Fish are comparatively long-living, mobile organisms which represents several trophic levels (levels in the food chain) and are dependent on various, different water habitats in the course of their development or their life cycle. Due to their distinctive habitat requirements, fish have been included in the EC-WFD as biological indicators for the structural variety of the surface bodies of water. High diversity of indigenous (local) species of fish typical for the body of water indicates the good ecological quality of that water, according to the EC-WFD, i.e. the intactness of a water ecosystem and thus also its value for species and biotope protection. It must be taken into account that only with the verification of natural reproduction is the existence of a population verified. As a rule, the proof of a high number of species of fish is the basis for a positive assessment, since this – provided it is not caused by stocking measures – indicates the existence of a variety of available habitats and resources, and hence of great structural variety.
The existence of stable populations of endangered species of fish is also the basis for a positive assessment. As a rule, they have the greatest demands upon their habitats, and therefore are affected most strongly by negative influences. As a result, the endangerment level of a species in the current Red Data Book is suitable as an indicator for the protection-worthiness of a habitat.
The stated information on ichthyologic water assessment was processed for cartographic representation as follows, analogously to the first complete Berlin Environmental Atlas:
For any body of water to be examined, the species of fish present and the population of the species can be directly taken from the map, and the proven species have been represented in color code,
a) by level of endangerment, according to the current Berlin Red Data Book and (cf. Tab. 1, Wolter et al. 2003);
b) by their relative frequency in the body of water.
In order not to overload the color design, species frequency was represented in three classes: rare species are those verified only irregularly and as individuals, after repeated sampling; while frequent species are those present at all fish samplings in greater quantities. The third group includes those species which were caught regularly, but only in relatively low numbers. Their stock was assessed as stable; their frequency as low or moderate.
Also retained was a body-of-water-type-specific assessment of the bodies of water examined, with regard to numbers of fish species. In the first version, nine types had been established (Wolter & Vilcinskas 1993), based on the factors genesis, surface-area, networking, type and continuity of water supply, and settlement possibilities for fish species, and since these have proven useful, they have been retained. They are:
- river lakes
- natural inland lakes
- artificial inland lakes
- retention basins
- sewage-treatment plant discharge channels
Artificial lakes and rain retention basins constitute an ichtheologically independent type of body of water, since their fish occurrences is at least initially, and as a rule continually, based on stocking, so that their fish associations reflect neither a settlement history nor a body-of-water specific population development.
Sewage-treatment-plant discharge channels and rain retention basins are the bodies of water, flowing and standing respectively, with the highest level of melioration. Moreover, the former differ from the other types by a water level which remains relatively constant over the course of the year, while comparable creeks and trenches regularly dry out.
The category of small bodies of water includes all standing ponds, tarns, meres, kettle-holes and the like with an area of up to one hectare. All other categories are self-explanatory.
From the current catch data, the mean average value for the number of indigenous (local) species of fish was calculated for every type of body of water. Neozoans were not taken into account for purposes of conservation and species protection, to prevent stocked, non-indigenous species of fish from compensating for a possible impairment-caused lack of local species. A deviation above or below by this mean average value by one fish species was allowed, and the resulting range was defined as the mean fish-species inventory of the respective type of body of water to be expected in Berlin. The type- specific range limits are shown in the map legend.
As explained above, a high number of fish species implies a variously structured, ecologically valuable habitat, so that a count of fish species above the type-specific average was judged as positive, and one below it as negative.
Since the average of the attested number of fish species was determined and assessed separately for each type of body of water, the map also contains e.g. positively assessed sewage-treatment-plant discharge channels, provided that they contain more species of fish than other ones. This apparent absurdity of a positive evaluation for an extremely degenerated body of water arises from the comparison of bodies of water exclusively within each type category.
On the one hand, this shows that the type-specific mean average value method selected is unsuitable as an instrument for the assessment of the overall ecological integrity of bodies of water or their ecological condition, in accordance with the EC-WFD. It would be really absurd to apply a mean average value in this context.
On the other hand, the ichthyologic comparison of bodies of water as implemented enables the development of body-of-water-type-specific ichthyo-ecological potentials, which are required for the implementation of the EC-WFD.
For artificial or anthropogenically degraded bodies of water, the Water Framework Directive requires the definition of the best ecological potential, i.e. the best fish species association attainable under the available conditions of use and quality of the given body of water. Using the optimum condition as the point of departure, a good ecological potential is to be attained by 2015.
This is the advantage of the average number of fish species per body-of-water-type model, as represented on the map. Within the category of artificial and degraded bodies of water (rain retention basins, sewage-treatment-plant discharge channels, canals and artificial lakes), positive references can identified which refer to the possible ichthyologic potential of the respective body-of-water type.
However, the implementation of the EC-WFD will require further examinations to underpin these species figures with information on dominance and age structures of fish species, and to develop reference cenoses.