Fish are comparatively long-living, mobile organisms which represent 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 threatened species of fish is also grounds for a positive assessment. As a rule, they make the greatest demands upon their habitats, and therefore are affected most strongly by negative factors. As a result, the threat level of a species in the current Red Data Book is suitable as an indicator for the protection-worthiness of a habitat.
By contrast with previous editions, the 2014 Edition has undertaken no ichthyological assessment of the bodies of water based on the relationship between attested fish species and the average number of species per body of water type. However, the type assessment of the bodies of water and the numbers of fish species in each has been retained.
Also retained was the level of threat, according to the current Berlin Red Data Book (2013).
In the first version, nine types had been established (Wolter & Vilcinskas 1993), based on the factors genesis of the bodies of water, 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:
- streams
- river lakes
- natural inland lakes
- artificial inland lakes
- retention basins
- canals
- trenches
- sewage-treatment plant discharge channels
Artificial lakes and rain retention basins constitute an ichthyologically independent type of body of water, since their fish occurrences are at least initially, and as a rule continually, based on stocking, and thus 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.