Lakes and Watercourses

Fish
Introduction
Fish form an essential part of freshwater ecosystems and it is therefore
important to assess the status of fish communities and any changes in
them. A considerable amount is known about the life cycles and habitat
requirements of individual species, which means that the structure and
function of fish communities is also a useful tool for assessing whether
any changes have occurred in the habitat.
The assessments in this chapter are based on a large number of
biologically relevant parameters, weighed together to form an overall
index. All parameters indicate various kinds of change in the biota.
Hence, the overall index gives a mean indication of the status of the fish
community, whereas the individual parameters combine to form a profile
indicating the factors of particular importance to the status of the
community.
The system is to some extent founded on a tried and tested US system,
used to assess deviation from original status with the help of fish (Index
of Biotic Integrity – IBI). The original IBI and most subsequent applications
have classified impact in relation to unaffected references, which
requires good knowledge of the pristine fish fauna in each instance. An
approach of this kind is scarcely possible in Sweden. Although waters
directly affected by acidification, liming and point sources, for example,
can indeed be identified, it is not usually possible to distinguish waters
affected in other ways, eg, by changes in land use, introduction of nonnative
species etc. The reference values presented here therefore constitute
”typical” values for each parameter in relation to the two national
fish databases used, rather than values for ”pristine state”. The analyses
performed show that this approach is still well able to distinguish waters
displaying documented impact from evidently pristine waters.
Historical migrations of fish and interaction between species determine
their presence in lakes and watercourses. Regional adjustments
have therefore been made by describing parameters on the basis of local
conditions such as height above sea level and lake size. This approach
71