Interpretation of Monitoring Data - Aspects and Prospects

The present study estimates the decline in abundance of the American eel (Anguilla rostrata) using monitoring data series and contrasts these with the decline of the European eel (Anguilla anguilla). The declines are estimated using several different parametric and non-parametric statistics for both species. Irrespectively of the statistics used to estimate the declines, the study finds very strong declines for both species and where the European eel shows the stronger decline; as such these results reconfirm previous estimates. While the data sets demonstrate declines, there are differences in the estimates depending on the method used and which data subsets are included. These differences may be relevant as they may relate to a priori criteria set out by national and/or international conservation bodies. The study examines these differences and also provides estimates of the performances of the different methods using simulated data under a range of parameter settings. Finally, the study discusses the prospect for estimating future abundance levels and what is needed to identify a recovery of the species. One conclusion is that well-defined criteria for conservation actions should be agreed upon as early as possible, beforehand and not in hindsight.