Spatial Analysis of Twaite Shad, Alosa fallax (Lacepède, 1803), in the Southern North Sea: Application of Non-Linear Geostatistics as a Tool to Search for Special Areas of Conservation

15 pages, 8 figures, 1 tableThis study aims to evaluate the suitability of non-linear geostatistics and indicator kriging (IK) as a tool in environmental impact assessment and nature conservation, in particular to search for potential Special Areas of Conservation (SAC) for the endangered fish species twaite shad, Alosa fallax (LACEPÈ-DE, 1803) within the German Exclusive Economical Zone (EEZ) of the North Sea. To analyse the spatial distribution of this fish species, data on standardised biomass index (catch per unit effort, c.p.u.e., kg × 30 min-1) from 1996 to 2001 were used, regarding the third and fourth quarters of each year, respectively. Thereby we assume that the spatial distribution can be described as a time-invariant process. This assumption is supported by information on annual sampling effort, allocation of hauls and spatial distribution of the positive catches. All indicator variograms obtained for different c.p.u.e. cut-off values displayed distinct spatial structures, clearly indicating that the indicator variables were spatially autocorrelated. Gaussian models were fitted by least-squares methods and were evaluated with a goodness-of-fit statistic. Subsequently, IK was employed to estimate the probability of exceeding the c.p.u.e. cut-off values for the twaite shad in the investigation area. These were highest in the Weser- and Elbeestuary, probably because of migrations of twaite shad to and from estuaries at the time of investigation due to spawning, while within the German EEZ of the North Sea no such areas with increased probabilities could be discerned. Thus, although available data did not allow to identify and implement any SAC in the German EEZ, the methods employed here can be regarded as a promising management tool in biological conservation issuesIt is part of the PhD thesis of V. STELZENMÜLLER at the Carl von Ossietzky Universität, Oldenburg, Germany, supported by the Scholarship Program of the German Federal Environmental Foundation (Deutsche Bundesstifung Umwelt, Osnabrück; No. 20002/290). The German Federal Agency for Nature Conservation (BfN, Bonn, FKZ: 802 85 230, UFOPLAN 2002) funded a project evaluating a first version of this modelling approachPeer Reviewe