Abstract
Fishing is often size selective such that the likelihood of capture increases with body size. It has therefore been postulated that fishing could favour evolution of slower growth because smaller size would reduce exposure to fishing gear (e.g. [Ricker 1981][1]). A recent study by Swain et al . ([
Highlights
LACK OF INTERCEPT IS NOT TRIVIALNone of the regression models considered by SSH included an intercept. The logic is that if the environment does not change and the selection differential is zero, DL4 will not change and the intercept should be zero
Efficiency with which it turns food into body mass
To check robustness of the results presented by SSH, we estimated a range of alternative models for sliding windows of 10 successive cohorts
Summary
None of the regression models considered by SSH included an intercept. The logic is that if the environment does not change and the selection differential is zero, DL4 will not change and the intercept should be zero. Hidden assumptions are that all relevant environmental variables are included, the data are unbiased and DL4 is genetically uncorrelated with other traits under selection. To test whether these assumptions hold, we added an intercept to SSH’s favoured model (DdCS ); it was significantly different from zero (K0.98; pZ0.03) and S became insignificant ( pZ0.34). A model with Dd and an intercept C has a lower AIC value than SSH’s favoured model, and S is not significant for any other combination of variables This challenges the conclusion of SSH that the selection differential S was driving the change in length-atage 4. We argue that a significant C suggests a negative component to change in length that cannot be statistically ascribed to any of the three explanatory variables
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