The Nature of Scotland

NEWS
Positive and negative
Beaver activity can have both positive and negative effects on freshwater fish.
That’s the conclusion of the most thorough review ever undertaken of the effects
of beavers on freshwater fish.
The review was carried out for SNH by the University of Southampton. It’s
based on studying the scientific literature and surveying 45 fishery managers and
scientists, along with beaver ecology experts, mainly in North America where most
of the research has been carried out. The report discusses the implications of this
work for the European beaver in Scotland.
Among the negative effects are beaver dams, which can sometimes stop
fish moving up or down in narrower rivers and streams. There were also reports
of some spawning areas being lost immediately upstream of dams due to more
silt being present. These effects can change over time and the influence on fish
populations remains unclear.
Beavers can also have positive effects on fish. These include an increase in the
variety of habitats in streams and a greater area for the rearing and overwintering
of fish. There are also reports of an increase in invertebrates, such as insects, on
which fish feed. Beaver activity may also help to provide refuge during periods of
high or low water flows.
Overall, the positive effects were mentioned more frequently (157 times)
than the negative effects (102 times). In the survey of independent experts, the
responses to more than half (58%) of the questions suggested that the impact of
beavers on fish was positive, with 21% negative and the remainder neutral.
The report concludes that the negative impact of beavers on the movement
of fish and the availability of suitable spawning habitat can be at least offset by
the benefits. These include an increase in the variety of habitats and the resulting
increase in the abundance and productivity of fish, including salmon.
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The Nature of Scotland