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Annexe A<br />

_____________________________________________<br />

Maximum-likelihood is highly sensitive to outliers (Hilborn et Mangel 1997). To take into<br />

account that all observations are experimental measures, we implemented the Tukey’s<br />

biweight with the standard <strong>de</strong>viation between two mo<strong>de</strong>ls being multiply by a factor 3<br />

(Hilborn et Mangel 1997). This procedure limits the influence of outliers in the maximum<br />

likelihood search.<br />

We searched for the standard error on each parameter that is necessary to ren<strong>de</strong>r the fitted<br />

value of SD i . For this purpose, all the combinations of parameters are varied ±2 SE_p (SE_p<br />

indicates the standard error of parameter p). Using each combination, a y i vs. x i curve is<br />

calculated. The minimum (Min_y i ) and maximum (Max_y i ) values over all the parameter<br />

combinations are retained for each x i . The fit criteria is the weighted sum of squares over all n<br />

observations that permit to mo<strong>de</strong>l the best as possible the fitted standard <strong>de</strong>viation around<br />

each point:<br />

(( y −2 SD ) −( Min_ y )) ( y −2 SD ) −( Max_<br />

y<br />

i i i i i i<br />

)<br />

2 2<br />

( )<br />

n<br />

SSQ = ∑ i=<br />

1<br />

+<br />

y −2SD y −2SD<br />

i i i i<br />

5

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