The ichthyological typology of rivers in the Ardenne region, Belgium
Quantitative data about fish populations from a homogeneous biogeographical environment, the Belgian Ardenne rivera, have been subjected to statistical analysis. This analysis was designed to determine which statistical method(s) could be used to identify ichthyological groups (second-ordre hierarchy). Electro-fishing data were available for 140 sites ; elimination of polluted stations left 98 sites, and of these 84 were used for the statistical analysis. Four matrices were constructed frome the raw data : absolute and relative species frequencies ; absolute and relative species biomasses. Correlation coefficients were used together with a hierarchical ranking method, based on chaining and principal component analysis (PCA). Methodological variations involved the selected data basic, selection of distance, species position criteria and chaining method. Application of PCA to the species frequency data yielded a primary axis (33 % explained variance) which separated suprarheophilic and limnophilic species. The second, third and fourth axes explained 14, 10 and 8 % of the variance, respectively. Similarly, PCA analysis of the biomass data yielded a primary axis (25 % explained variance) separating suprarheophilic and moderate-water species. The second, third and fourth axes explained 13, 10 and 8 % al the variance. Projection of the structure in the 1-2-3 numerical sub-space (57 % of the information) is very expressive. Hierarchical analysis of the proximity of different species led to the définition of the following socio-ecological groups : - Rheophilic supra brown trout, bullhead medio minnow, stone-loach infra grayling, gudgeon, chub - Limnophilic supra common nose, dace, barbet, roach medio tench (with reserves) Hierarchical analysis of the proximity between stations confirms, in the 2-4 space of the first PC analysis, these same divisions. In conclusion, the PC analysis of the relative occurrence of the various species at each station reveals the biogeographical structures and their relations. The structural ranking of the torrential waters depends on a single parameter, strongly related to the flow velocity (in relation to the bed slope). The structural ranking of the moderate to slow-moving waters is strongly related to a limnophilic parameter, likely associated with two other unknown parameters (to be determined). A typological level determination is proposed for the rhitron and epipotamon, as are the extension limits for the defined typological zones.