos–total phosphorus relationship (R2 = 0.59, P < 0.001).

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.