Lakes and Watercourses

Class 1 in the overall index indicates that the fish fauna of the watercourse
consists of a large number of species with high diversity, many
fishes with a high proportion of salmonids having a high reproduction
level. A classification around class 3 indicates a watercourse close to the
median for Swedish watercourses. Class 5 indicates an ecosystem poor in
species and individuals and with an absence of salmonids.
Assessment of deviation from reference values
Note that deviation from reference values for lakes (Table 46) is not
calculated for the parameters ”Presence of species and stages sensitive to
acidification”, ”Proportion of tolerant fish species” and ”Proportion of
alien species”, the deviation can instead be obtained directly by comparing
the recorded value with the classes in table 46. In these cases the
value for class 1 is the result obtained in waters with no or insignificant
impact.
Class boundaries have been adjusted to existing parameters and background
data. With a few isolated exceptions, the 50th, 25th, 10th and 5th
percentiles have been used for one-sided parameters, which means that
50 per cent of the background data will fall within class 1. The 2nd, 5th,
10th and 25th percentiles and the 75th, 90th, 95th and 98th percentiles,
respectively, have been used for double-sided parameters. Here, too, 50
per cent of the background data falls within class 1.
Note that reference values for watercourses are only calculated for the
”Number of species” parameter. The parameters ”Weight/100 m 2 ” to
”Proportion of alien fish species” are calculated as described in the
footnote. In these cases the value for class 1 is the result obtained in
waters with no or insignificant impact.
Comments
The assessments must be based on fish surveys carried out in accordance
with the instructions given in the Swedish EPA Environmental Monitoring
Handbook. Hence, lakes having a surface area of less than 10 ha
should not be included. Assessment of current conditions and deviations
should also be made with a degree of insight. This is particularly so
where the absence of species and stages forms part of the assessment (eg,
proportion of species sensitive to acidification and proportion of alien
species). In exceptional cases, natural variations may lead to erroneous
interpretations. Assessment of lakes over 500 m above sea level should be
made with great caution owing to the absence of data on high altitude
lakes and lakes with predominantly salmonid assemblages. The background
data on watercourses in lowland areas is similarly limited. Help
with calculations may be obtained from the data host for fish (Institute of
Freshwater Research, National Board of Fisheries).
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