Sensitivity of some biological reference points to shifts in exploitation patterns and inputs uncertainty for three North Sea demersal stocks

This study examines the sensitivity of six biological reference points (BRI's), both to variability and uncertainty of their input parameters, and to shifts in exploitation patterns. These shifts arise from accounting for the trends in catchability in extended survivors analysis (XSA) tuning fleets derived from the "hybrid method". By using non-parametric resampling and Monte Carlo stochastic procedures, we test whether such shifts in exploitation patterns significantly affect reference point values and probability distributions, for various levels of variation of input parameters. This method is applied to North Sea plaice, sole and cod stocks. We show that accounting for catchability trend generally induces only slight changes in reference point mean values. In particular, it is mostly significant over the last decade, especially for plaice, but not when introducing medium levels of uncertainty for the natural mortality. (C) 2002 Elsevier Science B.V. All rights reserved.